Relationship of Elysia diomedea and E. crispata

October 6, 2008
From: Patrick Krug

Concerning message #21911:

Hi Bill,

In response to your question about common ancestors in Elysia, my lab is working on a molecular phylogeny of the Elysiidae - a "family tree" based on DNA sequences. I have discussed the E. crispata / E. clarki issue with Skip Pierce at length, but my work in this area is unpublished so I don't want to say much for right now.

One of my students did a population genetic study of E. crispata from across the Caribbean; it's one of a dozen species we are studying to see if the period of time larvae spend in the plankton determines how much gene flow occurs between localities. E. crispata has little potential for larval dispersal, and not surprisingly we found deep genetic separation between at least six regional populations. Each population is 6-8% different from the others at the commonly used COI gene, suggesting hundreds of thousands of years of isolation. Within populations, we cannot distinguish E. crispata and E. clarki type slugs genetically. I think they might be ecotypes arising through parallel evolution in each regional population, but much more work remains to be done to sort this out (and it's equally interesting either way).

In comparison, E. diomedea and E. crispata/clarki are about 15% different from each other at the COI gene, a typical value for sister species of Elysia. Put another way, E. diomedea is equally distant from Florida E. clarki - type and Bahamas E. crispata - type slugs.

Hope that helps (at least a little),
Pat

pkrug@calstatela.edu

Krug, P. J., 2008 (Oct 6) Relationship of Elysia diomedea and E. crispata. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/21944

Dear Pat,

Thanks for your response. I wasn't really trying to reopen the E. clarki - E. crispata debate, I was curious to know if any work had been done on E. diomedea. But as you have brought up the E. clarki question again I can't resist a few thoughts - although I think we talked about this in Bonn.

As I am sure you are aware my background as a morphologist makes me very wary of dismissing the major morphological differences between E. clarki and E. crispata as being caused by environmental differences. If they are the same species, do you mean the two 'ecotypes' are the result of some embryological interaction with the environment during each individuals development? Or do parents produce both 'morphs' in their egg masses and only one morph survives? Or is there no gene flow between E. clarki environments and E. crispata environments and some form of genetic drift has occurred?

Your scenario of the same ecotypes evolving independently throughout the Caribbean doesn't seem to be to be the most parsimonious explanation - are there other examples of this? And if parallel evolution is occuring shouldn't  the result of each parallel event be considered a distinct taxon? I'm afraid saying that even though there are morphological differences they must be ecotypes because the COI gene is the same in each 'type' seems a bit circular to me. Perhaps the COI gene is not the best indicator in this group? 

Best wishes,
Bill Rudman

Rudman, W.B., 2008 (Oct 6). Comment on Relationship of Elysia diomedea and E. crispata by Patrick Krug. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/21944

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