Tag Archives: phylogeny

Who came first? The comb or the sponge?

by Piter Kehoma Boll

The endless question is here again, but this time it appears to be settled. What animal group is the earliest of all? Who came first?

It is clear that there are five animal lineages that are usually regarded as monophyletic: sponges, placozoans, comb jellies, cnidarians and bilaterians. Let’s take a brief look at each of them:

Sponges (phylum Porifera) are always sessile, i.e., they do not move and are fixed to the substrate. They have a very simple anatomical structure. Their body is consisted of a kind of tube, having a large internal cavity and two layers of cells, an outer one and an inner one around the cavity. There are several small openings connecting the cavity to the outside, called pores, and one or more large cavities, called oscula (singular: osculum). Between the two cell layers there is a jelly-like mesohyl containing unspecialized cells, as well as the skeleton structures, including fibers of spongine and spicules of calcium carbonate or silica. Some species also secrete an outer calcium carbonate skeleton over which the organic part grows. Sponges lack muscles, nervous system, excretory system or any other kind of system. They simply live by beating the flagella of the choanocytes (the cells of the inner layer), creating a water flow entering through the pores and exiting through the osculum. The choanocytes capture organic particles in the water and ingest them by phagocytosis. All sponge cells can change from one type to another and migrate from one layer to another, so there are no true tissues.

porifera_body_structures_01

Body structures found in sponges. Picture by Philip Chalmers.*

Placozoans (phylum Placozoa) are even simpler than sponges, but they actually have true tissues. They are flat amoeboid organisms with two layers of epithelium, one dorsal and one ventral, and a thin layer of stellate cells. The ventral cell layer is slightly concave and appears to be homologous to the endoderm (the “gut” layer) of other animals, while the upper layer is homologous to the ectoderm (the “skin” layer).

701px-trichoplax_adhaerens_photograph

Trichoplax adhaerens, the only species currently in the phylum Placozoa. Photo by Bernd Schierwater.**

Comb jellies (phylum Ctenophora) resemble jellyfishes, but a closer look reveals many differences. Externally they have an epidermis composed by two layers, an outer one that contains sensory cells, mucus-secreting cells and some specialized cells, like colloblasts that help capturing prey and cells containing multiple cilia used in locomotion, and an inner layer with a nerve net and muscle-like cells. They have a true mouth that leads to a pharynx and a stomach. From the stomach, a system os channels distribute the nutrients along the body. Opposite to the mouth there is a small anal pore that may excrete small unwanted particles, although most of the rejected material is expelled through the mouth. There is a layer of jelly-like material (mesoglea) between the gut and the epidermis.

bathocyroe_fosteri

The comb jelly Bathocyroe fosteri.

Cnidarians (phylum Cnidaria) have a structure similar to comb jellies, but not as complex. They also have an outer epidermis, but this is composed by a single layer of cells, and a sac-like gut surrounded by epthelial cells (gastrodermis), as well as a mesoglea between the two. Around the mouth there is one or two sets of tentacles. The most distinguishing feature of cnidarians is the presence of harpoon-like nettle cells, the cnidocytes, which are used as a defense mechanism and to help subdue prey.

800px-cross_section_jellyfish_en-svg

Body structure of a cnidarian (jellyfish). Picture by Mariana Ruiz Villarreal.

Bilaterians (clade Bilateria) includes all other animals. They are far more complex and are characterized by a bilateral body, cephalization (they have heads) and three main cell layers, the ectoderm, which originates the epidermis and the nervous system, the mesoderm, which give rise to muscles and blood cells, and the endoderm, which develops into the digestive and endocrine systems.

500px-bilaterian-plan-svg

Basic bilaterian structure.

Traditionally, sponges were always seen as the most primitive animals due to their lack of true tissues, muscular cells, nervous cells and all that stuff. However, some recent molecular studies have put the comb jellies as the most primitive animals. This was highly unexpected, as comb jellies are far more complex than sponges and placozoans, which would suggest that muscles and a nervous system evolved twice in the animal kingdom, or that sponges are some weird simplification of a more complex ancestor, which would be very hard to explain. The nervous system of comb jellies is indeed quite unusual, but not so much that it needs an independent origin.

However, now things appear to be settled. A study published this month on Current Biology by Simion et al. reconstructed a phylogenetic tree using 1719 genes of 97 animal species, and applying new and more congruent methods. With this more refined dataset, they recovered the classical reconstruction that puts sponges at the base of the animal tree, a more plausible scenario after all.

But why other studies have found comb jellies as the most basal group? Well, it seeems that comb jellies have unusually high substitution rates, meaning that their genes evolve faster. This leads to a problem called “long branch attraction” in phylogenetic reconstructions. As DNA has only four different nucleobases, namely adenine, guanine, cytosine and thymine, each one can only mutate into one of the other three. When mutations occur very often, they may go back to what they were in long lost ancestor, leading to misinterpretations in the evolutionary relationships. That seems to be what happens with comb jellies.

So, it seems that after all the sponge indeed came first.

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References and further reading:

Borowiec ML, Lee EK, Chiu JC, & Plachetzki DC 2015. Extracting phylogenetic signal and accounting for bias in whole-genome data sets supports the Ctenophora as sister to remaining Metazoa. BMC Genomics 16: 987. DOI: 10.1186/s12864-015-2146-4

Littlewood DTJ 2017. Animal Evolution: Last Word on Sponges-First? Current Biology 27: R259–R261. DOI: 10.1016/j.cub.2017.02.042

Simion P, Philippe H, Baurain D, Jager M, Richter DJ, Di Franco A, Roure B, Satoh N, Quéinnec É, Ereskovsky A, Lapébie P, Corre E, Delsuc F, King N, Wörheide G, & Manuel M 2017. A Large and Consistent Phylogenomic Dataset Supports Sponges as the Sister Group to All Other Animals. Current Biology 27: 958–967. DOI: 10.1016/j.cub.2017.02.031

Wallberg A, Thollesson M, Farris JS, & Jondelius U 2004. The phylogenetic position of the comb jellies (Ctenophora) and the importance of taxonomic sampling. Cladistics 20: 558–578. DOI: 10.1111/j.1096-0031.2004.00041.x
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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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If you like flowers, you should love insects

by Piter Kehoma Boll

ResearchBlogging.orgEverybody likes flowers, right? They are so colorful and beautiful and usually have a wonderful scent. People love to have them in their gardens and women love to receive a nice flower bouquet from their boyfriends.

Some flowering plants, from left to right: Rosa ‘Hybrid Tea’, Pachystachys lutea and Zinnia elegans. All photos by Piter K. Boll (i.e. myself!)*

But why are flowers so beautiful? Of course the flowers seen above are derived from varieties artificially selected by humans to increase their beauty, but flowers in nature are wonderful too!

Naturally occuring flowers. From left to right: Oxalis sp., Ipomoea fimbriosepala and Zephyranthes robusta. All photos again by myself (Piter K. Boll)*

Surely that beauty is not intended to please people or whatever. That’s totally nonsense and just some religious people could have such a wrong idea. If plants have nice flowers, it must give them some advantage.

As everybody knows (at least I hope so), plants usually cannot move like animals, so they are condemned to stay still on their spot. That can mean a lot of trouble when you are looking for resources like water, light or basic elements like nitrogen. So evolution leads to the rising of amazing structures to make plants survive, like getting a hard stem to become taller or developing smaller or larger leaves, thorns, tendrils or even becoming carnivorous. But plants also need to reproduce and for that they need a mate, but since they are attached to the substrate, they have to find alternative ways to join their gametes.

Most primitive plants manage to do that through water or wind, just letting their reproductive structures go and hoping that they will reach their destination. As you can see, this method is not the best one, since fertilization occurs totally by chance. Besides that, these ways are limited regarding to the places where they are successful. A plant fertilized through water needs to be inside water or live close to the ground on places where it eventually get submersed; the same way, a plant which relies on the winds needs, of course, to be where the wind blows.

Moss (left) relies on water to reproduce, while conifers (right) need wind. Once more, photos by Piter K. Boll. *

Those methods, though limited, worked well enough for millions of years until sometime in the Cretaceous period, when a group of animals started to diverse astonishingly: the insects.

Insects are small and prolific. They have a hard outer skeleton of chitin, which prevents dehydration and injuries, and many of them learned to fly, so being able to cross large distances and colonize new habitats.

Insects existed, of course, at least since the Carboniferous. The most famous of them is the giant dragonfly Meganeura. But during the Cretaceous those groups that today are the most diverse started to appear in fossils: ants, bees, termites, butterflies, moths, aphids and grasshoppers. Beetles, the most diverse group of insects today (containing more species than all other arthropods together) are found in fossils since the Carboniferous, but almost went extinct during the Permian-Triassic boundary that marks the most terrible mass extinction on Earth. After this tragic event, they stayed more discrete until a boom in diversification in Cretaceous together with the already mentioned insects.

Well, all those insects needed to eat like anything else and started to feed on plants, including their pollen. That could have been a serious trouble, but plants managed to deal with it by modifying themselves in a way that the insects now became something useful to them. If insects were attracted to their pollen, why couldn’t they carry it to other plants, so assuring a more secure fertilization?
That’s exactly what plants made, but in order to attract insects even more to their reproductive organs, those started to increase in size and get nice colors. It all happened through natural selection of random mutations, of course. No one is assuming that plants or insects actually chose to change, that’s nonsense. What I’m trying to say (through a simpler way) is that those plants that were able to attach some of their pollen grains to insects, so that it reached other plants that the insect visited, were more successful to reproduce. The same way, those plants with more beautiful flowers attracted more insects and were also more successful to reproduce.

Anyway, that’s why we should thank insects for existing, since without them we wouldn’t have so nice flowers to decorate our lives. And if you like flowers but hate insects, well, you are being extremely unfair to nature.

I know that some may say “but I like butterflies. They are beautiful and cool and cute and they pollinate everything, so I just need to like these insects and not all those disgusting things.”
Oh, really? So you like butterflies? I’m sure you like this one:

Caterpillar of Agraulis vanillae. Photo by Bill Frank extracted from jaxshells.org

Most people like butterflies and hate caterpillars, but they are exactly the same thing. And actually these insects spend most of their life as a larva. Now just to satiate your curiosity, this is what that caterpillar looks like as an adult:

An adult of Agraulis vanillae on a head of Zinnia elegans. Photo by Piter K. Boll.*

But butterflies are not the only pollinators and not even the most common ones. Bees, as you know, are also very important and the main pollinators of many economically important plants, especially fructiferous ones. Wasps, flies, mosquitos, scorpionflies and moths are also important, but we cannot forget beetles.

Most basal and primitive angiosperms are pollinated by beetles, so that its more likely that these were the guys behind the appearance and diversification of flowering plants. There are many evidences for that, like an increasing diversity of angiosperms in fossil records being contemporaneous to an increase of beetle species.

Recently, some fossil flowers from the Turonian age (about 90 million years old) were found in Sayreville, New Jersey. Those were called Microvictoria svitkoana due to their astonishing similarity to the giant Amazon water lily, Victoria amazonica, even though much smaller in size.

Flower of Victoria amazonica, one of the most primitive angiosperms. It’s easy to notice that it still resembles somewhat a conifer cone. Photo by Frank Wouters, extracted from commons.wikimedia.org.

Despite primitive, it’s certainly a very beautiful flower, and it can only exist thanks to beetles of the genus Cyclocephala, like this one.

Cyclocephala hardyi, a beetle that pollinates Victoria amazonica. Photo extracted from ssaft.com/Blog/dotclear/

What do you think about it? It’s actually a cool pal, isn’t it? If you look closer, you may see that every insect is amazing in it own way, even cockroaches!

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References:

Béthoux, O. 2009. The Earliest Beetle Identified. Journal of Paleontology, 83 (6), 931-937 DOI: 10.1666/08-158.1

Crepet, W. L. 1996. Timing in the evolution of derived floral characters: Upper Cretaceous (Turonian) taxa with tricolpate and tricolpate-derived pollen. Review of Palaeobotany and Palynology, 90, 339-359 DOI: 10.1016/0034-6667(95)00091-7

Gandolfo, M. A., Nixon, K. C. and Crepet, W. L. 2004. Cretaceous flowers of Nymphaeaceae and implications for complex insect entrapment pollination mechanisms in early Angiosperms. PNAS, 101 (21), 8056-8060 DOI: 10.1073/pnas.0402473101

Seymour, R. S. and Matthews, P. G. D. 2006. The Role of Thermogenesis in the Pollination Biology of the Amazon Waterlily Victoria amazonicaAnnals of Botany, 98 (6), 1129-1135 DOI: 10.1093/aob/mcl201

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A Brief History of the Kingdoms of Life

by Piter Kehoma Boll

Since ancient times, living beings were classified as either plants or animals and Linnaeus retained this system in his great work Systema Naturae in the 18thcentury, where he divided nature in three kingdoms: Regnum Animale (animal kingdom), Regnum Vegetabile (plant kingdom) and Regnum Lapideum (mineral kingdom). This system was not intended to reflect natural relationships among living organisms, since Linnaeus was a Christian and believed that all life forms were created separately by God himself just as they are today, but was created to make the study of living beings easier.

Linnaeus and the two kingdoms of life. Painting by Alexander Roslin, 1775.

When the first unicellular organisms were discovered by Antoine van Leeuwenhoek in 1674, they were placed in one of the two kingdoms of living beings, according to their characteristics. It remained so until until 1866, when Ernst Haeckel proposed a third kingdom of life, which he called Protista, and included all unicellular organisms in it.

Haeckel and the three kingdoms. Photo by the Linnean Society, 1908.

Later, the development of optic and electronic microscopy showed important differences in cells, mainly according to the presence or absence of distinct nucleus, leading Édouard Chatton to distinguish organisms in prokaryotes (without a distinct nucleus) and eukaryotes (with a distinct nucleus) in a paper from 1925. Based on it, Copeland proposed a four-kingdom system, moving prokaryotic organisms, bacteria and “blue-green algae”, into the kingdom Monera. The idea of a ranking above kingdom came from this time and so life was separated in two empires or superkingdoms, Prokaryota (Monera) and Eukaryota (Protista, Plantae, Animalia).

Two empires and four kingdoms

Since Haeckel, the position of fungi was not well established, oscillating between kingdoms Protista and Plantae. So, in 1969, Robert Whittaker proposed a fifth kingdom to include them, the called Kingdom Fungi. This five-kingdom system remained constant for some time; Monera were prokaryotes; Plantae were multicellular autotrophs (producers); Animalia multicellular consumers; and Fungi multicellular saprotrophs (decomposers). Protista was like the  trash bag, where anything that doesn’t fit in the other 4 kingdoms was placed in.

Whittaker and the five kingdoms. Photography source: National Academy of Sciences: Robert H. Whittaker (1920—1980) – A Biographical Memoir by Walter E. Westman, Robrt K. Peet and Gene E. Likens.

With the dawn of molecular studies around 1970, significant differences were found inside the Prokaryotes, regarded, for example, to the cell membrane structure. Based on those studies, Carl Woese divided Prokaryota in Eubacteria and Archaeobacteria, emphasizing that the differences between those two were as high as the ones between them and the eukaryotes. This later gave rise to a new higher classification of life in three domains, Bacteria, Archaea and Eukarya.

Woese and the three domains. Photo from Photo from News Bureau – University of Illinois, given by IGB (Institute for Genomic Biology).

By the end of the 20th century, Thomas Cavalier-Smith, after intense study of protists, created a new model with 6 kingdoms. Bacteria and Archea were put together in the same kingdom, called Bacteria. Protists were divided in two kingdoms: (1) Chromista, including Alveolates (Apicomplexa, parasitic protozoa like Plasmodium; Ciliates and Dinoflagellates), Heterokonts or Stramenopiles (brown algae, golden algae, diatoms, water moulds, etc) and Rhizarians (like Radiolaria and Foraminifera), among others; and (2) Protozoa, including Amoebozoa (amoebas and slime moulds), Choanozoa (choanoflagellates) and a set of flagellated protozoa called Excavata. Glaucophytes, red and green algae were classified inside the kingdom Plantae.

Cavalier-Smith and his two new kingdoms. Photo from Department of Zoology – University of Oxford.

From the 21th century on, a phylogenetic approach to classify living beings has gained strength. After a lot of molecular analyses using different genes, the real evolutionary relationship among Eukaryotes is still not clear. However, the following groups are supported by most phylogenetic trees:

(1) Archaeoplastida (or Plantae): glaucophytes (Glaucophyta), red algae (Rodophyta) and green plants and algae (Viridiplantae)

(2) Chromalveolata: Stramenopiles or Heterokonta, haptophytes (Haptophyta), cryptomonads (Cryptophyta) and Alveolata.

(3) Rhizaria: Foraminifera, Radiolaria and some amoeboid protozoa

(4) Amoebozoa: amoebas and slime moulds

(5) Opisthokonta: animals, fungi, choanoflagelates

(6) Excavata: many flagellate protozoa. This group, however, isn’t as well supported as the other ones.

The current (maybe not so) well-established groups of organisms

So, as we can see, the Eukaryotes’ case is yet to be solved, but we hope that further molecular studies will help us understand better how the tree of life branches.

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References:

Baldauf, S. L. et al. 2000: A Kingdom-Level Phylogeny of Eukaryotes Based on Combined Protein Data. Science 290, 972-977.

Cavalier-Smith, T. 2004: Only six kingdoms of life. Proceedings of the Royal Society B 271, 1275-1262.

Rogozin, I. B. et al. 2009: Analysis of Rare Genomic Changes Does Not Support the Unikont–Bikont Phylogeny and Suggests Cyanobacterial Symbiosis as the Point of Primary Radiation of Eukaryotes. Genome, Biology and Evolution 1, 99-113.

Wikipedia. Kingdom (Biology). Available on-line in: <en.wikipedia.org/wiki/Kingdom_(biology)>. Acess on December 5th, 2011.

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