Population Structure and Gene Flow in Two Rare, Isolated Quercus Species: Q. hinckleyi and Q. pacifica

This research examines the population structure, genetic variability, and gene flow of two rare, isolated scrub oak species using DNA microsatellite analysis. Quercus hinckleyi C.H. Muller is a threatened species whose U.S. range is essentially limited to one county in West Texas. Quercus pacifica K. Nixon & C.H. Muller is an endemic species found on three of the eight Channel Islands off the coast of California. While one is isolated by land and the other by sea, both of these species exist on what are in effect islands: Q. hinckleyi has been identified at a handful of sites separated by Chihuahuan Desert terrain; Q. pacifica is found on three offshore islands, Santa Rosa, Santa Cruz and Santa Catalina, separated by open waters of the Pacific Ocean. Low levels of genetic variability, inbreeding, and limited gene flow are three possible threats to small, fragmented plant populations, potentially contributing to inbreeding depression and reduced fitness levels. This study found, however, that Q. hinckleyi is not genetically depauperate despite its rarity, although unique genets are reduced because of cloning. While there is some hybridization within the Q. hinckleyi population, there is no evidence of genetic swamping by sympatric species, Q. pungens and Q. vaseyana. Quercus pacifica has high levels of genetic variation and low levels of genetic differentiation among the three islands on which it is found, suggesting it originated from a common, genetically diverse ancestral form and that gene flow among islands has maintained genetic continuity over great distances. Comparison of Q. pacifica with two mainland species, Q. berberidifolia and Q. dumosa, indicates significant levels of genetic differentiation supporting the current reclassification of these three species.