Synaptonemal complex analysis of domestic sheep (Ovis aries) with Robertsonian translocations. I. Pachytene karyotype substaging of normal sheep.

Meiotic chromosome pairing behaviour in three normal sheep was surveyed by synaptonemal complex (SC) analysis at the electron microscope level. The SC karyotype of normal rams is composed of a sex pair, three metacentric or submetacentric pairs, and 23 acrocentric autosomal bivalents, with the total autosomal SC length ranging from 309 to 315 μm. Five nucleoli are terminally located on the three metacentric, and one large and one small acrocentric autosomal bivalents. XY morphology was used to divide pachytene into five substages. Although pairing abnormalities (mostly SC interactions) were recorded in an average of 16% of the spermatocytes, bivalent–XY association occurred in only 4.7% of the cells.Key words: sheep, synaptonemal complex, XY pair, pachytene substaging.

An ultrastructural pachytene karyotype for Puccinia graminis f.sp. tritici

The karyotype for Puccinia graminis f.sp. tritici was determined from reconstructions of electron micrographs of serially sectioned pachytene nuclei. Epifluorescence microscopy was used to select DAPI-stained, pachytene nuclei in teliospore protoplasts from which walls were mechanically removed. Selection increased the probability that pachytene nuclei could be found in the absence of morphological markers. Six pachytene nuclei were reconstructed from four geographically disparate North American isolates. One nucleus was used to obtain a computer-enhanced, three-dimensional reconstruction that could be rendered as rotatable colorized stereo pairs. A karyotype of n = 18 was determined for all six nuclei. The 18 bivalents varied only slightly in size, each ranging from 3.0 to 8.8% of the total length of the genome. Total genomic lengths also proved highly comparable among isolates. Centromeres were not found, precluding use of the centromeric index as an aid in karyotyping. Only the nucleolus-associated bivalent could be cross-correlated among the six reconstructed nuclei. Heterogeneity in length among, but not within, isolates was observed for this bivalent, suggesting that chromosome length may be polymorphic in P. graminis f.sp. tritici, despite a constant total number of chromosomes. Key words: chromosomes, heterobasidiomycete, synaptonemal complex, Uredinales.

The synaptonemal complexes of Caenorhabditis elegans: pachytene karyotype analysis of hermaphrodites from the recessive him-5 and him-7 mutants

The him-5 and him-7 mutants (high incidence of males) of Caenorhabditis elegans both showed increased rates of X chromosome non-disjunction (16% and 3%, respectively) but him-7 also had a high frequency of autosomal non-disjunction (34%). Synaptonemal complex (SC) karyotype analysis revealed a haploid chromosome number of six in each strain. Alterations in him-7 nuclear morphology were observed but there were no aberrations in SC structure that could account for the increased frequency of autosomal non-disjunction. However, the frequency of X-chromosome non-disjunction occurred at predicted rates on the basis of the number of disjunction regulator regions (DRRs) present on the SCs. The observation that the levels of X-chromosome non-disjunction were not influenced by the increase in the frequency of autosomal non-disjunction supports the notion that the X chromosome is subject to separate controls during meiosis. The him-7 mutant is nested within the rad-4 map region on linkage group V, however, SC analysis did not reveal the physical position on the chromosome because of synaptic adjustment.

Triplo-X hermaphrodite of Caenorhabditis elegans: pachytene karyotype analysis, synaptonemal complexes, and pairing mechanisms

Pairing of the three X chromosomes in the triplo-X strain of Caenorhabditis elegans occurs at pachytene in a two-by-two fashion such that one bivalent and one univalent are formed. The XX bivalent pairs synchronously with the autosomes and the univalent X remains in a similar chromatic state as the rest of the chromosomal complement. Normal tripartite synaptonemal complexes (SC) are formed between all bivalents. The univalent X lacks a SC and an axial core is not observed. The condensation of the univalent X in the triplo-X is different than in the male where the univalent X is heterochromatic. This real difference in condensation states of the chromatin may explain the fact that the univalent X is maintained in the male line yet it is easily lost in the triplo-X strain.