The silversword alliance is named for its most famous and visually striking members, the silverswords. The species of the clade belong to three genera: Wilkesia, Argyroxiphium, and Dubautia.[4] There are five species, including two species known as greenswords, in the genus Argyroxiphium, confined to the islands of Maui and Hawaiʻi, and two species of Wilkesia (iliau) on Kauaʻi. The bulk of the species are placed in the genus Dubautia, which is widespread on all the main islands.
Similar species frequently occur in the same habitat and are often difficult to tell apart. Hybrids frequently occur between Dubautia species, and between Dubautia and Argyroxiphium. As a result, there is some disagreement over the number of species, with modern sources giving between 28 and 33 species.[5][6][7][8][9][10]
Characteristics
Dubautia linearis
All members of the silversword alliance are perennials, but otherwise occupy a wide range of ecological niches.[11] The genus Dubautia is the most morphologically diverse of the three genera, containing cushion plants, shrubs, trees, and lianas; all Argyroxiphium and Wilkesia species are rosette-forming shrubs.[12][13]
Almost all of the members of the clade have longitudinal leaf venation, whereas leaf shapes are highly diverse.[14]Argyroxiphium and Wilkesia species have long, narrowly ligulate to linear leaves. Dubautia species show a wider range of leaf morphologies, but 2n = 14 species of Dubautia tend to have longer leaves than 2n = 13 members of the genus.[12]
Like other members of the Asteraceae, silverswords have composite inflorescences known as capitula, made up of numerous smaller flowers ("florets"). Dubautia and Wilkesia species lack ray florets, the outer flowers resembling petals present in many other Asteraceae species. Capitula can vary widely in size between silversword species, and can be made up of anywhere from two to 650 individual florets. In Argyroxiphium and Wilkesia, these capitula tend to be very large.[14] Almost all silversword species have sticky, glandular trichome-covered bracts surrounding the capitula, a trait shared with their Californian tarweed relatives.[14][15]
Evolution
Due to their isolated location and recent geologic origin, the Hawaiian Islands have high numbers of endemic plant and animal species, often originating through adaptive radiations.[16]
All Hawaiian tarweeds trace their lineage back to a species of Pacific coast tarweed, very similar to extant species like Carlquistia muirii,[11] which likely arrived on Kauaʻi first, around 5.1 Ma.[3] The last common ancestor of all extant members of the silversword alliance lived approximately 3.5±1.5 Ma.[3]
Species of Argyroxiphium and Wilkesia, and most species of Dubautia, have 2n = 14 chromosomes; several species of Dubautia are instead 2n = 13.[12] All of the silverswords are tetraploid, differing from the diploidy of their closest North American relatives.[17] How the silverswords' chromosome number arose is a matter of some uncertainty, but two major scientific theories have been proposed. One is that two ancestor species, one with n = 6 and one with n = 8 chromosomes hybridized, resulting in a n = 7 hybrid. The hybrid then, by allopolyploidy doubled its chromosome number spontaneously, leading to the resultant and extant 2n = 14 species. Alternatively, the modern chromosome number could have arisen from an ancestor like Anisocarpus scabridus, with a chromosome complement of n = 7, and then arisen by autopolyploidy, instead of needing to first hybridize.[18]
↑Carlquist, S., B. G. Baldwin, and G. D. Carr, eds. 2003. Tarweeds & Silverswords: Evolution of the Madiinae (Asteraceae). Missouri Botanical Garden Press.
↑Lawton‐Rauh, A.; Robichaux, R. H.; Purugganan, M. D. (2007). "Diversity and divergence patterns in regulatory genes suggest differential gene flow in recently derived species of the Hawaiian silversword alliance adaptive radiation (Asteraceae)". Molecular Ecology. 16 (19): 3995–4013. doi:10.1111/j.1365-294X.2007.03445.x. ISSN1365-294X. PMID17784920. S2CID18623958.
↑Friar, Elizabeth A.; Prince, Linda M.; Cruse-Sanders, Jennifer M.; McGlaughlin, Mitchell E.; Butterworth, Charles A.; Baldwin, Bruce G. (2008). "Hybrid Origin and Genomic Mosaicism of Dubautia scabra (Hawaiian Silversword Alliance; Asteraceae, Madiinae)". Systematic Botany. 33 (3): 589–597. doi:10.1600/036364408785679815. ISSN0363-6445. JSTOR40211922. S2CID86257628.
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