Abstract
Plantago lanceolata is a weedy species in California, known for its gyno- dioecious breeding system. Because male-sterile plants do not contribute genes via pollen, some mechanism must compensate for their evolutionary disadvantage compared to hermaphrodites. The proximate and ultimate factors responsible for the evolution and maintenance of the gynodioecious breeding system in this species in Northern California were investigated. High frequencies of male-steriles (i.e., females) were found most commonly in mesic coastal sites. Studies of the relative fecundities of females and her- maphrodites revealed that females produce 1.77 times the number of seeds produced by hermaphrodites over the flowering season. The reproductive superiority of females is not, however, manifest in lst-year plants. Thus, it is hypothesized that high frequencies of females are found in mesic sites where plants can exhibit the perennial habit and females can manifest their reproductive advantage. Electrophoretic comparisons revealed that females and hermaphrodites are genetically very similar and remarkably uniform. The relationship of this genetic uniformity to the role of gynodioecy as an outcrossing mechanism is discussed.
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