Hybridization in the Catalina Island Mountain Mahogany (Cercocarpus traskiae): RAPD Evidence

Abstract

The Catalina Island mountain mahogany is one of 10 species of shrubs and small trees (Lis 1992) comprising the genus Cercocarpus H. B. K (Rosaceae). Species in the genus are widely distributed throughout western North America and are delimited primarily on the basis of leaf characteristics (Mortenson 1973; Lis 1992). The Catalina Island mountain mahogany (C. traskiae Eastwood) is one of the most distinctive species in the genus due to its thick, coriaceous leaves and woolly pubescence on the leaf undersurface. The species is extremely rare, restricted to Wild Boar Gully on the southwest side of Santa Catalina Island in the Channel Islands off the coast of California (Fig. 1). When the population was first discovered in 1897, it consisted of over 40 individuals (Thorne 1967). Due to overgrazing by introduced mammalian herbivores (Coblentz 1980; Hobbs 1980), however, the population has been greatly reduced (Rieseberg et al. 1989). Cercocarpus traskiae is listed as endangered by the state of California and thus is afforded protection by California law (Smith & Berg 1988). Inexplicably, the species is not listed as endangered or threatened by the U.S. Fish and Wildlife Service, although it is under consideration for listing. In the late 1970s a detailed inventory of the Wild Boar Gully population was undertaken, and a total of eight Cercocarpus trees were discovered and tagged (Martin 1984). In addition, two of the trees (A, B; Fig. 1) were fenced to protect them from herbivores, and in 1985 the Catalina Conservancy constructed a larger fence around an area that included both trees, as well as about an acre of good habitat. The fencing effort was extremely successful, and by 1988 approximately 70 seedlings were observed within the fenced area (Rieseberg et al. 1989). Unfortunately, several of the adult plants resembled the island mountain mahogany, Cercocarpus betuloides Torrey & A. Gray var. blancbeae (C. Schneider) Little, in key morphological features such as leaf pubescence and leaf thickness or were immediate for these characters (descriptions of both species are given by R. Lis in Hickman [1993]). Cercocarpus betuloides var. blancbeae is only slightly more widespread than C traskiae, occurring on all of the Channel Islands except San Clemente. But the species is much more abundant than C traskiae on Santa Catalina and is known to occur in adjacent canyons, so the possibility of hybridization between the two species could not be dismissed. Furthermore, it is suspected that some of the seedlings might be of hybrid origin (T. Martin, personal communication). In an attempt to clarify the identity of morphologically ambiguous individuals, Rieseberg et al. (1989) undertook a study of allozymic variation and leaf anatomy of all the adult mahogany trees discovered in Wild Boar Gully. Of the eight adult trees in the Gully, three were anatomically intermediate between C. traskiae and C betuloides var. blancbeae, suggestive of hybridization. Analysis of 17 isozyme loci in C traskiae, as well as a nearby population of C betuloides var. blancbeae (Fig. 1), revealed one diagnostic isozyme polymorphism differentiating the two species. Based on this single isozyme polymorphism, five of the eight trees were diagnosed as C traskiae, one as C betuloides var. blancbeae, and two as hybrids. However, isozyme analyses of the 28 seedlings large enough to survive the loss of a single leaf revealed all to be pure C traskiae. The results from this study were not entirely satisfactory, however, because the classification of individuals as parental versus hybrid was based on a single locus. The use of a single diagnostic locus can ensure the correct identification of first-generation hybrids, but F2or later-generation hybrid or backcross progeny could easily be misdiagnosed as belonging to one of the parental classes. Thus, it was not surprising that two of the plants lacking a hybrid enzyme genotype were anatomically Paper submitted April 1994; revised manuscript accepted July 15, 1994.