Mating system of wild Phaseolus lunatus L. and its relationship to population size

Using isozyme variation in a naturally pollinated seed family for 10 wild Phaseolus lunatus L. ( Lima bean) populations, ranging in sizes from 10 to 60 reproductive individuals, we estimated levels of outcrossing (t) and parental inbreeding coefficient ( F). We also examined the relationship between outcrossing rate and population size. Average estimates of the single-locus outcrossing rate (t(s)) ranged from 0.024 to 0.246 ( mean = 0.091 +/- 0.065). Estimates of the multilocus outcrossing rate (t(m)) ranged from 0.027 to 0.268, and averaged 0.096 +/- 0.071. Inbreeding coefficients based on genotypic frequencies of maternal plants were positive and significantly greater than zero ( F = 0.504), suggesting an excess of homozygotes in all the populations studied. There was indirect evidence of nonrandom mating for outcrosses and this was mainly attributed to self-fertilisation since the averaged difference between t(m) and t(s), which provides a measure of biparental inbreeding, represents only 1% of the autogamy rate. No significant correlation was observed between outcrossing rate and population size. Estimates of t showed significant heterogeneity among populations and factors explaining this tendency are suggested.