Delayed Selfing as an Optimal Mating Strategy in Preferentially Outcrossing Species: Theoretical Analysis of the Optimal Age at First Reproduction in Relation to Mate Availability

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Abstract
The evolution of mating systems and that of life history have usually been modeled separately. However, they may be to some extent coupled in natural situations because they rely on the same phenotypic traits. Here, we focus on one of these traits, the age at first reproduction, in a species able to self- and cross-fertilize. When inbreeding depression is strong, self-fertile species preferentially cross-fertilize. However, outcrossing is not always possible when the availability of sexual partners is limited. The optimal reproductive strategy in this case would be to wait for a sexual partner for a certain period of time (the waiting time) and then switch to selfing if no mates have been encountered (reproductive assurance strategy). We predict the evolution of an optimal waiting time depending on the efficiency of resource reallocation to late fecundity, on the inbreeding depression, and on the instantaneous probability of encountering a partner versus dying. As a consequence of reduced mate availability, intermediate selfing rates can be generated in preferentially outcrossing populations, but they are lowered by the existence of a waiting time. Our model may thus explain low selfing rates observed in natural populations of many self-fertile, preferentially outcrossing plants or animal species.