Category Archives: worms

The land planarian community of FLONA-SFP and how it gets along

by Piter Kehoma Boll

(First of all, I wish it were Bolsonaro, that piece of diarrhea-shaped cancer, who were dying by fire instead of the Amazon forest.)

(Now let’s go to the post itself:)

The São Francisco de Paula National Forest (FLONA-SFP) is a protected area for sustainable use in southern Brazil. Its was originally covered by Araucaria forest but currently is composed of a mosaic of the native forest and plantations of Araucaria, Pinus and Eucalyptus trees. This protection area is one of the main study areas of Unisinos’ Planarian Research Institute, where I conducted my undergradate, Master’s and PhD studies.

After studying the land planarian community of FLONA-SFP for many years, we conclude that it includes a fairly large number of species. Take a look at some of them and how cool they are:

Obama ladislavii, the Ladislau’s leaf-like flatworm. Photo by Piter Kehoma Boll.*
Obama anthropophila, the brown urban leaf-like flatworm. Photo by Piter Keehoma Boll.*
Obama josefi, the Josef’s leaf-like flatworm. Photo by Piter Kehoma Boll.*
Obama ficki, the Fick’s leaf-like flatworm. Photo by Piter Kehoma Boll.*
Obama maculipunctata, the spotted-and-dotted leaf-like flatworm. Photo by Piter Kehoma Boll.*
Cratera ochra. The ochre crater flatworm. Photo by Piter Kehoma Boll.*
Luteostriata arturi, the Artur’s yellow striped flatworm. Credits to Instituto de Pesquisas de Planárias, Unisinos.**
Luteostriata ceciliae, the Cecilia’s yellow striped flatworm. Photo by Piter Kehoma Boll.*
Luteostriata pseudoceciliae. The false Cecilia’s yellow striped flatworm. Credits to Instituto de Pesquisas de Planárias, Unisinos.**
Luteostriata ernesti, the Ernst’s yellow striped flatworm. Photo by Piter Kehoma Boll.*
Luteostriata graffi, the Graff’s yellow striped flatworm. Photo by Piter Kehoma Boll.*
Supramontana irritata, the irritated yellowish flatworm. Photo by Piter Kehoma Boll.*
Pasipha backesi, the Backes’ shiny flatworm. Photo by Piter Kehoma Boll.*
Pasipha brevilineata, the short-lined shiny flatworm. Photo by Piter Kehoma Boll.*
Matuxia tymbyra, the buried Tupi flatworm. Photo by Piter Kehoma Boll.*
Choeradoplana iheringi, the Ihering’s swollen-throated flatworm. Photo by Piter Kehoma Boll.*
Choeradoplana benyiai, the Benya’s swollen-throated flatworm. Photo by Piter Kehoma Boll.*
Choeradoplana minima, the lesser swollen-throated flatworm. Photo by Piter Kehoma Boll.*
Cephaloflexa araucariana, the Araucaria’s bent-headed flatworm. Photo by Piter Kehoma Boll.*
Paraba franciscana, the Franscican colored flatworm. Photo by Piter Kehoma Boll.*
Paraba rubidolineata, the red-lined colored flatworm. Credits to Instituto de Pesquisas de Planárias, Unisinos.**
Imbira guaiana, the Kaingang bark-strip flatworm. Photo by Piter Kehoma Boll.*

Land planarians live in the leaf litter of forest soils and prey on other invertebrates. The 22 species shown above are the ones found in FLONA-SFP that are formally described but there are still some awaiting description. We could say that there are at least 30 different species coexisting in this protected area.

How can they all persist together? Isn’t there any sort of competition for food? Thinking of that, I conducted my master’s research investigating the diet of those and other land planarians. My results suggest that, although some species share many food items, most of them have a preferred food or an exclusive food item that could be considered what Reynoldson and Pierce (1979) called a “food refuge”.

Here is what we know about the FLONA-SFP’s species until now:

  • Obama ficki feeds on slugs and snails and seems to prefer large slugs;
  • Obama ladislavii feeds on slugs and snails and seems to prefer snails;
  • Obama maculipunctata feeds on slugs and snails with unknown preference;
  • Obama anthropophila feeds on slugs, snails and other land planarians, especially of the genus Luteostriata, and prefers the latter;
  • Obama josefi apparently feeds on other land planarians only;
  • All species of Luteostriata feed exclusively on woodlice;
  • Species of Choeradoplana apparently feed on woodlice and harvestmen;
  • Cephaloflexa araucariana apparently feeds on harvestmen;
Obama ladislavii capturing a slug. Photo by Piter Kehoma Boll.*

The diet of the remaining species is still completely unknown but, based on other species of the same genera, it is likely that species of Pasipha feed on millipedes, species of Paraba feed on slugs and planarians, and Imbira guaiana feeds on earthworms.

Luteostriata ernesti near some juicy woodlice. Photo by Piter Kehoma Boll.*

There are plenty of different invertebrate groups that share the leaf litter with land planarians. Despite the apparently simple anatomy of these flatworms, they were able to adapt to feed on different types of prey and have muscular and pharyngeal adaptations for that. And attempt to relate anatomical adaptations to the diet of land planarians was part of my PhD research. As soon as it is published, I’ll make a post about it. There are some nice results!

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More on land planarians:

Friday Fellow: Abundant Yellow Striped Flatworm

Friday Fellow: Ladislau’s Flatworm

Darwin’s Planaria elegans: Hidden, extinct or misidentified?

How are little flatworms colored? A Geoplana vaginuloides analysis

Obama invades Europe: “Yes, we can!

The fabulous taxonomic adventure of the genus Geoplana

The hammerhead Flatworms: Once a mess, now even messier

The New Guinea flatworm visits France: a menace

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References:

Boll PK & Leal-Zanchet AM 2015. Predation on invasive land gastropods by a Neotropical land planarian. J. Nat. Hist. 49: 983–994.

Boll PK & Leal-Zanchet AM 2016. Preference for different prey allows the coexistence of several land planarians in areas of the Atlantic Forest. Zoology 119: 162–168.

Leal-Zanchet AM & Carbayo F 2000. Fauna de Planárias Terrestres (Platyhelminthes, Tricladida, Terricola) da Floresta Nacional de São Francisco de Paula, RS, Brasil: uma análise preliminar. Acta Biologica Leopoldensia 22: 19–25.

Oliveira SM, Boll PK, Baptista V dos A, & Leal-Zanchet AM 2014. Effects of pine invasion on land planarian communities in an area covered by Araucaria moist forest. Zool. Stud. 53: 19.

Reynoldson TB & Piearce B 1979. Predation on snails by three species of triclad and its bearing on the distribution of Planaria torva in Britain. Journal of Zoology 189: 459–484.

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*Creative Commons License This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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Loneliness made me female: more on the hermaphrodite’s dilemma

by Piter Kehoma Boll

Some time ago I wrote about the conflicts of hermaphrodite organisms while they have sex, i.e., how both could be the male and the female at the same time, but that is usually not of their interest, especially if playing the female role would force you to end up with low-quality sperm for your eggs.

Two banana slugs ready to copulate. Photo by Andy Goryachev.

But sex is usually much more complex and how it occurs is usually shaped by environmental conditions, especially by the presence of competitors. In dioecious species, males usually compete for the females but is there a similar behavior applied to hermaphrodites?

According to the sex allocation theory, hermaphrodite organisms have to choose how much they invest in the male versus the female function. If you produce more eggs, thus preferring the female side, you end up producing less sperm and vice versa. So what should hermaphrodites do?

Sometimes there is competition. Photo by Wikimedia user Miekemuis.*

One way to try to take the best of this situation is allocating resources to the female or the male role according to what will give you more advantages in the current scenario. This would be determined mainly by the number, or the density, of individuals in the population.

When there are a lot of individuals, there is a lot of sperm, and being able to fertilize the eggs becomes more difficult. Thus, hermaphrodites would increase their investment in the male function to have a better chance against the sperm of the others. In other words, it is better to be a male when there are too many guys around you.

On the other hand, when finding a mate is rare, there is little sperm competition, so focusing on being a female is more advantageous. After all, the little sperm you produce is enough to fertilize the eggs of the few other individuals around there.

Most studies looking for evidence of the sex allocation theory found conflicting results. In many organisms, only one of the sexual functions changes according to population density, with either the number of eggs or the amount of sperm remaining the same and sometimes both functions are enhanced at the same time, going against the idea of a trade-off that the sex allocation theory predicts.

The problem may be simply a matter of how to look at things. Most studies focused on gamete production only, but sex is much more than that. One important part that has been neglected is sexual behavior. In order to test whether behavioral investment may show sex allocation differences, a recent study compared the investment of the hermaphrodite polychaete worm Ophryotrocha diadema in a female-related and a male-related behavior. According to their hypothesis, a low density of organisms would increase parental care, a female-related behavior, while a high density of organisms would increase motility in order to find a mate, a male-related behavior.

Two individuals of Ophryotrocha diadema. The yellow marks on the upper one are eggs. Photo by Viriginie Boutias. Extracted from http://leec.univ-paris13.fr/new/animals_en.html

And their hypothesis proved to be correct! Worms kept in pairs, i.e., with few mating opportunities due to the low density of individuals, moved less but took more care of the eggs. On the other hand, worms kept in groups, i.e., with more mating opportunities, moved more and did not take so much care of their eggs.

More than being nice evidence for the sex allocation theory, this study highlights the need to look beyond gamete production to assess sex allocation not only in hermaphrodites but in all organisms.

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Reference:

Picchi L & Lorenzi MC 2019. Gender-related behaviors: evidence for a trade-off between sexual functions in a hermaphrodite. Behav Ecol. doi: 10.1093/beheco/arz014

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Friday Fellow: Common Peanut Worm

by Piter Kehoma Boll

Leia em português

Today our fellow is a peculiar marine animal that is also a common food in China and Vietnam. Named Sipunculus nudus, or the common peanut worm, it is a member of the clade Sipuncula, usually called peanut worms.

A dead specimen of Sipunculus nudus found on the Mediterranean coast of France. Photo by Benoit Nabholz.*

As other peanut worms, the common peanut worm has considerably simple anatomy. Its body is consistent of basically two parts, a sac-like trunk and a proboscis, also called the introvert. The introvert is a retractile structure and, when the animal is not feeding, is pulled inside the trunk by a group of muscles. At the end of the introvert, when everted, there is a series of tentacles that takes the food, composed of detritus, into the gut.

The common peanut worm is commonly found burrowed into the substrate in intertidal waters all around the world, with its mouth directed upward. They may reach about 20 cm in length when the introvert is everted, with about 1/4 of this length being composed by the trunk.

As mentioned above, the common peanut worm is used as a food in China, especially in southern regions, and Vietnam. Although the species seems easy to be raised in captivity, currently most, if not all, harvest happens in the wild, which may lead to overexploitation and eventually a serious decrease in the populations.

A bucket full of peanut worms for sale in China. Photo by Wikimedia user Vmenkov.**

Molecular analyses have revealed that, contrary to what is currently considered, Sipunculus nudus is not actually a cosmopolitan species. There are at least four clearly distinct lineages that certainly correspond to four distinct species. Of those, only one is found in waters around Europe, from which the species was originally described. The other three lineages correspond to those found in China and Vietnam (and the one used as food), the Atlantic Coast of the Americas (from Brazil to the USA) and the Pacific Coast of the Americas (around Panama). Let’s hope that soon this taxonomic problem will be solved.

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References:

Du, X., Chen, Z., Deng, Y., Wang, Q. (2009) Comparative analysis of genetic diversity and population structure of Sipunculus nudus as revealed by mitochondrial COI sequences. Biochemical Genetics 47: 884. doi: 10.1007/s10528-009-9291-x

Kawauchi, G. Y., Giribet, G. (2013) Sipunculus nudus Linnaeus, 1766 (Sipuncula): cosmopolitan or a group of pseudo-cryptic species? An integrated molecular and morphological approach. Marine Ecology 35(4): 478–491. doi: 10.1111/maec.12104

Trueman, E. R., & Foster-Smith, R. L. (2009). The mechanism of burrowing of Sipunculus nudus. Journal of Zoology, 179(3), 373–386. doi:10.1111/j.1469-7998.1976.tb02301.x

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Think of the worms, not only of the whales, or: how a planarian saved an ecosystem

by Piter Kehoma Boll

Leia em português

Due to the massive interference of human practices on natural habitats during the past decades, ecosystem restoration has become a trend in order to try to save what is still savable. Unfortunately, the effort of ecologists and other experts alone is not enough to achieve that, and a larger section of the society needs to be engaged in helping reach the goals. In order to do so, it is common to appeal to the beauty and cuteness of endangered species, which usually include mammals and birds, since they are more likely to caught the public’s attention. However, most of the endangered species are invertebrates or other less charismatic beings, and they are often ignored even by biologists.

Hopefully, things are able to change on this matter. Recently the first ecosystem restoration directed to save an invertebrate was successful, and I am here to tell you about it.

The invertebrate in question is a freshwater planarian named Dendrocoelum italicum. It was discovered in 1936 in a cave in northern Italy named Bus del Budrio. Inside the cave, there was a small freshwater pool, about 5 × 5 m or little more, caused by a waterfall from a small stream coming through a narrow elevated corridor. The species is apparently found only in this pool and nowhere else.

There are no available photos of Dendrocoelum italicum, but it should look similar to the widespread Dendrocoelum lacteum seen here, but D. italicum lacks the eyes. Photo by Eduard Solà.*

During the 1980’s, a pipe was installed to divert the water from the stream to a nearby farm. The waterfall ceased to exist and the pool dried up permanently. The planarian survived in a very narrow rivulet that formed inside the cave and some small isolated ponds resulting from water drips. This critical condition of the population was discovered in 2016 by a research group from the University of Milan. They informed the administrators of the cave about the situation and, together, the team started to raise awareness about the situation of the cave among the citizens that benefitted from the reservoir formed by the diverted water, which made the farmer responsible for diverting the water agree to remove the artificial structure.

Image of the inside of the cave. Photo by Livio Mola. Extracted from https://www.naturamediterraneo.com/forum/topic.asp?TOPIC_ID=57050

The removal happened on December 3, 2016 after all the planarians occurring in the rivulet were collected and stored in plastic tanks inside the cave. When the waterfall was restored, it quickly started to fill the old pool again and, one day later, the planarians were released into the pool.

The ecosystem was monitored during the following two years until January 2018. The number of planarians varied greatly during the survey, but was not significantly larger after the restoration from what it was before. However, there was a significant increase in the population of a bivalve species, Pisidium personatum, and a small increase in the population of a crustaceon of the genus Niphargus. Additionally, annelids of the family Haplotaxidae, that were absent in the cave, appeared after restoration. Thus, it is clear that the ecosystem benefited from the reappearance of the pool.

Thanks to the efforts of those researchers, Dendrocoelum italicum now has a better chance to avoid extinction. However, this is not an isolated case. There are many cave-dwelling planarian species all around the world living under similar conditions, usually restricted to a single small pool inside a single cave. Many of those occur, or occurred, as D. italicum, in Italy, but the help came to late for some of them. For example, a closely related species, Dendrocoelum beauchampi, was discovered in 1950 in a cave in northwestern Italy named Grotta di Cavassola, but a recent survey found no planarians inside the cave, which seems to have suffered great alteration due to human activities. Similarly, the species Dendrocoelum benazzi was discovered in 1971 in central Italy in a cave named Grotta di Stiffe, but nowadays, with the cave open to turists and its water polluted, the planarians disappeared. It is very likely that both D. beauchampi and D. benazzi are now extinct. The situation is the same for other Italian species.

Out of Italy, a recently described species living a similar small environment is the Brazilian cave planarian Girardia multidiverticulata, which is known to occur in a small pool about 10 m² inside a cave named Buraco do Bicho in the Cerrado Biome.

Girardia multidiverticulata is a planarian species restricted a small 10 m² pool inside a cave in Brazilian cerrado. Credits to Souza et al. (2015)**

The case of Dendrocoelum italicum shows us it is possible to save small endemic populations of threatened habitats, but we need the help of the public. Let’s hope other ecosystems have a similar happy ending.

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References:

Manenti R, Barzaghi B, Lana E, Stocchino GA, Manconi R, & Lunghi E 2018. The stenoendemic cave-dwelling planarians (Platyhelminthes, Tricladida) of the Italian Alps and Apennines: conservation issues. Journal for Nature Conservation.

Manenti R, Barzaghi B, Tonni G, Ficetola GF, & Melotto A 2018. Even worms matter: cave habitat restoration for a planarian species increased environmental suitability but not abundance. Oryx: 1–6.

Souza ST, Morais ALN, Cordeiro LM, & Leal-Zanchet AM 2015. The first troglobitic species of freshwater flatworm of the suborder Continenticola (Platyhelminthes) from South America. Zookeys 470: 1–16.

Vialli PM 1937. Una nuova specie di Dendrocoelum delle Grotte Bresciane. Bollettino di zoologia 8: 179–187.

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Friday Fellow: Rowell’s Velvet Worm

by Piter Kehoma Boll

Velvet worms form an intriguing group of animals that are the sister group of arthropods and also the only animal phylum with only terrestrial species, although aquatic species are known from fossil records.

Today I decided to bring one velvet worm species to be our fellow. Scientifically known as Euperipatoides rowelli, I decided to give it the common name Rowell’s velvet worm.

61533_orig

A specimen of the Rowell’s velvet worm in the lab. Photo by Alan Couch.*

The Rowell’s velvet worm is found in south-east Australia inhabiting humid, temperate forests. They are small animals, with about 5 cm in lenght, and live in decaying wood, dwelling in crevices and feeding on small invertebrates, such as termites and crickets.

Logs are usually inhabited by groups of several individuals that live in a sort of social relationship and are composed of females, males and juveniles, with females being larger and occurring in larger numbers than males. A sort of hierarchical organization also seems to occur, with one female being dominant and followed in dominance by other females, with males and juveniles occupying the bottom of the pyramid. Prey capture often happens in group, and after a prey is subdued, the dominant female will eat first and only after being satiated she will allow other females to eat. Males and juveniles eat the remains left by the females.

26038_orig

Welcome to our log! Photo by Andras Keszei.**

New logs are colonized by wandering males. Those release feromones that attract more males and later females. Thus, newly colonized logs have a male-biased aggregation, but the number of females later surpasses that of males. It has been suggested that the initial aggregation of males helps them to attract females due to the increased concentration of feromones.

During reproduction, the male places spermatophores on the skin of the female, With the aid of the female blood cells, the body wall below the spermatophore is breeched and the sperm is released in the female’s body cavity, where it swims to the female reproductive tract.

Due to its abundance in south-east Australia, the Rowell’s velvet worm is an easily obtained species and is slowly becoming one more interesting model organism.

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References:

Barclay S, Ash JE, Rowell DM (2000) Environmental factors influencing the presence and abundance of a log-dwelling invertebrate, Euperipatoides rowelli (Onychophora: Peripatopsidae)Journal of Zoology 250: 425–436.

Barclay S, Rowell DM, Ash Je (2000) Pheromonally mediated colonization patterns in the velvet worm Euperipatoides rowelli (Onychophora)Journal of Zoology 250: 437–446.

Reinhardt J, Rowell DM (2006) Social behavior in an Australian velvet worm, Euperipatoides rowelli (Onychophora: Peripatopsidae)Journal of Zoology 250: 1–7.

Sunnucks P, Curach NC, Young A, French J, Cameron R, Briscoe DA, Tait NN (2000) Reproductive biology of the onychophoran Euperipatoides rowelliJournal of Zoology 250: 447–460.

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Friday Fellow: Wood Cricket’s Worm

by Piter Kehoma Boll

Last week I introduced the small wood cricket, so I will use it as an oportunity to introduce, today, one of its parasites, the Wood Cricket’s Worm Paragordius tricuspidatus.

800px-paragordius_tricuspidatus

Two individuals of the wood cricket’s worm. Photo by D. Andreas Schmidt-Rhaesa.*

The wood cricket’s worm is a member of the phylum Nematomorpha, commonly known as horsehair worms. The adults are free-living worms that inhabit freshwater bodies, especially rivers and streams and have a peculiar mating behavior in which many worms are “tied” to each other in a large knot, like a worm orgy. After mating is finished, the female lays its eggs at the edge of the water, on the ground, where they may eventually be ingested by wood crickets living nearby.

Inside the cricket, the egg hatches and the larvae starts to develop inside the cricket’s body cavity, filling it completely during its development. When the worm is ready to leave its host, it is able to control the host’s behavior, inducing it to jump into a water body, allowing the parasite to leave the cricket and go looking for a partner to mate, starting the cycle again.

Paragordius_tricuspidatus

Paragordius tricuspidatus (arrow) leaving the body of a wood cricket. Photo extracted from Ponton et al. (2006) (See references).

An interesting behavior of the wood cricket’s worm is its ability to escape from the body of a predator. Usually when a wood cricket jumps into the water and the worm is trying to leave the host, an aquatic predator, such as a fish or a frog, may end up eating the cricket, which would put an end to the life of the parasitic worm as well. Recently, however, it has been found that the worm is able to escape the predator’s body, usually through the mouth, when the cricket is eaten. This is the first known case of a parasite escaping a predator of its hosts.

We have to accept that parasitic worms have very adventurous lives.

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References:

Thomas, F.; Ulitsky, P.; Augier, R.; Dusticier, N.; Samuel, D.; Strambi, C.; Biron, D. G.; Cayre, M. (2003) Biochemical and histological changes in the brain of the cricket Nemobius sylvestris infected by the manipulative parasite Paragordius tricuspidatus (Nematomorpha)International Journal of Parasitology 33: 435–443.

Ponton, F.; Lebarbechon, C.; Lefèvre, T.; Biron, D. G.; Duneau, D.; Hughes, D. P.; Thomas, F. (2006) Parasite survives predation on its hostNature 440: 756.

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Male resistance: when females disappear and hermaphrodites don’t like you

by Piter Kehoma Boll

During the evolution of life, sex was certainly a great innovation. It allows organisms to reproduce while mixing their genes with that of another individual. Although it usually makes your offspring to have only half of your genes, which does not seem to be as great as an offspring that carries you as a whole into the next generation, there are certainly advantages in mixing. The most evident advantage is that your genes can combine with other versions and, as a result, produce a better team of genes than the one that you had. Even though each of your children carries only half of you, that half is more likely to survive than a child that carries you as a whole. In other words, sex gives the possibility for a population of genes (those that make up an individual) to get rid of some of the less efficient ones and replace them with better copies.

As you know, most sexual organisms make such a recombination by fusing two sexual cells, the gametes, and those are usually of two different kinds: a small one (the male) and a large one (the female).

In some species, each individual can only produce either male or female gametes, therefore being either a male organism or a female organism. In such species, sexual reproduction requires a male to mate with a female. This is the pattern found, for example, in most vertebrates and arthropods.

lossy-page1-799px-19_ixodes_ricinus_pareni-tif

A female (large) and a male (small) of the tick Ixodes ricinus mating. Image by Jana Bulantová.*

In other species, each individual can produce both male and female gametes, therefore being called a hermaphrodite. The advantage of such a system is that hermaphrodites can mate with any individual of their species, sometimes even with themselves! One of the main problems with hermaphroditism is when you decide to play only one role, which may lead to conflict during sex.

800px-weinbergschnecke_paarung

Two snails Helix pomatia (hermaphrodites) making love. Photo by Wikimedia user Jangle1969.**

Now what evolved first? Dioecious species (those having male and female individuals) or hermaphrodites (allso called monoecious species)? It’s hard to tell, but we can be sure that during evolution many lineages switched from one system to the other and back. And the coolest part is that such switches still happen today.

You may know that most flowering plants are hermaphrodites. Flowers usually have both male and female organs, although they are rarely able to fertilize themselves (self-fertilization). Among plants, the cases of dioecious species seem to be mainly due to some mutation that ended up partially sterilizing an individual. For example, a mutation could appear that makes the plant unable to produce male organs, thus becoming only female. Other individuals in the population that lack this mutation continue to be hermaphrodites, so we have an “unbalanced” species with two sexes, females and hermaphrodites, but no males. Although unusual at first, such a system can remain stable if reproduction occurs through cross-fertilization and not self-fertilization. As both females and hermaphrodites need pollen (which produces the male gametes) from other plants, they can coexist as long as the pollinator carries pollen to both sexes. The same happens if the sexes are male and hermaphrodite. As long as the pollinator carries the male’s pollen to hermaphrodite flowers, both sexes can do just fine.

584px-geranium_sylvaticum_enbla02

The plant Geranium sylvaticum includes hermaphrodites and females, but no males. Photo by Enrico Blasutto.**

Species composed of males and hermaphrodites are called androdioecious (from Greek andro-, man, male + di-, two + oikos, home, house; therefore “male in two “houses”, i.e., in two different kinds of organisms), while those composed of females and hermaphrodites are called gynodioecious (from Greek gyno-, woman, female; therefore “female in two different kinds of organisms).

Androdioecious and gynodioecious species occur among animals as well, but in this case their existance indicates something happening in the other direction, i.e., it is a transition from a dioecious species (with males and females) to a hermaphrodite species. And this is much more complicated that the other way round. Actually, it can get really, really bad for the “single-sex sex”.

This unbalanced sexual system in animals usually happens like this. There is a happily dioecious species with male and female individuals, but one day a new mutation appears and allows one of the sexes to produce both male and female gametes, thus becoming an hermaphrodite. However, such hermaphrodites are usually unable to play the role of the new sex while mating, i.e., they have the gametes, but not the tool to mate using them. Thus, the only way to use both gametes is to fertilize themselves.

One problem that comes from doing that is inbreeding. When you fertilize yourself, you are not increasing genetic diversity. On the contrary, you have very high chances of producing offspring with two copies to the same gene, thus decreasing genetic diversity. In order to continue to have recombination, you must mate with the single-sex individuals, which means you can only play the role of your original sex and your hermaphroditism is irrelevant. You are producing useless gametes. Or are you?

maleintrofig3b

A male and a hermaphrodite of the nematode Caenorhabditis elegans an androdioecious species. Credit to Worm Atlas.

The problem with inbreeding happens when an organism ends up with two copies of a deleterious gene, which is fairly common in species where cross-fertilization is the rule and such deleterious genes are maintained in the population through individuals with a single copy that is not enough to cause any trouble. That is why having kids with your parents, children of siblings is usually a bad idea. When a species evolves from a system of cross-fertilization to one of self-fertilization, inbreeding can be a serious problem at first, producing many descendants that will die soon. However, eventually this will “purge” the set of genes. If individuals only mate with themselves, the number of deleterious genes will sharply decrease after some generations and inbreeding will not be such a big problem anymore.

When this happens in a species with unbalanced sex, the single-sex individuals will be in trouble. Two androdioecious animals have been studied regarding this conflict, the nematode and model organism Caenorhabditis elegans and clam shrimps of the genus Eulimnadia, such as Eulimnadia texana. In both groups, the hermaphrodites do not seem to be very interested in mating with males. They have even lost most phenotypic clues that help males identify them as potential mates. The only thing left for the males is to insist, to look for hermaphrodites and force them to mate with them, but it is a hard battle. Even when mating does occur, the hermaphrodite usually discards the male’s sperm.

drylakeclam

A hermaphrodite (left) and a male (right) of the clam shrimp Eulimnadia texana. Credits to arizonafairyshrimp.com

The persistence of males in the population depends basically on their ability to fertilize hermaphrodites against their will and the sex-determination system of the species. When hermaphrodites produce males by self-fertilization, they are destined to remain for at least some time even if they cannot fertilize that much. Now if self-fertilization only produce hermaphrodites, the poor males have to be really persistent or otherwise they will soon perish.

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References and further reading:

Chasnov JR 2010. The evolution from females to hermaphrodites results in a sexual conflict over mating in androdioecious nematode worms and clam shrimp. Journal of Evolutionary Biology 23: 539–556.

Ellis RE & Schärer L 2014. Rogue Sperm Indicate Sexually Antagonistic Coevolution in Nematodes. PLoS Biol 12: e1001916.

Ford RE & Weeks SC 2018. Intersexual conflict in androdioecious clam shrimp: Do androdioecious hermaphrodites evolve to avoid mating with males? Ethology 124: 357–364.

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