In theory, male-sterility may be maintained in gynodioecious populations through genetic factors only. However, it is possible that ecological factors, such as different habitat requirements of the two sexual morphs, could contribute to or detract from the ability of male-sterile mutations to invade and spread in populations. Here I present results showing morph-specific pollinator limitation in a natural population of Lobelia siphilitica, such that females are more likely to set fruit than hermaphrodites when pollinator service is low. The difference in fruit set is apparently caused by differences in floral morphology: because their stamen tubes contain no pollen, female flowers are far more likely than hermaphroditic flowers to become receptive to pollination without several prior visits from pollinators serving to remove pollen. In addition, hermaphroditic progeny of females, that definitely carry the male-sterility factor, enter the pistillate phase more easily than progeny of hermaphrodites that may not carry the male-sterility gene. This difference may be a result of hermaphrodites from female lines containing less pollen than those from hermaphroditic lines, possibly causing less blockage of the style as it grows through the stamen tube. Thus, interactions between floral morphology and an ecological factor, limited pollinator service, may allow the maintenance or spread of male sterility in some populations of Lobelia siphilitica.

Key words: dichogamy, floral longevity, gynodioecy, Lobelia siphilitica (Lobeliaceae), male-sterility, pollinator limitation