Just as a general note about sponge mitochondrial DNAs - my own thesis has
been directed pretty exclusively to rates of mtDNA evolution in demosponges
and calcisponges, and they're tremendously highly conserved, for sure.
Hexactinellids are another story - I never really had enough time to get
data I was comfortable with, and they may be quite different.
That being said, I think mtDNA sequences may prove to be very valuable
tools for exploring the higher-level taxonomy of the Porifera. I've looked
at 16S, COII, COIII, and ATP6 pretty extensively, and the former three are
so highly conserved as to be pretty much useless even at the inter-order
level. That being said, the relative rates of sequence evolution between
different genes as found in the bilateral animals hold up in the sponges as
well - ATP6 is much more variable than the others (but only in relative
terms).
I always intended to explore the utility of these genes for high-level
phylogenetic work in sponges, but I'm finishing up and I just don't think
it's likely to happen. I have neither the time nor a broad enough sample of
organisms. That being said, the trees I've made with ATP6 DNA and amino
acid sequence data are simply lovely. Nicely resolved, with very high
boostrap levels (75-99%) even at the inter-family level, and congruent
between all the phylogeny inference algorithms I've used.
As well, since they're still very highly conserved it's pretty easy to make
universal primers for them.
I'll be putting all the sequence data into Genbank as soon as I get the
time.
Russ Watkins
Department of Molecular Biology and Biochemistry
Simon Fraser University
Burnaby, BC, Canada
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