Theory that meiosis evolved from mitosis

Theory that meiosis evolved from mitosis

Mitosis is the process in eukaryotes for duplicating chromosomes and segregating each of the two copies into each of the two daughter cells upon somatic cell division (that is, during all cell divisions in eukaryotes, except those involving meiosis that give rise to haploid gametes). In mitosis, chromosome number is ordinarily not reduced. The alternate theory on the origin of meiosis is that meiosis evolved from mitosis. On this theory, early eukaryotes evolved mitosis first, but lacked meiosis and thus had not yet evolved the eukaryotic sexual cycle. Only after mitosis became established did meiosis and the eukaryotic sexual cycle evolve. The fundamental features of meiosis, on this theory, were derived from mitosis.
     Support for the idea that meiosis arose from mitosis is the observation that some features of meiosis, such as the meiotic spindles that draw chromosome sets into separate daughter cells upon cell division, and processes regulating cell division employ the same, or similar, molecular machinery as employed in mitosis.
However, there is no compelling evidence for a period in the early evolution of eukaryotes during which meiosis and accompanying sexual capability was suspended. Presumably such a suspension would have occurred while the evolution of mitosis proceeded from the more primitive chromosome replication/segregation processes in ancestral bacteria until mitosis was established.
In addition, as noted by Wilkins and Holliday, there are four novel steps needed in meiosis that are not present in mitosis. These are: (1) pairing of homologous chromosomes, 
(2) extensive recombination between homologs; 
(3) suppression of sister chromatid separation in the first meiotic division; and 
(4) avoiding chromosome replication during the second meiotic division.
    They note that the simultaneous appearance of these steps appears to be impossible, and the selective advantage for separate mutations to cause these steps is problematic, because the entire sequence is required for reliable production of a set of haploid chromosomes.