HETEROSIS AND INTERCLONAL VARIATION IN THERMAL TOLERANCE IN UNISEXUAL FISHES.

Although interspecific hybridization in nature is not unusual, the progeny of such matings rarely are successful (Dobzhansky, 1970; Mayr, 1970). Even hybrid swarms are usually restricted to narrow zones where ranges of species abut. Three reasons for failure of hybrids are 1) they are usually competitively inferior to their progenitors, being adapted to neither parental niche; 2) any successful combinations of parental traits are usually broken up by genetic recombination when hybrids reproduce; and 3) hybrids frequently have low fertility and/or viability. The failure of hybrid progeny means that the energy expended on reproduction by hybridizing individuals is wasted and their fitness lowered; this is the impetus for the evolution of isolating mechanisms (Mayr, 1970). Thus, the fate of hybrids is very different in nature than it is under domestication. Domestic hybrids are usually the result of crosses between inbred lines, frequently of the same species (see for example Lerner, 1954). Their "hybrid vigor" may only be the restoration of normal levels of heterozygosity reduced by inbreeding. But whatever the genetic relationship of the parents, domestic hybrids may be preserved by artificial selection if they have some desirable trait, even if their fertility is low. Hybrids of the genus Poeciliopsis (Pisces: Poeciliidae) differ from both typical wild hybrids and domestic hybrids in several ways. They represent naturallyoccurring, self-replicating populations which frequently outnumber the parental species, and they occur over much of the range of the genus in the rivers of northwestern Mexico (Moore et al., 1970). Poeciliopsis hybrids are also all-female. Schultz (1961) suggested that all-femaleness is caused by the original hybridizing female (always P. monacha) possessing a "stronger" sex-determining mechanism than the hybridizing male (either P. occidentalis, P. lucida, or P. latidens). In any case, the mechanism perpetuating unisexuality is consistent; lines of unisexuals maintained in the laboratory for over 40 generations have consistently produced only female offspring. Poeciliopsis hybrids like other hybrids are highly heterozygous as a result of each hybridizing species contributing different alleles to the progeny. P. monacha-lucida, for example, is heterozygous at three times as many electrophoretically-sampled loci as members of the parental species, and P. monacha-occidentalis is heterozygous at five times as many loci (Vrijenhoek et al., 1977). These highly heterozygous genotypes are preserved in Poeciliopsis hybrids by two unusual reproductive systems, hybridogenesis and gynogenesis. Diploid unisexual hybrids reproduce by hybridogenesis (Schultz, 1969), in which segregation of chromosomes is not random. The monacha genome is transmitted intact to the haploid egg, while the paternal chromosomes are lost during meiosis (Schultz, 1969; Cimino, 1972a, b). Hybrids are sympatric with one of the three paternal species and mate with local males. Fertilization of the monacha egg by one of these males restores the hybrid genotype. All offspring are female and exhibit characters of both parents. They are phenotypically and genotypically equivalent to an F1 cross of P. monacha and the paternal species. The maternal chromosomes are, therefore, inherited clonally, while the paternal chromosomes are replaced in each generation. Every zygote