POTENTIAL ECOLOGICAL CONSTRAINTS ON THE EVOLUTION OF GYNODIOECY IN MIMULUS GUTTATUS: RELATIVE FECUNDITY AND POLLINATOR BEHAVIOR IN A MIXED-SEX POPULATION

Although plant breeding systems are evolutionarily labile, a shift from one system to another can involve formidable ecological obstacles. In order for gynodioecy to evolve from hermaphroditism, the carriers of a male-sterility mutation (i.e., females) must overcome two major hurdles: females must attract pollinators without offering pollen as a reward, and they must produce enough seeds to compensate for the loss of reproduction through the paternal route. We investigated the relative performance of female and hermaphroditic plants in a greenhouse study of Mimulus guttatus originating from a largely hermaphroditic population. Females had larger floral displays than did hermaphrodites, with 5% wider corollas and 34% more flowers open at one time. They also produced 22% more flowers over a lifetime and 25% more ovules per ovary than hermaphrodites, giving females 1.53 times more maternal reproductive potential. In a related experiment, bumblebees visited female plants just as frequently as hermaphroditic plants, even though the females offered no pollen reward. Although the relative advantages enjoyed by females may be insufficient to overcome the obstacles for the evolution of stable gynodioecy, they might be strong enough to explain the maintenance of genetic variation for male-sterility alleles often observed in natural populations of M. guttatus.