Dynamics of genotypic structure in clonal Rhododendron ferrugineum (Ericaceae) populations

Two populations of Rhododendron ferrugineum growing at subalpine level in t he Pyrenees (France) were studied in two sites (Bethmale and Mourtis). Iden tification and delimitation of genets were inferred from amplified fragment length polymorphism (AFLP) markers, along a closure gradient (from meadow to more closed heath) in each site. Surface and age of genets, genotypic di versity (Simpson's index D), 'proportion distinguishable' genotypes and gen etic relationships between genets were then estimated. Amplification of the 312 DNA samples with three selective primer pairs gave a mean of 98 detect able peaks (i.e. bands) per sample, with size ranging from 60 to 300 bp. In total 60% (Bethmale) and 70% (Mourtis) of the peaks were polymorphic, and a total of 31 and 23 multilocus genotypes were identified, in Bethmale and Mourtis, respectively. We inferred that pioneer genotypes began arriving 11 0 years ago mainly over a 40-year period in the Mourtis meadow and began ab out 130 years ago over a 100-year period in the Bethmale meadow After this pioneer stage, populations extended vegetatively. Two different patterns of genotypic dynamics can be identified. At Bethmale, population closure coul d have led to a dramatic loss of genets and to the selection of highly gene tically related genotypes. In contrast, at Mourtis, genotypic diversity and genet density did not change fundamentally along the closure gradient. How ever the range of genetic diversity diminished from the open to the closed situation, suggesting that thinning could have occurred in the past.