Evolutionary Significance

Abstract

In earlier sections, few points were made on the modes of origin and occurrence of the phenomenon of male sterility in flowering plants. Theoretically at least it has manifold evolutionary importance. A study of distribution patterns of the diverse sexual forms among both monomorphous and polymorphous groups is expected to throw some light on the intermediate steps in the evolution of dioecism from hermaphroditism if one might assume that dioecy originated from the bisexual ancestors through mutation and natural selection. In general, the students of sex determination problem have tended to favour the hypothesis of a trigger mechanism governing the shifts in sex tendencies of a potentially bisexual individual, that is, naively speaking, primordia in floral regions destined to develop both sexes are directed towards either sex. The synthetic dioecious strains obtained by Jo-nes (1934) and others in maize show with exactitude a genetic basis of the origin of dioecy (Lewis, 1940, 1942). On the contrary, the trigger found in case of Melandrium seems to illustrate the retrogession from dioecism to bisexuality. Westergaard (1958) has critically examined both of these cases on a comparative basis to show their relative efficiency and stability. It may be pointed out, however, that controversy about whether dioecy or hermaphroditism is the primitive condition loses meaning if a theory of the polyphyletic origin of Angiosperms was accepted.