Low genetic variation but high population differentiation in a common tropical forest tree species

Genetic variation and population differentiation were studied in an abundant, outcrossing tropical tree species, Pentaclethra macroloba, in Costa Rica. Population differentiation was analyzed on two spatial scales by collecting leaf material from 7 locations within a 1800-ha biological reserve and from 11 additional sites up to 70 km from the reserve. Genetic differences between age classes were investigated by collecting leaves from adults and seedlings at six locations within the reserve. Measures of genetic variation were determined using protein electrophoresis of leaf material. Five polymorphic and nine monomorphic loci were identified. The mean heterozygosity of P. macroloba populations was 0.074. Heterozygosity varied among populations, ranging from 0.045 to 0.135, with the lowest value found in the reserve and the highest in a distant population. Low variation in the reserve was largely due to fixation at two loci that were polymorphic outside the reserve. Within the reserve, seedling populations were less differentiated than adult populations, with multilocus Fst, values of 0.0189 and 0.0384, respectively. On larger geographic scales, however, population differentiation accounted for 21.9% of the total genetic variation. Nm, calculated from multilocus Fsv was negatively correlated with geographic distance among pairs of populations within the reserve (r = 0.53) and among populations throughout Costa Rica (r = −0.47). In combination, these results suggest that while P. macroloba may be panmitic within the reserve, gene flow is relatively restricted among sites that are scattered over a deforested landscape. Furthermore, in spite of the abundance of this species, genetic neighborhoods may be small due to asynchronous reproduction and isolation by distance. Allelic diversity is higher outside the reserve, and continuing deforestation may result in loss of genetic diversity, greater genetic differentiation, and genetic isolation of these populations. Se estudiaron la variabilidad genetica y la diferenciacion poblacional en una especie de arbol tropical abundante y outbreeding, Pentaclethra macroloba en Costa Rica. La diferenciacion poblacional fue analizada a dos escalas mediante la colecta de material de hojas en 7 localizociones dentro de una reserva biologica de 1800 ha y en 11 sitios adicionales localizodos a una distancia de hasta 70 km de la reserva. Se investigaron las diferencias geneticas entre clases de edad por medio de la colecta de hojas de adultos y plantulas en 6 localidades dentro de la reserva. Se determinaron medidas de variacion genetica usando electroforesis de proteinas del material de hojas. Se identificaron 5 loci polimorficos y 9 monomorficos. La heterocigosidad promedio de las poblaciones de P. macroloba fue 0.074. La heterocigocidad vario de 0.045 a 0.135 entre poblaciones observandose el valor minimo en la reserva y el maximo en una poblacion distante. La baja variabilidad en la reserva fue debida principalmente a la fijacion de dos locus que fueron polimorficos fuera de la reserva. Dentro de la reserva, las poblaciones de retonos estaban menos diferenciadas que las de adultos con valores de Fst de loci multiples de 0.0189 y 0.0384, respectivamente. Sin embargo, a escalas geograficas mayores, la diferenciacion poblacional represento un 21.9% de la variacion genetica total. Nm, calculado a partir de Fst de loci multiples estubo correlacionado negativamente con la distancia geografica entre pares de poblaciones dentro de la reserva (R = −0.53) y para poblaciones a lo largo de Costa Rica (R = −0.47). En combinacion, estos restultados sugieren que si bien P. macroloba podria ser panmictica dentro de la reserva, el flujo genetico entre sitios que estan dispersos en un paisaje deforestado esta relativamente restringido. Mas aun, a pesar de la abundancia de esta especie, los vecindarios geneticos podrian ser pequenos debido a la reproduccion asincronica y al aislamiento por distancia. La diversidad alelica es mayor fuera de la reserva y la deforestacion continua podria resultar en una perdida de diversidad genetica, mayor diferenciacion genetica y aislamiento genetico entre estas poblaciones.