During meiosis, the mechanisms responsible for homolog alignment, synapsis, and recombination are precisely coordinated to culminate in the formation of crossovers capable of directing accurate chromosome segregation. An outstanding question is how the cell ensures that the structural hallmark of meiosis, the synaptonemal complex (SC), forms only between aligned pairs of homologous chromosomes. In the present study, we find that two closely related members of the him-3 gene family in Caenorhabditis elegans function as regulators of synapsis. HTP-1 functionally couples homolog alignment to its stabilization by synapsis by preventing the association of SC components with unaligned and immature chromosome axes; in the absence of the protein, nonhomologous contacts between chromosomes are inappropriately stabilized, resulting in extensive nonhomologous synapsis and a drastic decline in chiasma formation. In the absence of both HTP-1 and HTP-2, synapsis is abrogated per se and the early association of SC components with chromosomes observed in htp-1 mutants does not occur, suggesting a function for the proteins in licensing SC assembly. Furthermore, our results suggest that early steps of recombination occur in a narrow window of opportunity in early prophase that ends with SC assembly, resulting in a mechanistic coupling of the two processes to promote crossing over.
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