Resolving these mysteries will likely provide decisive steps towards understanding why sex and recombination are found in the majority of eukaryotes. Mechanistic studies of meiosis have been carried out in different fields, such as cell biology, genetics and epigenetics, encompassing a wide range of eukaryotes. The precise function of the synaptonemal complex is not entirely understood [ 20 ]; one possibility is that it may serve to stabilize homologues during CO maturation. Some pairing mechanism must be advantageous to ensure proper segregation of homologues, but the origins and selective advantage of extensive pairing, synapsis, gene conversion and recombination remain poorly understood [ 24 ]. This genetic reshuffling reduces genetic associations within and between loci and is thought to be the basis of the success of sexual reproduction.
Abstract Meiosis is a key event of sexual life cycles in eukaryotes. While these simplifications are legitimate and useful in many cases, the wealth of mechanistic findings being uncovered points to a considerable number of evolutionary puzzles surrounding meiosis that have yet to be resolved. Resolving these mysteries will likely provide decisive steps towards understanding why sex and recombination are found in the majority of eukaryotes. Introduction In eukaryotic sexual life cycles, haploid cells fuse to give rise to diploids, before diploid cells are converted back to haploids in a process known as meiosis. Many other modern eukaryotes also increase and decrease their ploidy somatically, depending on growth stage or specific environmental stimuli [ 16 ]. DNA repair through homologous recombination and possibly reduction , ii which selective steps were involved in the assembly of the full cellular process, and iii why different forms of meiosis were perhaps less successful. Some pairing mechanism must be advantageous to ensure proper segregation of homologues, but the origins and selective advantage of extensive pairing, synapsis, gene conversion and recombination remain poorly understood [ 24 ]. A difficulty with this idea is that such masking may not be sufficient to favour diploidy in asexuals [ 17 ]. Extant eukaryotes share a set of genes specifically associated with meiosis, implying that it evolved only once before their last common ancestor [ 6 , 7 ]. This article has been cited by other articles in PMC. In support of this view, mating-type switching which can allow syngamy in haploid colonies has evolved multiple times in yeasts and involves domesticated mobile genetic elements [ 19 ]. Two scenarios for this have been proposed. The precise function of the synaptonemal complex is not entirely understood [ 20 ]; one possibility is that it may serve to stabilize homologues during CO maturation. Syngamy may have been favoured because it allows recessive deleterious mutations to be masked in diploids [ 1 , 12 ]. Non-homologous centromere coupling is also often observed at this stage, but the functional and evolutionary significance of this coupling is elusive [ 21 ]. However, these studies rarely focus on the evolutionary significance of meiotic mechanisms, rather mentioning them in passing and often in a simplified manner. The second scenario is that proto-meiosis evolved in response to the fusion of two haploid cells syngamy , as in standard modern eukaryotic sexual life cycles. The origins of meiosis The origin of meiosis through gradual steps is among the most intriguing evolutionary enigmas [ 1 , 2 ]. The first is that diploidy accidentally occurred by replication of the nuclear genome without subsequent cell division endoreplication [ 8 — 12 ], 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 [ 13 , 14 ], a variant of this scenario is that a proto-meiosis—endoreplication cycle evolved to switch between ploidy levels [ 5 ]. Identifying the selective scenario that led to its early evolution is difficult, but clues can be obtained by determining i which mitotic cellular processes were reused in meiosis e. We present the various evolutionary scenarios and selective pressures that have been proposed to account for these features, and we highlight that their evolutionary significance often remains largely mysterious. In evolutionary biology studies, meiosis is often simplified and represented by random assortment of chromosomes and recombination maps expressing the probability of recombination events between ordered loci, with little attention to the molecular and cellular details. We discuss the origin of meiosis origin of ploidy reduction and recombination, two-step meiosis , its secondary modifications in polyploids or asexuals, inverted meiosis , its importance in punctuating life cycles meiotic arrests, epigenetic resetting, meiotic asymmetry, meiotic fairness and features associated with recombination disjunction constraints, heterochiasmy, crossover interference and hotspots. Meiosis reduces a cell's chromosome number by half, while also creating new allele combinations distributed across daughter cells through segregation and recombination. This genetic reshuffling reduces genetic associations within and between loci and is thought to be the basis of the success of sexual reproduction.
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