Patterns of pollen stainability in polyploids of the genus Onosma (Boraginaceae)

Background and aims - The genus Onosma (Boraginaceae) is a taxonomically and phylogenetically very complex plant group, with several taxa of hybrid or polyploid origin. An extensive pollen viability survey within the genus could help to solve some evolutionary or taxonomic incertainties within the genus, but it has never been performed. The aim of the present study is to determine the degree of pollen viability of selected species of various ploidy levels and to investigate if microevolutionary processes, such as polyploidization and hybridization, are associated with variation in pollen viability. Methods - Pollen staining with lactophenol blue was chosen as an indirect method for determining pollen viability. Ploidy level information was obtained from an extensive literature search. In mixed populations, the ploidy level of individuals was determined based on flow cytometry screening. Key results - Considerable variation in pollen stainability is present among taxa of Onosma. Pollen stainability is high in most diploids (72-98% in mean), allotetraploids (90%) and allohexaploids (97%), but lower in autotetraploids (71%) and allotriploids (2-49%). At the population level, considerable variation in pollen stainability was found in the allotriploid O. arenaria (14-83%), while the taxonomically related allotetraploid O. pseudoarenaria is less variable (71-99%). Conclusion - This study shows the value of the staining method in the study of the genus Onosma. Low pollen stainability appears to be associated with autopolyploidization as well as recent hybridization processes suggesting the presence of microsporogenesis or microgametogenesis abnormalities. In contrast, pollen stainability is high in stabilized allopolyploids indicating proper meiosis with maintenance of homologous pairing of parental chromosomes and correct pollen development.