Genetic Differentiation of Two California Red Oak Species, Quercus parvula var. Shreveii and Q. wislizeni, based on AFLP Genetic Markers

Oaks are renowned for posing problems in defining species boundaries. One example is the case of the interior live oak complex that is usually taken to include two varieties of Quercus wislizeni from the Coast Ranges of California and the Sierra Nevada, and Q. parvula var. shreveii from the central coast of California. The latter taxon was separated from Q. wislizeni, and was recognized in The Jepson Manual, but still is not fully accepted, since its morphological separation is ambiguous. In an attempt to test differentiation of these two taxa, molecular genetic analysis was carried out using Amplified Fragment Length Polymorphism (AFLP) markers. A total of 202 individuals from 19 populations including Q. parvula var. shreveii, Quercus wislizeni, Q. kelloggii and Q. agrifolia were analyzed. Four primer sets generated 311 polymorphic band classes. Although species-specific markers were observed between Q. kelloggii and other members of the red oak group in California, fixed genetic differences were not apparent among Q. agrifolia, Q. parvula var. shreveii and Q. wislizeni. Cluster analysis and principal coordinate analysis revealed low levels of genetic differentiation among coast live oak, interior live oak and shreve oak. Cluster analysis also suggested that Q. agrifolia populations are more similar to populations of Q. parvula var. shreveii than to coastal populations of Q. wislizeni. This may reflect the relatively short time since divergence of these species. On the other hand, repetitive hybridization and introgression may have acted as a homogenizing factor for these genetic markers. The relatively high genetic similarity between Q. agrifolia and Q. parvula var. shreveii may have implications for the management of Sudden Oak Death in the limited range of the latter taxon.