Isozyme variability and biogeography of Nothofagus truncata (Fagaceae)

Abstract

Leaf extracts of 1706 Nothofagus truncata trees from 30 provenances between the Bay of Plenty and south Westland were separated by horizontal starch gel electrophoresis and isozyme variation was analysed at 15 enzyme loci. Most loci were monomorphic or possessed low levels of polymorphism, but phosphoglucose isomerase (PGI) was highly polymorphic with four common and widespread alleles occurring in characteristic frequencies in each of the species’ disjunct distribution ranges. Total genetic diversity was comparatively low (H_e = 0.054) and mostly accounted for by within-population variation (95.1%), presumably as a result of successive range contractions/expansions of forest vegetation during the glacial/interglacial cycles of the Pleistocene whereby less common alleles were lost by random genetic drift in small and fragmented populations. Genetic divergence between populations was highest in the northern North Island, probably because of the longer time of residence and comparative isolation of individual populations. Very little interpopulational variation in the southern North Island and northern South Island and the geographical restriction of different rare alleles on both sides of Cook Strait imply postglacial population growth and spread of local populations from coastal refugia in both regions. The small south Westland populations possess a particularly low genetic diversity and revealed no recent gene transfer from N. solandri var. cliffortioides. However, they are characterised by the most divergent allele frequencies of phosphoglucose isomerase which is thought to indicate in situ survival during the glacial maximum.