Age-related changes in the meiotic chromosomes of the nematode Caenorhabditis elegans.

Comparison of pachytene karyotypes from old and young wild-type hermaphrodites and males and the mutant him-5 were made following three-dimensional reconstruction of serial ultrathin sections. Age-related changes included: (1) differential condensation of chromatin with increased variance in length of chromosomes; and (2) increased variation in nuclear and nucleolar volume along with increased density of the nucleoplasm. Synaptonemal complex (SC) fine structure was not altered in the nuclei from older specimens. Attachment of only one end of the SC to the nuclear envelope (NE), common to all nematodes, was present at all ages in the wild-type hermaphrodite and male, however, clustering of the SC ends was present in nuclei from older him-5 hermaphrodites. Condensed chromatin along the SC formed a continuous mass except in those small regions where the chromatin had a granular appearance and was decondensed. Such regions, termed "Disjunction Regulator Regions" (DRR), have been implicated in the regulation of X-chromosome segregation (Goldstein, P., The synaptonemal complexes of Caenorhabditis elegans: Pachytene karyotype analysis of the Dp 1 mutant and disjunction regulator regions. Chromosoma, 93 (1985) 177-182). In the present study, it was observed that the number of DRRs in the nucleus change with aging. In the wild-type hermaphrodite and male, the rate of X-chromosome non-disjunction increases with age which correlates with a decrease in the number of DRRs to the point where they are absent in older males. In him-5, the DRRs increase in number with advanced age, which correlates with an observed decrease in the rate of X-chromosome non-disjunction.