Consequences of reproductive barriers for genealogical discordance in the European corn borer

Speciation involves the origin of trait differences that limit or prevent gene exchange and ultimately results in daughter populations that form monophyletic or exclusive genetic groups. However, for recently diverged populations or species between which reproductive isolation is often incomplete, gene genealogies will be discordant, and most regions of the genome will display nonexclusive genealogical patterns. In these situations, genome regions for which one or both species are exclusive groups may mark the footprint of recent selective sweeps. Alternatively, such regions may include or be closely linked to "speciation genes," genes involved in reproductive isolation. Therefore, comparisons of gene genealogies allow inferences about the genetic architectures of both reproductive isolation and adaptation. Contrasting genealogical relationships in sexually isolated pheromone strains of the European corn borer moth (Ostrinia nubilalis) demonstrate the relevance of this approach. Genealogies for five gene regions are discordant, and only one molecular marker, the sex-linked gene Tpi, has evidence for pheromone strain exclusivity. Tpi maps to a position on the sex chromosome that is indistinguishable from a major factor (Pdd) affecting differences in postdiapause development time. The major factor (Resp) determining male behavioral response to pheromone is also sex-linked, but maps 20-30 cM away. Exclusivity at Tpi may be a consequence of these linkage relationships because evidence from phenotypic variation in natural populations implicates both Pdd and Resp as candidates for genes involved in recent sweeps and/or reproductive isolation between strains.