YEAR‐TO‐YEAR CHANGES IN PHENOTYPIC GENDER IN A MONOECIOUS CUCURBIT, APODANTHERA UNDULATA

Year-to-year variation in phenotypic gender in the monoecious cucurbit, Apodanthera undulata Gray was investigated. Small plants produce no flowers. Larger plants produce only staminate flowers ("male" plants), while a somewhat greater threshold size is necessary for pistillate flower production (cosexual plants). Approximately 85% of the plants that bloomed did not change gender group between years. Two measures of phenotypic gender were used: prospective femaleness, a measure standardized to the population floral ratio, and morphological femaleness, an unstandardized measure. Femaleness of cosexes between years was positively correlated; r values were somewhat greater when using morphological femaleness values. Plants that opened only staminate flowers one year were likely to open only staminate flowers the next year. Similarly, cosexes were likely to be cosexes again the following year, with similar femaleness values. Beyond the threshold size for pistillate flower production, plant size was not correlated with femaleness. These patterns suggest that all plants are male until they reach a certain size and that plants in their cosexual phase may have an intrinsic femaleness tendency due to either genotype or microsite effects. APPROXIMATELY 20% of the flowering plants produce at least some unisexual flowers (e.g., monoecy, gynomonoecy, and andromonoecy Yampolsky and Yampolsky, 1922). Their allocation to male vs. female functions can vary within and/or between years with the production of different types of flowers (e.g., Abul Fatih and Bazzaz, 1979; Freeman, Harper, and Chamov, 1980; Freeman et al., 1981; Bierzychudek, 1982, 1984b; Condon and Gilbert, 1988), possibly in response to ecological factors through appropriate hormonal or other cues. Though it has been suggested that these variations are adaptive, there have been few comprehensive field studies of the ecological correlates, adaptive value, and fitness consequences of different patterns of gender allocation. Most studies have looked at gender allocation at only one or two levels: between seasons, for only one season or, more rarely, temporally during one season (e.g., Lloyd, Webb, and Primack, 1980; Lovett Doust, 1980; Received for publication 28 July 1987; revision accepted 25 April 1988. I thank A. Kodric-Brown, J. H. Brown, S. Buchmann, M. Condon, A. Harvey, B. Holmes, D. Lloyd, D. L. Venable, and anonymous reviewers for many useful comments on this manuscript. K. Iverson helped with field work. Financial support was provided by Sigma Xi, the National Academy of Science, the Appleton-Whittell Research Foundation, and the Closse/Delesalle/Duggan Foundation. This work is part of my Ph.D. dissertation. 2 Current address: Department of Botany, Ohio University, Athens, OH 45701. Primack and Lloyd, 1980a, b; Thomson and Barrett, 1981; Bertin, 1982; Bierzychudek, 1982, 1984a, b; Primack and McCall, 1986). More long-term studies are necessary to determine the types of gender distributions observed in natural populations and to assess the variation possible in gender. Consequently, I started an investigation of gender allocation in a perennial monoecious cucurbit, Apodanthera undulata, where male and female functions are clearly separated. The following scenarios for year-to-year gender allocation in perennial cosexual species are proposed. In many cases and as could be predicted from the size-advantage model (Ghiselin, 1969), younger and smaller plants would first produce no flowers and then only staminate flowers until they reach a certain size. Some plants may fluctuate between producing no flowers and only staminate flowers for a few years. This pattern or a similar one is found in a number of species (e.g., Benseler, 1975; Hibbs and Fischer, 1979; Bierzychudek, 1984b; Condon and Gilbert, 1988) and assumes that reproduction through male function is less resource-limited than through female function (Willson and Burley, 1983). Once plants reach a certain size and age, they produce pistillate flowers. There are 3 possible scenarios: A) Allocation to female function (i.e., femaleness) could increase continuously until the plant reached its maximum femaleness. This maximum could vary between individuals de-