Competition's Role in Shaping Cryptic Genetic Variation.

Cryptic genetic variation-heritable genetic variation that is only expressed under stressful or novel environments-can potentially fuel the evolution of novel traits. While previous work has demonstrated that novel environments can expose cryptic genetic variation, whether and how multiple environments interact to shape such variation in natural populations is poorly understood. To determine how multiple environments may modulate cryptic genetic variation, we used tadpoles of the Eastern spadefoot, Scaphiopus holbrookii (Sc. holbrookii). Species of Scaphiopus have previously been used as outgroups to the genus Spea, which has evolved a novel carnivorous tadpole morph specialized for a shrimp diet. Here, we assess whether shrimp-induced cryptic genetic variation in Sc. holbrookii tadpole traits varies as a function of conspecific competition. Across all traits measured, we found that shrimp-induced cryptic genetic variation only occurred under specific competitive conditions. Specifically, the shrimp diet revealed cryptic genetic variation in body size, gut length, and jaw area when tadpoles experienced high intraspecific competition. Surprisingly, across these same traits, the shrimp diet suppressed the expression of heritable variation under low competition, suggesting that moderately stressful conditions can limit the expression of heritable variation. In contrast to the other traits, the expression of heritable variation in tadpole tail depth was largely unaffected by diet or competition. Together, our results indicate that interacting environmental factors jointly modulate how and in what traits cryptic genetic variation may be expressed, thereby affecting its potential to drive novel trait evolution.