NATURAL SELECTION FOR A FRUIT DIMORPHISM IN PLECTRITIS CONGESTA (VALERIANACEAE)

Bright pink carpets of sea blush, Plectritis congesta (Lindt) DC. , are a conspicuous sight on rocky headlands and clearings in spring in southwestern British Columbia. Although the flowers of Plectritis exhibit considerable variation in color and morphology, the most remarkable polymorphisms exhibited by the plants are characters of the fruit (Dempster, 1958; Morey, 1962). The most conspicuous of the fruit polymorphisms is the presence or absence of a pair of well developed wings on the one-seeded, indehiscent fruit (Fig. 1). Individual plants bear only one kind of fruit. In all the populations we have thus far examined in British Columbia, plants of both forms occur. We have not found plants bearing fruit of intermediate form. The difference between the two fruit morphs is so striking that early taxonomists based several species on this character, as well as on other fruit polymorphisms in this and other North American taxa of the genus. Nielsen (1949), in her monograph of Plectritis, recognized that the species based on differences in fruit morphology often occurred together in the same colony, but in the absence of experimental studies she still gave varietal or specific status to various fruit and flower morphs. In a treatment of the California species of Plectritis, Dempster (1958) suggested that the fruit forms might represent a simple single gene genetic polymorphism. Although she found that wild collected fruits of one form occasionally produced plants bearing the opposite type of fruit, the frequency was low and she did not analyze the situation genetically. The most recent taxonomic treatment of the genus is by Morey (1959, 1962), who likewise did not attempt any genetic or biosystematic studies. We follow Morey's taxonomic treatment except that we regard P. congesta as monotypic, and we regard the small flowered, presumably autogamous P. congesta ssp. brachystemon (F. & M.) Morey as a distinct species, P. brachystemon F. & M. It occurs sympatrically with P. congesta without evidence of hybridization, and attempts to cross the two species in our laboratory have failed. We regard P. congesta ssp. nitida (Dyal) Morey as merely another fruit morph of P. brachystemon. Winged and wingless fruits also occur in P. brachystemon, but monomorphic as well as dimorphic populations of this species occur in B.C., unlike the situation in P. congesta. Unlike most of the taxa of Plectritis in California investigated by Dempster (1958) and Morey (1962), P. congesta has conspicuous flowers and is actively visited by pollinating insects, including bee flies and honeybees. The flowers are fragrant, with an odor much like red clover blossoms, and possess a nectariferous spur. They are strongly protandrous. At the time the anthers dehisce the stigma is unreceptive and located in the throat of the flower. Later, the style elongates and the stigma lobes spread (Fig. 2) . However, many flowers in each inflorescence are open at the same time so that geitonogamous pollinations are possible. The plants are self-compatible, and will self pollinate in the absence of insects, but when excluded from pollinators the level of autogamous seed set is considerably lower than the seed set in populations in the field. The breeding system of the species thus