In article nhC9d.29785$a41.4982@pd7tw2no,
Elaine Jackson wrote:
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?

No. Someone mentioned the many marine animals in which sperm and ova
are both released into moving water. In that case, sperm and ova can
at least be distinguished as such -- the ova are larger and contain
yolk, while the sperm are smaller, motile and more numerous. There are
organisms in which the gametes are identical, including the true slime
molds (myxomycetes). These creatures have a large (up to a meter in
diameter in rare cases) mobile diploid stage, which sporulates. Each
spore germinates into an amoeba-like haploid stage which can grow a
flagellum in a wetter environment. These haploid forms can multiply by
fission for many generations, but when two of different mating type
meet, they fuse, and produce a diploid plasmodium which can grow to
respectable size. The species we were working with had many mating
types -- we found about a dozen from a pretty small sample of
individuals. Myxomycetes were regarded as fungi at the time, but now
they've been relegated to the grab bag of protista of uncertain
affinity, a great place to look for critters with bizarre reproductive
biology.

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

It's plausible because DNA contains two strands, and because diploid
organisms have two sets of chromosomes, one from each parent, and
gametes are haploid (one set of chromosomes). In seed plants, the
haploid generation has been reduced to a few cells. In lower plants,
like mosses and ferns, the haploid generation is visible and
independent, but still secondary. In true slime molds, the two
generations are more or less equal in "importance". There are a lot of
critters, fungi, protista (IIRC including some algae), in which the
haploid is the main form and the diploid forms briefly and sporulates.
In many but not all of these critters, there are not only no genders,
but no heterogameity -- no sperm or eggs. All gametes are the same.

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

It would mean that every individual was a unique mating type, which is
indistinguishable from the common situation in which plants are not
self-fertile, which different species enforce by a wide variety
of methods.

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

Leaf through some college level intro biology texts to find some that
have an approach you like, to get a grasp of basic biology. Read some
of Steven Jay Gould's books of essays on biology and evolution. I
remember some essays described some very strange life cycles.
Scientific American publishes books with collections of articles on a
single topic. Perhaps there's one which will interest you. Most
universities allow members of the general public to use their
libraries, although you may have to pay a fee to be allowed to borrow
books. The Tree of Life project is a good place to find out about the
affinities of different groups of organisms, and each page has
references to the literature. http://tolweb.org

Several universities have free online self-study biology courses, and
many have the materials for conventional courses on the web.

A couple of things to remember when you study biology: One, nothing
makes sense without eveolution by natural selection. It's as
fundamental to biology as atomic theory is to chemistry. Two, the real
world is a very complex, messy place, and life on earth has had at
least 3.5 billion years to develop and adapt to the complexity, so just
about every generalization you can make will have exceptions. I
suppose I can add Three: humans, mammals and even vertebrates are
large, visible critters, but they are only a very small fraction of
species and the way they do things is not the standard for everything
else, even though far more is known about them than about any other
taxa. So try not to be anthropocentric, mammal-centric or even
vertebrate-centric!

I hope some other members of this group will make better and more
specific reading suggestions than I have.