What Is the Fate of DSBs in Balancer Heterozygotes?
Given that the DSBs that occur along the length of the FM7/X bivalent are not matured into crossovers, it becomes important to understand just how they might be repaired. Studies of exchange in females heterozygous for both FM7 and a ring-X chromosome (FM7/R(1) females) presented in McKim et al. [6] failed to show a substantial level of ring chromosome loss, as might be expected if DSBs were frequently processed to sister chromatid exchanges [38]. One possibility is that these events are repaired by either inter-homolog gene conversion events or by gene conversion events involving the sister chromatid. The effect of breakpoint heterozygosity on conversion is unclear. Sherizen et al. [1] found a greater than 6-fold reduction of inter-homolog conversion events near a breakpoint in translocation heterozygotes. Perhaps then, in the vicinity of the breakpoint, sister chromatid conversion events predominate, while at greater distances from the breakpoint repair by inter-homolog conversion events becomes more frequent.
However, Chovnick [39] found little or no effect on gene conversion when comparing conversion at the rosy locus in females with two normal sequence third chromosomes and in females heterozygous for a paracentric inversion that includes rosy. One possible explanation for this discrepancy might lie in the fact that the two breakpoints studied by Sherizen et al. [1] were within one numbered polytene division of the rosy locus, while both breakpoints of the inversion studied by Chovnick [39] were greater than four polytene units away from rosy. Indeed, the inversion studied by Chovnick [39] included two “boundary or pairing site” elements (see [1]) that might function to restore pairing and synapsis.