Meiotic and mitotic recombination in meiosis

Abstract

Meiotic crossovers facilitate the segregation of homologous chromosomes and increase genetic diversity. The formation of meiotic crossovers was previously posited to occur via two pathways, with the relative use of each pathway varying between organisms; however, this paradigm could not explain all crossovers, and many of the key proteins involved were unidentified. Recent studies that identify some of these proteins reinforce and expand the model of two meiotic crossover pathways. The results provide novel insights into the evolutionary origins of the pathways, suggesting that one is similar to a mitotic DNA repair pathway and the other evolved to incorporate special features unique to meiosis.

Keywords: meiotic recombination; mitotic recombination.

Figure 1

Models of meiotic double-strand break repair. (A) In the Szostak et al. (1983) model, recombination initiates with a double-strand break that is processed into an extended displacement loop (D-loop) and then a double Holliday junction structure. The double-Holliday junction is resolved into either a crossover or noncrossover with equal probability. (B) In the revised model of Allers and Lichten (2001), some extended D-loops are unwound by an anticrossover helicase to produce noncrossovers, and double-Holliday junctions are resolved by a procrossover resolvase into crossovers.

Figure 2

Two meiotic crossover pathways. In this unified model synthesizing ideas from several sources (Börner et al. 2004; De Muyt et al. 2012; Zakharyevich et al. 2012), a double-strand break is processed into an extended displacement loop (D-loop). In the “mitotic-like” pathway (blue, class II), the extended D-loop can be unwound by an anticrossover helicase to produce noncrossovers. In some cases a double-Holliday junction is generated and then resolved by unbiased resolvases into either crossover or noncrossover products; the crossovers that are formed are noninterfering. Another possible fate of this intermediate is dissolution by a helicase and topoisomerase, to produce noncrossover products (not shown). In the meiosis-specific crossover pathway (red, class I), an anti-anticrossover complex blocks the action of anticrossover helicases to promote formation of a double-Holliday junction intermediate, which is then acted upon by a procrossover resolvase to form interfering crossovers. A double-Holliday junction is presented as a key intermediate to fit the original models (Figure 1) and the detection of joint molecules with properties of double-Holliday junctions in physical assays (Collins and Newlon 1994; Schwacha and Kleckner 1994; Schwacha and Kleckner 1995). However, there are other models that posit additional or alternative intermediates, including single Holliday junctions and multichromatid joint molecules (Osman et al. 2003; De Muyt et al. 2012; Zakharyevich et al. 2012). Variations on the two-pathway model as presented here can accommodate other intermediates and less-common fates of double-strand breaks.