Hybrid Sterility

Abstract

Throughout the ages man’s attempts to combine the desirable traits of parental species in the progeny through experimental hybridization have been frustrated by several barriers of reproductive isolation. These barriers, which effectively restrict gene exchange between species, are built by a variety of mechanisms, all of which may be interrelated. Depending upon the developmental stage at which a particular isolating mechanism operates, interspecific hybridization may result in hybrid inviability, total or partial sterility of the hybrids, or reduced viability of the back-cross progeny. Examples of the first type of isolating mechanisms are sheep-goat hybrids (Berry, 1938; Hancock, 1964; Warwick et al., 1934) and ferret-mink hybrids (Chang, 1965) all of which perish either in embryonal or fetal stages (Chang and Hancock, 1967). Total sterility of the progeny has been the general trend in most of the equine hybrids (Benirschke et al., 1964; King et al., 1966) despite the exceptions which have been reported from time to time (Benirschke, 1967; Koulischer and Frechkop, 1966). Partial sterility in hybrids is often expressed as sex-restricted malfunctions, which are generally more pronounced in the heterogametic sex (Haldane, 1922). Thus, among the interspecific hybrids, the males generally die in ntero or grow to be sterile adults in mammals, whereas in birds, with a reversed sex determining mechanism, it is the female hybrids which are either at a reproductive or developmental disadvantage.