The biological significance of meiosis.

Four possible biological functions of meiosis are considered. First, the conventional view that it generates by recombination and sexual reproduction the genetic diversity on which natural selection can act. Second, that recombination at meiosis plays an important role in the repair of genetic defects in germ line cells. Third, that it is essential, at least in animals, for the reprogramming of gametes which give rise to the fertilized egg. Fourth, that it helps maintain the immortality of the germ line, possible by a process of rejuvenation involving the removal of faulty RNA and protein molecules, or by the elimination of defective meiocytes. A unified hypothesis is proposed which attempts to link these diverse functions. Evidence is now available which strongly indicates that the control of gene activity in higher organisms depends in part on the pattern of cytosine methylation in DNA, and that this pattern is inherited through the activity of a maintenance methylase. Epigenetic defects may arise by the loss of methyl groups which the methylase is unable to replace in somatic and also germ line cells, if de novo methylation cannot occur. There is also evidence that recombination at meiosis is largely confined to structural genes or adjacent DNA. It is proposed that the absence of a functionally important methyl group in a promotor or operater region produces a recombinator or signal for the initiation of recombination. The formation of hybrid DNA in this region then allows the lost methyl groups to be replaced by maintenance methylase activity. The removal of epigenetic defects by recombination during meiosis therefore becomes an essential part of a reprogramming and rejuvenation process. Assuming some epigenetic defects are nevertheless transmitted to the next generation, sexual reproduction and outbreeding would be advantageous because they provide the opportunity for their removal at the next meiosis. Inbreeding would be disadvantageous, because it increases the probability that epigenetic defects would become homozygous and could no longer be removed by recombination.