Transgenerational cues about local mate competition affect offspring sex ratios in the spider mite Tetranychus urticae

Abstract In structured populations, competition between closely related males for mates, termed Local Mate Competition (LMC), is expected to select for female-biased offspring sex ratios. However, the cues underlying sex allocation decisions remain poorly studied. Here, we test for several cues in the spider mite Tetranychus urticae, a species that was previously found to adjust the sex ratio of its offspring in response to indicators of LMC. In particular, we investigate whether the offspring sex ratio of T. urticae females changes in response to 1) the current number of females laying eggs in the same patch, 2) the number of females in the patches of their mothers and 3) their relatedness to their mate. Females on patches with 1 (high LMC) or 15 (low LMC) other females produced similar sex ratios. Similarly, the offspring sex ratios of females mated with their brother did not differ with those of females mated with an unrelated male. In contrast, females produced a more female-biased offspring sex ratio if their mother laid eggs on a patch with 1 other female compared to 15 other females. Thus, the maternal environment is used as a cue affecting the sex allocation of daughters. We discuss the conditions under which the maternal environment can be a reliable predictor of inclusive fitness.