Cross fertilization in vivo and in vitro between three species of vesper mice, Calomys (Rodentia, Cricetidae)

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Abstract
Cross fertilization was tested between oocytes of Calomys callidus and spermatozoa from C. callidus, C. musculinus and C. laucha by both in vivo and in vitro insemination. After in vivo and in vitro insemination, respectively, percentages of oocytes fertilized were 68.8 and 46.6 (C. callidus × C. callidus), 20.3 and 12.7 (C. callidus × C. musculinus), 26.7 and 4.3 (C. callidus × C. laucha). Thus, the percentages obtained after in vitro insemination were always lower than those obtained with in vivo insemination. It was found that 23.9% and 44.4% of two‐cell hybrid embryos were present in oviducts 30 hr after in vivo insemination of C. callidus females with C. musculinus or C. laucha spermatozoa, respectively. At a later stage (56 hr postinsemination), development did not progress further, and abnormal embryos were found both at 30 and 56 hr postinsemination, suggesting some kind of cleavage arrest or degeneration of the embryos. We suggest that fertilization is not strictly species‐specific, at least among the species we studied, but that there are some factors that reduce the efficiency of interspecific fertilization.