Ant-mediated seed dispersal alters pattern of relatedness in a population of Trillium grandiflorum

Seed dispersal creates the initial spatial distribution of individuals in a population and in conjunction with the mating system influences spatial pa tterns of relatedness. This spatial template of related individuals sets th e stage for all subsequent density-dependent and frequency-dependent intera ctions. In this study we document how ant-mediated seed dispersal affects t he number and relatedness of seeds in both dispersed and undispersed aggreg ations and how these patterns influence seedling emergence in the long-live d perennial, Trillium grandiflorum. Experimental hand-pollinations in two y ears demonstrated that selfing is extremely rare and suggested that self-in compatibility (SI) is a likely explanation. Our multi-locus outcrossing est imate (t(m) =1.05 +/- 0.056) confirms this result and also suggests that se eds within a fruit are likely to have the same pollen parent. Thus a highly outcrossing mating system is the initial determinant of relatedness among seeds within a fruit. We tracked uniquely coded, radiolabeled seeds from 30 and 40 fruits in 1991 and 1992, respectively, to determine how dispersal a lters this initial relatedness of seeds. Of the 335 and 876 seeds labeled i n these two years, we recovered 63% and 76% of the seeds postdispersal and found that 19% and 23% of the recovered seeds were dispersed >10 cm from th e maternal parent in the first and second years, respectively. In both year s, ant-mediated dispersal reduced the number of seeds near the maternal par ent. However, the effect of seed dispersal on the number of seeds in aggreg ations varied among years. Anti-mediated dispersal increased the number of seeds in dispersed aggregations in the first year and decreased the number in the second year. The average seed dispersal distance also differed betwe en years: 2.41 m (+/-0.33) vs. 0.53 m (+/-0.06) in years 1 and 2, respectiv ely. Ant-mediated seed dispersal decreased the probability of a seed having a sibling as its nearest neighbor postdispersal by between one-third and o ne-half. In contrast, seedling emergence was related to neither dispersal n or seed aggregation size in our study. However, the fitness effects of disp ersal may be important later in the life cycle of this long-lived species a nd as such were undetected. One scenario is that plants derived from seeds dispersed out of their sibling relatedness group may gain minority advantag e both in terms of mating success (if the population is SI) and other frequ ency-dependent processes like disease resistance.