A fully fertile intergeneric hybrid derivative from Argyroxiphium sandwicense SSP. macrocephalum DUBAUTIA MENZIESII (Asteraceae) and its relevance to plant evolution in the Hawaiian Islands

Achenes collected from natural putative F 1 intergeneric hybrids involving Argyroxiphium sandwicense DC. ssp. macrocephalum (A. Gray) Meyrat and Dubautia menziesii (A. Gray) Keck were germinated and grown in cultivation. One of these apparent B 1 hybrids that approached D. menziesii in morphology exhibited greatly increased fertility over the F 1 (79% vs. 9% pollen stainability), and was used as a female parent to generate a B 2 progeny by backcrossing a second time to D. menziesii. The B 2 was surprisingly uniform. One individual that flowered was fully fertile (99% pollen stainability). Its floral and vegetative features were extremely similar to those of the recurrent parent, D. menziesii. Observations suggest that plants of similar origin may exist under natural conditions and that the close relatives, D. platyphylla (A. Gray) Keck and D. dolosa (Degener & Sherff) G. Carr, may have originated from the same or a similar intergeneric hybrid combination. Because many other analogous situations occur in the silversword alliance and elsewhere in the Hawaiian flora, it is concluded that hybridization is an evolutionarily important phenomenon, especially in unstable geological systems. Although the Hawaiian archipelago is a premier example of such instability, continents are not immune to natural perturbations. Therefore, hybridization, as a mechanism that greatly extends the size of gene pools available for recombination, should also be expected to play an important evolutionary role in continental floras.