An Empirical Test of Baker's Law: Dispersion Favors Increased Rates of Self-Fertilization in Caenorhabditis elegans Open Access

Milligan, William (2017)

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Baker's law predicts that population dispersion will select for uniparental reproduction, since dispersed individuals capable of self-fertilization will avoid mate-limitation due to reproductive assurance. However, Baker's law has been challenging to test empirically because controlled tests are often unfeasible. Here, we test Baker's law under controlled settings using C. elegans and agar plates with three different spatial arrangements of E. coli colonies that induce no, low, and high dispersion. Unlike its close dioecious relatives, C. elegans appears to rarely outcross in the wild populations, instead primarily reproducing via self-fertilization. Wild populations of C. elegans maintain few males, and hermaphroditic C. elegans instead fertilize their own eggs. The boom-to-bust ecology of C. elegans in conjunction with Baker's law suggests a possible mechanism for selfing alleles to invade the obligately sexual (outcrossing) ancestor of C. elegans. To empirically test the validity of this mechanism and of Baker's law, we created mixed-mating C. elegans populations consisting of 10% wild-type hermaphroditic C. elegans and 90% obligately outcrossing C. elegans. If selfing is favored, then we expect selfing to increase in frequency and more so in high dispersal treatments if Baker's law prediction is accurate. Both strains were from the same genetic background to mimic the evolution of a spontaneous mutation allowing for self-fertilization. For 18 cycles, the populations were permitted to disperse every two weeks. Self-fertilization invaded all replicate populations across each dispersal treatment. However, increased dispersal selected for greater invasion rates of self-fertilization. Thus, population dispersion can select for the invasion of selfing-alleles into a predominantly outcrossing population, which may explain why C. elegans rely on selfing to such an extent. Overall, we provide empirical support for a key prediction of Baker's law: frequent dispersal favors the evolution of increased rates of self-fertilization.

Table of Contents

Introduction 1

Methods 5

Figure 1 6

Results 9

Figure 2 9

Table 1 10

Table 2 10

Table 3 11

Figure 3 12

Discussion 13

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