Ploidy variation and genetic diversity in Dichroa

Abstract

Recent evidence suggests a close genetic relationship between Hydrangea macrophylla (Thunb.) Ser. and D. febrifuga Lour., which supports previous morphological and DNA sequence data. This relationship was confirmed by the production of fertile intergeneric hybrids. We characterize the genetic diversity of available D. febrifuga plants, both cultivars and wild-collected taxa, as breeding material to improve H. macrophylla. Relatively high genetic diversity is seen among D. febrifuga, which splits into two main clusters. We also document considerable differences in genome size when compared with previously characterized D. febrifuga. Dichroa versicolor (Fortune) D.R. Hunt plants were also included and data suggest that D. versicolor could be a hybrid between H. macrophylla and D. febrifuga, similar to the intergeneric hybrids produced by recent breeding efforts. Because native H. macrophylla plants do not overlap extensively with D. febrifuga populations, we tested Hydrangea indochinensis Merr. as a possible parent because endemic H. indochinensis populations overlap regions where D. febrifuga and D. versicolor have been collected. However, results suggest that H. indochinensis does not share a genetic background with D. versicolor. Taxonomic revision of Dichroa is warranted, especially because we document several more intergeneric hybrids from self-sown, open-pollinated sources.