I was wondering: How did the first sexually reproducing organisms come about? I've read a lot about asexual reproduction in the first cells that stemmed from the proteinoids and first DNA/RNA in the Earth's early atmosphere etc. but how did sexual reproduction come into the equation?
This message has been edited by Mission for Truth, 05-11-2004 03:44 PM
This is from the top of my head, let me know if you want more specific information.
First of all, bacteria participate in the swapping of DNA, so even the most simplest of life forms is capable of what we would call "reproduction". However, with bacteria this is usually limited to small bits of DNA instead of half the genome, as is seen in mammals for instance. Also, bacteria absorb extraneous DNA from their environment as well, and can incorporate this DNA into their chromosomes. This is called "horizontal gene transfer". The production of gametes (eggs and sperm) may have been a evolved pathway towards more specific DNA transfers, that is keeping DNA isolated to the same species instead of casting DNA "to the wind" so to speak.
Secondly, we also see that many species are not strictly separated into male and female. Take earthworms for example, they produce both sperm and eggs, and fertilize each other during mating. Once the apparatus is there, it wouldn't be too difficult for the sexes to be separated permanently.
So, with species alive today we see a gradation between simple DNA swapping---> gamete swapping ----> sexually separate species. There are probably many intermediate steps between these three, but the pathway seems to be well evidenced.
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.
quote:Just a small point LM about earthworms. As I understand it gonochory (separate sexes) is the primitive condition in metazoans.
Thanks for the correction.
quote: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.
So is sex controlled by the environment or by a genetic disposition (such as sex chromosomes in mammals)? Even in mammals, hormones control gonad development, however the timing of hormonal release is a mixture of genetic predispositions and mother hormonal feedback (IIRC). In shrimp, is it more of an environmental pressure or a genetic predisposition that decides gonadal development, be it sequential or simultaneous hermaphroditism?
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
quote: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.
Using antibiotic resistance as an example, bacterial recombinanation with exogenous DNA is an important mechanism. Plasmids carrying resistance genes can cross SPECIES barriers, even to the point of recombinantion between Gram positives and negatives (one of the major divisions in microbiological species). Some species have sexi pilli, which are transmembrane bridges that allow the transfer of small sections of DNA and plasmids. Even within the simplest organisms, horizontal gene transfer seems to be an important evolutionary and adaptational mechanism. Sexual reproduction in metazoans seems to be the next step towards taking advantage of recombination. That we see sexually dimorphic organisms (either simultaneous or sequential) is an important step that avoids the "chicken and egg" problem that separate sexes would pose as a first step in sexual evolution.
I've read, I believe that was by Richard Dawkins, maybe in the "selfish gene", that sexual reproduction is a stable evolutionary strategy, or that sexes are adaptative peaks that arises easily.
Seems that is really isogametes - equal gametes, both are "half" egg and sperm, with no specialization for reaching the other gamete or to just wait and select a certain gamete that reaches - that are a hard equilibrium. If a genetic condition produces gametes that instead of simply "float" randomly unworried to met other, rather have some strategy of reaching another gamete quicker that others that do not do it, that's a great advantage. This strategy may be simply producing more and tinier gametes, to start. At the same time, with some gametes being reached easily by those specializing in seek for another, there would be a advantage for just relaxing and staying in, wainting to be reached. But, better than just being "lazy", is making gametes that grant a good development for the formed zygote. This can be simply producing a smaller number, but better, more nutritious gametes. And then the peaks are formed, and any advantage that make the pre-sperm or pre-egg more efficient in being in their respective adaptations will spread very rapidly; the more isogametic-like a gamete is, the more disvantage he's in.
almost Every successful Organism has had a way to exchange DNA some way or another this is a survival technique. The more you can exchange the faster evolution occurs and the better Organisms adapt.
Sexual reproduction is very successful. however it isn't the same from species to species. For instance some species have eliminated the need to have both a male and female. Two females can work just as well.(some insects, amphibians , and reptiles)
This message has been edited by DC85, 05-15-2004 11:11 PM
Please note that I am a laymen, not a scientist. If you respond, please try to reason with my inferior gray matter.
I just recieved a journal dated June 22, 2004. I recieved it the day after I read this thread. It seems to argue against the basic tenant that reproductive capacity could evolve simustaneously on a macro scale. I will quote the magazive and give the source. It's only a few paragraphs that I hand typed.
"Complexity is especially evident when living organisms have complex parts that would be useless without other complex parts. Let us focus on the example of reproduction.
"According to evolutionary theories, living things continued to reproduce as they became ever more complex. At some stage, though, the female of a number of species had to develop reproductive cells requiring fertilization by a male with complementary reproductive cells. In order to supply the proper number of chromosomes to the offspring, each parent's reproductive cells undergo a remarkable process called meiosis, whereby cells from each parent are left with half the usual number of chromosomes.
"Of course, the same process would have been needed for other species. How, then did the "first mother" of each species become capable of reproducing with a fully developed "first father"? How could both of them have suddenly been able to halve the number of chromosomes in their reproductive cells in the manner needed to reproduce a healthy offspring with some characteristics of both parents? And if these reproductive featuees developed gradually, how would the male and female of each species have survived while such vital features were still only partially formed.
"In even a single species, the odds against this reproductive interdependence coming about by chance are beyond measuring. The chance that it arose in one species after another defies reasonable explaination. Can a theoretical process of evolution explain such complexity." --June 22, 2004 Awake
As someone said above it's nice to have a discussion free from the evc debate. If you don't answer it's fine, I don't want to disturb. What I read in the article makes sense to me.
What do you think. Can an evolutionary model such as an earthworm or shrimp explain the more complex dna halveing of species requiring male and female?
Please go simple on your expaination.
Edited for a couple of mistakes
This message has been edited by Rick Rose, 05-15-2004 11:33 PM
I think you'll find that's true of the majority of us here. If this stuff can be grasped by the likes of me, then there's absolutely no chance you'll have any problems with it.
The chance that it arose in one species after another defies reasonable explaination.
I don't know anything about chance or odds, but I do know that nothing in evolution predicts it happening "in one species after another."
All species with sexual reproduction share a common ancestor. So it only had to happen once - to that ancestor population.
Anyway there's plenty of organisms with intermediate reproductive situations, so I don't see that it takes anything but random mutation and natural selection to produce sexual reproduction. The powerful selective usefulness of sexual recombination has already been demonstrated, so what's missing, exactly? You've got the mechanism, the transitions, and the clear evolutionary benefit.
Can an evolutionary model such as an earthworm or shrimp explain the more complex dna halveing of species requiring male and female?
The "DNA halving" you refer to isn't any more complex in us than in earthworms or shrimp. Earthworms and shrimp are diploid - that is, each of their cells contains two copies of each chromosome - just like we are. Meiosis is normal cell division minus a step. (Sort of.)