Why Do Animals and Plants Reproduce Sexually?

Sexual reproduction is costly. Half of the individuals in a sexually reproducing species do not produce offspring and time and energy expenditures involved in searching for and selecting a mate are significant. Also, sexual reproduction may disrupt favorable gene combinations.

A hypothesis originally proposed more than 100 years ago asserts that sex allows natural selection to operate more effectively because it increases genetic variation. This hypothesis was recently put to the test by researchers at Imperial College London.

When supplied with sufficient nutrients and a benign environment, yeast cells reproduce asexually but when subjected to a harsh environment, cells produce haploid spores (spores containing only one copy of each chromosome) which can, when conditions improve, germinate and mate with spores of the opposite mating type.

To measure the benefits of sex, researchers engineered a yeast strain lacking certain genes necessary for meiosis so that the mutant yeast could not reproduce sexually but instead produced diploid spores (spores containing both copies of each chromosome) which were genetically identical to the parent and developed directly into new cells. They then subjected both the normal and mutant strains to repeated cycles of the benign environment (during which asexual reproduction occurred in both strains) followed by the harsh environment (to induce sporulation) simulating evolution over about 300 generations. The growth rates of the yeast cells were then assayed by comparing the growth rate of the new cells to that of the original ancestral strain. In the benign environment, no difference in growth rate was found between the sexual and asexual strains. In the harsh environment, both strains showed an increased growth rate, the asexual population showing an increase of 80% and the sexual strain 94%. The researchers conclude that the sexually reproducing strain's more efficient adaptation to the harsh environment was due to the beneficial effects of genetic recombination.

Ref: Nature 31-Mar-2005 Pg 638