CHROMOSOME-PAIRING IN INTERRACIAL HYBRIDS OF THE HOUSE-MUSK-SHREW (SUNCUS-MURINUS, INSECTIVORA, SORICIDAE)

Two chromosome races of the house shrew Suncus murinus that differ fro m each other for five Robertsonian translocations (8.17, 9.13, 10.12, 11.16, and 14.15), heterochromatic insertions in chromosomes 7 and X, and multiple rearrangements in the Y chromosome were crossed and then intercrossed in captivity to produce a hybrid stock. Electron-microsco pe analysis of synaptonemal complexes in fertile and sterile hybrid ma les was carried out. Meiosis in sterile males did not progress beyond pachytene and was severely disrupted. Meiotic arrest was not determine d by structural heterozygosity: heterozygotes for all variant chromoso mes distinguishing two parental races were found in both sterile and f ertile male hybrids. Fertile hybrids demonstrated an orderly pairing o f all chromosomes. In heterozygotes for Robertsonian fusions, complete ly paired trivalents were formed between the Robertsonian metacentrics and homologous acrocentrics. In heterozygotes for chromosome 7, bival ents with a small buckle were observed in a small fraction of pachyten e cells. No differences were found in the morphology and pairing patte rn of sex bivalents, composed of the X and Y chromosomes derived from the same or different parental races. Univalents, multivalents, and as sociations between X and Y chromosomes and autosomal trivalents, as we ll as associations of autosomal trivalents with each other, were obser ved in a small fraction of the pachytene cells of fertile males. Our r esults indicate that the system controlling male sterility in interrac ial hybrids of S. murinus is of genic rather than of chromosomal type.