THE SELECTIVE ADVANTAGE OF STAMINATE HETEROGAMY

Abstract

In a series of now classic experiments, Jones (1932) was able to demonstrate that as few as two mutations would suffice to convert Zea mays to dioecism. His studies were of particular interest in that they revealed that pistillate or staminate heterogamy could evolve with equal facility. On the basis of this information, there would seem to be no reason to anticipate that within dioecious angiosperms, one sex, more often than the other, should be characterized as the heterogametic sex. Yet a survey of the literature will indicate that in dioecious species, staminate individuals, to a very large extent, represent the heterogametic sex. Westergaard (1958), for example, refers to 13 families in which the genetics of sex determination is fairly well understood. In 11 of these families, all dioecious species studied were characterized by staminate heterogamy. Within a twelfth family, the Caryophyllaceae, some strains, but not all, of Silene otites may possess heterogametic pistillate individuals, while in Silene alba and S. dioica staminate plants are XY, pistillate XX. Fragaria (Rosaceae) is the only known genus in which all dioecious species studied definitely possess pistillate heterogamy. This preponderance of staminate heterogamy cannot be attributed to a common origin as might be the case among the Mammalia because, within the angiosperms, dioecism appears to be a short-lived and sporadic method of promoting outbreeding (Darlington, 1958). Westergaard (1958) considers that in a majority of cases, the evolution of dioecism has apparently taken place on the species level. It thus seems all the more remarkable that staminate heterogamy, if possessing no selective advantage relative to the alternative, should characterize so many examples of dioecism. However, recent studies, the results of which will be presented here in summarized form, have demonstrated that a mechanism, described by Correns (1928) and dependent upon staminate heterogamy, confers a positive selective advantage on a dioecious species possessing it. Working with Silene alba (Melandrium album), a dioecious species in which staminate plants are heterogametic, Correns saw that crosses in which small amounts of pollen were used produced approximately equal proportions of staminate and pistillate offspring. If, however, large amounts of pollen were used, the progenies contained a greater than expected proportion of pistillate plants. Correns reasoned that the pollen tubes containing an X (or pistillate determining) chromosome grew faster than did those containing a Y chromosome. Thus if large amounts of pollen were available, the faster growing X pollen tubes could reach and enter more than half the ovules while the Y pollen tubes were still penetrating the style. With only a little pollen available, the X pollen tubes would still be first in reaching the ovules but a sufficient number of eggs would remain unfertilized to accommodate the slower Y pollen tubes, resulting in approximately equal proportions of pistillate and staminate plants. The same mechanism was later observed in Rumex acetosa (Correns, 1928) and in Humulus japonicus (Kihara and Hirayoshi, 1932), both of which possess staminate heterogamy. The present study was designed to determine if this mechanism is of any significance