An empirical test of Baker's law: Dispersal favors increased rates of self-fertilization.

Baker's law is the observation that recently dispersed populations are more likely to be self-fertilizing than populations at the range core. The explanatory hypothesis is that dispersal favors self-fertilization due to reproductive assurance. Caenorhabditis elegans nematodes reproduce via either self-fertilization or outcrossing and frequently disperse in small numbers to new bacterial food sources. While C. elegans males facilitate outcrossing, males and outcrossing are rare in natural C. elegans populations. Here, we use experimental evolution to test if frequent dispersal selects for the invasion of self-fertilization into predominantly outcrossing populations. C. elegans dispersal often occurs in the dauer alternative life stage. Therefore, we tested the effects of dispersal on rates of self-fertilization in populations exposed to dauer-inducing conditions and populations maintained under standard lab conditions. Overall, we found that populations required to disperse to new food sources rapidly evolved substantially elevated rates of self-fertilization compared to populations that were not required to disperse in both dauer and non-dauer populations. Our results demonstrate that frequent dispersal can readily favor the evolution of increased selfing rates in C. elegans populations, regardless of life stage. These data provide a potential mechanism to explain the dearth of outcrossing in natural populations of C. elegans.