The origin and function of meiosis are currently not well understood scientifically, and would provide fundamental insight into the evolution of sexual reproduction in eukaryotes.
Two such haploid gametes, ordinarily arising from different individual organisms, fuse by the process of fertilization, thus completing the sexual cycle.
Transformation, like meiosis, is a complex process requiring the function of numerous gene products.
A key similarity between prokaryotic sex and eukaryotic sex is that DNA originating from two different individuals (parents) join up so that homologous sequences are aligned with each other, and this is followed by exchange of genetic information (a process called genetic recombination).
[6] In addition, G. intestinalis was recently found to undergo a specialized, sex-like process involving meiosis gene homologs.
[9] According to this theory, early eukaryotes evolved mitosis first, became established, and only then did meiosis and sexual reproduction arise.
Yet there is no compelling evidence for a period in the early evolution of eukaryotes, during which meiosis and accompanying sexual capability did not yet exist.
For instance, the budding yeast Saccharomyces cerevisiae (a single-celled fungus) reproduces mitotically (asexually) as diploid cells when nutrients are abundant, but switches to meiosis (sexual reproduction) under starvation conditions.
[11] The fission yeast Schizosaccharomyces pombe, treated with H2O2 to cause oxidative stress, substantially increases the proportion of cells which undergo meiosis.
[14] These examples, and others, suggest that, in simple single-celled and multicellular eukaryotes, meiosis is an adaptation to respond to stress.
[16] In Streptococcus mutans and other streptococci, transformation is associated with high cell density and biofilm formation.
[17] In Streptococcus pneumoniae, transformation is induced by the DNA damaging agent mitomycin C.[18] These, and other, examples indicate that prokaryotic sex, like meiosis in simple eukaryotes, is an adaptation to stressful conditions.
When only one strand of the DNA is damaged, the lost information (nucleotide sequence) can ordinarily be recovered by repair processes that remove the damaged sequence and fill the resulting gap by copying from the opposite intact strand of the double helix.
[2] In contrast, recombination between sister chromosomes cannot repair double-strand damages arising prior to the replication which produced them.
An argument against this hypothesis is that adequate repair mechanisms including those involving recombination already exist in prokaryotes.
The arrest of ooctyes at the four genome copy stage was proposed to provide the informational redundancy needed to repair damage in the DNA of the germline.
[27] The adaptive function of the DNA repair capability during meiosis appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility.
Thus, on this view,[28] an advantage of meiosis is that it facilitates the generation of genomic diversity among progeny, allowing adaptation to adverse changes in the environment.
Considerations such as this have led many investigators to question whether genetic diversity is a major adaptive advantage of sex.