Evolution of Reproductive Systems in the Gramineae

Abstract

From a simple hermaphrodite flower, and from a complex incompatibility system unique among the flowering plants, several breeding systems have evolved in the Gramineae.Self-compatibility is amy is a s oeci uncommon (ca.20 oa and gyn nodioecism i is much iis plo still (3-4 genera).Both are seen 23 m family.These states are also interpreted as responses to a and though they do generate cross-fertilization, they assist its evolution.Pathways for the evolution of these feeding systems are described.Apomixis and the breeding system s puc are discussed.Reproductive biology in the Gramineae begins at the transition in the shoot apex from leaf production to the initiation of inflorescence primordia and the later development of floral structures.These have been well described for numerous grasses (Barnard, 1955(Barnard, , 1957(Barnard, , 1964;;Bonnett, 1966;Sharman, 1960), and are mediated by photoperiod.The review of Evans (1964) elegantly reveals data on the interplay of daylength, temperature, and vernalization on inflorescence develpment.Floral induction and initiation may occur in the season of flowering (Evans, 1964), or in the season preceding inflorescence emergence (Mark, 1965; Hodgson, 1966).Inflorescence emergence is temperature or daylength dependent (Cooper, 1952; Connor, 1963; Heslop-Harrison, 1961).Temperature also controls anthesis, and later the release of pollen from anthers.Photoperiod, however, has other effects on the reproductive cycle.It may, for example, affect the frequency of cleistogamy in facultatively cleistogamous grasses (Langer & Wilson, 1965), or in a facultative apomict the frequency of apomictic or sexual embryo sacs (Knox, 1967; Knox & Heslop-Harrison, 1966), or depress maleness (Heslop-Harrison, 1959), or reduce the number of florets in the male inflorescence of Zea mays (Moss & Heslop-Harrison, 1968), or control protandry and protogyny (Emerson, 1924).Although these environmental influences are considerable, genetic influences may cause transient male and female sterility, or restore lost fertility in part or in whole, or promote redistribution of the sex-forms.A combination of both genetic and physiological factors may introduce difficulties into an interpretation of the breeding system of any grass.