Phenotypic Variety in Seed Germination Behavior: The Ontogeny and Evolution of Somatic Polymorphism in Seeds

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Abstract
Germination polymorphism is commonly found among seeds in bulked samples. The source of this variation in behavior often derives from differences between seeds produced within the same clutch (i.e., from the same mother). The ontogeny of these somatic polymorphisms is discussed with particular reference to the Leguminosae. Five generalizations emerge: differences in germination behavior are frequently associated with differences in the rate of seed development; polymorphism in seed size is frequently correlated with polymorphic behavior; polymorphism within a clutch may occur between or within fruit; the type or ratio of types of seed produced may vary with time and season; the degree of somatic polymorphism present in a clutch may be heritable. A general model of seed development is described which can account for these 5 features of within-clutch diversity in germination behavior. The term somatic heterochrony is suggested for the process which involves differential independent rates of ripening in the tissues of seeds. Somatic heterochrony may be the cause of germination polymorphism in all species in which seed dormancy is the product of an immature embryo and/or a developing influence of the seed coats on germination. A large number of angiosperm families fall into these categories. Somatic heterochrony may produce within-clutch diversity under the influence of natural selection. The same sources of variation may influence within-clutch and between-clutch (or interpopulation) differences in germination behavior. Given how easily seed development may be perturbed and produce changes in germination response, there is danger in assuming that the diversity in seed behavior produced by somatic heterochrony is automatically adaptive. Some limitations of the model are mentioned.