Estimating Genetic Erosion in Threatened Conifers: The Example of Picea chihuahuana Martínez

Abstract

The necessity of genetic diversity for evolution and the relationship between heterozygosity and population fitness are important arguments for conserving genetic diversity. The loss of genetic diversity can be detrimental to the short-term viability of individuals and populations, and to the evolutionary potential of populations and species. Genetic erosion can be defined as the permanent reduction in richness or evenness of common local alleles or as the loss of combinations of alleles over time in a defined area. 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 Chihuahua spruce (Picea chihuahuana Martinez), which is endemic to Mexico, is an excellent model for estimating genetic erosion. The species occurs in about 40 isolated relict populations in the Sierra Madre Occidental, in the northwest of the country. Here, we will review a study assessing the degree of genetic erosion that was evaluated in five populations of P. chihuahuana M. in the State of Durango (Mexico), by comparing genetic diversity across diameter classes (which were assumed to be a surrogate for age classes). In the two largest populations, there was a moderate loss of genetic diversity at AFLP loci from older trees to saplings, and to young seedlings. Significant genetic erosion was only detected in the very small population of San Jose de las Causas (SJ). Hence, if genetic diversity at AFLP loci reflects diversity in the whole genome, genetic erosion per se does not explain the relict status of Chihuahua spruce, except for very small populations, such as SJ. However, further researches with candidate genes are necessary to assess the putative loss of evolutionary potential in these stands. Activities that increase population size should be helpful to preserve genetic diversity.