A cis-acting locus that promotes crossing over between X chromosomes in Caenorhabditis elegans.
This study reports the characterization of a cis-acting locus on the Caenorhabditis elegans X chromosome that is crucial for promoting normal levels of crossing over specifically between the X homologs and for ensuring their proper disjunction at meiosis I. The function of this locus is disrupted by the mutation me8, which maps to the extreme left end of the X chromosome within the region previously implicated by studies of X; A translocations and X duplications to contain a meiotic pairing site. Hermaphrodites homozygous for a deletion of the locus (Df/Df) or heterozygous for a deletion and the me8 mutation (me8/Df) exhibit extremely high level of X chromosome nondisjunction at the reductional division; this is correlated with a sharp decrease in crossing over between the X homologs as evidenced both by reductions in genetic map distances and by the presence of achiasmate chromosomes in cytological preparations of oocyte nuclei. Duplications of the wild-type region that are unlinked to the X chromosome cannot complement the recombination and disjunction defects in trans, indicating that this region must be present in cis to the X chromosome to ensure normal levels of crossing over and proper homolog disjunction. me8 homozygotes exhibit an altered distribution of crossovers along the X chromosome that suggests a defect in processivity along the X chromosome of an event that initiates at the chromosome end. Models are discussed in which the cis-acting locus deleted by the Dfs functions as a meiotic pairing center that recruits trans-acting factors onto the chromosomes to nucleate assembly of a crossover-competent complex between the X homologs. This pairing center might function in the process of homolog recognition, or in the initiation of homologous synapsis.
Villeneuve
A M
AM
Department of Developmental Biology, Stanford University School of Medicine, California 94305.
D000818:Animals; D017173:Caenorhabditis elegans / Q000235:genetics*; D002874:Chromosome Mapping; D003434:Crossing Over, Genetic; D012734:Disorders of Sex Development / Q000235:genetics; D005260:Female; D017238:Genes, Helminth; D006579:Heterozygote; D008297:Male; D008540:Meiosis / Q000235:genetics; D008957:Models, Genetic; D009154:Mutation; D009865:Oocytes / Q000648:ultrastructure; D011995:Recombination, Genetic; D013090:Spermatocytes / Q000648:ultrastructure; D013489:Suppression, Genetic; D014178:Translocation, Genetic; D014960:X Chromosome