Evidence for an extreme bottleneck in a rare Mexican pinyon: Genetic diversity, disequilibrium, and the mating system in Pinus maximartinezii

Maxipinon (Pinus maximartinezii Rzedowski), which is confined to a single p opulation of approximately 2000 to 2500 mature trees, covers about 400 ha i n southern Zacatecas, Mexico. Genetic diversity measured by expected hetero zygosity was 0.122, which is moderate for pines. However, percentage polymo rphic loci was low, 30.3%. The fixation index (F) of 0.081 indicated only s light heterozygote deficiency. Mating system analysis indicated a significa nt but low level of selfing; the multilocus outcrossing rate, t(m), was 0.8 16. The mean of single locus estimates, t(s), was smaller (0.761), perhaps suggesting mating among relatives, although the difference between t(m) and t(s) was not statistically significant. The most striking features of maxipinon's genetic structure were that no po lymorphic locus had more than two alleles and most alleles at polymorphic l oci were at intermediate frequencies. This is in contrast to other pines, w hich often have three to five or more alleles at some loci and in which the distribution of allele frequencies is U-shaped, most alleles being present at frequencies less than 10% or greater than 90%. A population with only t wo alleles per locus and at intermediate frequencies could occur if the pop ulation had been reduced to an extreme bottleneck and then expanded rapidly before random drift modified allele frequencies. A novel origin from a hyb ridization event would also explain the results. Significant gametic disequilibrium was detected at several pairs of loci in both maternal and paternal gametes. The presence of disequilibrium is in a greement with an origin from an extreme bottleneck, perhaps even a single s eed. Furthermore, it demands that the event be relatively recent. The numbe r of generations, as calculated from the observed mean disequilibrium, sugg ested that maxipinon derived from an extreme bottleneck four to five genera tions ago, which is less than 1000 years in this species.