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

Weeping pinon (Pinus pinceana) has a restricted and fragmented range, trees are widely scattered within populations, and reproduction is limited. Neve rtheless, genetic diversity was high; based on 27 isozyme loci in 18 enzyme systems, unbiased expected heterozygosity averaged 0.174. Differentiation also was high (F-sr = 0.152), reflecting isolation between southern, centra l, and northern fragments of the range. Among populations in the northern f ragment, F-sr 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 l ower, 0.99-1.66, indicating a degree of genetic isolation. Multilocus outcr ossing rates (t(m)) ranged from 0.836 in the south to 0.897 in the north. T herefore, selling is low but statistically significant. The equilibrium inb reeding coefficient (F-e) calculated from t(m) was in good agreement with o bserved inbreeding coefficients, suggesting that weeping pinon may be near equilibrium with respect to inbreeding and selection against selfed trees. Weeping pinon was variable at all loci polymorphic in maxipinon (Pinus maxi martinezii) and, therefore, qualities as a possible progenitor of maxipinon . Because of the high level of diversity, reasonable levels of gene flow wi thin the northern fragment of weeping pinon's range, high rates of outcross ing, and, perhaps, only weak selection against inbred trees, protection in reserves would be a viable option for conservation.