EVOLUTION OF THE SEED PLANTS AND INCLUSIVE FITNESS OF PLANT TISSUES.

Abstract

Here we put forward a new interpretation of the significance of some major features of vascular plant reproductive biology. These features are the integuments of maternal tissue surrounding the offspring, which distinguish seed plants from vascular cryptogams, and the (usually) triploid endosperm produced by double fertilization, which characterizes angiosperms. Maternal integuments and double fertilization are generally regarded as key innovations in plant evolution, and are usually given as the most fundamental characters which distinguish seed plants and angiosperms respectively (Cronquist, 1968; Takhtajan, 1969; Bold, 1973; Foster and Gifford, 1974; Stebbins, 1974; Sporne, 1975; Darlington, 1976). Our interpretation draws together an evolutionary-ecology model on maternal investment, inclusive-fitness calculations from population genetics, and information from comparative plant morphology and embryology. Each of these elements of our interpretation is well-known within the relevant discipline, but the elements have not previously been combined and applied to interpreting plant evolution.1 In this paper we first summarize how mother plants support their offspring in different groups of vascular plants. We show that this support is committed later, with respect to meiosis and fertilization, in more advanced groups of plants. Second, we describe available interpretations, to which our interpretation is an alternative, of the integuments and of double fertilization. Deficiencies of the available interpretations are pointed out, and the points to be explained by our interpretation are defined more exactly. Third, we argue that the advantage achieved by deferring investment in more advanced groups is that mothers can cause a limited total investment to be directed selectively to better offspring genotypes. Fourth, we show that the pattern of investment allocation which maximizes inclusive fitness for the mother is not the same as that which maximizes inclusive fitness for any one offspring. Fifth, we put forward our interpretation, which briefly is that the integuments and endosperm allow the mother to retain control of how her investment is allocated, while deferring the investment until offspring with genotypes different from her own (and therefore with potentially conflicting interests) have been created. Finally, we consider some relatively minor features of comparative plant reproductive biology, which are relevant to assessing our interpretation and other published interpretations.