Endemic dwarf birchBetula apoiensis(Betulaceae) is a hybrid that originated fromBetula ermaniiandBetula ovalifolia

Abstract

Betula apoiensis (Betulaceae), a tetraploid endemic to Mount Apoi in Hokkaido, is endangered in Japan. To reveal the origin of this species, the internal transcribed spacers and 5.8S gene of nuclear ribosomal DNA (ITS + 5.8S) and a microsatellite with its flanking region of nuclear DNA (Bp09) were sequenced and their haplotypes were determined for 12 Betula species in the Sakhalin, Hokkaido and Honshu Islands. The most parsimonious trees of ITS + 5.8S and Bp09 haplotypes had different topologies and no clades were consistent between the two trees. In both ITS + 5.8S and Bp09, B. apoiensis shared identical haplotypes with Betula ermanii and Betula ovalifolia , and had haplotypes that appeared to have been derived from each of the two species. This result suggests that B. apoiensis originated from hybridization between B. ermanii and B. ovalifolia.

Phylogenetic relationships in Betula (Betulaceae) based on AFLP markers

“…However, we could not unambiguously resolve relationships among these groups. Due to the extensive hybridization and introgression within the genus Betula, good support for the relationships is not always to be expected, although bootstrap support was generally higher for the AFLPderived groups compared to groups identified based on ITS sequences (Li et al 2005;Nagamitsu et al 2006), and AFLP was able to provide resolution on clades that were unresolved with ITS data. Apart from differences in support and resolution, congruence between ITS data and AFLP data was high for the genus Betula.…”

“…Although ample morphological variation exists in characters such as leaf shape, bark color, and shape of the catkins, attempts to reconstruct species relationships using morphological characters failed to produce a reliable classification. The occurrence of polyploidization (Nagamitsu et al 2006), hybridization, and introgression (Palme et al 2004;Thórsson et al 2001;Williams and Arnold 2001) and the fact that morphological characters may have evolved independently more than once in Betula (Li et al 2005) may account for this. Up to now, Betula taxonomy had been studied using morphological characters, flavonoid composition, and nuclear and chloroplast DNA sequences.…”

“…Hybridization and introgression are common in situations where the natural distributions of birch species overlap, for example among the European birch species B. pendula, B. pubescens, and B. nana (Palme et al 2004). Moreover, several of the recognized Betula species have a hybrid origin (Nagamitsu et al 2006). Hybrids generally show a morphology intermediate between the parental species but are not always morphologically distinct as a group (Thórsson et al 2001).…”

“…For example, the chloroplast matK sequences examined by Järvinen et al (2004) differentiated only three North American species from the other species. Moreover, trees constructed from different nuclear DNA markers display incongruences, e.g., nuclear ribosomal internal transcribed spacers (ITS) versus microsatellite sequences (Nagamitsu et al 2006) and ITS versus ADH sequences (Järvinen et al 2004;Li et al 2005).…”

“…Given the difficulties with morphological characters in reconstructing species relationships within the genus Betula, alternative markers were explored, e.g., flavonoid composition (Keinänen et al 1999), nuclear deoxyribonucleic acid (DNA) sequences (Järvinen et al 2004;Li et al 2005;Nagamitsu et al 2006), a microsatellite (Nagamitsu et al 2006), and chloroplast DNA sequences (Järvinen et al 2004). These markers have provided useful information on the evolution of the genus, but relationships between species remain largely inconclusive due to their limited variation.…”