Genetic Structure of Age Classes in Camellia Japonica (Theaceae)

Abstract

Abstract. Camellia japonica L. (Theaceae), an insect-and bird-pollinated, broad-leaved evergreen tree, is widely distributed in Japan and the southern Korean peninsula. The species has a relatively even age distribution within populations, which may influence the spatial genetic structure of different age classes relative to species with typical L-shaped age distributions. To determine whether the internal spatial genetic structure found in seedlings and young individuals carries over into adults, we used allozyme loci, F-statistics, spatial autocorrelation statistics (Moran's I), and coancestry measures to examine changes in genetic structure among seven age classes in a population (60-m ϫ 100-m area) in southern Korea. In seedlings, weak but significant positive values of Moran's I-statistics and coancestry measures were found for distances less than 14 m, which is consistent with a mechanism of limited seed dispersal combined with overlapping seed shadows. This spatial structure, however, dissipates in older age classes, and in adults genetic variation has an essentially random spatial distribution. Morisita's index of dispersion of individuals in each age class showed that seedlings and juveniles are more highly clustered than are older individuals. These results suggest that self-thinning changes the spatial relationships of individuals, and thus genotypes. A multilocus estimate of F ST (0.008) shows a small but statistically significant difference in allele frequencies among age classes. In summary, intrapopulation genetic structure within and among age classes of C. japonica was significant but weak. Despite presumably limited seed dispersal, weak spatial genetic structure in juveniles suggests overlapping seed shadows followed by self-thinning during recruitment. The present study also demonstrates that studies of spatial genetic structure focusing on limited numbers of generations may not be sufficient to reveal the entire picture of genetic structure in populations with overlapping generations. Theoretical models have shown that the genetic structure of local populations is affected by various nonequilibrium processes (Slatkin 1977;Wade and McCauley 1988). Genetic and demographic factors, such as pollen and seed dispersal from neighboring parents, past major reproductive events, selection, and other processes determining the life history of a species, influence population genetic processes (Jain and Bradshaw

Fine-scale spatial genetic dynamics over the life cycle of the tropical tree Prunus africana

“…It further indicates that the extent of gene flow is large for all life stages, and that no historic events have led to a fundamental alteration of processes across life stages. This is in line with other studies that have also shown that genetic diversity is similar across life stages within populations (for example, Chung et al, 2003;Jacquemyn et al, 2006). The levels of genetic diversity in our study were similar to those of another study on P. africana in Kakamega Forest (Farwig et al, 2008) and to those of European Prunus species (Stoeckel et al, 2006, P. avium: H e : 0.448-0.918) but higher than the diversity in an Asian Prunus species (Pakkad et al, 2003, P. cerasoides: H e : 0.292-0.689).…”

“…Restricted gene flow is probably one of the most prevalent factors influencing fine-scale spatial genetic structure (SGS), that is, a decrease in relatedness with increasing spatial distance between individuals (Vekemans and Hardy, 2004). It results in spatial aggregations of siblings in close vicinity of the mother trees and, thus, in high offspring's SGS (for example, Chung et al, 2003).…”

“…As only few individuals from the seed rain survive, SGS is likely to weaken in older stages (Chung et al, 2003). This effect is expected even if mortality is random in space (random thinning).…”

“…Thus, SGS analyses on pooled life stages of trees can be misleading. Hence a multistage perspective can lead to a higher resolution of the processes underlying the formation of SGS in trees (Chung et al, 2003;Pardini and Hamrick, 2008). Only a few studies that assessed SGS from a multistage perspective incorporated the initial seed rain stage in their analyses (for example, Jones and Hubbell, 2006;Zhou and Chen, 2010).…”

Estimating dispersal from short distance spatial autocorrelation

“…However, in some respects this may be similar to what occurs across life stages in populations that have strong intraspecific competition (although it should be noted that N e would also be decreasing in many cases). It has been observed in many tree species that relatively large amounts of autocorrelation are found in seedlings, less in juveniles and still less in reproductive adults and older age classes (Epperson and Alvarez-Buylla, 1997;Chung et al, 2003).…”