Flavonoids as Indicators of Hybridization in a Mixed Population of Lava-colonizing Hawaiian Tarweeds (Asteraceae: Heliantheae: Madiinae)

Abstract

Dubautia scabra (DC.) Keck subsp. scabra (n = 14) and D. ciliolata (DC.) Keck subsp. glutinosa G. Carr (n = 13) are colonizers of relatively recent lava flows on the island of Hawaii. In areas where populations of these subspecies meet, hybrids commonly form. The flavonoid profiles of the parental taxa differ substantially. Dubautia scabra subsp. scabra lacks external flavonoid agly- cones, whereas D. ciliolata subsp. glutinosa produces a series of flavones, flavonols, and flavanones. This pattern is consistent throughout the genus: the n = 14 taxa lack external flavonoid aglycones, while the n = 13 taxa generally produce them. The hybrids' external flavonoid profiles generally include a subset of the compounds produced by D. ciliolata subsp. glutinosa, and the variability in these profiles suggests that backcrossing toward this parent may be occurring in the population of hybrids. Both parental subspecies produce vacuolar flavonoid glycosides. The glycoside profile of D. scabra subsp. scabra is more complex than that of D. ciliolata subsp. glutinosa, and the hybrids produce profiles combining the structural types of the parents with shared or novel sugar substi- tution patterns. Thus, the hybrids exhibit a degree of complementation in flavonoid glycoside