During meiosis, crossovers are essential to orient and segregate homologous chromosomes. Disruption of condensin I or condensin II, SMC-family complexes that structure chromosomes for cell division, perturbs the number and distribution of meiotic crossovers (COs). In C. elegans, wild type animals have just one CO per chromosome; in animals depleted of condensin, some chromosomes have two or three. This increase in COs is strongly correlated with an extension of the chromosome axis and an increase and shifted distribution of CO precursors (double-strand breaks, DSBs). In many organisms, COs are resolved from DSBs by one of two pathways: the ZMM family of proteins or the Mus81-Mms4 endonuclease. The ZMM pathway is thought to be the dominant method of generating COs in C. elegans, but it is not known whether the extra COs in condensin-depleted animals are resolved by this pathway. In animals treated with gamma-irradiation, CO number increases but the numbers of a cytological marker for COs, COSA-1 foci, do not differ from non-irradiated controls (Yokoo et al 2012). Animals depleted of subunits in condensin I (
dpy-26/+), condensin II (
kle-2/+), or both, have more COs than wild-type animals, but retain wild-type levels of COSA-1 foci. Therefore, condensin mutants have COs that are marked by COSA-1 and not marked by COSA-1.
To determine whether the unmarked COs are resolved by the MUS-81 pathway, we examined the suppression of COs by
mus-81 in condensin mutants. In
mus-81;
dpy-26/+ and
mus-81;
kle-2/+ animals, we observed no suppression by
mus-81. However, in
mus-81;
kle-2/+;
dpy-26/+ animals, we did observe some suppression. In
kle-2/+;
dpy-26/+ animals, 20% of X chromosomes had double COs and 2% had triple COs, while in
mus-81;
kle-2/+;
dpy-26/+ animals, 6% of X chromosomes had double COs and none had triple COs (p value=.00001), similar to levels observed in
dpy-26/+. Condensin depletion may induce repair by MUS-81 through changes in chromosome structure, formation of excess DSBs, or a reduction in condensin complexes. To better understand the conditions required to invoke this alternate CO pathway, we are examining whether DSBs induced by gamma irradiation use MUS-81 for their resolution as COs, and whether the requirement for MUS-81 is caused by excess DSBs or exogenous damage from irradiation.