Sperm numbers vary between inter- and intra-population matings of the grasshopper Chorthippus parallelus

Persistent postmating, prezygotic reproductive isolation between populations

Studying reproductive barriers between populations of the same species is critical to understand how speciation may proceed. Growing evidence suggests postmating, prezygotic (PMPZ) reproductive barriers play an important role in the evolution of early taxonomic divergence. However, the contribution of PMPZ isolation to speciation is typically studied between species in which barriers that maintain isolation may not be those that contributed to reduced gene flow between populations. Moreover, in internally fertilizing animals, PMPZ isolation is related to male ejaculate-female reproductive tract incompatibilities but few studies have examined how mating history of the sexes can affect the strength of PMPZ isolation and the extent to which PMPZ isolation is repeatable or restricted to particular interacting genotypes. We addressed these outstanding questions using multiple populations of Drosophila montana. We show a recurrent pattern of PMPZ isolation, with flies from one population exhibiting reproductive incompatibility in crosses with all three other populations, while those three populations were fully fertile with each other. Reproductive incompatibility is due to lack of fertilization and is asymmetrical, affecting female fitness more than males. There was no effect of male or female mating history on reproductive incompatibility, indicating that PMPZ isolation persists between populations. We found no evidence of variability in fertilization outcomes attributable to different female × male genotype interactions, and in combination with our other results, suggests that PMPZ isolation is not driven by idiosyncratic genotype × genotype interactions. Our results show PMPZ isolation as a strong, consistent barrier to gene flow early during speciation and suggest several targets of selection known to affect ejaculate-female reproductive tract interactions within species that may cause this PMPZ isolation.

Reproductive Costs for Hybridizing Female Anasa tristis (Hemiptera: Coreidae), but No Evidence of Selection Against Interspecific Mating

Individuals of different species sometimes mate in nature, and such behavior often carries costs, such as wasted gametes and inviable offspring. One context in which interspecific mating commonly occurs is when closely related species come into secondary contact. Here, we tested whether reproductive isolation is greater in an area of recent secondary contact than in allopatry for two closely related insect species, and we examined whether mating between individuals of these two species constitutes reproductive interference. In Florida, two species of squash bugs (Hemiptera: Coreidae: Anasa tristis DeGeer and Anasa andresii Guérin-Méneville) have been secondarily sympatric for ≥80 generations, and male A. andresii copulate with female A. tristis. Because hybridization is often costly for females, we predicted that secondarily sympatric females would be less likely to mate with heterospecifics than would allopatric females. We found no evidence of recent selection on reproductive isolation: females from both populations were equally likely to mate with heterospecifics, and heterospecific males did not make more mating attempts than conspecifics to achieve copulations. However, female A. tristis paired with heterospecifics produced many fewer eggs and offspring than females paired with conspecifics, and this did not differ according to whether females were from allopatric or sympatric populations. Our findings show that reproductive barriers between these species existed before secondary contact. We suggest that habitat use may limit encounter frequency, and that female choice, multiple mating, and postcopulatory processes may reduce costs for females. Consequently, we suggest that mating systems and ecological factors mediate the effects of reproductive interference.

Chemical cues from females trigger male courtship behaviour in grasshoppers

Gomphocerine grasshoppers use species-specific calling songs for sex recognition and mate attraction. In two closely related species, Chorthippus biguttulus and C. mollis, acoustic communication is the only experimentally characterized communication channel that elicits male courtship behaviour. However, courtship in these species involves extensive close-range interactions that are likely to be mediated by other signalling modalities, in particular chemical cues. We developed a bioassay to determine if female cuticular hydrocarbons (CHCs) act as chemical cues that induce courtship behaviour, and if males assess variation in CHCs to determine whether or not to court a female. The results of this approach provide evidence that grasshopper males use species- and sex-specific information from CHC signals and respond with a courtship song to the CHC profile of conspecific females but not to the CHC profile of heterospecific females and conspecific males. We conclude that males of C. biguttulus and C. mollis use multimodal channels for mating decisions, based on both acoustic and olfactory cues. We discuss various factors that might favour the evolution of male choosiness in grasshoppers.

Wolbachia induced cytogenetical effects as evidenced in Chorthippus parallelus (Orthoptera)

The cytoplasmic incompatibility induced by the bacterial endosymbiont Wolbachia is attributed to chromatin modification in the sperm of infected individuals and is only 'rescued' by infected females after fertilization. Chorthippus parallelus is a grasshopper with 2 subspecies that form a hybrid zone in the Pyrenees in which this Wolbachia-generated cytoplasmic incompatibility has recently been described. The analysis of certain cytogenetic traits (sex chromosome-linked heterochromatic bands, nucleolar organizing region expression, spermatid size and morphology, and number of chiasmata formed) in pure and hybrid Chorthippus parallelus that are infected and not infected by this bacterium indicates that the infection affects some of these traits and, in the case of the spermatids, reveals a synergism between the infection and the hybrid condition. These results are interpreted as being secondary effects of the chromatin modification induced by Wolbachia which thereby support this model of modification/rescue. The possible effects of these cytogenetic variations on affected individuals are also considered.