Asymmetrical homogamy and unequal sex ratio from reciprocal mating-order crosses between Chorthippus parallelus subspecies

Abstract

To study the evolution of this Pyrenean hybrid zone following postglacial secondary contact, a crossing programme was performed involving sequential mating of single females by males of both subspecies in both orders. Progeny were scored as embryos using Acridine orange and C-banding to identify their male parent. This revealed a number of significant departures from straightforward inheritance. There was an excess of pure over hybrid progeny. This homogamy is produced at fertilization but may have been determined before through sperm preference or competition. There was also sperm precedence with the second mating producing most of the progeny. This may be due to sperm ageing or differential storage and utilization by the female. The fundamental sex ratio in the progeny was biased towards males. Of particular interest is the finding that homogamy is markedly greater when C.p. parallelus is the mother, suggesting some form of differential biochemical recognition. This asymmetry should have important consequences for the dynamics of the hybrid zone. All these results are discussed in the light of the divergence of the two subspecies and the mixing of their genomes in the hybrid zone.