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Martínez spruce (Picea martínezii T.F. Patterson) is a conifer currently passing through a bottleneck, reduced to a few relict populations totaling less 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 outcrossing rates (tm) and 95% confidence intervals were only 0.399 (0.197 < tm < 0.601) for the smallest population and 0.589 (0.475 < tm < 0.703) or 0.685 (0.465 < tm < 0.905), depending on year, for the largest. These are among the lowest rates of outcrossing observed in conifers. The fixation indices for the two populations were -0.058 and 0.121, less than expected for such high levels of selfing. Expected heterozygosity, unbiased He, 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, FST, was 2.4%. Nm, the number of migrants per generation, was about 1.00 or 10.17, depending on the method of estimation. The time since the two populations were 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 conservation.

Fragmentation and reduction in population size are expected to reduce genetic diversity. However, examples from natural populations of forest trees are scarce. The range of Chihuahua spruce retreated northward and fragmented coincident with the warming climate that marked the early Holocene. The isolated populations vary from 15 to 2441 trees, which provided an opportunity to test whether census number is a good predictor of genetic diversity. Mean expected heterozygosity, H , based on 24 loci in 16 enzyme systems, was 0.093 for 10 sampled populations, which is within the range reported for conifers. However, estimates varied more than twofold among populations and H was closely related to the logarithm of the number of mature trees in the population (r = 0.93). Diversity among populations, F , was 24.8% of the total diversity, which is higher than that observed in almost all conifer species studied. Nei's genetic distance, D, was not related to geographic distance between populations, and D̄ was 0.033, which is higher than estimates for most wide-ranging species. Most populations had excess homozygosity and the fixation index, F , was higher than that reported for all but one species of conifer. Nm, the number of migrants per generation, was 0.43 to 0.76, depending on estimation procedure, and is the smallest observed in conifers. The data suggest that populations of Chihuahua spruce have differentiated by drift and that they are effectively isolated. The results illustrate how a combination of paleontological observation and molecular markers can be used to illuminate recent evolutionary events. Multilocus estimates of outcrossing for two small populations were zero (complete selfing) and 0.153, respectively, which are in striking contrast to the near complete outcrossing observed in most conifers. The high fixation index and a high proportion of empty seeds (45%) suggest that inbreeding may be a serious problem for conservation of Chihuahua spruce.

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