INTERSPECIFIC HYBRIDIZATION AND GENETIC DIVERGENCE IN MILKWEED BUGS (ONCOPELTUS: HEMIPTERA: LYGAEIDAE).

Abstract

Hybridization is generally considered of relatively minor importance in animal evolution (Mayr, 1963, 1970; White, 1973, 1978), primarily because most hybrid forms are sterile. It can, however, have a significant evolutionary role if it results in introgression. Furthermore, interspecific hybridization in fish and insects has produced viable, fertile forms with clonal reproductive mechanisms (Schultz, 1973; Drosopoulos, 1978), although the evolutionary potential of these asexual forms is, as yet, uncertain (see Ghiselin, 1974; Williams, 1975; Maynard Smith, 1978). In some species of milkweed bug (Oncopeltus) interspecific pairs are frequently found in nature and, in some cases, hybrid individuals have been collected (Chaplin, 1973; Root and Chaplin, 1976; Blakley, 1977; O'Rourke, 1979; J. D. Baldwin, Univ. of Iowa, pers. comm.). It is possible, therefore, that hybridization and introgression may be important factors in the evolution of this genus. In an earlier study, O'Rourke (1979) systematically hybridized three species of milkweed bug, 0. cingulifer, 0. sandarachatus, and 0. unifasciatellus. Oncopeltus cingulifer females failed to produce any progeny in matings with males of either of the other two species, while 0. unifasciatellus females had only minimal success in matings with 0. sandarachatus males. All other F1 matings had between 21% and 27% hatching and backcrosses 34% to 55% hatching. Interestingly, not all F1 progeny from these crosses displayed the expected intermediate phenotype, but instead strongly resembled one or the other parental phenotype. The unusual inheritance of phenotypes persisted when phenotypically intermediate F1 progeny were backcrossed to either parental species. Three phenotypic classes were observed in the progeny: both ancestral parental phenotypes, as well as the expected intermediates. Cytological studies of progeny indicate the chromosomal markers are consistent with the external phenotype, e.g., individuals with a sandarachatus phenotype, regardless of parent, are homozygous for the sandarachatus chromosomes (O'Rourke, 1979). The irregular patterns of inheritance in these species and their hybrids are surprising and merit further study, particularly using phenotypic characters, such as allozymes, with known genetic and simple modes of transmission. In this paper we report an attempt to replicate O'Rourke's (1979) work on 0. sandarachatus and 0. cingulifer and extend it by hybridizing these species with other members of the subgenus Erythrischius: 0. fasciatus, 0. aulicus, and 0. longirostris. The specific questions to be addressed are as follows: 1) Is the unusual reproductive mechanism in sandarachatus Y x cingulifer d crosses reported by O'Rourke repeatable, (a) using independently derived strains of the same subspecies, 0. c. cingulifer, from the same island (Trinidad); (b) using strains of 0. c. cingulifer from sources other than Trinidad; and' (c) using the other subspecies 0. cingulifer antillensis, which occurs in the northern Caribbean? 2) Do other interspecific hybrids have unusual inheritance patterns? 3) Is the production of viable hybrids predictable on the basis of genetic divergence between species as measured by differences in electromorphs?