SPEC: versicolor origins

From: Bill Shear <BILLS@hsc.edu>

On the origins of versicolor:  It evidently is not just a simple hybrid
between setosa and virginica.  That cross has to my knowledge never been
made successfully in a controlled setting.  The hypothesis that versicolor
is a setosa x virginica hybrid (or the reverse) is based on the fact that
many versicolor populations have a chromosome count of 108, while virginica
usually has 70 chromosomes, and setosa, 38.  Add the two, and voila!
However, pollen and ovule are haploid ( with half the characteristic
number), so a setosa/virginica hybrid would actually be expected to be
35+19=54.  This chromosome number has never been found in versicolor
populations.  So the story is that either the gametes were unreduced, or
the chromosome number of one of the (rare?) hybrids spontaneously doubled.

Also, versicolor will cross very easily with virginica, but not with setosa
(does anyone know of a setosa/versicolor hybrid?), and as noted, the
setosa/virginica cross also has not been made (any new info on this?).

So, though I'm as guilty as anyone in retailing this nice story in my book
(on p. 108, appropriately enough), it seems to me questionable and is more
like what Steve Gould calls a "just-so story" rather than a real
explanation.  Let's call it a hypothesis.

Perhaps molecular comparisons of the DNAs of the three species could settle
the question.  This could probably be done quite easily, given modern
techniques.  Maybe it has been done already!

There are some folks working with these molecular tools on the Hexagonae
complex.

Bill Shear
Department of Biology
Hampden-Sydney College
Hampden-Sydney VA 23943
(804)223-6172
FAX (804)223-6374
email<bills@hsc.edu>




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