Tuesday, 2 July 2013

Sex- A Gene's Eye View

Speaking of the birds and bees, sex is ubiquitous. But what remains elusive is why that is the case; why it persists as a 'popular' means of reproduction and an engine of evolutionary change. The standard Darwinian picture is that reproduction results in more progeny that the environment can sustain, intraspecific diversity is abundant, resulting in the 'struggle for existence'. Verily, descent with inherited modification results and speciation follows, creating new reproductively isolated yet interbreeding populations (species). The phylogenetic prevalence of sexual reproduction demands an explanation; what evolutionary advantages does sex offer over asexual breeding? Taking a gene's eye view, the concepts of accumulated mutation, genetic drift and negative epistasis should be taken into account; but superficially, the disadvantages and costs of sex seem easier to recognise and fathom than the actual evolutionary benefits. Maynard Smith illustrated the so called 'two-fold cost' of sex; whereby in organisms that reproduce sexually, males simply contribute genes to the progeny but a mutation causing females to breed asexually will ultimately triumph as its prevalence doubles at each and every generation. Secondly, the whole point of sex is the union of male and female gametes which might be ecologically taxing if population densities are low, distinct from asexual organisms that don't encounter such a burden. Progeny arising from sexual reproduction often become troubled by recombinational loads, whereby mutually adapted genes become separated via recombination, resulting in a decline of overall fitness. So what are the benefits of sex? In brief, the evolutionary advantages are both direct and indirect and their recognition is made complicated by sexual selection and anisogamy (variation in sizes of male and female gametes). Nevertheless, the filamentous fungus Emericella nidulans demonstrates slower decline in fitness relative to its asexual counterparts when its progeny are exposed to rates of accumulated mutation, such is an example of a 'selectional arena', whereby strict selection against offspring that carry lethal mutations is maintained. Volvox carteri, a multicellular green alga, may be induced into sexual reproduction via increasing the levels of reative oxygen species (ROS), which may hint a relationship between free radicals and the early origin of sex in eukaryotic life forms. Indirect advantages of sex seem to reinforce the reaction to directional selection, where there is linear phenotypic change; the Fisher-Muller hypothesis holds that sexual recombination accelerates the ability of a species of adapt by stringing together beneficial mutations from different origins, sex also protects against 'mutational meltdown' (in which asexual life forms may accumulate irreparable deleterious mutations) via background selection by separating deleterious alles from beneficial mutations. This so called 'Muller's ratchet' is also observed in segmented bacteriophages in lowering and alleviating the load of deleterious alles. Negative epistasis caused by sexual recombination may contribute greatly to the architecture and intricacy of the genome: such is a decline in the fitness of two entangled alleles which paves the way for future removal of potential hosts for deleterious mutations from the population and deploys a sexual advantage.

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