Will the most primitive animal group please stand up
In my post few days back we have looked into the biological classification of animals based on 18s RNA study and in this one we see what changes are in store when you do phylogenomics analysis.Modern evolutionary biology has come a long way from Linnaeus days to molecular phylogenetics (Field et al 1988 )(In this method: rRNA is the most conserved (least variable) gene in all cells For this reason, genes that encode the rRNA (rDNA) are sequenced to identify an organism’s taxonomic group, calculate related groups, and estimate rates of species divergence) and now to the era of phylogenomics ( Blair et al 2002 BMC evolutionary biology)
(sequences from more number of genes are used from each species for making phylogenetic trees.)
For the past century, the use of detailed descriptions of animal adult morphology ,embryology and sequences from 18s DNA has been at the heart of the study of evolutionary relationships among distant groups.Based on the phylogenetic trees obtained from these methods placed sponges(Porifera) at the base of the tree meaning sponges could be representing the group of the most primitive animals on this planet.Just to remind that sponges occupies the place in diploblastic organisms along with Ctenophores and Cnidaria.
Recently in the april issue of Nature Cassey Dunn, Mark Martindale ,Gonzalo Giribet with others published their results of phylogenomics analysis done on total of 39.9 Mb of expressed sequence tags including more than 150 genes from 29 animals belonging to 21 phyla, including 11 phyla previously lacking genomic or expressed-sequence-tag data.The result was something which none expected by placing ctenophores as the sister group to all other animals included in the analysis, including sponges meaning ctenophores split off in evolution from other animals and went on its own path before sponges.This finding challenges the traditional view of the base of the tree of life, which honored the lowly sponge as the earliest diverging animal.This result was something hard to believe at first because Sponges are simplest organisms one could find on earth whereas Ctenophores are more complex organisms with slightly developed nervous system and other developed tissues.
But The placement of ctenophores (comb jellies) as the sister group(the base of animal tree) to all other sampled metazoans is strongly supported in all thier analyses.The presence of the relatively complex comb jelly at the base of the tree of life suggests that the first animal was probably more complex than previously believed
The comb jelly could only have achieved its apparent seniority over the simpler sponge via one of two new evolutionary scenarios: 1) the comb jelly evolved its complexity independently of other animals, after it branched off onto its own evolutionary path; or 2) the sponge evolved its simple form from more complex creatures–a possibility that underscores the fact that “evolution is not necessarily just a march towards increased complexity.” “This scenario would provide a particularly dramatic example of that principle.”– Cassey Dunn
Even though their is strong evidence ctenophores being the most primitive animal group on earth authors believe that it could be seen as a provisonal result till more data is generated from placozoa and other sponges.If further evidence is generated in this regard then it would be of great implication in early animal evolution proving either sponges lost many of its genes and became more simplified or Ctenophores evolved into more complex organism after splitting away from rest of animals.Till then the confusion remains for the most primitive animal group.I hope it stays the old way with sponges occupying the base of tree because its difficult to digest, an organism(sponges) losing so many genes and become so simple after descending from a more complex ancestor.But sometimes examples of Xenoturbella and Ciona in deuterostomes makes me believe sponges can also behave in this manner.Forget its getting more complex,Hope more data on various sponges come out soon and i am also looking forward for this article from Martindale’s lab–Pang, K., and Martindale, M.Q., Homeodomian gene expression in the developing ctenophore, Mnmiopsis leidyi. Develop. Genes Evol. In Press.
Broad phylogenomic sampling improves resolution of the animal tree of life.
Dunn CW, Hejnol A, Matus DQ, Pang K, Browne WE, Smith SA, Seaver E, Rouse GW, Obst M, Edgecombe GD, SÃ¸rensen MV, Haddock SH, Schmidt-Rhaesa A, Okusu A, Kristensen RM, Wheeler WC, Martindale MQ, Giribet G.
Nature. 2008 Apr 10;452(7188):745-9. (435)