The California Red Oaks: a Pandora's Box
Richard S. Dodd1, Zara Afzal-Rafii2, and Vladimir Douhovnikoff3
Although in most cases oak species are distinct and readily identifiable in the field, species boundaries may break down locally due to a combination of recent speciation followed by slow rates of morphological divergence and of hybridization among closely related taxa. These two processes appear to have occurred in the coastal woodlands of California, leading to confusion in the identification of some trees that may be showing variable symptoms of infection by Phytophthora ramorum. Recognition of Quercus parvula var shrevei (Shreve oak) as a distinct taxon from Q. wislizeni var wislizeni (interior live oak) has not received full acceptance by the scientific community because of variable and overlapping morphological traits. Our morphological, biochemical and molecular data indicate that these two taxa are differentiated. However, the molecular data suggest a recent divergence that has not yet been accompanied by full reproductive isolation. The habitat preferences of Shreve oak and interior live oak are distinct and are reflected in the very marked divergence in biochemical characters that we have shown are indicators of adaptation to habitat stress. We suggest that Shreve oak and interior live oak have arisen from a recent common ancestor that may have diverged due to isolation in different glacial refugia. Because of the lack of full reproductive isolation, hybridization can be locally common between these two species and also with sympatric populations of Quercus agrifolia (coast live oak). Preliminary trials in which we have inoculated coast live oak, Shreve oak and interior live oak with Phytophthora ramorum, suggest that interior live oak is less susceptible than the two other species. If true, introgressed populations of Shreve oak and coast live oak with interior live oak genes may prove less susceptible than pure populations. We are testing this in in vitro inoculation studies.
1Department of ESPM, 145 Mulford Hall, University of California, Berkeley CA 94720; (510) 643-1635; dodd@nature.berkeley.edu
2CNRS, Université d'Aix-Marseille III, 13397 Marseille, France
3Department of ESPM, 145 Mulford Hall, University of California, Berkeley CA 94720
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