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NUMBER 2: OXIDATIVE STRESS. "Another hypothesis is that pairing and recombination initially arose as a way to repair mutational damage caused by increased oxidative stress due to rising atmospheric oxygen or endosymbiosis." http://biorxiv.org/content/biorxiv/early/2016/04/28/050831.full.pdf View in LinkedIn

DIPLOID TO HAPLOID. "Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. This process is required to produce egg and sperm cells for sexual reproduction. During reproduction, when the sperm and egg unite to form a single cell, the number of chromosomes is restored in the offspring." https://lnkd.in/eTsFK2C View in LinkedIn

NUMBER 1: RETROTRANSPOSONS. "One hypothesis is that meiotic pairing and extensive homologous recombination in meiosis evolved to avoid the burden and consequences of non-allelic ectopic recombination in the large genomes of early eukaryotes, which have repetitive sequences. Such sequences might have been related to the spread of retrotransposons in early eukaryotes, of which many types are very ancient in eukaryotes but absent in bacteria and archaea." https://lnkd.in/e2tp_G5 View in LinkedIn

SEXUAL REPRODUCTION requires two parents which share equal amounts of genetic information in the offspring. Meiosis creates the germ cells. It is a strategy used by most of the animal kingdom and by many plants, whose flowers are their sexual organs. View in LinkedIn

UBIQUITOUS HOMOLOGOUS RECOMBINATION. "Most evidence suggests that homologous recombination evolved long before meiosis, as it occurs in all domains of life and involves proteins that share strong homology, but the origins and selective advantage of synapsis, gene conversion and recombination remain poorly understood." http://biorxiv.org/content/biorxiv/early/2016/04/28/050831.full.pdf View in LinkedIn

GENE CONVERSION. "During meiosis, crossover recombination results from the pairing of homologous chromosomes and the formation of DNA double-strand breaks (DSBs), followed by synapsis, a process in which homologues become tightly paired along their length involving a protein structure known as the synaptonemal complex. DSBs are then repaired as either crossovers (hereafter ‘COs’), resulting in the exchange of large chromatid segments, or gene-conversion events." http://biorxiv.org/content/biorxiv/early/2016/04/28/050831.full.pdf View in LinkedIn

SCENARIO 2b. "In a variant of this scenario, early syngamy evolved as a result of ‘manipulation’ by selfish elements (plasmids, transposons) to promote their horizontal transmission." WOW. https://lnkd.in/e2tp_G5 View in LinkedIn

SCENARIO 2. "Proto-meiosis evolved in response to the fusion of two haploid cells (“syngamy”), as in standard modern eukaryotic sexual life cycles. Syngamy may have been favoured because it allows recessive deleterious mutations to be masked in diploids." https://lnkd.in/e2tp_G5 View in LinkedIn

SCENARIO 1. "Diploidy accidentally occurred by replication of the nuclear genome without subsequent cell division (“endoreplication”), and that returning to haploidy was selected for to correct this. Because either haploidy or higher ploidy levels may be favoured in different ecological situations, a variant of this scenario is that a proto-meiosis–endoreplication cycle evolved to switch between ploidy levels." https://lnkd.in/e2tp_G5 View in LinkedIn

EARLY EVENT. "A form of reductional cell division (a.k.a ‘proto-meiosis’) probably evolved in early asexual unicellular eukaryotes." https://lnkd.in/e2tp_G5 View in LinkedIn