Genome Size and Pollen Viability as Taxonomic Criteria: Application to the Genus Hosta

Abstract: Genome size (C values) and pollen viability staining were applied as new criteria to investigate the taxonomy of the genus Hosta Tratt. (Hostaceae). Nearly all species of the genus Hosta have the same basic chromosome number (2n = 2x = 60). However, the nuclear DNA contents, as measured by flow cytometry with propidium iodide, could be demonstrated to range between 17.2 to 26.6 pg. This implies that the largest genome contains roughly 10¹⁰ more base pairs than the smallest. Therefore, nuclear DNA content is a very relevant taxonomic trait that can be measured simply by flow cytometry. In addition, differences in overall DNA composition were demonstrated by comparing to DAPI fluorescence. In general, genome size data confirmed the division into three subgenera. The geographical distribution of genome sizes indicates the migration pattern of Hosta throughout East Asia. The species belonging to the mainly Korean subgenus Bryocles, with a low nuclear DNA content (17.2 - 19.3 pg), can now largely be distinguished from the mainly Japanese species of the subgenus Giboshi (21.3 - 26.5 pg). The exception is H. longissima, that with only 19.6 pg provides a nice example of a decrease in DNA content. On the mainland, as well as on Honshu, species with increased and decreased DNA content have evolved independently. The usefulness of pollen viability to detect hybrids in Hosta was demonstrated in a large series of artificial crosses between bona fide species. Consequently, pollen viability was measured in all available Hosta described as species. Several had low pollen viability and were concluded to be hybrids. Morphology and DNA content confirmed this in most cases. The resulting 23 species approximate the number of Hosta species that follows from the combined studies by Fujita (1976¹⁸) on the Japanese species and Chung (1991 a¹¹) on the Korean species.