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! ;-)