You can show that there is in fact a significant evolutionary loss caused by having sex, and by undergoing the characteristic processes of sex: In other words, a gene for eliminating sex, and reproducing asexually, should sweep through populations.
Given the cost of doing so, there must be some tremendous evolutionary advantage to having sex, one that is strong enough to outweigh the big twofold cost of having sex. If you have more offspring, perhaps some of them would have the right genetic combinations to withstand infections or other environmental challenges. This could select for genes that produce sexual reproduction, allowing some offspring to survive.
Now, however, a new paper in Science by Nadia Singh et al. Here are two individuals of Drosophila melanogaster, the study organism, having sex. The male, with the black abdomen, is to the right:. What Singh et al.
He was a true genius, and here he is about the time he did that experiment note the label, which I recall Sturtevant wrote himself:. In the case of Singh et al. One could test that as well by seeing if infection by other organisms, like tiny worms, would also have the same effect. Another experiment using injection of heat-killed bacteria showed no effect, so live bacteria are required to initiate the production of more-diverse progeny.
One is mitotic recombination: Ergo, if the appearance of recombinant offspring increases after this, it must be transmission distortion. We have no idea how this is done. And, as with the bacteria, the female larvae that successfully fought off the wasp infections grew up after pupating to produce a higher proportion of recombinant offspring than control, uninfected larvae.
Box plots illustrating distribution of recombination fractions in D. The median is marked with a black line; the first and third quartiles are rep- resented as lower and upper edges of the box, respectively. The whiskers extend to the most extreme Fruit fly asexual reproduction advantages point no farther from the box than 1 times the interquartile range. Recombination fraction is shown estimated over the entire day egg-laying period.
Or, to say it yet another way, genes for sexual reproduction are advantageous because they happen to be present in those individuals that have other genes allowing them to better survive infections.
The paper of Singh et al. But, contra that, there are many cases of sexually-reproducing species having some asexual reproduction, so this reverse evolution can and does happen. Is infection that pervasive, and such a strong selective force? Or are there other factors that select for sexual reproduction? Here are some questions I have that would extend this paper, which—make no Fruit fly asexual reproduction advantages about it—is very good.
Over generations, this can lead
Fruit fly asexual reproduction advantages a pretty strong increase in recombination you can also select for decreases. Further, you can also select for increased recombination between a specific pair of genes on a single chromosomeleaving the rest of the genome with no change.
Some theorise that endangered animals are being forced to diversify their methods of reproduction. The linear arrangement of six sex-linked factors in Drosophila, as shown by their mode of association. Journal of Experimental Zoology, 14, 43— Sturtevant and Bridges are heroes of mine. While Painter recognised the utility of polytene chromosomes, it was Bridges that made the useful map. At the risk of nit-picking, ebony and rough are on chromosome 3!
Very interesting paper AND blog post, Jerry. Indiana University about somewhat similar experiments done in C.
Could you comment briefly about how these two papers compare? Another thing I am curious about is whether recombination per se has been shown to increase fitness in individuals in a population.
We know that inbreeding definitely leads to accumulation and expression of recessive traits, but has some work been done on the beneficial effects of recombination? Fascinating article and commentary! One of the most interesting posts ever.
I wonder about something else, though.
Is the cost of sex really 2x, even in hermaphrodites like snails? In a normal population of organisms, the vast majority of nucleotides are not polymorphic. So in fact after sex, each parent is still Fruit fly asexual reproduction advantages to transmit some very large fraction of its genome to its offspring, even though some of those genes get there by a circuitous route via the member of the opposite sex who inherited them from some common ancestor of both male and female, or via new mutations.
So the cost of sex, for hermaphrodites at least, should be quite a bit less than 2x in most populations, unless I am missing something.
In fact, the cost of sex is zero if all members of the population are genetically identical. It depends on the hermaphrodite whether they experience a 2-fold cost of sex. The hermaphrodites that I know best earthworms, some snails must still mate with another individual and so these only pass on half of their genes.
But I expect their are other hermaphrodites that can mate with themselves and pass on all of their genes. I cannot think of any example, however.
There would be no cost at all if the population had no genetic diversity. The cost must be a decreasing function of genetic diversity, and should approach zero as diversity approaches zero.
It should not be 2x. Think about the limiting case of zero diversity.