Sex for Fitness
Reproductive sex can be messy and sterile, difficult and easy, energy-consuming and relaxing, complex and simple. In other words - sex is manic. On the other hand, asexual reproduction appears significantly less so and in some species is nothing more physically involved than the budding off of a duplicate from a parent cell.
From a purely reproductive standpoint, asexual reproduction is superior in its efficiency. However, from a survival standpoint sexual reproduction ensures that we and other sexual organisms continue to endure changes to our environment and the development of disease by adapting genetically to meet the challenge. Asexual organisms suffer and sometimes die off.
The primary difference between the two sex styles is that sexual reproduction basically encourages the incorporation of new gene sequences and reshuffles the new gene sequences with the old gene sequences for creating genetically unique progeny. In the process, mutations are often lost.
Asexual sex, on the other hand, is more similar to the idea of cloned copies of a parent. This in turn leads to significantly less genetically unique progeny at the cost of carrying a slowly growing mutational burden inherited from previous generations and passed onto the next.
This basic lesson in reproductive biology was made especially poignant in a recently published study in the Journal of Evolutionary Biology that compared the rate of adaptive molecular evolution of sexually reproducing plants over asexually reproducing plants for a gene that protects both sexual types from a shared disease.
In the study, 32 species of Evening Primrose wildflowers were put to the test as to whether sexual species of Evening Primrose were evolutionarily fitter than asexual species of the same plant type. Of the 32 species, 16 were sexually reproducing plants and 16 were asexually reproducing plants. Both sexual types possess a gene that makes the enzyme chitinase, which the plants need to identify and fight off diseases such as powdery mildew.
Although all of the plant species studied possess the same gene for chitinase, some have slightly different variants in DNA sequence referred to as alleles. You can think of alleles as being similar to the way knots are tied. A knot is a knot is a knot, but the way a knot is tied can differ and can make the difference as to how well the knot does its job of staying secured.
Sequence analysis and some predictive mathematical modeling of the chitinase gene from each of the 32 species of Evening Primrose revealed that the asexual Evening Primrose species were relatively unchanged genetically (and therefore nearly identical to a much older, earlier ancestor species the two sex types have in common) in comparison to the sexual species. The sexual species possessed 4 regions within the gene that had slight allelic differences in their DNA code.
Furthermore, when looking at the expression of the chitinase enzyme in response to disease from a powdery mildew fungus, the sexual species of Evening Primrose with their modified gene yielded significantly more of the protective enzyme than the asexual species did. As a result of the chitinase expression differences, the sexual plants survived more readily, were healthier and produced more flowers than the asexual plants.
According to a news release by Michigan Tech where lead author Erika Hersch-Green performed the study, “This suggests that common pathogens of evening primrose plants are an important selective agent for these plants,” says Hersch-Green. “Furthermore, molecular changes that increase chitinase expression and reduce disease damage are likely to be very good for the plants…this is in line with the theory that sex provides an evolutionary advantage to organisms.”
The results of the study provides a simple example of how that the evolutionary theory successfully predicts that sexual reproduction provides evolutionary advantages over asexual reproduction by reducing mutational load and increasing the progeny’s adaptive potential. More to the point, sex for fitness offers hope for mankind just as much as it does for plants. Just don’t forget to bring flowers or you may find yourself living an asexual lifestyle.
Image Source: Courtesy of MorgueFile
Reference: “Adaptive molecular evolution of a defense gene in sexual but not functionally asexual evening primroses” Journal of Evolutionary Biology; 15 May 2012 DOI: 10.1111/j.1420-9101.2012.02542.x; E. I. Hersch-Green, H. Myburg and M. T. J. Johnson.