Abstract
Reports of species crosses among rodents have been infrequent in the literature and in most of these only small numbers were involved. A general phenomenon of such crosses has been the presence of heterosis. This has been manifested under several forms such as large size of the hybrids, longevity and increased fecundity. Another criterion might be the frequency of stillborn young since, as Feldman 1 concluded in the case of rats, stillbirths are due at least partly to impaired vigor of the female. Thus the paucity of stillbirths among the offspring of hybrid females would be a relative measure of augmented vigor. The present note presents some data on this point.
The forms used in this cross were the Little inbred strain of dilute brown non-agouti Mus musculus and a stock of Mus bactrianus, the original animals of which were captured near Peiping, China, in 1926. They are small, light-colored, white-bellied murids (genetically, intense black agoutis) intermediate between the subspecies gansuensis Satunin and tantillus Allen, 2 on the whole nearer the former. They mate readily with musculus and the offspring of both sexes are fertile. The first generation hybrids are intermediate in body weight although in such skeletal measurements as skull length, femur length and tibia length they usually equal or exceed the larger parent. Heterosis is markedly manifested in the increased litter size of the females, their litters averaging 6.4 ± 0.12 as opposed to 5.4 ± 0.04 for the straight dilute brown non-agouti musculus and 4.6 ± 0.11 for bactrianus.
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