Revista Mexicana de Biodiversidad 81: 745 - 758, 2010 Distribution, genetic structure, and conservation status of the rare microendemic species, Guaiacum unijugum (Zygophyllaceae) in the Cape Region of Baja California, Mexico Distribución, estructura genética y conservación de la especie microendémica Guaiacum unijugum (Zygophyllaceae) en la región de los Cabos de Baja California, México. Ross A. McCauley*1, Aurea C. Cortés-Palomec, and Ken Oyama Centro de Investigaciones en Ecosistemas, Universidad Nacional Autónoma de México. Antigua Carretera a Pátzcuaro No. 8701, Col. Ex-Hacienda de San José de la Huerta, 58190 Morelia, Michoacán, México. 1Current address: Department of Biology and Agriculture, Fort Lewis College, 1000 Rim Drive. Durango, Colorado, 81301 USA. *Correspondent: [email protected]; [email protected] Abstract. Guaiacum unijugum is a rare shrub endemic to a 70 km stretch of coastline extending east from San José del Cabo in Baja California and is the least well-known of the 4 species of Guaiacum in Mexico. To increase our knowledge of this species and assess its conservation status we surveyed the extent of occurrence using both herbarium material and field work, assessed levels of genetic diversity, determined its phylogenetic relationships, and completed an evaluation of risk of extinction (MER). Herbarium material identified 5 known localities of occurrence with field work verifying the continued persistence of 4 of these with an additional site discovered. Genetic analysis across the small range using 17 microsatellite loci showed very low levels of genetic diversity with a mean expected heterozygosity (HE) of 0.162 over all polymorphic loci. Most loci were found to be monomorphic and genetic divergence was small, maintained by the presence of rare private alleles in widely-separated populations. Phylogenetic analysis indicated a sister group relationship to G. coulteri along the Pacific coast suggesting vicariance for the origin and occurrence of G. unijugum. The unique evolutionary history coupled with current small population sizes warrants increased conservation via listing as a critically endangered species. Key words: Baja California Sur, conservation genetics, endemic species, Guaiacum unijugum, MER, Mexico, Zygophyllaceae. Resumen. Guaiacum unijugum es un arbusto endémico en un área de aproximadamente 70 km en la región de Los Cabos, Baja California Sur, siendo la menos estudiada de las 4 especies de Guaiacum en México. Para incrementar nuestro conocimiento sobre esta especie y determinar su estatus de conservación se realizó un censo de sus poblaciones determinándose su estructura genética, su relación filogenética con otros miembros del género y se calculó su riesgo de extinción (MER). La revisión de material de herbario, confirmó la presencia de 4 poblaciones a las que se sumó el hallazgo de 1 más. Mediante el uso de 17 loci de microsatélites se encontraron bajos niveles de diversidad genética con un promedio de heterocigosidad (HE) de 0.162. La mayoría de los loci fueron monomórficos y la diferenciación genética entre poblaciones fue pequeña y atribuida a la presencia de alelos raros y únicos en algunas poblaciones. El análisis filogenético indicó que el de G. unijugum es un grupo hermano de G. coulteri de la costa del Pacífico en México, lo cual sugiere que la distribución de G. unijugum es el resultado de vicarianza. Su historia evolutiva y la presencia de poblaciones pequeñas indican la necesidad de considerarla como especie en peligro de extinción. Palabras clave: Baja California Sur, genética de la conservación, especie endémica, Guaiacum unijugum, MER, México, Zygophyllaceae. Introduction native to the American dry tropics ranging from Sonora and the Florida Keys to northern Venezuela. The genus has a long history of human utilization. It was likely The genus Guaiacum, often known commonly as used as a medicinal plant for centuries by native peoples “Lignum Vitae” or “Palo santo”, is a group of 6 species of the Caribbean and was adopted by Europeans by the late 15th century for the treatment of syphilis (Munger, Recibido: 29 septiembre 2009; aceptado 06 febrero 2010 1949). It has been used in a variety of other medicinal 16-566.indd 1 18/11/2010 09:52:49 p.m. 746 McCauley et al. - Guaiacum unijugum in Baja California treatments and the heartwood resin is currently used for and establishment is however unknown. The relationship the production of kits for detecting hidden gastrointestinal of G. unijugum to the remainder of the genus in Mexico bleeding (Grow and Schwartzman, 2001; Oldfield, 2005). is additionally uncertain. Porter (1963) hypothesized that Its wood additionally has been of high value. It is one G. unjugum was closely related to G. coulteri extending of the hardest and most dense woods in trade and has a along the coast from Sonora to Oaxaca and that it could self-lubricating quality making it useful for mechanical represent only a restricted subspecies or variety of this purposes such as wooden bearings and marine propeller- more widespread taxon. Its unique leaf structure and shaft bushings (Oldfield, 2005). isolation by more than 200 km from the remainder of the Overexploitation in conjunction with habitat loss genus have led to its continued recognition. and a slow rate of regeneration has led to the listing of The isolation and rarity of G. unijugum has apparently most Guaiacum species on various state, national, and limited its use in medicinal treatments and wood international lists of endangered species. In Mexico, production. While it is likely that G. unijugum possesses Guaiacum sanctum and G. coulteri are classified as species many of the same chemical and wood characteristics as under special protection (SEMARNAT, 2002) and both are the other species of Guaiacum, it is not known to have classified in the IUCN Red List with G. sanctum listed as ever been exploited commercially. Such lack of use has endangered (EN C2a) and G. coulteri listed as lower risk led to G. unijugum not being recognized on national or conservation dependent (LR/cd) (IUCN, 2007). Protection international lists of threatened or endangered species. regulating international trade in all parts and derivatives is As the genus Guaiacum is a group of high conservation afforded by the 2003 listing of all species of Guaiacum in concern across its entire range, the assessment of Appendix II of the Convention on International Trade in conservation status for all of the species is equally Endangered Species of Wild Fauna and Flora (CITES) due important. Thus in this study we aim to provide a set to the inability to differentiate timber from among different of basic information regarding G. unijugum including species (CITES, 2003; UNEP-WCMC, 2007). a detailed survey of its precise distribution, an analysis One of the rarer and least known species of the genus of genetic diversity and differentiation, assessment of Guaiacum is G. unijugum Brandegee, a rare endemic of phylogenetic relationship, and a criteria-based analysis of the Cape Region of Baja California. The species was first extinction risk using the Method for Evaluation of Risk of observed and collected by T. S. Brandegee in September of Extinction for Mexican Wild Species (MER; SEMARNAT, 1890 while surveying the flora of southern Baja California 2002). under the auspices of the California Academy of Sciences. He initially identified the shrub as Guaiacum sanctum L., a large tree native to tropical forests from southeastern Materials and methods Mexico to Costa Rica, although his notes at the time expressed doubts regarding this placement (Brandegee, 1891). The species, while showing similarities to G. Herbarium, field observations and tissue sampling. sanctum, differed by its much smaller stature and in Herbarium material was sought from a large number of having only 1 or 2 pairs of oblique leaflets, indicating to North American herbaria with the collections at DEK, Brandegee its uniqueness and thus suggesting designation HCIB, MEXU, SD and UC housing specimens of G. as a discrete species (Brandegee, 1915). unijugum. These were consulted to compile the known Guaiacum unijugum is a divaricately branched shrub distribution of the species (See Appendix for listing of growing to about 2 meters in height on coastal dunes and all reviewed specimens). In August of 2007 a survey of sandy washes. It has small pinnately compound leaves the region was undertaken based on the information from with generally one or rarely 2 pairs of leaflets. It produces previous collection localities as a guide to delimit the region 5-merous flowers with light blue petals during mid- of species occurrence and determine the persistence of the summer, maturing into a yellowish-brown capsule with species in these locations. Areas comprising the potential generally one reddish-orange arillate seed by late summer vegetation type near previous collection localities were to early fall (Porter, 1963) (Figure 1). searched for individuals of G. unijugum. To determine Due to the rarity of G. unijugum little is known abundance, qualitative observations of the number of adult regarding the basic biology of the species. Pollination individuals were made in each collection locality with sites is likely facilitated by bees which have been observed demarcated as zones in which individuals of G. unijugum visiting flowers of G. coulteri (R. McCauley, personal could be encountered within 0.5 km of each other. Leaf observation) which are almost identical in both color and material was sampled from all observed adult plants in structure. Further information regarding the reproduction each population and placed in silica-gel desiccant for later 16-566.indd 2 18/11/2010 09:52:49 p.m. Revista Mexicana de Biodiversidad 81: 745 - 758, 2010 747 Figure 1. Vegetative and floral morphology of G. unijugum. A. Typical opposite pinnately compound leaves with a single pair of leaflets. B. Atypical leaves having 2 pairs of leaflets. C. Habit showing divaricate branches with leaves densely clustered on short lateral branchlets. D. View showing dense fruiting branch.