Abstract
Pollination patterns within Quercus lobata and interspecific hybridization between Q. lobata and Quercus douglasii were studied in a coastal central California mixed woodland. We first identified hybrids by means of microsatellite genotyping and assignment tests. Hybrids were rare, both among adults (4 of 190, 2.1%) and among acorns collected from Q. lobata trees (6 of 392, 1.5%). These low rates of hybridization at both early and late life history stages suggest that fertility barriers, rather than natural selection against hybrids, limit hybridization between these two species. However, hybrid adults, although rare, may facilitate gene flow between the two species. Acorns collected from a hybrid tree had both Q. lobata pollen donors (11 of 30, 37%) and Q. douglasii or hybrid pollen donors (19 of 30, 63%). After removing hybrid acorns from the analysis, we used paternity assignment to track pollination patterns within Q. lobata. Of 108 acorns, only 32 (30%) were assigned to candidate pollen donors within 200 m of the maternal tree, indicating that the majority of effective pollen travels more than 200 m. Individual trees had acorn crops with many different sires and an average effective number of pollen donors (Nep) of 219 per tree. Indirect methods using correlated paternity estimated mean pollination distances of ∼100 m and mean Nep of 5.2 per tree, values much lower than those derived directly from paternity assignments.
Highlights
In plants, pollination patterns are a critical component of the ecological and evolutionary dynamics of plant populations
We identified adults and acorns that were the result of hybridization between valley oak, Quercus lobata, and blue oak, Quercus douglasii
For the six loci scored in both species, 33 of the 88 alleles observed in Q. lobata were not observed in Q. douglasii, and most of the shared alleles occurred at different frequencies between the two species
Summary
Pollination patterns are a critical component of the ecological and evolutionary dynamics of plant populations. Rates and patterns of effective pollen movement will determine the size of a plant’s reproductive neighborhood, the connectivity of populations, and the potential effects of habitat fragmentation (Levin and Kerster 1974; Loveless and Hamrick 1984; Smouse and Sork 2004). In addition to characterizing the distance and direction of pollination patterns, microsatellite analysis allows precise estimates of rates of hybridization. In the genus Quercus (oak), hybridization is common, and reproductive barriers between species appear to be weak (Whittemore and Schaal 1991; Muir et al 2000; Williams et al 2001). We characterized both inter- and intraspecific pollination patterns in a mixed stand of oaks. We characterized intraspecific pollination within one species, Q. lobata, using
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