Influence of female cuticular hydrocarbon (CHC) profile on male courtship behavior in two hybridizing field crickets Gryllus firmus and Gryllus pennsylvanicus

Abstract

Background: The hybridizing field crickets, Gryllus firmus and Gryllus pennsylvanicus have several barriers that prevent gene flow between species. The behavioral pre-zygotic mating barrier, where males court conspecifics more intensely than heterospecifics, is important because by acting earlier in the life cycle it has the potential to prevent a larger fraction of hybridization. The mechanism behind such male mate preference is unknown. Here we investigate if the female cuticular hydrocarbon (CHC) profile could be the signal behind male courtship.

Results: While males of the two species display nearly identical CHC profiles, females have different, albeit overlapping profiles and some females (between 15 and 45%) of both species display a male-like profile distinct from profiles of typical females. We classified CHC females profile into three categories: G. firmus-like (F; including mainly G. firmus females), G. pennsylvanicus-like (P; including mainly G. pennsylvanicus females), and male-like (ML; including females of both species). Gryllus firmus males courted ML and F females more often and faster than they courted P females (p < 0.05). Gryllus pennsylvanicus males were slower to court than G. firmus males, but courted ML females more often (p < 0.05) than their own conspecific P females (no difference between P and F). Both males courted heterospecific ML females more often than other heterospecific females (p < 0.05, significant only for G. firmus males).

Conclusions: Our results suggest that male mate preference is at least partially informed by female CHC profile and that ML females elicit high courtship behavior in both species. Since ML females exist in both species and are preferred over other heterospecific females, it is likely that this female type is responsible for most hybrid offspring production.

Keywords: Hybrid zone; Introgression; Mate choice; Pre-mating barrier; Speciation.

Fig. 1

Clustering of CHC for females. Principal components for relative peak proportion for all crickets, labeled by sex and female CHC categories generated through clustering analysis. Categories include cluster “F” (predominantly G. firmus females, n = 29), cluster “P” (only G. pennsylvanicus females, n = 23), and cluster “ML” (females with male-like CHC profiles, n = 32 for G. firmus and n = 40 for G. pennsylvanicus)

Fig. 2

Courtship success by male species, crossing type and female CHC category. Estimated proportion of successful courtships by male species, crossing type (conspecific or heterospecific) and male-like female CHC category (ML, blue) or not (red) from mixed effects binomial logistic regression model with 95% confidence intervals

Fig. 3

Kaplan Meier (KM) curve of time to courtship for G. firmus males (a) and for G. pennsylvanicus males (b) by crossing type (Con = conspecific; Hetero = heterospecific) and male-like female CHC category. The KM curve estimates the probability of not yet successful courting at a given time. The time to courtship differs significantly by crossing type and male-like CHC category for G. firmus males (p < 0.0001), but not for G. pennsylvanicus males

Fig. 4

Principal component analysis of CHC for all individuals. a Relative peak proportion and b composition. Green represents G. pennsylvanicus and orange represents G. firmus. Sex is shown with male and female symbols