Meiosis of T70H translocation trisomic male mice

Abstract

Meiosis of T70H/+,Ts(1¹³)70H translocation trisomic male mice has been studied using C-banded preparations of multivalents at the first meiotic division, marker chromosomes at the second meiotic division, and sucrose-spread pachytene spermatocytes for the observation of synaptonemal complexes. During zygotene and pachytene the three marker chromosomes, 13¹ (long) and 1¹³ (2 x, small) associate with the chromosomes 1 and 13 to form either a Chain of three plus bivalent (CIII+II), a Chain of four plus univalent (CIV+I) or a Chain of five (CV). During pachytene, the 1¹³ univalent of the CIV+I configuration shows association with the sex chromosomes in the sex vesicle while for CV, the unpaired segment of chromosome 1 can do so. — The frequencies of the multivalent configurations during late diplotenemetaphase I were CIII+II, 47.6%, CIV+I, 34.5% and CV, 14.4%. However, among late diplotene-early diakinesis cells, the frequency of CIV+I was 12.9% while in (late) spermatocytes with contracted bivalents it was 45.7%. — The notion that proximity of the interstitital chiasma of chromosome 13 to the centromere affects the chances of non-disjunction for these centromeres within the multivalent (i.e., adjacent 2 segregation) has been strengthened by the observed adjacent 2 frequency of 13.0% in the translocation trisomics compared to the 26.3% found in T70H/+ translocation heterozygotes with more proximal chiasmata. Thus, for reciprocal translocations between acrocentric chromosomes, a proximally located chiasma in an interstitial segment enhances the chance of adjacent 2 segregation. — Other parameters of irregular chromosome behaviour at anaphase I, such as equational separation of chromosome 1¹³ into chromatids and non-disjunction of normal bivalents, were increased in the translocation trisomics when compared with T70H/+ translocation heterozygotes, especially among adjacent 2 segregating cells.