Lineage sorting and phylogeography in Lithocarpus formosanus and L. dodonaeifolius (Fagaceae) from Taiwan

Gene genealogy of the cpDNA atpB-rbcL noncoding spacer was reconstructed to assess the phylogeographic pattern of two closely related oaks, Lithocarpus formosanus (Skan) Hayata and L. dodonaeifolius (Hayata) Hayata (Fagaceae). High levels of nucleotide and haplotype diversities but low levels of genetic differentiation between species and among populations were detected. The result is consistent with the paraphyly of this cpDNA spacer in both species suggested by a neighbor-joining analysis. A minimum spanning network of the cpDNA haplotvpes identified two major clades, A and A′ (consisting of clades of B, C, and D). No clades were confined to either species or single populations. Clades B and C did not occur in two smaller populations of L. dodonaeifolius. The apportionment of haplotypes between species and populations indicated a lineage sorting of the cpDNA noncoding spacer. On the contrary, RAPD fingerprints revealed limited ongoing gene flow and significant genetic differentiation between species and among populations. Given low possibilities that seeds disperse across a long geographic range in the modern vegetation, high Nm values, estimates of number of migrants per generation deduced from the seed-carried organelle DNA marker are likely to represent historical migrations. A migrant-pool model explains the heterogeneous composition of the organelle DNA within populations and the low differentiation among populations. According to geological evidence, during the deglaciation period common ancestral populations were possibly forced to migrate into refugia at local peaks. Invading and adapting to habitats of different elevations, two oaks flower with a lag interval of about half a month, which may have triggered the reproductive isolation and speciation. Ancestral polymorphic alleles, however, prolonged the lineage sorting period within populations and species. Given a relatively short evolutionary duration since isolation, high genetic heterogeneity has made the attainment of coalescence improbable.