Experimental hybridization reveals biased inheritance of the internal transcribed spacer of the nuclear ribosomal DNA in Begonia x taipieiensis

Unidirectional hybridization was previously confirmed in Begonia x taipeien sis C.-I Peng, a naturally occurring hybrid resulting from B. formosana x B . aptera, in Taiwan. To understand the inheritance of ribosomal DNA in unid irectional hybridization, experiments were conducted using B. formosana and B. aptera as ovule and pollen donors, respectively. The internal transcrib ed spacer region (ITS) of nuclear ribosomal DNA was amplified from the arti ficial hybrids, parental species, and natural hybrids. In contrast to the s ingle type of ITS in the parental species, multiple sequences were cloned f rom both natural and artificial hybrids. A split decomposition network base d on ITS nucleotide variation revealed that all but one (clone B14) of the hybrid sequences were "phylogenetically" closely related to B. formosana. A pparently, in such unidirectional hybridization, maternal DNA provided most of phylogenetic information. In the hybrid sequences, in addition to addit ive polymorphisms inherited from maternal (38.1%) and paternal (30.1%) plan ts, a novel nucleotide composition (31.8%) was also detected. The "new" cha racters are seen as noise in phylogenetic inference. They were probably obt ained via intramolecular recombination, as gene conversion was not detected . The occurrence of genetic recombination appeared to be nonrandom, with a higher frequency in the ITS1 (3.14%) and ITS2 (3.42%) regions than in the 5 .8S RNA gene (2.22%). Given the lack of sexual recombination in B. X taipei ensis and short time span, unequal crossing-over likely contributed to the heterogeneity of the ITS composition in the nuclear genome. Although the st erile hybrids have not attained their own lineage independent from the pare ntal species, a high level of genetic diversity is transmitted asexually an d maintained in these plants.