This is good stuff. I'd like to ask a couple follow-up questions if I may:

Is it correct to say that, in any scheme in which there are exactly two kinds of
gametes, one of them makes the journey to the other, and the latter nourishes
the embryo?

Are two-gender systems more prevalent? (It seems plausible they would be most
efficient, 2 being the smallest integer bigger than 1.)

If the idea of mating types is overgeneralized in the way you indicate, what
absurdity results?

Can you recommend any good references for finding out more about this kind of
thing?

Thanks again for your help.

Peace

wrote in message
.. .
| In article c4p9d.660794$gE.331977@pd7tw3no,
| Elaine Jackson wrote:
| Thank you for your help. Actually I was already familiar with a few of the
| examples you cite. (One or two I'd been aware of for some time, and
| naturally I
| tried to answer this question with my own resources first.) But here's what
| I'm
| wondering: I know that certain things like eyes and wings have evolved
| independently in different species. Did sex evolve independently in plants
and
| animals? And if it did, in what sense is it the same thing in those two
cases?
|
| It depends on how you define sex. If you mean exchange of genetic
| material between individuals, it goes back to the bacteria.
|
| If you mean heterogameity, i.e. the condition in which there are two
| kinds of gametes, large ones that are mostly sessile and contain
| nutrition for the embryo and small motile ones that consist mostly of a
| nucleus and a means of propulsion, I don't know how many times it was
| invented independently. It almost certainly wasn't present in any
| common ancestor of animals and plants, because AFAIK, it doesn't occur
| in single celled organisms.
|
| If you mean separate male and female organisms, this condition has
| developed innumerable times independently in animals. As has been
| explained by others, plants have a system called alternation of
| generations, so even dioecious species don't really have male and
| female individuals in the animal sense.
|
| If you mean genetic sex determination, AFAIK, very little is known
| about this is in invertebrates. In vertebrates, it's standard in
| mammals, birds and frogs (dunno about other amphibians). It occurs in
| some fish and some reptiles, but even within a taxon as small as a
| family some species may have GSD, some may have environmental sex
| determination, and some may have a combination. It's not necessarily
| implemented with different chromosomes, either. It may be controlled
| by one or more individual genes. (It's actually like that in mammals,
| but less visible -- there's a single gene called TDF - testis
| determining factor -- which is normally only on the Y chromosome, but
| it can get translocated to the X, producing XX males.) In mammals,
| frogs, and some fish males are heterochromatic (XY) while females are
| homochromatic (XX). The reverse is true in birds. So this trait too
| has arisen numerous times even in just the vertebrates.
|
| Note that some seed plants (and fungi) have mating types, of which
| there are usually more than two. Plants of the same mating type are
| infertile with each other. Sweet cherries are a well known example --
| they have at least eight mating types. I suppose these mating types
| could be regarded as sexes in a very restricted sense, and this could
| be overgeneralized to any plant that isn't self-fertile.
|
| (Incidental question: What characteristics have plants and animals
| inherited from their common ancestors?)
|
| How to be a eucaryotic cell. This takes up a substantial part of the
| genome, and is the reason behind the tag line that we share half our
| genes with the banana, which is more apparent in some individuals than
| in others! ;-)