Cytonuclear Disequilibrium in Chrysochus Hybrids Is Not Due to Patterns of Mate Choice

Abstract

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, > or = 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 < 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.

Cryptic gametic interactions confer both conspecific and heterospecific advantages in the Chrysochus (Coleoptera: Chrysomelidae) hybrid zone

“…Nearly all individuals in the hybrid zone have parental or F1 hybrid multilocus genotypes, but rare exceptions suggest that the completion of speciation was either very recent or has not yet happened (Peterson et al 2005a). Because of the promiscuous nature of these beetles (adults mate daily during their 6-8 week lifespan (Dickinson 1995)), patterns of mate choice in this system have proven remarkably easy to study, both in the lab and the field (Dickinson 1995; Peterson et al 2005aPeterson et al , b, 2007Monsen et al 2007). Previous work has demonstrated that mixed populations exhibit a weak pattern of positive assortative mating (Peterson et al 2005b), due at least in part to male mate choice that is mediated by the cuticular hydrocarbon profiles of females ).…”

Viability and fertility of hybrid New Zealand tree wētā Hemideina spp. (Orthoptera: Anostostomatidae)

“…A bimodal hybrid zone in two species of chrysomelid beetles (Chyrsochus cobaltinus and C. auratus) also involves stronger post-zygotic barriers than pre-zygotic barriers (Peterson et al 2005), so the association of assortative mating and bimodal hybrid zones has exceptions. A later study of these same beetles also showed a significant sex-bias in the production of offspring (most had mtDNA haplotypes and hence mothers from one species), despite mating occurring in both directions in the wild, and offspring in both sex-pairings being produced in equal numbers and with equal viability in laboratory crosses at the first instar (Monsen et al 2007). The proposed explanation was asymmetric post-mating pre-zygotic barriers, or possible asymmetric inviability later in development.…”

The effects of heterospecific mating frequency on the strength of cryptic reproductive barriers

“…To evaluate the effects of cryptic barriers on the genetic composition of hybridizing populations, we combined our estimates of the strength of cryptic reproductive barriers with previously published observed species abundances, heterospecific mating frequencies and maternal species identification using F1 mtDNA haplotypes all taken from a focal hybrid zone site (Monsen et al., ; Peterson et al., 2005a). First, we estimated the mean heterospecific mating frequency of auratus and cobaltinus females at our focal site over three independent surveys by directly observing mating pairs (Table S4).…”

“…We conducted all crosses in the summers of 2004 ( cobaltinus females) and 2005 ( auratus females). We randomly assigned virgin beetles to mating treatments that mimicked the wide range of heterospecific mating frequencies observed in the hybrid zone, in which heterospecific mating frequency for a given species is negatively correlated with its relative abundance (Monsen et al., ; Peterson et al., 2005a). The design of the mating treatments varied slightly between years due to differences in the availability of virgin females, but we restricted comparisons between species to treatments with ten total mates and shared heterospecific mating frequencies: 0, 0.3, 0.5 and 0.7 (in bold below).…”

“…In this study, we crossed females of both species to varying numbers of conspecific and heterospecific males and assessed female lifetime reproductive fitness to: (a) determine whether females benefit from multiple matings with heterospecific males, (b) evaluate the effect of heterospecific mating frequency on the effectiveness of PMPZ barriers and (c) determine whether these interactions can explain a previous finding from a focal hybrid zone population, where the great majority of F1 hybrids were shown to be the offspring of cobaltinus females despite more than half of the heterospecific mating pairs having auratus females and cobaltinus males (Monsen et al., ). Our results demonstrate the importance of considering field‐based estimates of heterospecific mating frequency in studies of PMPZ barriers.…”