Reproductive and Pollination Biology of the Endemic Hawaiian Cotton, Gossypium tomentosum (Malvaceae)

Gossypium tomentosum is a cotton species endemic to the Hawaiian Islands. We studied several aspects of its reproductive biology, including potential pollinators, floral biology, and diurnal and seasonal flowering phenology. Flower visitors were observed in G. tomentosum populations on O‘ahu, Kaho‘olawe, and Maui. Primary visitors were introduced species, honeybees and carpenter bees, both of which were pollinating the flowers. No native bee species were seen visiting flowers. In examining floral biology we found that in some cases 10% of flowers had styles that were as short as the anthers or were recurved toward the anthers. In the greenhouse, in the absence of pollinators, these flowers were the only ones that set fruit. Flowering of G. tomentosum commences in January and February, following the rainy season, peaks in May, and may continue into August and September. In one year, after higher than average precipitation during the rainy season, there was a greater abundance of flowering, and flowering persisted later into the year. Transgenic varieties of commercial cotton, G. hirsutum, are grown in Hawai‘i and are interfertile with G. tomentosum. Honeybees and carpenter bees are also known pollinators of commercial cotton. Because these pollinators are long-distance foragers, we estimate that transgenic cotton fields would have to be greater than 10 km from a G. tomentosum population to prevent gene flow. A major concern about the long-term persistence of many plant species is the decline in insect pollinators resulting from human activity (Beismeijer et al. 2006). Endemic plant species on islands are especially vulnerable because of population reduction and fragmentation that may no longer support their pollinators and because of the impact of introduced pollinators on native pollinator species (Cox and Elmqvist 2000). Gossypium tomentosum Nuttall ex Seeman (Malvaceae), locally referred to as ma‘o, is a cotton species endemic to the Hawaiian Islands. It historically has been relatively common in welldrained volcanic loams on the leeward side of the major islands of the archipelago except Hawai‘i and Kaua‘i (there is some evidence that it was once present on Kaua‘i) (Stephens 1964). Many populations of this species have disappeared due to coastal development. An additional concern for G. tomentosum is that Hawai‘i is commonly used as a nursery for the improvement of commercial cotton, with which G. tomentosum is fully interfertile (Meyer and Meyer 1961; J.F.W., pers. obs.). Commercial cotton consists primarily of Upland cotton (G. hirsutum) and to a lesser extent Pima cotton (G. barbadense). The majority of the cotton raised in nurseries is transgenic G. hirsutum (Bt cotton and Roundup-Ready cotton). Cross-pollination of G. tomentosum with commercial cotton could compromise the genetic integrity of the species, and the introgression of transgenes might alter intraand interspecific competitive relationships, confer an advantage that could lead to new weedy species, or in some other way alter evolutionary trajectories (Ellstrand et al. 1999). Pacific Science (2010), vol. 64, no. 1:45–55 doi: 10.2984/64.1.045 : 2010 by University of Hawai‘i Press All rights reserved 1 The Environmental Protection Agency provided financial support for this research. Manuscript accepted 29 January 2009. 2 Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, Iowa 50011. 3 Corresponding author (e-mail: [email protected]). There have been no studies documenting the pollinators of G. tomentosum. Swezey (1935), in a survey of insects associated with G. tomentosum, collected two species of megachilid bees (nonnative species) from flowers. Stephens (1964) never saw any visitors to G. tomentosum flowers. He did observe honeybees and carpenter bees in the vicinity of some G. tomentosum populations, but those bees were visiting other plant species. He speculated, however, that infrequent visits by honeybees might have been responsible for producing two hybrid populations between G. barbadense and G. tomentosum he observed on O‘ahu. Fryxell (1979) speculated that the native pollinator might be a moth, based on the following reasoning. First, he assumed that there must be a native pollinator because the fact that the style was exerted meant that no self-pollination could occur and therefore a pollinator would be needed for seed set and the persistence of the species. Second, he noted the absence of a dark spot at the base of the petals of G. tomentosum flowers; this spot is found in almost all other members of the Gossypieae, the cotton tribe, and serves as a nectar guide for bees. The absence, he said, indicated a nonbee pollinator. He also noted that G. tomentosum flowers stay open through the night, which indicates a pollinator active in the evening, such as a moth. This makes it different from the commercial cottons, and indeed all wild species of Gossypium, in which flowers open in the morning. The seasonal flowering phenology of G. tomentosum has also not been well studied. Stephens (1964) suggested that there may be little seasonality to flowering and indicated that some individuals could be found in flower at any time during the year. The purpose of our study was to identify the pollinators of G. tomentosum and characterize aspects of its reproductive biology including diurnal and seasonal phenology and