Abstract
Spo11 is the topoisomerase-like enzyme responsible for the induction of the meiosis-specific double strand breaks (DSBs), which initiates the recombination events responsible for proper chromosome segregation. Nineteen PCR-induced alleles of SPO11 were identified and characterized genetically and cytologically. Recombination, spore viability and synaptonemal complex (SC) formation were decreased to varying extents in these mutants. Arrest by ndt80 restored these events in two severe hypomorphic mutants, suggesting that ndt80-arrested nuclei are capable of extended DSB activity. While crossing-over, spore viability and synaptonemal complex (SC) formation defects correlated, the extent of such defects was not predictive of the level of heteroallelic gene conversions (prototrophs) exhibited by each mutant. High throughput sequencing of tetrads from spo11 hypomorphs revealed that gene conversion tracts associated with COs are significantly longer and gene conversion tracts unassociated with COs are significantly shorter than in wild type. By modeling the extent of these tract changes, we could account for the discrepancy in genetic measurements of prototrophy and crossover association. These findings provide an explanation for the unexpectedly low prototroph levels exhibited by spo11 hypomorphs and have important implications for genetic studies that assume an unbiased recovery of prototrophs, such as measurements of CO homeostasis. Our genetic and physical data support previous observations of DSB-limited meioses, in which COs are disproportionally maintained over NCOs (CO homeostasis).
Highlights
An important function of meiosis is to precisely segregate one copy of each chromosome into cells that become gametes
Hypomorphic spo11 alleles were isolated by screening transformants carrying PCR-derived mutant SPO11 sequences for defects in gene conversions and spore viability (Materials and Methods)
Genetic analysis of spo11 hypomorphs must take into account that mutants with severe defects will generate mostly dead spores due to nondisjunction of nonrecombinant chromosomes [26]
Summary
An important function of meiosis is to precisely segregate one copy of each chromosome into cells that become gametes. This accurate division is accomplished by presenting the meiosis I spindle with homolog pairs that have been linked by crossovers (COs), so that partner chromosomes can segregate from each other. A single CO can be sufficient for proper segregation Those COs that occur too close or too far from the centromere can be detrimental [1,2,3]. It is essential for the meiotic cell to control the number and distribution of COs
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