The impact of local extinction on genetic structure of wild populations of lima beans (Phaseolus lunatus) in the Central Valley of Costa Rica: consequences for the conservation of plant genetic resources

Plant populations may experience local extinction and at the same time new populations may appear in nearby suitable locations. Species may also colonize the same site on multiple occasions. Here, we examined the impact of local extinction and recolonization on the genetic structure of wild populations of lima beans (Phaseolus lunatus) in the Central valley of Costa Rica. We compared genetic diversity from the samples taken from the populations before and after extinction at 13 locations using microsatellite markers. Locations were classified according to the occurrence of extinction episodes during the previous five years into three groups: 1) populations that experienced extinction for more than one year, and were later recolonized (recolonized), 2) populations that did not experience local extinction (control), and 3) populations that did not experience local extinction during the study, but were cut to experimentally simulate extinction (experimental). Our data did not show a clear tendency in variation in allele frequencies, expected heterozygosity, and effective number of alleles within and between groups of populations. However, we found that the level of genetic differentiation between samples collected at different times at the same location was different in the three groups of populations. Recolonized locations showed the highest level of genetic differentiation (mean Fst= 0.2769), followed by control locations (mean Fst= 0.0576) and experimental locations (mean Fst= 0.0189). Similar findings were observed for Nei’s genetic distance between samples (di,j= 0.1786, 0.0400, and 0.0037, respectively). Our results indicate that genetic change in lima beans depends on the duration and frequency of local extinction episodes. These findings also showed that control populations are not in equilibrium. Implications of these results for the establishment of conservation strategies of genetic resources of lima beans are discussed. Rev. Biol. Trop. 56 (3): 1023-1041. Epub 2008 September 30.

Las poblaciones de plantas pueden experimentar extinción local, y al mismo tiempo, pueden surgir a sus alrededores nuevas poblaciones. Algunas especies pueden colonizar el mismo sitio en múltiples ocasiones. Aquí examinamos el impacto de la extinción local y recolonización en la estructura genética de poblaciones silvestres del frijol lima (Phaseolus lunatus) en el valle Central de Costa Rica. Comparamos la diversidad genética de muestras tomadas en poblaciones, antes y después de la extinción, en 13 sitios, usando marcadores de microsatélite. Según los episodios de extinción durante los cinco años previos, clasificamos los sitios así: 1) poblaciones que han experimentado extinción por más de un año, y después han recolonizado (recolonizado), 2) poblaciones que no han experimentado extinción local (control), y 3) poblaciones que no han experimentado extinción local durante el estudio, pero fueron cortadas experimentalmente, simulando una extinción (experimental). Nuestros datos no mostraron una clara tendencia en la variación de las frecuencias alélicas, heterozigosidad, o número efectivo de alelos en y entre grupos de poblaciones. Los niveles de diferenciación genética entre muestras recolectadas en diferentes momentos en el mismo sitio fueron diferentes en los tres grupos de poblaciones. Los sitios recolonizados mostraron el mayor nivel de diferenciación genética (Fst = 0.2769), seguidos de los sitios control (Fst= 0.0576) y sitios experimentales (Fst= 0.0189). Obtuvimos resultados similares en la distancia genética Neis entre muestras (d i,j = 0.1786, 0.0400, y 0.0037, respectivamente). Los cambios genéticos en los frijoles lima dependen de la duración y frecuencia de los episodios de extinción local. Las poblaciones "control" no están en equilibrio. Las implicaciones de estos resultados para el establecimiento de estrategias de conservación de los recursos genéticos de habas se encuentran en discusión.

Variación del tamaño de frutos y semillas en 38 poblaciones silvestres de Phaseolus lunatus (Fabaceae) del Valle Central de Costa Rica / Variation in fruit and seed size from 38 wild populations of Phaseoulus lunatus (Fabaceae) from Central Valley, Costa Rica

We studied the morfological diversity in fruits and seeds in 38 wild populations of Phaseolus lunatus var. lunatus (lima beans) in the central valley of Costa Rica. In order to do so, measured the length and width of the fruits and the length, width and thickness of seeds. We also calculated the ratio between these traits and determined the weight of 100 seeds. In general, we found significant variation between populations for all variables. When we grouped the 38 populations into eight geographical regions within the study area, we found significant differences between regions. However, the levels of variation between populations within geographical regions was larger than that found between geographical regions. These findings suggested that there is no clear relationship between these variables and the geographical grouping established in this study. The implications of these findings for the establishment of strategies for in situ conservation of wild populations of lima beans are discussed.