Importance of Sexual Reproduction

The crucial step in sexual reproduction, which provides the contrast with asexual reproduction, is the fusion of nuclei derived from different individuals. If the individuals involved in a mating have different genotypes, the fusion nucleus will be heterozygous and the products of the meiotic division can be recombinant genotypes. Thus, in one sexual cycle, new combinations of characters can be created in the next generation for selection. Consequently, the most common 'explanation' for sex is that it promotes genetic variability through out-crossing and that variability is needed for the species to evolve to deal with competitors and environmental changes. There is plenty of evidence to show that asexual lineages change little in time and that out-crossing certainly does promote variability in a population, which enables the organism to survive environmental challenges (Hurst and Peck, 1996; Burnett, 2003).

This, though, is a 'group selectionist' interpretation. It argues that variation generated in an individual meiosis benefits the group or population to which the individual belongs. Yet current theory prefers to emphasise that selection acts on individuals (Carlile, 1987; Dawkins, 1989). A feature that is advantageous in selection must be so because of benefit to the individual itself or its immediate progeny. As noted above, an alternative interpretation of the selective value of a sexual cycle suggests that repair of damaged DNA is the crucial advantage of meiosis (Bernstein et al., 1985). It is argued that bringing together genomes from two different individuals enables DNA damage in one parental chromosome, caused by mutation or faulty replication, to be repaired by comparison and recombination with the normal chromosome provided by the other parent. Genetic fitness would be increased but only when out-crossing ensures heterozygosis. Even an incomplete sexual cycle might be of advantage in this case.

Gene mutations can be recessive and damaging, and different mutations are likely to occur in different mitotically generated cell lines. Just the formation of the diploid (or heterokaryon in most Basidiomycota) by out-crossing will benefit the mated individual if recessive adverse mutations are masked by non-mutant ('wild-type') alleles in the nuclei of the other parent. Out-crossing might also give rise to heterozygous advantage, where the heterozygous phenotype is better than either of its homozygous parents. This has been demonstrated frequently in plants and animals, and also in Saccharomyces cerevisiae (James, 1960).

Clearly, the genotype of the parental mycelium makes a crucial contribution to the genetics of the progeny population, but to produce a progeny population the parental mycelium must first produce a crop of fruit bodies and to do that it must grow into and through the substratum to capture, translocate and accumulate sufficient nutrients to support the formation of what can be massive multicellular structures.

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