ABSTRACT
Let us fi rst answer the questions in the title of this section. A hybrid is a mixture, an offspring of the cross(es) between genetically different organisms. Hybrids may be treated as well as individuals having a mixed ancestry. In a certain sense, a heterozygote at one or more loci is a hybrid individual. So, the cross P1: A1A1 B2B2 × A2A2 B3B3 gives the hybrid F1 = FH: A1A2 B2B3. Hybridization is a process through which hybrids occur. We should recognize the difference between the simple intra-deme crosses and crosses between different lines, demes and species. True hybrids are normally thought to be the offspring of more distant parents. The meaning of distant here is quite conditional, depending on the particular organism and its normal system of breeding. Beyond F1 hybrids, other types of hybrids may develop: F1 × F1 = F2, F1 × P1= Fb and so on. Hybridization may be either natural or artifi cial. In this chapter we will deal mostly with the former. Hybrids need not be exactly intermediate to the parental types, but depending on the complexity of the cross can be closer to one of them (Fig. 10.1.1). We will look at more details on this point later. It is accepted normally that fi tness (W) is lower in hybrids in comparison to parental forms (Fig. 10.1.1), but this rule does not always hold. Sometimes the reverse is true: hybrid vigor can occur. However, these cases are beyond our theme, and belong largely to the artifi cial crosses between inbred
lines or brood stock kinds. The ability and propensity of taxonomically distinct fi shes to interbreed and produce viable hybrid offspring are now well established. Schwartz (1972; 1981) compiled nearly four thousand references dealing with the natural and artifi cial hybridization of fi shes. Much earlier Hubbs (1955) reviewed the literature on natural hybridization among North American fi shes.
