MCM8 Is Required for a Pathway of Meiotic Double-Strand Break Repair Independent of DMC1 in Arabidopsis thaliana

Abstract

Mini-chromosome maintenance (MCM) 2–9 proteins are related helicases. The first six, MCM2–7, are essential for DNA replication in all eukaryotes. In contrast, MCM8 is not always conserved in eukaryotes but is present in Arabidopsis thaliana. MCM8 is required for 95% of meiotic crossovers (COs) in Drosophila and is essential for meiosis completion in mouse, prompting us to study this gene in Arabidopsis meiosis. Three allelic Atmcm8 mutants showed a limited level of chromosome fragmentation at meiosis. This defect was dependent on programmed meiotic double-strand break (DSB) formation, revealing a role for AtMCM8 in meiotic DSB repair. In contrast, CO formation was not affected, as shown both genetically and cytologically. The Atmcm8 DSB repair defect was greatly amplified in the absence of the DMC1 recombinase or in mutants affected in DMC1 dynamics (sds, asy1). The Atmcm8 fragmentation defect was also amplified in plants heterozygous for a mutation in either recombinase, DMC1 or RAD51. Finally, in the context of absence of homologous chromosomes (i.e. haploid), mutation of AtMCM8 also provoked a low level of chromosome fragmentation. This fragmentation was amplified by the absence of DMC1 showing that both MCM8 and DMC1 can promote repair on the sister chromatid in Arabidopsis haploids. Altogether, this establishes a role for AtMCM8 in meiotic DSB repair, in parallel to DMC1. We propose that MCM8 is involved with RAD51 in a backup pathway that repairs meiotic DSB without giving CO when the major pathway, which relies on DMC1, fails.

Effects of XRCC2 and RAD51B mutations on somatic and meiotic recombination in Arabidopsis thaliana

“…Recombinational repair of the majority of meiotic DSB (non‐crossover) thus does not meet one or both of these conditions. That Arabidopsis is able to repair meiotic DSB using sister chromatids is clearly seen in the analysis of haploid meioses by the Mercier lab (Crismani et al ., ), and recent results confirm the frequent use of sister chromatids for meiotic DSB repair in yeast (Goldfarb and Lichten, ; Hyppa and Smith, ). The second requirement for interhomologue crossing over is the choice of a pathway, leading to a resolution of the recombination intermediate and resulting in a crossover (CO), rather than a non‐crossover (NCO).…”

“…Data from budding and fission yeasts show that both sister and non‐sister chromatids can be used as templates for repair of meiotic DSB, although the relative use of one or the other remains uncertain (Schwacha and Kleckner, , ; Goldfarb and Lichten, ; Hyppa and Smith, ; Pan et al ., ; Pradillo and Santos, ). In plants, the use of the sister chromatid to repair meiotic DSB is suggested by the lack of chromosome fragmentation in achiasmate Arabidopsis dmc1 mutants (Couteau et al ., ; Pradillo and Santos, ; Kurzbauer et al ., ; Pradillo et al ., ), and clearly occurs in the meiosis of Arabidopsis haploids (Crismani et al ., ). Whether or not this is also so in wild‐type (WT) diploid plants is of course uncertain; however, in the case of meiotic crossover formation (and in G1‐phase mitotic cells), the template sequence certainly lies on the homologous sister chromosome.…”

Meiotic Recombination in Arabidopsis Is Catalysed by DMC1, with RAD51 Playing a Supporting Role

“…Many studies specifically implicate the meiosis-specific DMC1 protein in meiotic crossing-over recombination with the homologous chromosome [2], [9], [46], as illustrated by the different meiotic phenotypes of Arabidopsis rad51 and dmc1 mutants. Absence of RAD51 in Arabidopsis meiosis leads to defects in chromosome pairing and synapsis, and to extensive chromosome fragmentation at meiotic zygotene/pachytene [28]–[32], [47]. Arabidopsis dmc1 mutant plants have a strikingly different meiotic phenotype, with synapsis defects and absence of chiasmata leading to the random segregation of intact univalent chromosomes [27], [29]–[32], [47].…”

“…Absence of RAD51 in Arabidopsis meiosis leads to defects in chromosome pairing and synapsis, and to extensive chromosome fragmentation at meiotic zygotene/pachytene [28]–[32], [47]. Arabidopsis dmc1 mutant plants have a strikingly different meiotic phenotype, with synapsis defects and absence of chiasmata leading to the random segregation of intact univalent chromosomes [27], [29]–[32], [47].…”

Meiotic Recombination: The Essence of Heredity

“…Suppression of the rec/mcm8 crossover defect by mutation of the BLM helicase ortholog, MUS309, led Sekelsky and colleagues to propose that mei-MCM is the functional analog of the MutSg complex, which is absent from the Drosophila genome (Kohl et al 2012). Consistent with this idea, absence of MCM8 or MCM9 in species that possess MutSg confers a general defect in DSB repair, but not the crossover-specific phenotype seen for Drosophila mei-MCM mutants (Lutzmann et al 2012;Crismani et al 2013).…”