The relation between the genetic determination and the ecological significance of the seed wing in Spergularia media and S. marina

Plants with winged seeds and plants with unwinged seeds occur in Spergularia media and in S. marina. The degree of development of the seed wing is more differentiated in S. media than it is in S. marina, seeds with a narrow wing being of common occurrence in populations of S. media and absent in S. marina. In a stable environment the plants produce mainly winged seeds and in unstable habitats unwinged ones. Since 5. marina occurs only in unstable types of environment, plants with winged seeds are very rarely encountered in this taxon. In S. media ‘winged’ is dominant over ‘unwinged’, but in S. marina ‘unwinged’ is dominant over ‘winged’. The considerable difference in dominance is correlated with differences in selective pressure: in populations of S. media the predominating selection pressure acts in favour of the phenotype with winged seeds and the genetic factors for ‘winged’ are dominant, whereas in populations of S. marina the selective pressure favours the phenotype with unwinged seeds and the genetic factors for ‘unwinged’ are dominant over ‘winged’. In S. media the phenotypic character ‘winged’ is most probably determined by two factors, one of which is epistatic, and in S. marina this characteristic is presumably brought about by polymery involving two factors. In populations of S. media plants with narrowly winged seeds may arise as the result of genetic recombination, which individuals may have a high survival value in certain environments, but in S. marina such phenotypes – which have hardly any adaptive significance or none at all – do not arise. The degree of outbreeding in populations of S. media is estimated at 8 to 15 per cent, in populations of S. marina at I to 2 per cent. The different genotypic determination of the degree of wing development of the seed and the different rates of outbreeding and inbreeding are most probably attributable to the different ecological situations in which the two species occur in nature.