The mating system and genic diversity in Martinez spruce, an extremely rare endemic of Mexico's Sierra Madre Oriental: an example of facultative selfing and survival in interglacial refugia

Martinez spruce (Picea martinezii T.F. Patterson) is a conifer currently pa ssing through a bottleneck, reduced to a few relict populations totaling le ss than 800 trees. We used isozyme markers to analyze the mating system and survey the level of genic diversity in two populations. The mating system was characterized by a high frequency of selfing. The multilocus outcrossin g rates (t(m)) and 95% confidence intervals were only 0.399 (0.197 < t(m) < 0.601) for the smallest population and 0.589 (0.475 < t(m) < 0.703) or 0.6 85 (0.465 < t(m) < 0.905), depending on year, for the largest. These are am ong the lowest rates of outcrossing observed in conifers. The fixation indi ces for the two populations were -0.058 and 0.121, less than expected for s uch high levels of selfing. Expected heterozygosity, unbiased H-e, based on 22 loci in 13 enzyme systems, was 0.121 and 0.101 in the two populations. The proportion of the total genic diversity between populations, F-ST, was 2.4%. Nm, the number of migrants per generation, was about 1.00 or 10.17, d epending on the method of estimation. The time since the two populations we re isolated was estimated from Nei's genetic distance as only 150 to 15 000 years, which is consistent with a hypothesis of population collapse during late Pleistocene or Holocene warming. We discuss the implications for cons ervation.