Estimating genetic erosion using the example of Picea chihuahuana Martinez

Genetic erosion can be defined as a permanent reduction in richness or evenness of common local alleles or the loss of combination of alleles over time in a defined area. This process can be detrimental to the short-term viability of individuals and populations, the evolutionary potential of populations and species, and the direct use of genetic resources. Various international and intergovernmental organizations and networks have therefore recognized the need to assess and monitor plant genetic erosion in order to prevent such effects. The rare tree species Picea chihuahuana Martinez, which is endemic to Mexico, is an excellent model for estimating potential genetic erosion. The species occurs in about 40 often isolated relict populations in the Sierra Madre Occidental. In this study, the degree of genetic erosion was evaluated in five populations of P. chihuahuana M. in the State of Durango (Mexico), by comparing the genetic diversity in diameter classes (as a surrogate variable for age classes). The results of the study demonstrate a moderate loss of genetic diversity at some amplified fragment length polymorphism (AFLP) loci from the older trees to saplings and to young seedlings in the two largest populations. Significant genetic erosion was only detected in a very small population, named San Jos, de las Causas (SJ). Hence, if genetic diversity at AFLP loci reflects diversity in the genome on the whole, genetic erosion per se does not explain the relict status of Chihuahua spruce, except in very small populations, such as SJ, which comprised of ca. 120 individuals.