Phylogeny, biogeography, and processes of molecular differentiation in Quercus subgenus Quercus (Fagaceae)

Quercus is one of the most abundant and economically important genera of wo ody plants in the Northern Hemisphere. To infer phylogenetic relationships within Quercus subgenus Quercus, chloroplast DNA (cpDNA) restriction sites and nucleotide sequences of the internal transcribed spacers (ITS) and the 5.8S coding region of the nuclear ribosomal DNA repeat were obtained for 44 individuals, including 25 species, intraspecific samples, and three outgro ups. Separate parsimony analyses of each data set showed that individual ge ne trees were congruent and often complementary in supporting clades that g enerally corresponded to previously recognized taxonomic groups. Only one i nstance of strongly supported gene tree incongruence was detected and this anomalous pattern was explained best by ancient introgression of cpDNA acro ss sectional boundaries. Simultaneous parsimony analysis of the pruned data sets supported the recognition of the strictly Eurasian section Cerris and resolved a novel hypothesis for the major infrageneric groups (Cerris- (Lo batae- (Protobalanus + Quercus sensu stricto))). The biogeographic hypothes is that all major oak lineages evolved locally at middle latitudes within t he general distribution of their fossil ancestors was fully supported. This set of relationships also suggested a New World origin for the widespread white oaks of the Northern Hemisphere (section Quercus s. s.). For both dat a sets, inter- and intraspecific sampling within section Protobalanus showe d little correspondence to morphological species. Greater cladistic structu re among the samples was obtained by cpDNA restriction sites and two well-d elimited plastomes types comprising a total of 15 distinct haplotypes were resolved. Haplotypes of 2 of the peripheral species in this species complex occupy terminal portions of one of the plastome clades, suggesting a more recent origin relative to those of more widespread species. The phylogeogra phy of the two divergent plastome types suggested a north-south pattern, co nsistent with a Late Tertiary disjunction in the ancestral distribution of section Protobalanus. (C) 1999 Academic Press.