I found this thread rather late, but feel I must step in. The evolution of sex is very interesting to me and has been part of my academic career. In reading through this thread I have seen a great deal of misunderstanding on both sides regarding both the origin and terms involved in the evolution of sexual systems. My Masters thesis was on the evolution of crustacean sexual systems. As part of that I was required to review an enormous body of literature on the evolution of sex, varying sexual systems, etc. I am in no way trying to insult prior posters on this thread, just trying to clarify some info. I apologize for the length of this post, but am going to start from the beginning. What is sex?
Sex, by definition, is the coupling of two haploid cells. In advanced multicellular species, like us, this involves the coming together of sperm and egg, each containing half of the genes of the parent to form a new individual. But sex precedes multicelluarity. Sex began with unicellular organisms. Bacterial exchange of DNA has been discussed so I am not going to talk about it but am going to focus on eukaryotic sexual reproduction. I am going to discuss true sex and how it happens. We have a wonderfully diverse world of protists (protoctysts to botanists out there) that show every stage of sexual reproduction.
The most primitive eukaryotes are in the kingdom Protista (or Protoctista). Unlike most animals, plants, and fungi, a good number of protists spend their lives as haploid, possessing only one set of chromosomes. Most of these reproduce mitotically, doubling their chromosomes to the diploid number then splitting to form 2 haploid individuals. This is really all of the hardware needed for sex.
Early Sex
In some protists that do not have sex (only asexually reproduce) it has been observed that these will consume each other during times of starvation. Usually one individual will envelop the other. In these species it has been observed that sometimes the nucleus of the eater will fuse with the nucleus of its dinner, forming a short-term diploid individual! As these creature ”want’ to be haploid, this fusion causes the cell to divide into two individuals. Now the individuals that form are not the same as before, some mixing occurs. Now the cannibal in this scenario is the loser. It went for a meal, and instead returned as a slightly different being. Now for the would-be food the situation was a win. True it is not exactly unharmed, but it went from a zero survival chance to escaping relatively intact. It is not hard to imagine how in a population that faced starvation often would select for those individuals who could survive being eaten in this way. In the protists where this system is observed it is relatively rare (not the cannibalism, but being able to survive it) as these live in the gut of termites. A termite’s gut is a fairly benign place. What is described here is sexual in nature but not true sex. It might better be described as accidental sex.
The next level of sex observed in protists is much more common. This one occurs in many well-known protists. In this case two haploid individuals fuse with each other (but with no pretense of trying to consume the other). Their nuclei fuse to form the diploid phase. This diploid may then split into two new individuals, both with jumbled genes from the original. In some the diploid form undergoes mitosis before returning to the haploid phase, making 4 individuals out of two. But why do this? One advantage in some forms is that the diploid phase is resistant to environmental stresses. In Chlamydomonas, discussed below, the diploid stage is called a zygospore and forms when the cells are faced with (guess what?) starvation, drying, osmotic stress, cold, heat, etc. The zygospore remains diploid and protected by thick wall. When conditions return to normal the cells shed the cell wall, mitose, then split into the haploid phase.
With Chlamydomonas and relatives we start seeing something unique and wonderful. Instead of just any two random individuals being able to come together they come in two slightly different morphotypes, called Mt+ and Mt-. While not male and female, these morphotypes exhibit some of the characteristics seen in true male and female gametes.
Sex and multicelluarity
Multicellular organisms have probably evolved multiple times within the protists, plants, animals, and fungi. The simplest form of this involves cloned identical cells in balls, sheets, clusters, etc. Obviously every cell cannot fuse with another individual’s cells. It makes biological sense for some cells from the group detach and find another such cell to fuse with. A very common trend in multicellular organisms is to have the ”mature’ form be entirely diploid. Haploid cells are only those which are destined to fuse with another cell, thus being true gametes. The advantage of this is that the individual genotype is not lost (remember when these single cells fused and separate neither daughter cell was exactly the same as the parents). This way only a few cells are ”sacrificed’ while the rest remain pure.
So, we now have a multicellular diploid organism that is capable of producing haploid products that can leave and fuse with others. So what factors lead to separate sexes? Sperm, as you know, are essentially a haploid nucleus with a motile apparatus. Eggs are a haploid nucleus surrounded by varying amounts of cytoplasm and typically organelles that keep the cell running for a long time (sperm are nearly always short lived). The question is, what makes it advantageous to have this system rather than relatively identical gametes (called isogamy).
When two gametes fuse to form a haploid individual, more than just the nuclei fuse. The cytoplasm and organelles also mix. In species like Chlamydomonas this starts a ”war’ within the cell. The nuclear DNA plays nice and fuses, but the organelles compete (only so much room) and eventually the organelles (mitochondria and chloroplasts) of one or the other parents dominate. As these have their own DNA the winner is the parent cell who gets of their nuclear DNA spread and all of their cytoplasmic DNA spread. So two selective winning scenarios arise. One, make your gametes as big as possible to hold as much cytoplasmic DNA as possible. But there is a limit to how big these can be. The other is to ”accept’ the losing scenario and make your gametes as small as possible and just go for spreading your nuclear DNA.
So there will be a strong selective force on some members of a species to have small gametes, and others to have large ones. Because size of gametes limits mobility, selection also favors the small gametes to become more motile. So now we have true sperm and eggs.
Derivations of the new sexual system
This is where I hope to not offend previous posters. Hermaphrodites (possessing male and female gametes) are not ancestral to gonochoristic (possessing only male or female gametes) species. The primitive (least derived) system in all multicellular species is gonochory. We find this throughout multicellular Eukaryotes.
In animals (the group I study) this is a constant. Wherever we find hermaphroditism it is secondarily derived from true gonochory. In mollusks we find that all primitive mollusks are obligate gonochores, while advanced forms (such as pulmonates or land snails and slugs) are hermaphrodites. In pulmonates we hit the ultimate advancement, the ability to self-cross. This is, that in a pinch they can combine their own sperm and eggs.
In crustaceans we see the same trend. In nearly all crustacean groups gonochory is the rule. In primitive Crustacea, like barnacles, we see many examples of obligate outcrossing hermaphrodites (meaning they cannot self fertilize). But in that group we also see an amazing thing called pseudohermaphroditism. This is where the main population (visible) is female. Males exist only as larvae and parasitize females to become only an internal testis producing sperm. I believe that this will be found to be the rule rather than the exception in future research in barnacles. Someone (I am not going to look) made a comment that "All barnacles do..." I am sorry to report that there is nothing ALL barnacles do, espcially sexually.
In decapod crustaceans gonochory tends to rule. All true crabs (Infraorder Brachyura) are obligate gonochores, as are lobsters and crayfish (infraorders Palinura and Astacidea). In true shrimps (Infraorder Caridea) we start seeing variations of this. Sequential hermaphroditism has evolved several times in shrimp lineages. This is where individuals start as one sex and change to another with time. In many shrimp it is starting as males then turning female when large. This is called protandry. It makes good biological sense. Sperm is ”cheap’, you can make a lot of it when very small. Eggs are expensive, the bigger you are, the more you can hold (shrimp, like all pleocyemate decapods, brood their young on the female’s abdomen). So because of this shrimp commonly start life as small males then change to female as they mature. In an increasing number of one group of shrimp it is apparent that they do this but also keep male function, becoming simultaneous hermaphrodites!
Some refs: (first two are mine)
Baldwin AP (2002). Behind the green operculum: the sex lives of mollusks. Of Sea and Shore Magazine 25(1): 17-19.
Baldwin AP and Bauer RT. (2003). Growth, survivorship, life-span, and sex change in the hermaphroditic shrimp Lysmata wurdemanni (Decapoda: Caridean: Hippolytidae). Marine Biology 143: 157-166.
Margulis L and Sagan D (1997). What is sex? Simon and Schuster Editions. New York, New York. 256 pp.
Edited by Lithodid-Man, : Clarified 'decapods' as 'pleocyemate decapods'. I was wrong to claim all decapods as the dendrobranchiate decapods do not brood embryos.
Wanda: To call you stupid would be an insult to stupid people. I've known sheep who could outwit you. I've worn dresses with higher IQs, but you think you're an intellectual, don't you, ape?
Otto: Apes don't read philosophy.
Wanda: Yes they do, Otto, they just don't understand it.
"A Fish Called Wanda"
™ Version 4.2