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Genetic diversity and the mating system of a rare Mexican piñon, Pinus pinceana, and a comparison with Pinus maximartinezii (Pinaceae)¹

²Institute of Forest Genetics, Pacific Southwest Research Station, USDA Forest Service, 2480 Carson Road, Placerville, California 95667 USA; ³Departamento Forestal, Universidad Autónoma Agraria Antonio Narro, Buenavista, Saltillo, Coahuila, México; ⁴Service de Génétique et d'Amélioration des Arbres Forestieres, Centre National de la Recherche Forestiere, Charia Omar Ibn El Khattab, B.P. 763 Rabat, Morocco; ⁵Programa Forestal, Instituto de Recursos Naturales, Colegio de Postgraduados en Ciencias Agricolas, Montecillo, México, C.P. 56230, México; and ⁶Centro de Genética Forestal, Universidad Autónoma Chapingo, Apartado Postal No. 37, Chapingo, México, C.P. 56230, México

Weeping piñon (Pinus pinceana) has a restricted and fragmented range, trees are widely scattered within populations, and reproduction is limited. Nevertheless, genetic diversity was high; based on 27 isozyme loci in 18 enzyme systems, unbiased expected heterozygosity averaged 0.174. Differentiation also was high (F_ST = 0.152), reflecting isolation between southern, central, and northern fragments of the range. Among populations in the northern fragment, F_ST was only 0.056, and the number of migrants per generation (Nm) was 4.21, which should preclude fixation. Nm between central and southern populations or between them and populations in the northern fragment was lower, 0.99–1.66, indicating a degree of genetic isolation. Multilocus outcrossing rates (t_m) ranged from 0.836 in the south to 0.897 in the north. Therefore, selfing is low but statistically significant. The equilibrium inbreeding coefficient (F_e) calculated from t_m was in good agreement with observed inbreeding coefficients, suggesting that weeping piñon may be near equilibrium with respect to inbreeding and selection against selfed trees. Weeping piñon was variable at all loci polymorphic in maxipiñon (Pinus maximartinezii) and, therefore, qualifies as a possible progenitor of maxipiñon. Because of the high level of diversity, reasonable levels of gene flow within the northern fragment of weeping piñon's range, high rates of outcrossing, and, perhaps, only weak selection against inbred trees, protection in reserves would be a viable option for conservation.

Key Words: endangered species • fitness • fragmentation • genetic distance • genetic structure • isozymes • pollen allele frequencies • selfing

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