Does heterogeneity in opsin genes demand selection pressure

C.J.L.Wolf C.J.L.Wolf at ncl.ac.uk
Thu May 3 16:07:12 EST 2001


Just a quick question re: the evolution of opsin genes.

I'm writing an essay on colour vision in which I hope to include a few
paragraphs on the evolution of opsins (part of the photoreceptor
pigment). Several isoforms of the opsin found in L/M-cones are found in
wild populations of Marmosets and some other New-World monkeys, each with
a different spectral sensitivity. Opsin spectral sensitivity is known to
be very labile, so in the c. 40 million years since New-World marmosets
split from our ancestors I assume many different isoforms would have
arisen through point mutation.

Male monkeys have only 1 copy of the gene, as it is on the X-chromosome.
The spectral sensitivity of the monkey opsin may vary within a certain
range without any impact on the viability of the monkey (i.e. are equally
well adapted to its environment). Female monkeys, that have 2 alternative
copies of the gene, may express 2 different isoforms due to mosaicism
giving them enhanced colour vision.

My question is whether, as is often stated, we must invoke positive
selection pressure for the female monkeys that have enhanced colour vision
in order to account for the maintainance of several opsin isoforms in the
population. If the hardy-weinberg equilibrium applies to genes on the
X-chromosome, presumably the proportion of individuals in the population
that carry each alternative opsin gene should not change from generation
to generation so there's no need to search for a reason for the
continuation of established heterogeneity in the population. But I'm sure
I remember vaguely some maths in a pop-science book that said that a
million years from now we'd all end up with the same surname, as the
statistics of surname inheritance tend to select names out at random. As
an embryo is more likely to carry an X chromosome from its mother than its
father, presumably similar stats should apply?

If no selection pressure is necessary to explain the maintainance of the
alternative alleles in the population, would it still be a prerequisite
for their establishment in a reasonable proportion of individuals? In
fact, only 3 alternative forms of the pigment are present in Marmosets.

I'm not studying genetics, and so feel a little out of my depth. I'd be
grateful also if anyone could suggest references I could try.


Kit Wolf

Sent by C.J.L.Wolf at ncl.ac.uk
BMedSci student, Dept of Physiology,
University of Newcastle upon Tyne.

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