How Did the First Sexually Reproducing Organisms Arrive?

Just a small point LM about earthworms. As I understand it gonochory (separate sexes) is the primitive condition in metazoans. That is to say that hermaphroditism (both sequential and simultaneous) seems to be derived from the gonochoristic condition rather than the other way around. If you look at the developmental aspect of gonads in metazoans you see how hermaphroditism can independantly develop. For example, caridean shrimps have a structure called an ovitestis that is literally an ovary in the front and a teste posteriorly. In most shrimp one or the other portion doesn't develop at all. In those species which are sequential or simultaneous hermaphrodites hormones cause the ovarian portion to grow and the testicular portion to either remain functional or atrophy. What is important is that all primitive shrimp are gonochores while a few (about 35 species to date, probably many more) shrimp are protandric hermaphrodites or simultaneous hermaphrodites.Ghiselin MT (1969)The Evolution of Hermaphroditism among Animals. Quarterly Review of Biology 44(2):189-208.

Great question, LM. I attempted to answer this question for my MS work, only succeeded in creating a host of new questions. Really sex allocation (I suppose "sex-phase" allocation) in hermaphroditic shrimp (sequential specifically, simultaneous hermaphroditic shrimp are all sex-changers as well so the same rules apply) has three hypothetical proximate causes. The first is called environmental sex determination but I prefer the term socially mediated sex change to distinguish it from the second which is physical environmentally induced sex change. The first is when sex change is triggered by the social composition of nearby conspecifics. The second is when physical parameters (season, day-length, tides) trigger sex change. The last is genetic where sex change occurs at a pre-determined time in the life of the shrimp.I personally don't believe in socially mediated sex change in shrimp (although well-documented in fish). Some excellent experiments by Marliave et al. (1993) determined that the size/age of sex change in one species of pandalid shrimp was nearly identical to that of its maternal parent and seemingly random with respect to sex ratio in local population. This strongly infers a genetic trigger. My work with Lysmata wurdemanni suggests that sex change occurs during a seasonal window such that if an individual recruits too late it will remain male-phase until the next year and switch to the sim. hermaphrodite phase at a much larger size than an individual recruited early in the spring (Baldwin and Bauer, 2003). Zupo (2001) provides evidence that sex change in a grass shrimp is triggered by the presence of particular diatom in the diet. To be completely fair, however, a more recent paper on Lysmata wurdemanni (from my old lab, in fact) strongly suggests a social trigger. I haven't seen that paper yet but am not giving up my position unless they have adressed some serious experimental design issues (as well as some other things).I hope this isn't way too off-topic. Evolution of sex change --> Sex allocation theory --> Evolution of sexual reproduction. One of my areas of research is invertebrate sexual biology, and I think it has great relevance to the understanding of evolutionary principles.Baldwin AP and RT Bauer (2003)Growth, survivorship, life-span, and sex change in the hermaphroditic shrimp Lysmata wurdemanni (Decapoda: Caridean: Hippolytidae). Marine Biology 143: 157-166.Marliave JB, Gergits WF, Aota S (1993) F sub(10) pandalid shrimp: Sex determination; DNA and dopamine as indicators of domestication; and outcrossing for wild pigment pattern. Zoo Biology 12(5): 435-451.Zupo V (2001) Influence of diet on sex differentiation of Hippolyte inermis Leach (Decapoda: Natantia) in the field. Hydrobiologia 449: 131-140.This message has been edited by Lithodid-Man, 05-12-2004 06:32 PM

I find this topic to be of great interest, and it is nice to discuss a topic away from the EvC 'debate'. Sex is fascinating from a natural selection viewpoint because it is seemingly contrary to Darwinian selection. What I mean is that it would seem that asexual reproduction would be the rule in nature if the goal was to put as many of your own genes into the next generation. Sexually reproducing species put only half of their genes into their offspring. In a numbers game we would expect that asexual reproduction would quickly replace sexual reproduction. That we don't see that in eukaryotes suggests that factors of sexual reproduction have their own selective advantage. When we see asexual reproduction in metazoans it is typically a colonial establishment life-history following sexual reproduction (as in corals) or a species living under highly unusual circumstances.The most obvious and probably most important reason for sex is recombination. This is not saying that some unicellular eukaryote 3.7 BYA decided it needed to recombine. But the fact that some of them (unicellular eukaryotes) do shows there is a reason to comingle. I think that external factors such as virisoids selected for indivdiduals with varying genomes over those who selfishly cloned. From that point factors such as parental investment selected differentiation of sexes. No cites on this one, just an idea.This message has been edited by Lithodid-Man, 05-14-2004 05:07 AM

First, just for fun, here is an article I wrote for Of Sea & Shore magazine on mollusk reproduction that details major aspects of sexual biology and then talks about specific mollusk reproductive strategies (entitled "Behind the green operculum"). Be forewarned, it is not from a peer-reviewed journal and was written to amuse and inform. also, it is by far the worst written piece I have ever submitted (I had less than two weeks two write it from when it was suggested). Check out: http://www.ofseaandshore.com/greenoperculum.htm.I think Extremophile's comments are most relevant to this topic. A large number of protists have what are called isogametes. To reiterate, these are gametes that are merely packets of DNA contained in a membrane with varying amounts of supportive cellular contents. In those species that have pure isogamy there is no such thing as male or female. Within the phyla of protists we see all stages of the evolution of sexual reproduction. The most primitive is hologamy. This is where two single celled organisms combine, exchange DNA, then split and then asexually produce clones. This is probably the first eukaryotic "sex". The flaw with it is that genetically both parents are dead. The resultant beings are a mix.The next step is to meet a conspecific, clone yourself with a minimal amount of cytoplasm and a full complement of DNA and let that 'minimee' comingle with your partner's progeny and resort into two individuals, leaving the parents intact. At this step we have true isogamy. Once we have isogamy then selective factors as mentioned by Extremophile take over and lead to separate sexes as we see them today.I think it is important to remember that our definition of male and female in a biological sense is only related to gamete size. Mammal sexes are defined by XX and XY chromosomes, but this is not the case in most organisms. Males are those individuals who produce motile gametes that tend to have reduced cytoplasm and associated organelles. Females have large gametes with supportive organelles and a large amount of cytoplasm.Because of the great diversity of reproductive patterns we see in protists, even within different phyla, I think it is likely that sexual reproduction evolved separately, possibly several times within the existing multicellular kingdoms or their immediate protist ancestors.