Barriers to gene exchange in hybridizing field crickets: the role of male courtship effort and cuticular hydrocarbons

Abstract

BackgroundPre-zygotic barriers often involve some form of sexual selection, usually interpreted as female choice, as females are typically the choosier sex. However, males typically show some mate preferences, which are increasingly reported. Here we document previously uncharacterized male courtship behavior (effort and song) and cuticular hydrocarbon (CHC) profiles in the hybridizing crickets Gryllus firmus and G. pennsylvanicus. These two species exhibit multiple barriers to gene exchange that act throughout their life history, including a behavioral barrier that results in increased time to mate in heterospecific pairs.ResultsWe demonstrated that male mate choice (as courtship effort allocation) plays a more important role in the prezygotic behavioral barrier than previously recognized. In gryllids females ultimately decide whether or not to mate, yet we found males were selective by regulating courtship effort intensity toward the preferred (conspecific) females. Females were also selective by mating with more intensely courting males, which happened to be conspecifics. We report no differences in courtship song between the two species and suggest that the mechanism that allows males to act differentially towards conspecific and heterospecific females is the cuticular hydrocarbon (CHC) composition. CHC profiles differed between males and females of both species, and there were clear differences in CHC composition between female G. firmus and G. pennsylvanicus but not between the males of each species.ConclusionAlthough many barriers to gene exchange are known in this system, the mechanism behind the mate recognition leading to reduced heterospecific mating remains unknown. The CHC profiles might be the phenotypic cue that allow males to identify conspecifics and thus to adjust their courtship intensity accordingly, leading to differential mating between species.

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

“…Hybrid zone cobaltinus males appear to differentiate heterospecific females from conspecific females based on their cuticular hydrocarbon profiles (Peterson et al., )—chemicals that insects commonly used to identify potential mates and that can contribute to premating barriers between species (e.g. Havens & Etges, ; Maroja et al., ; Rundle & Dyer, ).…”

Replicated evolutionary divergence in the cuticular hydrocarbon profile of male crickets associated with the loss of song in the Hawaiian archipelago

“…Selection on male CHCs is likely to have been particularly acute in the Hawaiian archipelago due to the evolutionary loss of a major channel of sexual communication in these crickets, the male calling and courtship songs. Given that CHCs serve the dual functions of desiccation resistance and mate choice and are thereby subject to both ecological divergence and divergence due to sexual selection, CHCs may be particularly important traits for the processes of reproductive isolation and speciation (Higgie et al, 2000;Blows, 2002;Chung et al, 2014;Maroja et al, 2014).…”

Chemical cues mediate species recognition in field crickets

“…Given their general importance in mate choice in crickets, we might also expect CHCs to play an important role in species recognition. There are a few studies implying that this may be the case in G. integer, G. lineaticeps, and Allonemobius species (Otte and Cade, 1976;Paul, 1976;Leonard and Hedrick, 2009), and differences in CHC profiles have been found among species of rapidly diverging Laupala (Mullen et al, 2007(Mullen et al, , 2008, however, other than Maroja et al (2014), no studies have explicitly tested this prediction. Here, we investigate the role of chemical cues in species recognition between a pair of European field crickets.…”

“…Species of true crickets, which have been intensively studied in the context of reproductive isolation (reviewed by Veen et al, 2013), are rarely reported to use chemoreception for species recognition (but see Maroja et al, 2014). Like all other insect species, the exoskeletons of crickets bear lipid molecules known as cuticular hydrocarbons (CHCs).…”