The roles of the various potential ecological and evolutionary causes of spatial population genetic structure (SPGS) cannot in general be inferred from the extant structure alone. However, a stage-specific analysis can provide clues as to the causes of SPGS. We conducted a stage-specific SPGS analysis of a mapped population of about 2000 Trillium grandiflorum (Liliaceae), a long-lived perennial herb. We compared SPGS for juvenile (J), nonreproductive (NR), and reproductive (R) stages. Fisher's exact test showed that genotypes had Hardy-Weinberg frequencies at all loci and stage classes. Allele frequencies did not differ between stages. Bootstrapped 99% confidence intervals (99%CI) indicate that F-statistic values are indistinguishable from zero, (except for a slightly negative F IT for the R stage). Spatial autocorrelation was used to calculate f, the average kinship coefficient between individuals within distance intervals. Null hypothesis 99%CIs for f were constructed by repeatedly randomizing genotypic locations. Significant positive fine-scale genetic structure was detected in the R and NR stages, but not in the J stage. This structure was most pronounced in the R stage, and declined by about half in each remaining stage: near-neighbor f 0.122, 0.065, 0.027, for R, NR, and J, respectively. For R and NR, the near-neighbor f lies outside the null hypothesis 99%CI, indicating kinship at approximately the level of half-sibs and first cousins, respectively. We also simulated the expected SPGS of juveniles post dispersal, based on measured R-stage SPGS, the mating system, and measured pollen and seed dispersal properties. This provides a null hypothesis expectation (as a 99%CI) for the J-stage correlogram, against which to test the likelihood that post-dispersal events have influenced J-stage SPGS. The actual J correlogram lies within the null hypothesis 99%CI for the shortest distance interval and nearly all other distance intervals indicating that the observed low recruitment, random mating and seed dispersal patterns are sufficient to account for the disappearance of SPSG between the R and the J stages. The observed increase in SPGS between J and R stages has two potential explanations: history and local selection. The observed low total allelic diversity is consistent with a past bottleneck: a possible historical explanation. Only a longitudinal stage-specific study of SPGS structure can distinguish between historical events and local selection as causes of increased structure with increasing life history stage. Two major factors are thought to contribute to the extent of spatial population genetic structure (SPGS) either within or among plant populations: the patterns and distances of pollen and seed dispersal (Ennos 1994) and the extent of local selection (Hedrick 1986). The predicted effects of various patterns of pollen and seed dispersal are well understood (Slatkin 1985; Ennos 1994), as are the theoretical consequences of local selection (Felsenstein 1976; Slatkin 1985). However, the relative importance ...
JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. Seed dispersal creates the initial spatial distribution of individuals in a population and in conjunction with the mating system influences spatial patterns of relatedness. This spatial template of related individuals sets the stage for all subsequent density-dependent and frequency-dependent interactions. In this study we document how ant-mediated seed dispersal affects the number and relatedness of seeds in both dispersed and undispersed aggregations and how these patterns influence seedling emergence in the long-lived perennial, Trillium grandiflorum. Experimental hand-pollinations in two years demonstrated that selfing is extremely rare and suggested that self-incompatibility (SI) is a likely explanation. Our multi-locus outcrossing estimate (tm = 1.05 + 0.056) confirms this result and also suggests that seeds 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 alters this initial relatedness of seeds.Of the 335 and 876 seeds labeled in 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 the maternal parent in the first and second years, respectively. In both years, ant-mediated dispersal reduced the number of seeds near the maternal parent. However, the effect of seed dispersal on the number of seeds in aggregations varied among years. Antmediated 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 between years: 2.41 m (+0.33) vs. 0.53 m (+0.06) in years 1 and 2, respectively. Ant-mediated seed dispersal decreased the probability of a seed having a sibling as its nearest neighbor postdispersal by between one-third and one-half. In contrast, seedling emergence was related to neither dispersal nor seed aggregation size in our study. However, the fitness effects of dispersal may be important later in the life cycle of this long-lived species and as such were undetected. One scenario is that plants derived from seeds dispersed out of their sibling relatedness group may gain minority advantage both in terms of mating success (if the population is SI) and other frequency-dependent processes like disease resistance.being especially common (van der Pijl 1972, Howe and Smallwood 1982). From the plant's perspect...
Abstract. The roles of the various potential ecological and evolutionary causes of spatial population genetic structure (SPGS) cannot in general be inferred from the extant structure alone. However, a stage-specific analysis can provide clues as to the causes of SPGS. We conducted a stage-specific SPGS analysis of a mapped population of about 2000 Trillium grandiflorum (Liliaceae), a long-lived perennial herb. We compared SPGS for juvenile (J), nonreproductive (NR), and reproductive (R) stages. Fisher's exact test showed that genotypes had Hardy-Weinberg frequencies at all loci and stage classes. Allele frequencies did not differ between stages. Bootstrapped 99% confidence intervals (99%CI) indicate that F-statistic values are indistinguishable from zero, (except for a slightly negative F IT for the R stage). Spatial autocorrelation was used to calculate f, the average kinship coefficient between individuals within distance intervals. Null hypothesis 99%CIs for f were constructed by repeatedly randomizing genotypic locations. Significant positive fine-scale genetic structure was detected in the R and NR stages, but not in the J stage. This structure was most pronounced in the R stage, and declined by about half in each remaining stage: near-neighbor f ϭ 0.122, 0.065, 0.027, for R, NR, and J, respectively. For R and NR, the near-neighbor f lies outside the null hypothesis 99%CI, indicating kinship at approximately the level of half-sibs and first cousins, respectively. We also simulated the expected SPGS of juveniles post dispersal, based on measured R-stage SPGS, the mating system, and measured pollen and seed dispersal properties. This provides a null hypothesis expectation (as a 99%CI) for the J-stage correlogram, against which to test the likelihood that post-dispersal events have influenced J-stage SPGS. The actual J correlogram lies within the null hypothesis 99%CI for the shortest distance interval and nearly all other distance intervals indicating that the observed low recruitment, random mating and seed dispersal patterns are sufficient to account for the disappearance of SPSG between the R and the J stages. The observed increase in SPGS between J and R stages has two potential explanations: history and local selection. The observed low total allelic diversity is consistent with a past bottleneck: a possible historical explanation. Only a longitudinal stage-specific study of SPGS structure can distinguish between historical events and local selection as causes of increased structure with increasing life history stage. Two major factors are thought to contribute to the extent of spatial population genetic structure (SPGS) either within or among plant populations: the patterns and distances of pollen and seed dispersal (Ennos 1994) and the extent of local selection (Hedrick 1986). The predicted effects of various patterns of pollen and seed dispersal are well understood (Slatkin 1985;Ennos 1994), as are the theoretical consequences of local selection (Felsenstein 1976;Slatkin 1985). However, the relative i...
The relative importance of age and size as predictors of demographic parameters such as survivorship or reproductive status is generally unknown for herbaceous perennials, due primarily to the difficulty in estimating the age of herbaceous plants. We investigated the relationship between age, size, and reproductive status in Trillium grandiflorum, a rhizomatous perennial herb in which age can be estimated. We measured the leaf area and rhizome volume and estimated the age of plants in a study population that included reproductive and nonreproductive individuals. Reproductive plants were significantly older ( = 22.8 years) than nonreproductive plants ( = 13.3 years). Reproductive plants also had significantly larger total leaf areas and rhizome volumes. Total leaf area, rhizome volume, and age were positively correlated in both groups. Reproduction in this population occurred once plants reached a threshold leaf area or rhizome volume. Both measures of plant size, i.e., total leaf area and rhizome volume, were better predictors of plant reproductive status than was plant age.
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