CHROMOSOMAL HETEROZYGOSITY AND FERTILITY IN-HOUSE MICE (MUS-MUSCULUS-DOMESTICUS) FROM NORTHERN ITALY

Following the discovery of over 40 Robertsonian (Rb) races of Mus musc ulus domesticus in Europe and North Africa, the house mouse has been s tudied extensively as an ideal model to determine the chromosomal chan ges that may cause or accompany speciation. Current models of chromoso mal speciation are based on the assumption that heterozygous individua ls have a particularly low fertility, although recent studies indicate otherwise. Despite their importance, fertility estimates for the hous e mouse are incomplete because traditional measurements, such as anaph ase I nondisjunction and germ cell death, are rarely estimated in conj unction with litter size. In an attempt to bridge this gap, we have ta ken advantage of the house mouse hybrid zone in Upper Valtellina (Lomb ardy, Italy) in which five Rb races interbreed. We present data on the fertility of naturally occurring (''wild-caught'') hybrids and of off spring from laboratory crosses of wild-caught mice (''laboratory-reare d''), using various measurements. Wild-caught mice heterozygous for on e fusion were more infertile than predicted from past studies, possibl y due to genic hybridity; laboratory-reared heterozygotes carrying sev en or eight trivalents at meiosis I and heterozygotes carrying one pen tavalent also had low fertilities. These low fertilities are especiall y significant given the probable occurrence of a reinforcement event i n Upper Valtellina.