A high rate of self-fertilization and increased seed fertility of homostyle primroses

Abstract

There has been extensive theoretical study of the evolution of self-fertilization in hermaphroditic or monoecious flowering plants because self-compatible species with high rates of self-fertilization are often derived from outcrossing ancestors^1–4. Such studies have shown that a gene causing an increased rate of self-fertilization has an advantage because the mother transmits genes via both pollen and egg to selfed progeny⁵, and thereby evades the ‘cost of meiosis’^6–8. Selfing also may be advantageous if the amount of pollen available to fertilize ovules by outcrossing is limited^1,2,4,6. However, these advantages are offset, at least partially, if the fitness of selfed progeny is lower than that of outcrossed progeny, due to inbreeding depression^1,2,6,7,9,10. Biogeographical evidence suggests that reduced pollinator activity may be sufficient to counteract this disadvantage and permit the evolution of selfing in populations in ecologically marginal areas^1–4, but direct evidence is usually lacking. Here we show that the self-fertile homostyle variant of the primrose Primula vulgaris, which reaches high frequencies in certain populations¹¹, experiences a fertility advantage, in terms of seeds per plant, as a result of its high rate of self-fertilization.