Genetic analysis of conflicts arising during development of seeds in the Angiospermophyta

Abstract

The genetic differences that exist among seeds developing on the same maternal sporophyte in the Angiospermophyta can lead to conflicts as the seeds absorb maternal resources. To analyse the outcome of these conflicts, a population-genetic model is developed to give first, second and third order conditions for initial increase of a mutant that affects interactions among relatives in a diploid, diallelic, random-mating population. Conflicts within families of developing seeds are then considered as a particular case of the model, by supposing that there exists a mutant which, when expressed in the endosperm, causes the seed to receive a greater share of maternal resources. Such seeds then have ‘over-consumer’ phenotypes, in contrast to their ‘under-consumer’ sibs. Conditions for initial increase of the mutant are obtained for nine major types of embryo sac and three kinds of gene expression. It is shown that all dominant mutants start to increase. However, the set of recessive mutants that can start to increase is restricted to those that give the over-consumer a large fitness advantage and, or, a low rate of loss of fitness to under-consumers. Behaviour of dosage-threshold mutants is more complicated, depending more on the number of megaspores that contribute to the embryo sac and the contribution of embryo sac nuclei to the endosperm. By using these results, an attempt is made to interpret some features of the genetic system of endosperm. Explanations for double fertilization lack the generality one would wish to account for such a ubiquitous feature in the Angiospermophyta. The double dosage of maternal genes arising from Polygonum -type embryo sac development has little effect on conflicts within families. However, the occurrence of tetrasporic types of endosperm can be seen as the outcome of selection on maternal sporophytes, which eliminates conflicts among their progeny.