Ejaculate investment differs by population, but not wing morph or perceived sperm competition risk, in Pacific field crickets.

Conspicuous sexual signals are frequently under selection from conflicting sources, such as natural versus sexual selection and precopulatory versus postcopulatory sexual selection, which may act in the same or different directions. The Pacific field cricket (Teleogryllus oceanicus) is undergoing rapid evolutionary trait loss: mutations that render males obligately silent (the "flatwing phenotype") have become established on several islands of Hawai'i. Females strongly discriminate against silent males, yet flatwings do gain matings. In this study, we take advantage of this natural system to understand how investment in ejaculate quality under high and low perceived sperm competition risk (manipulated using acoustic cues) differs between wild type males and flatwings, which obligately adopt alternative reproductive tactics (ARTs). We replicated our study in two islands with different flatwing frequencies because female mating rate and therefore sperm competition risk is likely higher in O'ahu than Kaua'i. We measured testes mass and gene expression for three seminal fluid proteins (SFPs) that affect paternity success, sperm viability, and female propensity to remate. Compared to Kaua'i males, O'ahu males showed higher residual testes mass and expression of ToSfp011, which increases male sperm viability and paternity success, and reduces mate searching behaviour by females. We found no effect of immediate sperm competition risk or wing morph on any of the SFPs or testes mass. Differential SFP expression and residual testes mass in Hawaiian populations that likely differ in mating rate are compatible with the predictions of sperm competition theory.