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Re: Metazoan gene count

Thinking out loud here, and without necessarily looking things up:

    The news item also mentions a similar gene count for Arabidopsis,
meaning that this would have to be the gene number for all eukaryotes - it
seems likely these days that the divergence between animals and plants is
either close to or at the first divergence in the eukaryotic crown
(Cavalier-Smith 2002, Simpson & Roger, 2002).
    Teleost fish have a genome duplication relative to other vertebrates
(Mulley & Holland, 2004 - just today, in fact). So the number of genes would
be, a priori, doubled. I'm currently working in fish genetics, and I know
that in at least some of these (i.e. those our lab is currently working on)
both duplicates are functional, and have functionally diverged since
duplication. So fish would be expected to have considerably more genes than
    I don't think that there's likely to be some set gene number that all
eukaryotes share - just one that the three species in the news item share.

    Cavalier-Smith, T. 2002. The phagotrophic origin of eukaryotes and
phylogenetic classification of Protozoa. International Journal of Systematic
and Evolutionary Microbiology 52: 297-354.
    Mulley, J., & P. Holland. 2004. Small genome, big insights. Nature 431:
    Simpson, A. G. B., & A. G. Roger. 2002. Eukaryotic evolution: Getting to
the root of the problem. Current Biology 12 (20): R691-R693.

    Also in today's Nature - it's not a dinosaur, but the images are just
way too cool:

    Siveter, D. J., M. D. Sutton, D. E. G. Briggs & D. J. Siveter. 2004. A
Silurian sea spider. Nature 431: 978-980.

"Pycnogonids (sea spiders) are marine arthropods numbering some 1,160 extant
species. They are globally distributed in depths of up to 6,000 metres, and
locally abundant; however, their typically delicate form and
non-biomineralized cuticle has resulted in an extremely sparse fossil record
that is not accepted universally. There are two opposing views of their
phylogenetic position: either within Chelicerata as sister group to the
euchelicerates, or as a sister taxon to all other euarthropods. The Silurian
Herefordshire Konservat-Lagerstätte in England ( 425 million years (Myr) BP)
yields exceptionally preserved three-dimensional fossils that provide
unrivalled insights into the palaeobiology of a variety of invertebrates.
The fossils are preserved as calcitic void in-fills in carbonate concretions
within a volcaniclastic horizon, and are reconstructed digitally. Here we
describe a new pycnogonid from this deposit, which is the oldest adult sea
spider by 35 Myr and the most completely known fossil species. The large
chelate first appendage is consistent with a chelicerate affinity for the
pycnogonids. Cladistic analyses place the new species near the base of the
pycnogonid crown group, implying that the latter had arisen by the Silurian

    The 3D images really show just how bizarre pycnogonids are. Offhand,
though, I think they're a little too complacent about pycnogonids being
chelicerates - they don't actually test it, and their assumption is based
solely on the chelate head appendages.


        Christopher Taylor

On 21/10/04 5:13 am, "Phil Bigelow" <bigelowp@juno.com> wrote:

> http://e.usatoday.com/a/tBBdt2MAQc50zATjebZAQoyZa-m/usat37
> The story talks about humans.  However, if one reads between the lines,
> it appears that this "phenomenon" applies to nearly all(?) metazoans.
> Curious, isn't it, that evolution stopped adding genes to metazoan
> chromosomes when the magic number of 25,000 genes was reached.  Past that
> point, evolution only played with the genes already present in the
> chromosome.  Who would have guessed that evolution could count?
> What I would like to know is, what is the phylogenetic topology of "junk"
> DNA in metazoans?  Do humans have more "junk" DNA than, say, earthworms?
> <pb>
> --
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