HYBRID BREAKDOWN BETWEEN TWO HAPLODIPLOID SPECIES: THE ROLE OF NUCLEAR AND CYTOPLASMIC GENES

Abstract

A central question in evolutionary biology concerns the population and genetic processes by which new species arise. Here, the genetic basis of hybrid breakdown between two haplodiploid species, Nasonia vitripennis and N. giraulti is investigated. Hybridization between the two species is normally prevented by microorganisms that cause bidirectional incompatibility. However, after elimination of microorganisms, F[ hybrids females are readily produced (due to haplodiploidy, males develop from unfertilized eggs and are therefore not hybrids). F[ hybrid females are viable and fecund, but recombinant (haploid) F 2 male offspring suffer from severe hybrid breakdown (larval and pupal mortality). This is typically interpreted as evidence for the existence of different coadapted gene complexes in the two species, which are broken up by recombination. F 2 recombinant eggs were rescued by fertilization with the complete chromosome complement from either species, supporting the view that hybrid lethality genes tend to be recessive. Negative epistatic interactions occur between nuclear genes of the two species, and between cytoplasmically inherited factors (cytoplasmic genes) of giraulti and nuclear genes of vitripennis. Interactions between nuclear genes and cytoplasmic genes are asymmetric. Experiments clearly demonstrate that the latter incompatibility is not due to maternal-effect genes, but to cytoplasmically inherited elements. Nuclear-mitochondrial interactions are possibly involved. The problem of speciation-how new species arise-is a central question in evolutionary biology. A key element in speciation is the development of barriers against gene flow between populations. Most theoretical and empirical studies of speciation have focused on the role of conventional nuclear genes. The classical view of the evolution of postzygotic reproductive isolation under the biological species concept (see Otte and Endler 1987) argues that reproductive isolation is achieved by fixation of a set of interacting genes or incompatibility genes in different populations (Dobzhansky 1937; Muller 1942). This view is supported by a number of empirical studies in plants (e.g.