Genetic diversity within a declining natural population of Ferocactus histrix (DC) Lindsay PATRICIA CASTRO-FELIX,* VERÓNICA CAROLINA ROSAS-ESPINOZA,† BRENDA DÍAZ-CÁRDENAS,* LAURA IZASCUM PÉREZ-VALENCIA,* FRANCISCO MARTÍN HUERTA-MARTÍNEZ† and ANNE SANTERRE* *Departamento de Biología Celular y Molecular, and †Departamento de Ecología, Centro Universitario de Ciencias Biológicas y Agropecuarias, Universidad de Guadalajara, Zapopan, México Abstract Ferocactus histrix is a barrel cactus that is widespread in Mexico. A population located in Llanos de Ojuelos, a semiarid zone representative of many disturbed regions in north- central Mexico, was studied. Over a period of 10 years (1997 to 2007), the average number of individuals decreased from 21.95 to 3.53 plants per 300 m 2 . A change in population size structure was also registered over this period of time. In 2008, a plot selected on the basis of plant abundance was established within the population and a genetic analysis was conducted with ISTR and ISSR markers. This analysis revealed low levels of genetic diversity (expected heterozygosity (HE) = 0.073, Shannon index (I) = 0.113 and HE = 0.178, I = 0.271, respectively) compared with those of most studied cacti species. The genetic diversity between the different life stages was also evaluated, and a gradual decrease in levels of genetic variation was observed from adults to juveniles and seedlings (HE = 0.130, I = 0.192 to HE = 0.103, I = 0.157). These differences, however, were not signifi- cant. Loci fixation and a decrease in the frequency of rare alleles were observed in seedling and juvenile classes. The decline in genetic variation may be associated with recent bottlenecks experienced by the population of F. histrix. If the sizes of local popu- lations of F. histrix continue to decrease, genetic variation will be gradually lost, and the risk of extinction will increase. Keywords: bottleneck, ISSR, ISTR, life stage, within population. Received 19 March 2012; revision received 18 July 2013; accepted 1 August 2013 Introduction Population size and progeny production are two factors that have an important effect on population effective size (Ne) (Tremblay & Ackerman 2001) and on overall fitness (Harper 1977; Wesselingh et al. 1997). For populations in natural environments, fluctuation in population size is often the most important factor reducing Ne below the average actual size (Ellstrand & Ellam 1993; Frankham 1996). Variation in plant distribution within size classes also has an important effect on Ne and fitness, because reproductive success increases with increasing plant size (Harper 1977; Pino et al. 2002; Metcalf et al. 2003). Even if the estimation of Ne is complicated, it can be indirectly inferred through monitoring changes in population size and structure and in the rate of genetic variation (Luikart et al. 1998; Garza & Williamson 2001). The amount of genetic variation in a population depends on the action of evolutionary forces. Variation generated by mutations decreases through selection and genetic drift (Frankham 1996). A sudden decrease in population size (bottleneck) can lead to important changes in the genetic parameters of the population (Zhang et al. 2004). Bottlenecks increase genetic drift at a rate that is inversely proportional to the population size (Wright 1968; Nei et al. 1975; Leberg 1992); thus, a sudden decrease in population size can lead to fixation of alleles (Zhang et al. 2004). Severe population bottlenecks reduce the pool of possible mates, increasing inbreeding. The smaller the Ne, the higher the probability that genetic drift causes allele fixation in a population (Reed & Frankham 2003). In the study of plants, there are a few published studies that address both the analysis of demographic parameters and the distribution of genetic diversity among size classes or life stages (Linhart et al. 1981; Eguiarte et al. Correspondence: Verónica Carolina Rosas-Espinoza Email: verorosas75@hotmail.com Plant Species Biology (2013) ••, ••–•• doi: 10.1111/1442-1984.12028 © 2013 The Society for the Study of Species Biology