Origins of Hexapoda
kinman2 at YAHOO.COM
Tue May 4 22:45:24 CDT 2004
Among those who believe Collembola are not directly related to forktails and insects (and there are many such workers), there seems to be three main hypotheses:
(1) P.N. Lawrence, who believes they are related to amphipods. Doubts have been expressed about this option.
(2) others who believe collembolans are sister group to Branchiopod crustaceans (supported by an 18S rRNA analysis in 1997 by Spears and Abele).
(3) collembolans related to ostracods (my new hypothesis).
Unfortunately, some of Spears and Abele's (1997) trees show a polyphyletic Ostracoda, so this makes me wonder just how reliable 18S rRNA sequences are in solving this particular problem. The preponderance of evidence convinces me that Collembola evolved from a crustacean group. I still see no good reason to abandon my ostracod hypothesis, but if I do, Brachiopoda would be a good second choice. On the other hand, Insecta (ectognathous hexapods) probably evolved from some group of malacostracan crustaceans. If they have separate crustacean sister groups, then two or more origins of hexapods will obviously make Hexapoda polyphyletic. And for those who might be wondering, if forktails (diplurans) are not sister group to the ectognathous Insecta, then they may be sister group to another malacostracan taxon, but I am not ready to tackle that one until I get the collembolans placed.
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