Cytogenetic implications of artificial hybrids between the hawaiian silversword alliance and North American tarweeds (Asteraceae: Heliantheae—Madiinae)

Two vigorous transoceanic, bigeneric hybrids, Dubautia laevigata (n = 14) × Raillardiopsis muirii (n = 8) and [Dubautia knudsenii × laxa] (n = 14) × Madia bolanderi (n = 6), and one vigorous transoceanic trigeneric hybrid, Dubautia scabra (n = 14) × [M. bolanderi × R. muirii], between mainland tarweeds and Hawaiian silversword allies were artificially produced and subjected to cytogenetic analysis. In addition to univalents, ≈46–80% of the microsporocytes scored from these hybrids exhibited from one to four bivalents. However, some of the bivalents scored in the second bigeneric hybrid represented infragenomic association of Dubautia chromosomes. Stainable pollen of these hybrids ranged from 4.4 to 49%, mostly comprising large, tetracolporate, apparently diploid grains. The functionality of such grains was demonstrated in the primary hybrid M. bolanderi × R. muirii that was used to produce the trigeneric hybrid, and suggests the possible mode of origin of the Hawaiian genome via allopolyploidy. Illustrations of parents and F1s indicate that the hybrids produced in this study generally exhibit intermediate character states. However, the phenotypes of the “ray” flowers in hybrids between discoid and radiate species were noticeably unpredictable; in one case intermediacy appeared to be expressed largely in quantitative terms, while in two others intermediacy appeared to be expressed largely in qualitative terms.