Population genetics of the tropical epiphytic orchid, Laelia rubescens
Trapnell, Dorset Waters
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Insights into the long-term survival potential of plant species occupying remnant habitat fragments require an understanding of the processes responsible for the distribution of genetic variation within species. This research project examines the partitioning of genetic variation at multiple spatial scales, as well as patterns of mating and gene movement in the epiphytic orchid, Laelia rubescens, in the disturbed, seasonally dry tropical forest of Costa Rica. Threedimensional fine-scale genetic structure was examined in three large populations. In larger clusters multiple genets/cluster were common with discrete clusters containing up to nine genotypes. Spatial autocorrelation analyses indicated significant positive genetic structure at distances =45 cm. This result is likely due to the establishment of discrete clusters by vegetative reproduction as well as the establishment of sexually derived progeny within and near maternal clusters. Full-sib paternity analyses were used to examine mating patterns and to directly measure pollen flow between populations. Gene flow between populations was responsible for 34% of the capsules produced in 1999 and 2000. Pollen moved mean distances of 279 m and 519 m in the two years (maximum = 1034 m). The marked increase in pollen dispersal distances in 2000 was related to a substantially increased floral display. Smaller populations, that more closely resemble those in undisturbed forest, displayed higher gene flow rates than the large populations that characterize disturbed sites. Therefore higher levels of gene flow between lowdensity populations occupying undisturbed habitats is predicted. Contributions of pollen and seed movement to overall gene flow were investigated at multiple spatial scales. Nuclear (allozyme) genetic structure was low at every spatial scale indicating high overall rates of gene flow. Chloroplast RFLPs displayed greater structure but similar patterns. Both genomes displayed nonsignificant isolation-by-distance. At every spatial scale, pollen movement was responsible for greater gene dispersal than seed flow. Seed dispersal was highly localized with a large proportion of the seeds establishing within the maternal population. However once the wind-dispersed seeds escape the natal site they can move considerable distances. This species appears to be well suited to distributing its genes across an anthropogenically altered landscape.