For an orderly segregation of homologous chromosomes during meiosis I, each chromosome must recognize, pair and recombine with its correct partner. Once homologs recognize each other, they colocalize along their lengths and a proteinaceous structure known as the synaptonemal complex (SC) assembles between them. Meiotic recombination takes place in this context resulting in the formation of chiasmata, physical links that hold homologs together upon SC disassembly. The SC is composed of two axes, one on each homolog, and a central region that links the axes. Our goal is to understand the mechanisms of chromosome pairing and their relation to later meiotic events such as synapsis and recombination. We identified a new pairing mutant,
me84, based on its lack of chiasmata at the diakinesis stage. Upon initial observation, DAPI-stained chromosomes at the pachytene stage appeared normal, with parallel tracks indicating the presence of intimately aligned and synapsed chromosomes. Surprisingly, FISH experiments revealed a severe pairing defect at this stage, demonstrating the occurrence of extensive non-homologous synapsis; close observation of chromosomes also revealed the presence of pairing partner switches. Both axial components (HIM-3) and central region components (SYP-1 and SYP-2) load onto chromosomes and form linear structures similar in appearance to those seen in wild type. However, SYP-1 and SYP-2 are also found on some unsynapsed regions of chromosomes, and they persist longer (compared to wild type) on the unattached diakinesis chromosomes.
me84 is a loss-of-function mutation in F41H10.10 which encodes a HIM-3 paralog. Whereas HIM-3 is required to load SYP-1 and SYP-2 onto chromosomes, however, our results suggest that F41H10.10 plays a very different role, in preventing SYP-1 and SYP-2 from loading inappropriately between non-homologous chromosomes. Although
me84 mutants apparently lack interhomolog crossovers, RAD-51 foci reflecting nascent recombination events both appear with normal kinetics and diminish as prophase progresses, suggesting that the
me84 mutant may also be defective in inhibiting the use of sister chromatids as recombination partners. We also examined SYP-1 and SYP-2 localization in the
chk-2mutant in which homologs never pair. Although HIM-3 loads onto chromosomes with normal kinetics in
chk-2 mutants, loading of SYP-1 and SYP-2 is substantially delayed and restricted to a subset of chromosome segments. We are investigating how the CHK-2 protein kinase and the HIM-3 paralog F41H10.10 coordinate SC assembly with homolog recognition.