A COMPARISON OF MEIOSIS AND GAMETOGENESIS IN HOMOZYGOTES AND SIMPLE ROBERTSONIAN HETEROZYGOTES OF THE COMMON SHREW (SOREX-ARANEUS) AND HOUSE MOUSE (MUS-MUSCULUS-DOMESTICUS)

Chromosome pairing at prophase I of meiosis was orderly in both shrews and mice. In heterozygotes for simple Robertsonian rearrangements a t rivalent forms between the metacentric and two homologous acrocentric chromosomes. In shrews only 36% of trivalents had a small side arm for med by the pairing of the centromeric regions of the acrocentrics, whe reas in mice 100% of trivalents exhibited a distinct side arm. No diff erence was found between homozygotes and heterozygotes of either speci es, in the percentage of cells in which there was an association betwe en the autosomes and sex chromosomes. The level of germ cell death was not significantly greater (6%) in heterozygotes of the common shrew c ompared to homozygotes. In the house mouse, heterozygotes had a signif icantly greater (18%) level of germ cell death over homozygotes. In bo th species, however, single simple Robertsonian heterozygotes would ha ve near normal fertility.