Ted Hoffman

After reading plenty of ISCID papers and spending too much time at ARN, I looked at the opposite sex yesterday and wondered.

I can grant abiogenesis - but how did these sexes come to be - still Natural Selection and Randon Mutation? Or is there another known theory/mechanism out there?

How the heck does random mutation create a male/female?
Plus a penis? And a breast?

Ted Hoffman

And what about sperms and the whole reproductory mechanism? Any info on the evolution of the reproductory system would be useful.

Godless Dave

I don't have the specifics on when and how sex first appeared, but I'd like to point out that men do have breasts, but the mammary glands don't work, and the clitoris is basically a very small penis without the plumbing. The male reproductive organs are basically the female organs turned inside out.

Ted Hoffman

Very interesting GD, very interesting.

RufusAtticus

1. Asexual reproduction (cell division) -> sexual reproduction (cell fusion) of isogametes (cells of the same size.)
2. Isogamy -> anisogamy (egg & sperm)
3. anisogamy -> dioecy (specialization for either egg or sperm)
4. internal fertilization
5. recreational sex

Ted Hoffman

RA, thanks for your response. It appears to me however that you dont find my question very serious(no 5. above). I'd like to assure you that I am asking it in all seriousness. Thanks for your answer. Now I have to go find out the meaning of those words in 2. and 3.
RBH? PZ? anyone?

RufusAtticus

IM, just chalk it up to me needing to eat lunch.

Itzpapalotl

Here are some interesting papers on the evolution of sexes:

RESOLVING THE PARADOX OF SEX AND RECOMBINATION (pdf)

Origins of the machinery of recombination and sex
(full text free)

Comparative evidence for the evolution of genitalia by sexual selection (pdf)

genitals evolve extremely rapidly

THE RAPID EVOLUTION OF REPRODUCTIVE PROTEINS (pdf)

reproductive proteins evolve extremely rapidly.

Abalone lysin: the dissolving and evolving sperm protein (pdf)

A specific example of reproductive protein evolution.

An interesting model of reproductive protein evolution is presented in this paper:

G. Sander Van Doorn, Pieternella C. Luttikhuizen, and Franz J. Weissing. Sexual selection at the protein level drives the extraordinary divergence of sex-related genes during sympatric speciation. Proc. R. Soc. Lond. B (2001) 268, 2155-2161.

abstract:

"An increasing number of molecular studies are indicating that, in a wide variety of species, genes directly
related to fertilization evolve at extraordinarily high rates.We try to gain insight into the dynamics of this
rapid evolution and its underlying mechanisms by means of a simple theoretical model. In the model,
sexual selection and sympatric speciation act together in order to drive rapid divergence of gamete
recognition proteins. In this process, intraspecific competition for fertilizations enlarges male gamete
protein variation by means of evolutionary branching, which initiates sympatric speciation. In addition,
avoidance of competition for fertilizations between the incipient species drives the rapid evolution of
gamete recognition proteins. This mechanism can account for both strong stabilizing selection on gamete
recognition proteins within species and rapid divergence between species. Moreover, it can explain the
empirical ¢nding that the rate of divergence of fertilization genes is not constant, but highest between
closely related species."

Doubting Didymus

What you're looking for is an intermediate form that bridges the gap between asexual cloners and obligatory sexual reproducers.

I've just tried to look up chytridiomycete life cycle diagrams on google, but not much luck I'm afraid. If you own or have access to any biology textbooks in local libraries you should look up fungus life cycles, especially chytridiomycota and zygomycota. Their life cycle is pertinent here, because they CAN sexually reproduce, but they dont HAVE to, having asexual budding/spore methods at their disposal as well. What this demonstrates is that there is no unbridgable gulf between cloners and sexual reproducers. A population could start out asexual and gradually become more and more reliant on sexual reproduction until they lose the asexual abilities. A very broad and smooth spectrum of possible forms therfore exists that bridges asexual and sexual organisms.

lpetrich

RufusAtticus:
(1) Asexual reproduction (cell division) -> sexual reproduction (cell fusion) of isogametes (cells of the same size.)

Multiplication by division, sometimes called "mitosis", is essentially universal among one-celled organisms and the cells of multicelled organisms.

Sexually-reproducing protists alternate between a haploid (X) and a diploid (XX) phase; they can reproduce asexually in one or both phases. Diploid-to-haploid is meiosis, haploid-to-diploid is cell fusion.

How the haploid/diploid cycle got started is obscure, but some protists do something similar that may provide some clues: conjugation, where two protists meet and exchange genetic material.

However, meiosis looks a bit like normal division that went too far:

Normal: XX -> XXXX -> XX,XX
Meiosis: XX -> XXXX -> XX,XX -> X,X,X,X

The next question is why more than one sex? Protists and fungi typically have lookalike "mating types" or sexes ("isogamy") -- sometimes a large number of them. The only way to distinguish them without looking at their genes and receptor proteins is to see which ones refuse to fuse with which other ones.

This is likely to be an anti-inbreeding adaptation; such adaptations are common. However, the haploid/diploid cycle does not require this mechanism, and the ancestral sexually-reprodcing protist most likely had had only one sex.

(2)Isogamy -> anisogamy (egg & sperm)

If a multicellular organism tries to reproduce by emitting gametes (sex cells), it has a problem -- individual cells tend to be small. One can get around that by making the gametes bulky, but that slows them down. But if one kind of gamete scrimps on food, it can become fast again -- and allow the other kind to become very bulky and stationary. Thus, sperm cells and egg cells.

This has happened at least twice (animals and plants).

(3)anisogamy -> dioecy (specialization for either egg or sperm)

That is common in the animal kingdom; animal hermaphrodites are rare (I know of land snails and earthworms). However, plants are often hermaphrodites (flowers with stamens and pistils).

(4)internal fertilization

This is an adaptation to living on dry land; it helps keep the gametes from drying out. It has been invented several times in the animal kingdom and at least once among plants (ancestor of seed plants).

However, some aquatic animals, like sharks, also perform internal fertilization.

(5)recreational sex

It may be a side effect of being social, though our species is oversexed by the standards of many species, even many social species.

More specifically, one thing that could bring it about is sperm competition -- several males mating with the same female will leave their sperm in the same place, and sperm cells from different males can compete with each other. One way of doing that is to produce a large number of them; that strategy is used by chimps, which have relatively large testicles. Among gorillas, however, a male monopolizes several females, and his sperm thus does not have to do much competition. And gorillas have relatively small testicles. Our species, curiously, is about halfway in between.