Selection caused by self-fertilization I. Four measures of self-fertilization and their effects on fitness

Abstract

The standard models of selfing in seed plants consider only the ovules, which are assumed to have a constant selfing rate. It has recently become clear, however, that hermaphrodite or monoecious populations frequently show sexual asymmetry (nonconstant pollen:ovule fertilities among individuals). Such asymmetry usually results in pollen selfing rates which differ from those for the ovules and are frequency-dependent even for constant ovule selfing rates. A recent study of selfing rates for all gametes of an individual is extended here to include four selfing rates (for ovules, pollen, all gametes, and zygotes), and simple mathematical relationships linking the four rates are obtained. Unlike earlier models of selfing, it is not assumed that the ovule selfing rate is constant, but instead that this rate, like all the others, is determined by the mobility of the pollen, which in turn is determined by the floral biology and ecology. It is found that all four selfing rates are usually frequency-dependent. The selfing rate for all gametes (the combined selfing rate) is usually intermediate between those for the ovules and pollen, and the zygotic rate is usually the smallest of the four. The exceptions to the above statements occur for relatively extreme situations, such as complete selfing for pollen or ovules, no selfing, or sexual symmetry. Three modes of selfing are considered: prior (PS), competing (CS), and delayed (DS) self-fertilization. It is shown that if there are at least two types with different selfing rates in the population, then the ranking of their selfing rates may depend upon the frequencies of the types (for the combined and the zygotic rates), may be frequency-independent (ovule rate), or may be dependent or independent, according to the mode of selfing (pollen rate). The effects of the various influences on the amount of selfing are by no means negligible. Thus a numerical study shows pollen selfing rates for one type which vary from 0.09 to 0.96, according to its frequency. Another numerical result shows a change in combined selfing rate from 0.13 to 0.86, depending solely on the mode of selfing. Results for Scots Pine show that an ovule selfing rate of 0.5 was accompanied by a combined rate of 0.143. The population selfing rate is not the same as the mean of individual selfing rates, and can only be obtained if female fitnesses as well as ovule selfing rates are known for each type. Previous models of selfing have failed to distinguish between the effects of increased selfing and increased pollen fertility, with the result that increased selfing always resulted in greater fitness. In the present models the two effects are distinguishable, and it is found that increased selfing may result in increased or decreased fitness, depending also on population density and on a form of pollen density. Thus the old dogma that in the absence of viability and fertility selection increased selfing always results in increased fitness is finally refuted, and the importance of the influence of ecological parameters on selfing and fitness is emphasized, since population density and pollen density influence the selfing rates.