Sex and the Single Cell
Let's talk about sex. It is by far the most widely employed mode of reproduction in nature, and yet, even now, no one knows why. The alternatives to sex are relatively simpler, more energy efficient, and, in many cases, safer. Despite its obvious disadvantages, sex is an evolutionary path to which most species have stuck, and in the last few months, research has shown that the advantages are not as mysterious as we had thought.
Basically, sex uses a process called meiosis, a type of cell division, to create sperm and egg cells. When these join, the individual produced has a genetic makeup that is different from either parent. This genetic variability in the offspring is one of the key differences between sexual and asexual reproduction. In reproduction without sex, only one parent is needed and the individuals produced are clones, sharing the exact same genetic blueprint as their parent.
Many plants, insects, and lower organisms use asexual reproduction to create their hoards of young and are very happy living the single life. No need for a mate means they save time sniffing about and they lose dangers of intercourse: two distinct advantages over sex.
There are also organisms that can switch between sexual and asexual reproduction, depending on their needs. Volvox carteri is one such species. This green algae grows in colonies in temporary ponds that fill with water each spring and dry out during the summer. Aurora M. Nedelcu, lead author of the paper, "Sex as a response to oxidative stress," has been studying what turns these guys on.
"Sex evolved as a way to deal with stress and its consequences," Nedelcu states. According to Nedelcu, asexual Volvox can nroamlly switch to sexual reproduction in the presence of DNA damaging oxidants. These oxidants usually appear during the late summer when the water becomes warmer.
In their experiments, Nedelcu and her team simulated late-summer conditions, placing culture plates full of Volvox into water baths that contained a dye that fluoresces in the presence of oxidants. Heating the water baths to 111 degrees Farenheit (42.5 degrees Celsius), Nedelcu's team was able to measure the amount of fluorescence, and therefore, the amount of oxidants present. They discovered that the heated culture plates contained twice as many oxidants as the unheated plates, suggesting that it is the hot summer conditions that induce oxidant formation in the water, which in turn, triggers the switch to sexual reproduction.
Richard E. Michod, coauthor of the paper, adds, "We're the first to show that oxidants are responsible for sex in this organism. This is the first demonstration that stress turns on sex-inducer genes."
Other recent experiments have shown that the genes for sex can be lost through evolutionary processes. Christopher Eckert, a biologist at Queen's University in Kingston, Ontario, is coauthor of a recent study that looked at northern populations of the plant species Decodon verticillatus and how they have evolved to reproduce without sex.
"Our genetic analysis shows that northern plant populations acquire mutations that disable sex itself, a trait central to the biology of almost all higher organisms," Eckert asserts. Their study showed that environmental conditions in the north, where the plants were taken, were harsh enough that any genetic variability acquired through sex was actually harmful to the plants; in order to survive as a population, the plants must remain highly adapted to their environment through each successive generation. Since sex seemed to disrupt this process, sex was rooted out of the population over time.
The switch was permanent.
"Sex is a complex trait, involving an intricate series of developmental processes. It is highly unlikely that sex would be restored through mutation and selection in asexual lineages," says Marcel Dorken, co-author of the paper. Dorken goes on to state that the effects of Muller's Ratchet and environmental change due to forces such as global warming, these plants may have a harder road ahead, "Complete asexuality is thought to be an evolutionary dead end, with substantial environmental change leading to the demise of asexual lineages...[through] reduced fitness, or even the local extinction of plants in asexual populations of Decodon verticillatus. If this occurred, these populations could be invaded and replaced by sexually fertile populations from further south in the species' range."
Though the results of their study suggest that complex traits can be eroded more quickly than previously thought, it does not mean that all species are on the verge of jumping on the asexual bandwagon.
Rather, sex, like anything else, is simply a means to an end: a way to produce offspring from generation to generation. As humans, we have never had a choice in the matter. Sex is the only way we have ever been able to reproduce as a species. With the advances in human cloning, this might change; however, the thought that sex can become obsolete or can bounce in and out of style is a little disconcerting. In the end, cloning just doesn't sound as fun.
References and Suggested Reading
Dorken M E, Neville KJ and CG Eckert. (2004). Evolutionary vestigialization of sex in a clonal plant: selection versus neutral mutation in geographically peripheral populations. Proceedings of the Royal Society of London. 271:2375-80.
Freeman S and JC Herron. Evolutionary Analysis, 3rd Edition. Upper Saddle River, New Jersey: Pearson Education Inc., 2004.
Kitchener V. (2003). No sex please, we're ostracods. Nature Reviews Genetics. 4:334.