I would be grateful if somebody could help me out here as Bioinformatics is not my strong card.
Regarding the recent Nature publication
New deep-sea species of Xenoturbella and the position of Xenacoelomorpha
I query the authors’ explanation; to wit…
The sister group relationship between Nephrozoa and
Xenacoelomorpha supported by our phylogenomic analyses implies that the last common ancestor of bilaterians was probably a benthic, ciliated acoelomate worm with a single opening into an epithelial gut, and that excretory organs, coelomic cavities, and nerve cords evolved after xenacoelomorphs separated from the stem lineage of Nephrozoa.
My problem arises with their placement of Ctenophora on their own phylogenetic tree as the “more primitive out-group” (for lack of better words on the spur of a rushed moment). Myself, I always considered Ctenophora as bilateral – in this case more primitively bilateral which IMHO should root the bilateran tree… which of course begs more than one question upon rereading their analysis.
Forget Ctenophores – what about Cnidarians!? Some taxonomists argue that Cnidarians are descendents of ancient bilateral coelomates and not the other way around. Biologists have known since the 1920s that Cnideria had a directive axis which gave them right and left-hand sides. Volker Schmidt goes on to argue that non-radially organized hydrozoan larvae have an anterior concentration of sensory and ganglionic nerve elements, suggesting that a fundamental genetic toolkit for the establishment of bilateral and polarized anatomies was already present before the Cnidaria-Bilateria divergence. Volker Schmidt goes so far as to suggest that diploblastic status of adult Cniderians is derived and that true mesoderm can be even be detected during Cniderian embryogenesis. OK – I concede that last argument is particularly contentious… but you get my drift.
I am partial to the notion the UrBilateran that subsequently gave rise to “Protostomes” & Deuterostomes and was itself coelomate with possessed a dorsal nerve chord. Any subsequent acoelomy and pseudocoelomy was derived… ditto ventral nerve chords. But hey… now I am being really contentious!
Review is an uncertain process at best. There are much worse lapses than that every day.
I remain grateful for your patient intercessions and correction of my naivite. TIA for answering my queries
This just in
Frankly, I wonder if the authors are making two categories of assumptions:
1 – Function as determined by gene expression may have little to do with overt morphology. For example : Box Jellyfish can pursue evasive prey by using their eyes. At a minimum, this behaviour implies an arc-reflex predicated on a central nervous system. Textbook definitions of nerve-nets may be specious. Box Jellyfish ganglia may not be clustered at an anterior end or bundled into cords; but their neural organization clearly is “central” even while spatially dispersed.
2 – Is it not possible the UrBilateran had a single dorsal ( or perhaps ventral ) nerve cord which later gave rise to derived lineages which lost the ancestral “cord bundling” and cephalization? …
… or alternatively, later lineages may have increased the number of cords as a later derivation. For example, should not the ganglia comprising a central nerve ring of a Sea Cucumber be considered the ANTERIOUR focal point of a central nervous system no differently than the more rudimentary ring of ganglia that comprises the “brain” of an Annelid?
The conclusion of convergent evolution to my jaundiced eye seems somewhat forced.
Any feed back greatly appreciated
… again hoping anybody can weigh in and help me out
Biologists have known since the 1920s that Cnideria larva have a directive axis which confer right and left-hand sides. Volker Schmidt even went on to argue that non-radially organized hydrozoan larvae also have an anterior concentration of sensory and ganglionic nerve elements, suggesting that a fundamental genetic toolkit for the establishment of bilateral and polarized anatomies was already present before the Cnidaria-Bilateria divergence.
Maybe Box Jellyfish are descendants of such an UrBilateran mentioned above.