The formation of haploid cells (gametes) from diploid precursor cells during meiosis is essential to maintain a constant number of chromosomes from generation to generation in sexually reproducing species. Haploids arise in meiosis because there are two nuclear divisions but only one round of replication. The major problem is to ensure that exactly one copy of each chromosome pair is inherited by each haploid cell. This requires that homologs, or more precisely homologous centromeres, segregate from each other at the first meiotic division and that sister centromeres segregate at the second meiotic division. In most species homolog segregation requires formation of a physical connection between homologs. This connection is detected genetically as a crossover or microscopically as a chiasma (pl., chiasmata). Meiotic recombination also forms new combinations of alleles, thereby speeding the evolution of species. Thus, recombination plays a dual role in meiosis, with both immediate and long-term consequences.
Almost from the time of their discovery a century ago, meiotic crossovers and chiasmata were known to be non-randomly distributed along chromosomes. Crossovers do not occur independently: a crossover in one interval decreases the likelihood of a crossover in a nearby interval, a phenomenon called crossover interference, the first recognized control (Box 1). Crossovers are rare in and around centromeres, because their occurrence there interferes with proper chromosome segregation. Crossovers too far from the centromere (i.e., near the telomere) less effectively direct proper segregation, and in at least some species crossing over is reduced near the telomeres.
One key to understanding these controls has come from studies of the mechanism of crossing over, which is initiated by the formation of lesions in one of the interacting DNA molecules. Double-strand breaks (DSBs) in DNA can initiate crossing over in two well-studied species, the budding yeast Saccharomyces cerevisiae
and the very distantly related fission yeast Schizosaccharomyces pombe
, although other lesions, such as single-strand breaks (nicks), have not been excluded (Box 2). DSBs are made by a meiosis-specific topoisomerase-like protein Spo11 (called Rec12 in S. pombe
, ) in conjunction with several “meiotic break proteins”, which, like Spo11, are essential for both DSB formation and meiotic recombination () [1
]. A Spo11 ortholog appears to be present in all species that undergo meiosis, making it likely that DSBs are important for meiotic recombination in all species.
Proteins involved in meiotic recombination.
Meiotic recombination initiation in the fission yeast S. pombe
At a DSB, the 5' ends are digested away (resected), and the resultant 3'-ended single-stranded (ss) DNA invades an intact duplex at a point of extensive nucleotide sequence identity. Base-pairing between the two interacting DNA molecules forms hybrid DNA (). If the hybrid DNA contains one or more mismatches stemming from a genetic difference between the two parents, mismatch correction can produce three copies of one allele and only one of the other, a phenomenon called gene conversion or non-reciprocal recombination, an exception to Mendel's rule of 2:2 inheritance. Gene conversion can also arise from DNA synthesis that replaces the resected DNA. Resolution of the hybrid DNA intermediate can reciprocally recombine alleles flanking the hybrid DNA region to form a crossover. Alternatively, resolution can leave the flanking alleles in the parental configuration to form a non-crossover. Gene conversion can thus be accompanied by either a crossover or a non-crossover, both of which are forms of reciprocal recombination.
The formation of DSBs and their repair provide multiple levels for control of recombination. Gene conversion frequencies vary greatly along chromosomes, and not surprisingly DSBs were first detected at hotspots of gene conversion, sites that convert at a frequency higher than the genome average. Genome-wide studies have shown that DSB formation is far from random in both budding and fission yeasts, and DSB hotspots and gene conversion hotspots appear to be coincident. Since DSB formation occurs after replication, a DSB can be repaired by interaction with its sister chromatid or with either chromatid of the homolog. Until recently, it was assumed that DSB repair in meiosis occurred only with the homolog, because only crossovers between homologs can properly direct homolog segregation (). Studies in both yeasts unexpectedly show, however, that DSBs can be repaired with either the sister or one of the two chromatids of the homolog [3
]. A further surprise is that resolution to crossover vs.
non-crossover can be regulated in response to the total number of DSBs in the cell, as observed in budding yeast [5
]. Here, we discuss factors that influence DSB formation and repair, including two recently described phenomena - crossover homeostasis [5
] and crossover invariance [6