Outcrossing increases resistance against coevolving parasites.

Despite substantial costs, biparental sex is the dominant mode of reproduction across plant and animal taxa. The Red Queen hypothesis (RQH) posits that coevolutionary interactions with parasites can favor biparental sex in hosts, despite the costs. In support of the RQH, previous studies found that coevolutionary interactions with virulent bacterial parasites maintained high outcrossing rates in populations of the androdioecious nematode host Caenorhabditis elegans . Here we test three non-mutually exclusive mechanisms that could explain how coevolving parasites maintain outcrossing rates in C. elegans hosts: 1) short-term parasite exposure induces plastic increases in the hosts' propensity to outcross, 2) hosts evolve increased outcrossing propensity in response to selection imposed by coevolving parasites, and 3) outcrossed offspring incur less parasite-mediated fitness loss than selfed offspring, increasing host male frequencies and opportunities for outcrossing. We find no evidence that parasites cause plastic or evolved changes in host outcrossing propensity. However, parental outcrossing significantly increases survival of host offspring in the F2 generation when exposed to a coevolving parasite. Hence, coevolving parasites maintain outcrossing in host populations by selecting against selfed offspring, rather than by inducing changes in the propensity to outcross.

Outcrossing in Caenorhabditis elegans increases in response to food limitation.

Theory predicts that organisms should diversify their offspring when faced with a stressful environment. This prediction has received empirical support across diverse groups of organisms and stressors. For example, when encountered by Caenorhabditis elegans during early development, food limitation (a common environmental stressor) induces the nematodes to arrest in a developmental stage called dauer and to increase their propensity to outcross when they are subsequently provided with food and enabled to develop to maturity. Here we tested whether food limitation first encountered during late development/early adulthood can also induce increased outcrossing propensity in C. elegans. Previously well-fed C. elegans increased their propensity to outcross when challenged with food limitation during the final larval stage of development and into early adulthood, relative to continuously well-fed (control) nematodes. Our results thus support previous research demonstrating that the stress of food limitation can induce increased outcrossing propensity in C. elegans. Furthermore, our results expand on previous work by showing that food limitation can still increase outcrossing propensity even when it is not encountered until late development, and this can occur independently of the developmental and gene expression changes associated with dauer.