Outcrossing and Sex Function in Hermaphrodites: A Resource-Allocation Model

Publisher Website
Google Scholar
Abstract
Genetic estimates of outcrossing rates in seed plants usually measure the ovule or seed outcrossing rate only. However, there is good evidence for population variation in pollen fertility, which may result in sharply different ovule and pollen outcrossing rates. Therefore, a new combined ovule and pollen outcrossing rate, which depends upon the concept of successful gametes, is proposed. This rate estimates the number of ovules and pollen grains which take part in crossing, as a proportion of all successful ovules and pollen grains (fitness value). Fertility variation also causes unequal male/female functioning of hermaphrodites, and the degree of female function (functional sex) is defined as the number of successful ovules as a proportion of all successful gametes. This is a number between 0 (for males) and 1 (for females), and seldom equals 0.5 for non-fully-selfed hermaphrodites. All of these quantities are frequency dependent, since a common component (the number of successful cross-fertilizing pollen grains) is frequency dependent because of the variation in ovule and pollen fertility. These quantities are studied by means of a one-locus two-allele model of male/female resource allocation, with dominance, in which all genotypes have the same reproductive resources. For 0 < .sigma. < 1, in which .sigma. is the ovule selfing rate, the phenotype with the greater pollen fertility has the greater combined outcrossing rate. For polymorphic populations and 0 < .sigma. < 1, all genotypic combined outcrossing rates are increasing functions of the frequency of the more ovule-fertile phenotype. For .sigma. < 1, functional sex is an increasing and fitness a decreasing function of the frequency of the more pollen-fertile phenotype. Optimal resource allocation for a genotype is defined as that which results in the fixation of that genotype. The genotypic proportion of resources devoted to seeds (Rij) is optimally 1/2 for .sigma. = 0, and increases with .sigma.. Optimal combined outcrossing and functional sex are similarly defined. Population dynamics can be described in terms of resource allocation as follows: If one genotypic R value is less than and the other greater than 1/2(1 + .sigma.), then .**GRAPHIC**. the mean seed resource allocation in equilibrium populations, equals 1/2(1 + .sigma.), whereas if both R''s are < or > 1/2(1 + .sigma.), .**GRAPHIC**. equals the genotypic value nearest to 1/2(1 + .sigma.). Unlike the classical selection model, there may be a polymorphism despite dominance, and mean population fitness may decrease with selection, since polymorphic equilibrium mean fitness is an increasing function of .sigma.. The mean combined outcrossing rate and functional sex equal the ovule outcrossing rate 1 - .sigma. and 1/2, respectively, in equilibrium populations.