Category Archives: Evolution

Friday Fellow: Blue whale

by Piter Kehoma Boll

We’ve talked about the cutest and the leggiest, so now it’s time to introduce the largest, at once.

I think most of us know already that the largest animal ever is our beloved blue whale, Balaenoptera musculus. It can reach 30 m in length and weigh more than 180 tonnes. It’s really big, but probably not as big as many people think. There are some popular legends, like that the heart of a blue whale is the saze of a car or that a human could swim inside its aorta, which are not actually true.

It's almost impossible to find a good photo of the entire body of a blue whale. Afterall, it's huge and lives underwater!

It’s almost impossible to find a good photo of the entire body of a blue whale. Afterall, it’s huge and lives underwater!

But what else can we say about the blue whale? It is a rorqual, a name used to designate whales in the family Balaenopteridae and, as all of them, its main and almost exclusive food is krill, a small crustacean very abundant in all oceans. And krill needs to be abundant in order to provide the thousands of tonnes that all whales in the oceans need to eat every day. A single blue whale eats up to 40 million krill in a day, which equals to roughly 3.5 tonnes. A blue whale calf (young) is born measuring around 7 m in length and drinks around 500 liters of milk per day!

Blue whales were abundant in nearly all oceans until the beginning of the 20th century, when they started to be hunted and were almost extinct. Nowadays, the real population size is hard to estimate, but may encompass as few as 5,000 specimens, much less than the estimated hundreds of thousands in the 19th century. Due to such a drastic reduction in the population, the blue whale is currently listed as “endangered” in IUCN’s Red List.

But let's see a blue whale in all of its blueness.

But let’s see a blue whale in all of its blueness.

Occasionally, blue whales can hybridize with fin whales (Balaenoptera physalus) and perhaps even with humpback whales (Megaptera novaeangliae), a species classified in a different genus! Some recent genetic analyses, however, indicate that the Balaenoptera genus is polyphyletic and the blue whale may become known as Rorqualus musculus.

Different from other whales, blue whales usually live alone or in pairs, but never form groups, even though they may sometimes gather in places with high concentrations of food.

Like other cetaceans, especially other baleen whales, the blue whale sings. The song, however, is not as complex and dynamic as the ones produced by the related humpback whale. An intriguing fact that was recently discovered is that the frequency of the blue whale song is getting lower and lower at least since the 1960s. There is no good hypothesis to explain this phenomenon yet, but several ones have been proposed, such as the increase in background noise due to human activities or the increase in population density due to the decrease in whaling.

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References:

Hassanin, A.; Delsuc, F.; Ropiquet, A.; Hammer, C.; van Vuuren, B. J.; Matthee, C.; Ruiz-Garcia, M.; Catzeflis, F.; Areskoug, V.; Nguyen, T. T.; Couloux, A. 2012. Patter and timing of diversification of Cetartiodactyla (Mammalia, Laurasiatheria), as revealed by a comprehensive analysis of mitochondrial genomes.  Comptes Rendus Biologies, 335: 32-50.

Mellinger, D. K.; Clark, C. W. 2003. Blue whale (Balaenoptera musculus) sounds from the North Atlantic. Journal of the Acoustical Society of America, 114(2): 1108-1119.

Wikipedia. Blue whale. Available at: <https://en.wikipedia.org/wiki/Blue_whale&gt;. Access on January 27, 2016.

 

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The tegu lizard and the origin of warm-blooded animals

ResearchBlogging.org by Piter Kehoma Boll

Warm blood is the popular way to refer to endothermy, the ability that certain animals have to maintain a high body temperature by the use of heat generated via metabolism, especially in internal organs. Mammals and birds are the only extant groups in which all representatives are endothermic, but some fish also have this feature.

Tunna fish are truly endoothermic fish, similar to mammals and birds.

Tunna fish are truly endothermic fish, similar to mammals and birds. Photo by opencage.info**

In order to maintain a high body temperature, endothermic animals need a much higher amount of daily food than ectothermic animals (the ones that rely on environmental sources to adjust their body heat). There must be, therefore, a considerable advantage in endothermy to explain such a increased consumption of resources. The advantages include the ability to remain active in areas of low temperature and an increase in efficienty of enzimatic reactions, muscle contractions and molecular transmission across synapses.

The origin of endothermy is still a matter of debate and several hypothesis have been erected. The main ones are:

1. A migration from ectothermy to inertial homeothermy and finally endothermy.

According to this hypothesis, animals that were initially ectothermic grew in size, becoming inertially homeothermic, i.e., they retained a considerable constant internal body temperature due to the reduced surface area in relation to the their volume. Lately, selective pressures forced those animals to reduce in size, which made them unable to sustain a constant internal temperature and therefore their enzimatic, muscular and synaptic efficiency became threatened. As a result, they were forced to develop an alternative way to maintain a high body temperature and acquired it through endothermy.

Initially considered a plausible explanation due to the body size of the ancestors of mammals in fossil record, new phylogenetic interpretations caused a complete mix of large-bodied and small-bodied animals, so that currently fossils don’t support this idea anymore.

2. A large brain heating the body

The brain in endothermic species produces much more heat than any other organs. This led to the assumption that maybe a large brain generating heat was the responsible for the later development of full endothermy. However, evidence from both exant and extinct species point to the opposite. It seems more reasonable that a large brain evolved after endothermy and not the opposite.

3. A nocturnal life needs more heat

This idea states that the development of endothermy happened as a way to allow animals to be active during the night. The fact that most primitive mammals appear to have been nocturnal seems to support this hypothesis, but in fact many extant nocturnal mammals actually have a lower body temperature than diurnal mammals. Other aspect that counts against this hypothesis is that the ancestors of mammals already showed evidences of an increase in body temperature despite the fact that they most likely were not nocturnal.

4. Heat to help the embryos to develop

As you may know, in many ectothermic vertebrates, such as reptiles, eggs need to be incubated at a constant temperature in order to develop adequately. Endothermy, therefore, could have evolved as a way to allow parents to incubate the eggs themselves and have a higher control on temperature stability. One fact that support this theory is the dual role of thyroid hormones in reproduction and in the control of metabolic rate.

Endothermy may have evolved to incubate eggs at a constant temperature.

Endothermy may have evolved to incubate eggs at a constant temperature. Photo by Bruce Tuten**

5. Aerobic capicity leading to the heating of internal organs

According to this hypothesis, endothermy evolved after the increase of aerobic capacity, i.e., the first thing to happen was to increase the ability of muscles to consume oxygen in order to release energy, which helped the animal to move faster, among other things. This increased aerobic capicity was attained by increasing the number of mitochondria in muscle cells, which led to higher body temperature in the muscules and consequently a higher visceral temperature. Despite fossils indicating that mammal ancestors developed morphological adaptations indicating increased aerobic capacity, it is not possible to afirm that endothermy was not already present in those species.

Very recently, it has been found that the tegu lizards (Salvator merianae) from South America increase their body temperature during the reproductive season, achieving as much as 10°C above the environment temperature at night. Thus, it seems that they are able to increase heat production and heat conservation in ways similar to the ones used by fully endothermic animals.

The tegu lizard Salvator merianae is a facultative endotherm.

The tegu lizard Salvator merianae is a facultative endotherm. Photo by Jami Dwyer.

As such an increase in body temperature happens during the reproductive cycle, it supports the hypothesis of endothermy evolving to assist the development of embryos, as explained above. Also, it indicates that ectotherms may engage in temporary endothermy and perhaps permanent endothermy may have evolved by using this path.

Further studies on the tegu lizards are needed to clarify this interesting phenomenon and expand our knowledge on endothermy evolution in mammals and birds.

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References:

Kemp, T. (2006). The origin of mammalian endothermy: a paradigm for the evolution of complex biological structure Zoological Journal of the Linnean Society, 147 (4), 473-488 DOI: 10.1111/j.1096-3642.2006.00226.x

Tattersall, G., Leite, C., Sanders, C., Cadena, V., Andrade, D., Abe, A., & Milsom, W. (2016). Seasonal reproductive endothermy in tegu lizards Science Advances, 2 (1) DOI: 10.1126/sciadv.1500951

Wikipedia. Endotherm. Available at: <https://en.wikipedia.org/wiki/Endotherm&gt;. Access on February 1, 2016.

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Endosperm: the pivot of the sexual conflict in flowering plants

ResearchBlogging.org by Piter Kehoma Boll

The theory of sexual selection, based on the idea that there are conflict of interests between males and females, is quite recognized, but almost entirely focused on animals, especially dioecious animals, i.e., animals in which males and females correspond to separate individuals. Meanwhile, hermaphroditic animals and other organisms, such as plants, are usually ignored, but does hermaphroditism or “non-animalism” prevent the occurrence of sexual selection?

The peacock is one of the most famous examples of how sexual selection can drive the evolution of dioecious species. Photo by Oliver Pohlmann.

The peacock is one of the most famous examples of how sexual selection can drive the evolution of dioecious species. Photo by Oliver Pohlmann.

In the last decades, hermaphroditic animals started to be investigated more deeply concerning sexual conflict as a considerable evolutionary force in these organisms. For example, some studies demonstrated that many hermaphrodites, during copulation, fight to play the role of male, or female, in something called “gender conflict” (which DOES NOT HAVE ANYTHING TO DO with any social aspect of the word “gender”. Here it refers to the sexual role that a hermaphroditic organisms plays during sex).

In plants, on the other hand, the subject is much less explored, especially due to the lack of direct interaction between the two mating organisms. Reproductive strategies in plants were seen, for a long time, as a mean to ensure the supposedly difficult task to unite male and female gametes when one is a sessile organism, i.e., an organism unable to move. After all, this disadvantage forces these organisms to develop special techniques that guarantee the transport of gametes through the environment. With such a relevant problem to assure that sex will happen, it seems absurd to think that plants could yet afford to choose with whom to get laid.

737px-Bee_merry_(8199716283)

Plants need external agents, such as wind, water or animals, to carry their gametes. Photo by psyberartist (flickr.com/people/10175246@N08).*

So far, the most approached point about sexual selection in plants is related to mechanisms developed by the female part to avoid the ovule to be fertilized by pollen of the same individual (the so-called self-fertilization) or of incompatible individuals (such as pollen of another species or of a close relative, because yes, incest can be a taboo even for plants). Another studied mechanism is related to the prevention of future attempts of fertilization once the zygot has been formed, as an already fertilized flower is not interested in receiving more and more pollen grains.

The passive travel of pollen from the male part to the female one gives us the impression that the male part cannot carry out any intersexual selection. After all, once the pollen arrives at a flower, it cannot leave, so its only chance is to try fertilization in any case, even if it is on an incompatible organism. This also highlights the fact that competition between pollen grains may occur on the female part, on a real race to see who gets first to the ovule. This competition may be controlled by the female part by changings in pollen receptivity.

Pollen_competition

When a pollen grain reaches the female part of a flower, it has no option but to germinate, creating a pollen tube that grows towards the ovule. In this picture, three pollen tubes are running towards the ovule and one of them has a clear advantage over the others. It may be because it arrived first or because the female part changed its receptivity to accept this specific grain more eagerly than the others.

An intriguing aspect in angiosperm reproduction is the phenomenon of double fertilization. When a pollen grain falls onto the female organ, it germinates, originating a long tube that grows towards the ovule, the so-called pollen tube. The pollen tube carries with it two male gametes: one of them will fertilize the egg cell, giving rise to the zygote that will form the embryo, and the other fertilizes the central cell, an auxiliary cell that accompanies the egg, giving rise to a second zygot that forms the endosperm, a tissue that feeds the embryo during its development.

double_fertilization

In the double fertilization of angiosperms, the pollen tube carries two male gametes to the ovule. One of them will fertilize the egg cell, leading to the embryo, and the other will fertilize the central cell, originating the endosperm.

Since the egg and the central cell, as well as both male gametes, are genetically identical, the endosperm is also identical to the embryo and may be seen as an altruist that sacrifices itself to assure the survival of its sibling. The evolutionary origin of the endosperm and its adaptive advantage remain subjects of much discussion and without much solution. The situation is yet more complicated because, in most angiosperms, the endosperm is triploid, having a duplicate maternal material because the central cell has two nuclei. In other words, the endosperm has two copies of the maternal genes and one copy of the paternal genes (configuration 2m/1p), while the embryo is an ordinary organism, having one copy of the maternal genes and one copy of the paternal genes (configuration 1m/1p).

Several hypothesis on the reason that led to the rising of this selfless triploid sibling have been raised and are usually based on different interpretations on the sequence of the events that happened during the evolution of the group. Functionally, the endosperm works are the female gametophyte of other plants, which is, in these, responsible for nourishing the developing embryo. The female gametophyte is the “mother” of the embryo, just like the pollen grain (male gametophyte) is the “father”. The plants with the flowers are, therefore, the embryo’s grandparents. Crazy, isn’t it? But that’s the rule for plants. One generation of large organisms (the sporophyte), gives rise to a generation of tiny organisms (the gametophyte), which in turn will “mate” to generate new large organisms.

Going back to the subject, the functional similarity between the endosperm and the female gametophyte seems to favor the hypothesis that the endosperm was initially a maternal tissue (having, therefore, an original configuration 1m/0p or 2m/0p) and the paternal intromission happened later. On the other hand, the phenomenon of double fertilization is also found in Gnetales (supposedly the closest group to angiosperms) and, in these, double fertilization originates two identical embryos. In addition, basal angiosperms also have diploid endosperms, with a single copy of chromosomes from each parent (1m/1p). This scenario points to a primitive situation of two embryos, in which one of them was deviated to the role of endosperm.

Here we need to include one more important concept in biology: genome imprinting. It is a phenomemon in which genes are differently expressed depending on the parent from which they came; and it is usually seen are a consequence of sexual conflict. What happens is that paternal cells may be silenced in some cells, so that the organism expresses, in those cells, only features inherited through the mothers. The opposite may also happen.

It is assumed that, in angiosperms, the paternal side benefits from the production of large endosperms that provide more nutrients to the embryo, so that there is interest both to express genes leading to a higher accumulation of resources coming from the mother and to silence genes that limit growth. In contrast, the maternal side would attempt to limit the nutrients destined to a single endosperm, as the excess of investment would compromise its future reproductive success. It is better for the mother to invest a little in each endosperm than to invest everything in a single one. Therefore, the maternal side would express genes that control the amount of resources invested in each embryo while inhibiting genes inducing an increased growth.

In such a scenario with genome imprinting, the increased expression of genes by duplication may be seen as a female strategy to counterattack a male attempt to express genes responsible for resource allocation. The paternal plant would express genes for resource collection, while the maternal plant, with two copies of its material in the endosperm, would express genes leading to a contrary response in higher intensity, trying to stop the paternal influence. Such a phenomenon has been attested in corn seeds, where 2m/0p endosperms are smaller than 2m/1p endosperms. As we can see, there is a fight between males and females even among plants!

In angiosperms, fertilization involves the direct interaction of five distinct organisms belonging to three generations: female sporophyte (maternal plant), masculine gametophyte (pollen grain), female gametophyte (ovule), embryo and endosperm. Each one of these organisms has an interest that may be contrary to one or more interests of the others, leading to a complex interaction still poorly defined and in which the endosperm certainly constitutes the most intriguing point and may be the consequence of certain strategies and, at the same time, lead to the emergence of new ones.

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References and further reading:

Alcock J (2001) Animal Behavior, 7th edn. Sinauer Associates, Sunderland

Arnqvist G, Rowe L (2005) Sexual Conflict: Princeton University Press, Princeton, N. J

Baskin, J., & Baskin, C. (2015). Pollen (microgametophyte) competition: an assessment of its significance in the evolution of flowering plant diversity, with particular reference to seed germination Seed Science Research, 25 (01), 1-11 DOI: 10.1017/S0960258515000033

Beale, K., & Johnson, M. (2013). Speed dating, rejection, and finding the perfect mate: advice from flowering plants Current Opinion in Plant Biology, 16 (5), 590-597 DOI: 10.1016/j.pbi.2013.08.005

Becraft, P. (2012). Endosperm Imprinting: A Child Custody Battle? Current Biology, 22 (3) DOI: 10.1016/j.cub.2011.12.043

Cailleau, A., Cheptou, P., & Lenormand, T. (2009). Ploidy and the Evolution of Endosperm of Flowering Plants Genetics, 184 (2), 439-453 DOI: 10.1534/genetics.109.110833

Charnov EL (1979) Simultaneous hermaphroditism and sexual selection. PNAS 76:2480–2484.

Davies NB, Krebs JR, West SA (2012) An introduction to behavioural ecology, 4th edn. Wiley-Blackwell, Oxford

Dresselhaus, T., & Franklin-Tong, N. (2013). Male–Female Crosstalk during Pollen Germination, Tube Growth and Guidance, and Double Fertilization Molecular Plant, 6 (4), 1018-1036 DOI: 10.1093/mp/sst061

Fetscher AE (2001) Resolution of male-female conflict in an hermaphroditic flower. Proc R Soc B 268:525–529. doi: 10.1098/rspb.2000.1395

Friedman WE (1995) Organismal duplication, inclusive fitness theory, and altruism: understanding the evolution of endosperm and the angiosperm reproductive syndrome. PNAS 92:3913–3917. doi: 10.1073/pnas.92.9.3913

Friedman WE (1998) The evolution of double fertilization and endosperm: an “historical” perspective. Sex Plant Reprod 11:6–16. doi: 10.1007/s004970050114

Friedman WE (2001) Developmental and evolutionary hypotheses for the origin of double fertilization and endosperm. Comptes Rendus de l’Académie des Sciences – Series III – Sciences de la Vie 324:559–567. doi: 10.1016/S0764-4469(01)01326-9

Grossniklaus U, Spillane C, Page DR, Köhler C (2001) Genomic imprinting and seed development: endosperm formation with and without sex. Curr Opin Plant Biol 4:21–27. doi: 10.1016/S1369-5266(00)00130-8

Haig D, Westoby M (1989) Parent-Specific Gene Expression and the Triploid Endosperm. Am Nat 134:147–155.

Haig D, Westoby M (1991) Genomic Imprinting in Endosperm: Its Effect on Seed Development in Crosses between Species, and between Different Ploidies of the Same Species, and Its Implications for the Evolution of Apomixis. Phil Trans R Soc B 333:1–13. doi: 10.1098/rstb.1991.0057

Härdling R, Nilsson P (1999) Parent-Offspring and Sexual Conflicts in the Evolution of Angiosperm Seeds. Oikos 84:27–34. doi: 10.2307/3546863

Lankinen, A., & Madjidian, J. (2011). Enhancing pollen competition by delaying stigma receptivity: Pollen deposition schedules affect siring ability, paternal diversity, and seed production in Collinsia heterophylla (Plantaginaceae) American Journal of Botany, 98 (7), 1191-1200 DOI: 10.3732/ajb.1000510

Leonard JL (1990) The Hermaphrodite’s Dilemma. J Theor Biol 147:361–371. doi: 10.1016/S0022-5193(05)80493-X

Maruyama, D., Hamamura, Y., Takeuchi, H., Susaki, D., Nishimaki, M., Kurihara, D., Kasahara, R., & Higashiyama, T. (2013). Independent Control by Each Female Gamete Prevents the Attraction of Multiple Pollen Tubes Developmental Cell, 25 (3), 317-323 DOI: 10.1016/j.devcel.2013.03.013

Mazer SJ (1987) Maternal investment and male reproductive success in angiosperms: parent-offspring conflict or sexual selection? Biol J Linn Soc 30:115–133. doi: 10.1111/j.1095-8312.1987.tb00293.x

Prasad NG, Bedhomme S (2006) Sexual conflict in plants. J Genet 85:161.

Schärer, L., Janicke, T., & Ramm, S. (2015). Sexual Conflict in Hermaphrodites Cold Spring Harbor Perspectives in Biology, 7 (1) DOI: 10.1101/cshperspect.a017673

Spira TP, Snow AA, Whigham DF, Leak J (1992) Flower Visitation, Pollen Deposition, and Pollen-Tube Competition in Hibiscus moscheutos (Malvaceae). Am J Bot 79:428–433. doi: 10.2307/2445155

Winsor JA, Peretz S, Stephenson AG (2000) Pollen competition in a natural population of Cucurbita foetidissima (Cucurbitaceae). Am J Bot 87:527–532

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Biological fight: the case of artificial stimuli in behavior research

by Piter Kehoma Boll

ResearchBlogging.org The study of animal behavior is an important approach to understand several aspects on the ecology and the evolution of living beings, both from the analyzed animals themselves and the species with which they interact. For example, understanding how a bee recognizes a flower as a food source and how it approaches it may explain a lot about the physiology and the evolution of the flower and vice-versa, thus clarifying why such a combination of characters is the one that is found in the current population.

As with virtually any type of study in biology, a research may be done with sampling or experiments. By sampling you obtain non-manipulated information directly from the environment. You collect or observe a small sample of the whole and infer the general situation of the population based on it. On the other hand, in an experiment you manipulate the environment and watches how the organisms will react to the different stimuli presented to them and, from this, you develop your conclusion.

For example, if you want to know what a species of frog eats, you may find out by sampling, observing some frogs in the wild while they feed or capturing some and examining their stomach contents. You may also offer them different kinds of food, either in the environment or in the lab, and observe how the frogs reacts to each one.

Thus, in experiments you control the stimuli the species receives from the environment. This is the point where things start to get nasty. May the stimuli have artificial elements, i.e., elements that cannot be found by the animal in its habitat?

The opinions about it are divergent and recently led to a “formal fight” published in the journal Ethology:

On one side is a group of researchers from several universities around the world (Hauber et al., 2015) that defends the use of artificial stimuli to analyze behavior. They use as a model the studies on the rejection of eggs of parasitic birds by parasitized birds, a well-studied phenomenon.

First, let us contextualize this phenomenon briefly:

Several bird species, mainly cuckoos, do not incubate their own eggs. Instead of doing it, they lay them in the nests of birds of other species and hope that the poor creatures incubate and later feed the chicks as if they were their own. As a result, natural selection favors cuckoos whose eggs are more similar to the ones of the parasitized bird and also favors the parasitized birds that better distinguish their eggs from the ones of the intruders. It is a typical evolutionary race.

Find the intruder. The similarity between the egg of the parasite and the parasitized can vary greatly. Photos by wikipedia user Galawebdesign (left)* and by Grüner Flip (right).

Find the intruder. The similarity between the egg of the parasite and the parasitized can vary greatly. Photos by wikipedia user Galawebdesign (left)* and by Grüner Flip (right).

In experimental studies on egg rejection by parasitized birds, it is common to use artificial eggs that exaggerate features of natural eggs. This includes, for example, changing color and size in order to understand which is the most relevant for the bird to recognize the eggs as being yours or not. However, can we trust the results of such experiments using artificial elements?

Haubert et al. (2015) think that we can. Their arguments in favor of the use of such artificial stimuli are the following:

  1. Real eggs of the studied species are difficult to get in large quantities and could cause significant impacts over the populations if used. So, artificial eggs ensure the integrity of populations.
  2. It is difficult to get a set of natural eggs similar enough to allow the necessary repetitions to validate the test. After all, a result is only considered valid if it is recorded several times in face of the same stimulus. Artificial eggs allow identical copies and, thus, true repetitions.
  3. Natural eggs vary in several aspects at the same time, such as color, size, form, texture… In artificial eggs it is possible to control these aspects and allow only one to show free variation, so isolating the influence of each one during the recognition by the bird.
  4. A variation beyond the ones found in the wild may help to find populations with different degrees of perception of strange eggs and consequently where are the sites of higher selective pressure.
Original eggs of the parasitized species painted to exaggerate color features. Photos by István Zsoldos. Extracted from Moskát et al. 2010.

Original eggs of the parasitized species painted to exaggerate color features. Photos by István Zsoldos. Extracted from Moskát et al. 2010.

Not everyone looks so favorably to such an unrestrained use of artificial stimuli. Soon after the opinion of Hauber et al. we find the reply of David C. Lahti (2015) who faces all by himself the “artificialist” army. Lahti shows some aversion to such exaggerate use of artificial elements that many times are not used in a responsible manner.

Suggesting a more restrict use of artificial elements, he argues the following:

  1. Our perception of the environment is different from the one of the species we are studying. For instance, a bird sees a much wider range of colors than we do. When we paint an artificial egg black and white in order to simulate a natural black and white egg, we don’t know whether the bird really sees both eggs with the same colors. So, while we suppose that the eggs look similar by our perception, the reality from the bird’s point of view can be very different.
  2. When we try to create a set of artificial eggs that vary in only one aspect, such as the size of the spots on the shell, for instance, in order to control the influence of this stimulus only, we always end up including secondary stimuli that are not measured, such as the paint used to make the spots. If the birds shows a different response to eggs with small spots (natural ones) when compared to eggs with large spots (artificial ones), how can we know that the difference was not caused by the perception of the paint, either chemically or visually, by the animal? It would be necessary to perform tests that would discard this possibility, but it does not happen usually.
  3. Exaggerated artificial stimuli may go beyond the species’ range of recognition. An egg with a color too different from any color variation found in the environment could cause the bird not to see it as an egg, which would lead to problems in the interpretation of the results.

Concerning this last argument, Hauber et al. emphasize that is important to take care on a priori interpretations on the species behavior. That is to say, we cannot guess what the bird is thinking. The fact that the bird removes the parasite’s eggs from the nest or not does not mean that it is capable of recognize the egg as an intruder, or even as an egg. The way the bird interprets the stimulus is not as important as its response to it.

Therefore, we can conclude that artificial stimuli can be advantageous and in several circumstances they are the only available alternative. It is important, however, to take care with their use and try to be sure that secondary features, generally neglected, are not considered important by the animal.

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References:

Hauber, M.; Tong, L.; Bán, M.; Croston, R.; Grim, T.; Waterhouse, G.; Shawkey, M.; Barron, A.; & Moskát, C. 2015. The Value of Artificial Stimuli in Behavioral Research: Making the Case for Egg Rejection Studies in Avian Brood Parasitism Ethology, 121 (6), 521-528 DOI: 10.1111/eth.12359

Lahti, D. 2015. The Limits of Artificial Stimuli in Behavioral Research: The Umwelt Gamble Ethology, 121 (6), 529-537 DOI: 10.1111/eth.12361

Moskat, C.; Ban, M.; Szekely, T.; Komdeur, J.; Lucassen, R.; van Boheemen, L.; & Hauber, M. 2010. Discordancy or template-based recognition? Dissecting the cognitive basis of the rejection of foreign eggs in hosts of avian brood parasites Journal of Experimental Biology, 213 (11), 1976-1983 DOI: 10.1242/​jeb.040394

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Pooping to evolve: how feces allowed us to exist

by Piter Kehoma Boll

ResearchBlogging.orgBillions of years ago, when the first lifeforms appeared on Earth, our planet was very different from what it is today. Oxygen, so essential for our survival, was not present in the atmosphere.

Thanks to the appearance of the first photosynthetic bacteria, the so-called Cyanobacteria or blue-green algae, our atmosphere started to accumulate oxygen. As you may know, photosynthesis is a process by which plants and other photosynthetic organisms convert water and carbon dioxide into oxygen and organic compounds.

Oxygen is a very reactive element, so it can easily interact with other compounds and is great to burn organic matter to release energy. Without oxygen, heterotrophic life, such as animals, would not be able to use large quantities of energy and therefore would have never been able to achieve large size.

As you may also know, animals most likely appeared in the oceans and only much later conquered the land. However, oxygen produced by photosynthesis accumulates mainly in the atmosphere and not in the oceans. Today, only 1% of the global oxygen is found in the oceans, and it was even worse during the first million years of multicellular life. Do you know why?

The most primitive animals alive today are sponges, which are quite different from other animals. They usually have a hollow body with several pores, which function as tiny mouths through which water carrying small planktonic organisms and other organic matter is pulled inwards and later released by a large opening on the top of the body. So the main thing sponges do is mixing water and extracting a small amount of organic matter from the water column. Their feces, when returning to the water, are not very different in size from the organic matter they initially ingested.

Sponges ingest organic particles and release organic particles. They are not very efficient in removing organic matter from water.

Sponges ingest organic particles and release organic particles. They are not very efficient in removing organic matter from water.

Thus, in a sponge-only world, the water column was possibly always crowded with dissolved organic matter. This was a feast for bacteria, which are always eager to decompose organic matter and, while doing so, they consume large amounts of oxygen. Therefore, water with high amounts of organic matter increases bacterial activity and turns the environment anoxic, i.e., without oxygen. As a result, there was no oxygen available to allow animals to become large.

Despite not growing very much, animals were still evolving, of course, and eventually the bilaterian animals appeared. Bilaterian animals have a bilateral symmetry and, the most important feature in this story, a gut. It means they ingest food, digest it, process it and later eliminate the rests as… poop! In the gut, feces become compact as fecal pellets and sink much quickly to the bottom of the ocean, cleaning the water column from organic matter and drastically reducing bacterial activity. With no bacteria decomposing in the water column, the oxygen levels rapidly started to increase, allowing animals to grow and things like fish to evolve.

Bilaterian animals produce compact fecal pellets which sink to the bottom, cleaning the water column.

Bilaterian animals produce compact fecal pellets which sink to the bottom, cleaning the water column.

If animals had never started to poop, we most likely would have never been able to arise in this world. Long live the poop!

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References:

Holland, H. (2006). The oxygenation of the atmosphere and oceans. Philosophical Transactions of the Royal Society B: Biological Sciences, 361 (1470), 903-915 DOI: 10.1098/rstb.2006.1838

Turner, J. T. (2002). Zooplankton fecal pellets, marine snow and
sinking phytoplankton blooms. Aquatic Microbial Ecology, 27, 57-102

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Filed under Ecology, Evolution, Paleontology

Acoelomorpha: A Phylogenetic Headache

ResearchBlogging.orgby Piter Kehoma Boll

Take a look at these guys:

The green worm Symsagittifera roscoffensis (Graff, 1891). Photograph by Vincent Maran. Extracted from doris.ffessm.fr

Green worms Symsagittifera roscoffensis (Graff, 1891). Photograph by Vincent Maran. Extracted from doris.ffessm.fr

It’s a member of the group Acoelomorpha, animals which are still a puzzle in phylogeny. That means no one knows for sure where in the animals’ evolutionary tree they are exactly.

But first, let’s take a look about what makes an acoelomorph.

Those little guys are small worms, usually measuring less than 1 mm in length and living in marine or brackish waters or as symbionts. There are two groups of acoelomorphs: Acoela and Nemertodermatida. Acoels are the simplest ones; they have a mouth, but lack a gut, so that the food ingested goes directly to the internal tissues. In Nemertodermatida, there is a blind gut, i.e., with only one opening, like in the primitive cnidarians or in flatworms. In fact, they were initially classified as flatworms, but several features later challenged their position inside this phylum. The main differences are:

  • Acoelomorphs have an epidermis (“skin”) with cilia whose roots are interconnected in a hexagonal pattern, while other flatworms have independent cilia.
  • Acoelomorphs lack protonephridia (primitive kidney-like organs) and all other groups of animals have at least one of those or more complex organs with similar function.
  • While flatworms and all other protostomes (arthropods, annelids, mollusks, nematodes…) have ventral nerve cords and deuterostomes (chordates, echinoderms…) have dorsal ones, in acoelomorphs there are several nerve cords distributed radially along the body length.
Distribution of nerve chords in Acoelomorpha, Protostomia and Deuterostomia. Picture by myself, Piter K. Boll.

Distribution of nerve chords in Acoelomorpha, Protostomia and Deuterostomia. Picture by myself, Piter K. Boll.

Analyzing such features, it seems obvious that Acoelomorpha is a basal group of bilateral animals and may be the reminiscent of a primitive group of animals later almost completely extinct by their most complex descendants, the true protostomes and deuterostomes. The original radially-distributed nerve cords were simplified in dorsal or ventral ones in higher groups, but remained radial in Acoelomorpha.

Several phylogenetic studies indicate that Acoelomorpha is indeed a basal group of bilateral animals. They also lack several important Hox genes (responsible for determining body plan and organs’ distribution in animals) and it is quite unlikely that they would have lost most of them by secondary simplification.

Another group of simple animals, the Xenoturubellida, was sometimes proposed as a sister group for Acoelomorpha. Their proximity would be explained by several shared features, mainly the simple nervous system, the lack of a stomatogastric (mouth-gut) system, the structure of the epidermal cilia and the unusual fact that, in both groups, epidermal degenerated cells as resorbed in the gastrodermis.

A Xenoturbella worm. Photograph extracted from bioenv.gu.se/english/staff/Hiroaki_Nakano_eng/

A Xenoturbella worm. Photograph extracted from bioenv.gu.se/english/staff/ Hiroaki_Nakano_eng/

The group Xenoturbellida, however, has been placed in Deuterostomia in some molecular studies and recently Philippe et al. 2011 proposed that Acoelomorpha would also belong to the the Deuterostomia! But how could such a thing be possible when they obviously have primitive and unique features, like the radially placed nerve cords? The group’s explanation is that Acoelomorpha, as well as Xenoturbellida, have a sequence of microRNA (miR-103/107/2013) which is exclusive to Deuterostomia, so they would also be deuterostomes.

But wait a minute! What do they mean by “exclusive to Deuterostomia”? It means that that microRNA sequence is found in deuterostomes, but not in protostomes. Now think with me. We have 4 bilateral groups here: Acoelomorpha, Xenoturbellida, Deuterostomiaa and Protostomia. If you look at them this way, we can see that the statement “miR-103/107/2013 is exclusive to Deuterostomia” is false. The truth is that this sequence is absent in Protostomia, but present in all other groups. Wouldn’t it be more logical to think that, instead of deuterostomes acquiring this sequence, what really happened is that it was a primitive microRNA and protostomes have lost it?

If you consider Xenoturbellida and Acoelomorpha inside Deuterostomia, you have to assume that they passed through a huge simplification, and you maintain the radial nerve cords unexplained. Now if you think of them as primitive groups, the only thing necessary is to analyze protostomes as having lost a microRNA sequence. Quite a simpler explanation which doesn’t let open gaps.

Phylogenetic position of Acoelomorpha and Xenoturbellida according to Philippe et al. 2011. It means that (1) the bilaterians' ancestor had a complex set of Hox Genes; (2) miR-103/107/2013 appeared in an ancestor of true Deuterostomes+Xenacoelomorpha (Xenoturbellida + Acoelomorpha); (3) Xenacoelomorpha passed through a huge loss of Hox genes, loss of most internal organs and a mysterious set of radial nerve cords appear. Very complicated.

Phylogenetic position of Acoelomorpha and Xenoturbellida according to Philippe et al. 2011. It means that (1) the bilaterians’ ancestor had a complex set of Hox genes, being a complex animal; (2) miR-103/107/2013 appeared in an ancestor of true Deuterostomes+Xenacoelomorpha (Xenoturbellida + Acoelomorpha); (3) Xenacoelomorpha passed through a huge loss of Hox genes, loss of most internal organs and a mysterious set of radial nerve cords appear. Very complicated.

Position of Xenacoelomorpha as a basal group. It means that (1)

Phylogenetic position of Acoelomorpha and Xenoturbellida according to Boll et al. 2013 (that’s me!) based on a review of previous studies, as a basal group. It means that (1) the bilaterian’s ancestor was a simple animal, with a simple set of Hox genes, having miR-103/107/2013 and radial nerve cords; (2) the set of Hox genes became more complex and the nerve cords where simplified to either dorsal or ventral; (3) miR-103/107/2013 is lost in Protostomia. Way simpler.

You can read more in the references listed below.

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References:

Boll, P., Rossi, I., Amaral, S., Oliveira, S., Müller, E., Lemos, V., & Leal-Zanchet, A. (2013). Platyhelminthes ou apenas semelhantes a Platyhelminthes? Relações filogenéticas dos principais grupos de turbelários. Neotropical Biology and Conservation, 8 (1), 41-52 DOI: 10.4013/nbc.2013.81.06

Egger, B., Steinke, D., Tarui, H., De Mulder, K., Arendt, D., Borgonie, G., Funayama, N., Gschwentner, R., Hartenstein, V., Hobmayer, B., Hooge, M., Hrouda, M., Ishida, S., Kobayashi, C., Kuales, G., Nishimura, O., Pfister, D., Rieger, R., Salvenmoser, W., Smith, J., Technau, U., Tyler, S., Agata, K., Salzburger, W., & Ladurner, P. (2009). To Be or Not to Be a Flatworm: The Acoel Controversy. PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005502

Hejnol, A., Obst, M., Stamatakis, A., Ott, M., Rouse, G., Edgecombe, G., Martinez, P., Baguna, J., Bailly, X., Jondelius, U., Wiens, M., Muller, W., Seaver, E., Wheeler, W., Martindale, M., Giribet, G., & Dunn, C. (2009). Assessing the root of bilaterian animals with scalable phylogenomic methods. Proceedings of the Royal Society B: Biological Sciences, 276 (1677), 4261-4270 DOI: 10.1098/rspb.2009.0896

Moreno, E., Nadal, M., Baguñà, J., & Martínez, P. (2009). Tracking the origins of the bilaterian
patterning system: insights from the acoel flatworm. Evolution & Development, 11 (5), 574-581 DOI: 10.1111/j.1525-142X.2009.00363.x

Mwinyi, A., Bailly, X., Bourlat, S., Jondelius, U., Littlewood, D., & Podsiadlowski, L. (2010). The phylogenetic position of Acoela as revealed by the complete mitochondrial genome of Symsagittifera roscoffensis. BMC Evolutionary Biology, 10 (1) DOI: 10.1186/1471-2148-10-309

Philippe, H., Brinkmann, H., Copley, R., Moroz, L., Nakano, H., Poustka, A., Wallberg, A., Peterson, K., & Telford, M. (2011). Acoelomorph flatworms are deuterostomes related to Xenoturbella. Nature, 470 (7333), 255-258 DOI: 10.1038/nature09676

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Where did milk come from? The mysterious origin of lactation in mammals.

by Piter Kehoma Boll

ResearchBlogging.org In previous posts, I talked about two scientists who introduced “revolutionary” ideas to explain certain aspects in evolution, contradicting what other specialists use to say. But they come up with such unlikely explanations and use to ignore anything that says something contrary to their ideas that no one takes them seriously. You can read about them here and here.

Today, however, I will talk about another scientist who had interesting and innovative ideas, but much more likely and based on many more evidences. His name is Olav T. Oftedal and the topic is the origin of lactation.

As everybody knows, mammals are characterized mainly by the fact that females nourish their young through mammary secretions, the thing we know as “milk”. But how did this trace evolved in the mammalian lineage?

A cat (Nani) with her babies. Photo by Piter Kehoma Boll

A cat (Nani) with her babies. Photo by Piter Kehoma Boll*

Charles Darwin was one of the first ones to think of an explanation. Based on marsupials, Darwin concluded that mammals evolved from animals which carried eggs in a pouch and nourished them by cutaneous secretions. His idea was also inspired by seahorses, were eggs are carried in an abdominal pouch by the male and it was though that they might be nourished by cutaneous secretions too. However the subsequent discovery that the platypus, one the most primitive mammals, lays shelled eggs and have no pouch marked the ruin of Darwin’s theory on the origin of lactation. But… did it indeed?

In a paper published in 2002 in the Journal of Mammary Gland Biology and Neoplasia, Olav T. Oftendal raised some interesting questions and proposed an explanation to the origin of lactation very similar to the one proposed by Darwin one and a half century  before.

As it is known, mammals evolved from a lineage of amniotes called synapsids, from which they are the only extant group, so that everything about their ancestors must be assumed through fossil record.

Mammals’ ancestors are usually called “mammal-like reptiles”, but such a comparison is far away from what they really were. The animals we call reptiles – turtles, snakes, lizards and crocodiles – are sauropsids, so as birds, and they diverged from synapsids soon after the origin of amniotes and were probably very different creatures after some million years.

Let’s start thinking about what’s an amniote. The clade “Amniota” comprises all extant tetrapods other than amphibians, and they are characterized by the presence of a set of membranes surrounding the embryo which assisted in nutrient mobilization, gas exchange and storage of embryonic waste products, as well as in protecting it from the environment in a fluid-filled sac. But how were the eggs of those early amniotes?

When we think of an egg, we use to think of a bird’s egg, with a calcified shell, but this was probably not the case in early amniotes. The eggs of current egg-laying mammals lack a calcareous layer and it is likely that a calcified shell evolved only in the sauropsid lineage.

An ordinary chicken egg is the first think to come to mind for most people when they heard the word "egg". Photo by Sun Ladder, extracted from commons.wikimedia.org

An ordinary chicken egg is the first think to come to mind for most people when they heard the word “egg”. Photo by Sun Ladder*, extracted from commons.wikimedia.org

Eggs without a calcified shell rarely fossilize, and in fact there are no fossil eggs from Carboniferous, Permian and early Triassic, i.e., during all the history of non-mammalian synapsids.

But what’s so good in calcified shells? Well, a calcareous layer helps to prevent water loss from the embryo to the environment, whether to a reduction of relative humidity in the environment or a difference in temperature. Reptiles with non-calcified egg shells use to bury their eggs in damp soil where relative humidity approaches 100%, so dealing with the water loss problem. The increase of aridity from the end of the Permian to the Triassic may have favored the evolution of calcified eggs, but still there is no evidence of synapsid eggs. Could they have buried them in damp soil just as some reptiles do today? It’s possible, but…

Birds and mammals have developed a common feature: endothermy, i.e., “warm blood”. As we all know, birds use that to incubate eggs at elevated temperature, so speeding up the embryo development. It’s quite possible, and likely, that synapsids did the same. But as it was said, the difference in temperature between the egg and the environment also leads to water loss. In birds, the calcified shell solves this problem, but synapsids didn’t seem to have had this advantage.

Observing non-calcified eggs in snakes and lizards, it is possible to notice that they increase in mass by 10 to 100% after being laid. This happens due to water uptake from the environment and, as the shells is not rigid, the egg can also increase in volume, something that cannot happen in rigid-shelled eggs.

According to water uptake, eggs can be classified in endohydric and ectohydric. Endohydric eggs contain all water the embryo needs for the development when it is laid, while ectohydric eggs must take up water from the environment. In non-calcified eggs, it depends mainly of the eggs size. Large eggs may have enough water to sustain the embryo even with some water loss, since their surface:volume ratio is low, but the same cannot be applied to small eggs. And as it is known, the first mammals, and so their direct ancestors, were quite small creatures.

So what we have is an animal with non-calcified egg prone to dehydration and a high body temperature that can help to speed up the embryo development. How to join these two things?

The egg-laying mammal echidna and some marsupials have an abdominal pouch to retain eggs or newborns. It is known that the short-beaked echidnas incubate their eggs in the pouch. Female platypuses don’t have a pouch, but they incubate their eggs by curving their flat tail so that it touches the abdomen, forming a pouch-like enclosure. Inside this pouch, cutaneous secretions from the female’s abdomen would maintain the eggs moist, so preventing dehydration.

An egg of an echidna inside her pouch. (I found this picture on the internet without information about the author and stuff).

An egg of an echidna inside her pouch. (I found this picture on the internet without information about the author and stuff).

A problem in retaining eggs in a pouch is that it constrains the mobility of the parent. To solve this problem, eggs could stay some time inside the pouch to get hydrated and then be removed until it were necessary to rehydrate. This could let the female free from carrying the eggs along all the time and also increase the clutch size beyond the number of eggs that can fit in the pouch. This situation would have favored those females with more cutaneous secretions.

Once the young hatch from the eggs, they could also use the secretion as a source of food, which would lead to a progressive increase in the secretion’s nutritious value until it reached the status of milk.

It makes sense, doesn’t it? It does not contradict any kind of evidence about mammalian evolution and is based on true evidence through fossils and comparison with many kinds of extant groups.

– – –

References:

Oftedal, O. T. 2002. The origin of lactation as a water source for parchment-shelled eggs. Journal of mammary gland biology and neoplasia, 7 (3), 253-66 PMID: 12751890

Oftedal, O. T. 2002. The mammary gland and its origin during synapsid evolution. Journal of mammary gland biology and neoplasia, 7 (3), 225-52 PMID: 12751889

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