We investigated patterns of cytonuclear disequilibrium between nuclear allozyme loci and partial mitochondrial COI and COII restriction fragment length polymorphism patterns within a population of hybridizing chrysomelid beetles and assessed to what degree the genotype frequencies of F1 hybrids were consistent with patterns of mate choice or endosymbiont infection. We document that in this population, ≥50% of the heterospecific pairs at a given time are composed of Chrysochus auratus females and Chrysochus cobaltinus males, suggesting that at least half of the F1 hybrids should possess the C. auratus mitochondrial genotype. However, we found that the majority (89%) of F1 hybrids possessed C. cobaltinus mtDNA (P \u3c 0.001). The lack of evidence for Wolbachia infection in these highly promiscuous beetles, coupled with the fact that F1 hybrids of both cross types do exist, indicates that endosymbionts are an unlikely explanation for the discrepancy between cytonuclear genotype frequencies and behavior. We argue that cytonuclear disequilibrium at this focal Chrysochus hybrid site is likely due to a strong directional bias in postmating prezygotic barriers in this system. The results presented here underscore the importance of combining both field and molecular data in studies of cytonuclear disequilibrium and point to the dangers inherent in attributing patterns of cytonuclear disequilibrium to assortative mating
