To Pat and Elizabeth and Laurie and Elaine and all the other women who so enriched my life
This is no joke. Studies on the human genome showed that the Y chromosome, the one that determines maleness, is accumulating mutations at a much more rapid rate than the other chromosomes, which makes it conceivable that in a few short millions of years it would lose its functions and become totally useless. Does this spell the end of the human race? Not to worry, we are told. We know now how to clone a woman’s egg without fertilization, in effect getting a high fidelity copy of the mother. What about the males? Well, we males apparently outlived our usefulness; get used to it. So sex, banish the thought, will become superfluous as well? Now wipe the smirk off your face and think for a moment. Males need females to reproduce; they can’t do it by themselves. But from an evolutionary point of view, in most species males contribute little to the offspring except genes. On the other hand, there are species in which females can reproduce asexually, and they are doing just fine, thank you. In fact, they can grow at double the rate of a population that reproduces sexually.
What is sex good for?
This question occupied evolutionists for many years. The most common explanation is surprising: sexual reproduction allows the species to survive parasites. How does it work?
Any species has a genetic makeup that forms a normal distribution curve. Let me explain: each chromosome has 2 copies of a gene in a given location. For instance, one gene can determine the hair color brown, and the other black. These two genes are called alleles. Most of the population will have some combination of these alleles. But a small minority may have a mutation of one allele, say red hair, instead of the black hair allele. This situation is true for every genetic trait. A parasite is adapted to infect the most common set of alleles in the host population. But the small minority of alleles for which the parasite is not adapted for will not get infected, which of course has a significant survival value. With time this rare population of resistant individuals will become the predominant form, and the parasite will then fail. But not so fast: the very same process then happens on the parasite side. A small minority of the parasite population will have a genome whose alleles make it successful in attacking the newly resistant host form. And so on and on…This process is called co-evolution. Reminiscent of cyber-warfare, where a program is attacked by a virus or a worm (these names are not just a coincidence); programmers design defenses; hackers design countermeasures to penetrate the defenses, and so on, and so on…
So where does sex fit into this biological warfare? When an egg is fertilized the chromosomes undergo extensive rearrangement, not unlike shuffling a deck of cards. Outcome: new combinations of alleles, completely different from the original set. Now the parasite is stumped; check one for the newly reorganized host genome. But the parasite has its own outlying minor population, which can attack the new genome. And so the cycle repeats through the co-evolutionary game of measure -countermeasure. What about the offspring of asexual reproduction? They are basically clones of their mother, no shuffling of their alleles. And the parasite that had afflicted their mother has no problem attacking them as well. This accounts for the observation that species that can reproduce primarily asexually will switch to sexual mode of reproduction when the population is subjected to parasitic or other environmental assaults that threaten its survival. It allows a minor population to arise that is better suited to withstand the new onslaught.
What’s the proof?
Nice theory, you might say. And indeed, many field observations are consistent with the theory. But such studies are, well, consistent with –but do not prove causation. We are not being picky; the environment is so rich and complex that one could also come up with an alternative explanation.
In the July 8, 2011 edition of Science magazine Morran and his colleagues provided the necessary proof. They used a tiny worm, a nematode called C. elegans. This nematode has many advantages as an experimental animal. It isfacultative sexual, which means that it is not locked into being sexual (which is called obligatory sexual); it can switch back and forth between sexual and asexual modes. Also, It has a lifespan measured in days, so they could study many generations without spending the rest of their career on a single experiment (and forget about that tenure promotion). The worms can also be kept frozen for a long time, and then thawed and brought back to life. And finally, they have a natural parasite, the bacteria Serratia marcescens, that also have a short lifespan and can therefore be studied over many generation in a short time span. In the wild, sexual reproduction occurred at a frequency of about 20%. When they infected the nematodes with the bacteria, the frequency of sexual reproduction attained orgy-like proportions of 80-90%. They then allowed the host and parasite to co-evolve over several generations, and as the worms adapted the male frequency declined back to 20%. In another experiment they used mutant worms that were either obligate sexual or obligate asexual, none of that wishy-washy facultative sexual business. When infected with Serratia marcescence the obligate sexual did just fine; the obligate asexual became extinct.
So, all the feminists who rejoiced at hearing the news about the atrophy of the Y chromosome and the eventual disappearance of the male persuasion: not so fast. If not for sex, one insignificant cold virus could very quickly dispatch us all to the dustbin of history. And all you male chauvinists who think you are so important, think again: all you are good for is contributing some DNA; once you performed your sexual duty, you are essentially useless. What a sobering thought.
Dov Michaeli, MD, PhD is a basic researcher who writes at The Doctor Weighs In.