Silversword alliance

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Dubautia linearis Dubautia linearis Kalopa.jpg
Dubautia linearis

The silversword alliance, also known as the tarweeds, [1] refers to an adaptive radiation of around 30 species in the composite or sunflower family, Asteraceae. The group is endemic to Hawaii, and is derived from a single immigrant to the islands. For radiating from a common ancestor at an estimated 5.2±0.8 Ma, the clade is extremely diverse, composed of trees, shrubs, subshrubs, mat-plants, cushion plants, rosette plants, and lianas. [2]

Contents

The silversword alliance is named for its most famous and visually striking members, the silverswords. The species of the clade break down into three genera: Wilkesia, Argyroxiphium, and Dubautia. [3] There are three species of silverswords and two 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.

The genus Dubautia contains a wide variety of forms, including cushion plants, shrubs, trees, and lianas. [4]

Dubautia-silversword hybrid in Haleakala crater Dubautia silversword hybrid.jpg
Dubautia-silversword hybrid in Haleakalā crater

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

All members of the silversword alliance are perennials, but otherwise occupy a wide range of ecological niches. [11]

Evolution

All Hawaiian tarweeds trace their lineage back to a species of Pacific coast tarweed, very similar to extant species like Carlquistia muirii . [12] The last common ancestor of the silversword alliance was likely a mat and rhizome forming plant not more than .3 metres (0.98 ft) tall, with a chromosome number of 2n = 16, and perhaps another similar species. Species of Dubautia however have 2n = 14 chromosomes. 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. [13]

Related Research Articles

<i>Argyroxiphium</i> Genus of plants

Argyroxiphium is a small genus of plants in the family Asteraceae. Its members are known by the common names silversword or greensword due to their long, narrow leaves and the silvery hairs on some species. The silverswords belong to a larger radiation of over 50 species, including the physically different genera Dubautia and Wilkesia. This grouping is often referred to as the silversword alliance. Botanist P. H. Raven referred to this radiation as "the best example of adaptive radiation in plants".

<i>Argyroxiphium sandwicense <span style="font-style:normal;">subsp.</span> macrocephalum</i> Subspecies of flowering plant

Argyroxiphium sandwicense subsp. macrocephalum, the east Maui silversword or Haleakala silversword, is a rare plant, part of the family Asteraceae. The silversword in general is referred to as ʻāhinahina in Hawaiian.

<i>Wilkesia gymnoxiphium</i> Species of plant

Wilkesia gymnoxiphium, is a species of flowering plant in the family Asteraceae that is endemic to the island of Kauaʻi in Hawaiʻi. It is classified as endangered on the IUCN Red List. Wilkesia is one of three genera, with Argyroxiphium and Dubautia that are believed to be descendant from a single species related to the North American tarweed. The members of these three genera constitute what is called the silversword alliance, a group whose exceedingly close genetic heritage is not reflected in their exceptionally diverse morphologies.

<i>Argyroxiphium sandwicense</i> Species of plant

Argyroxiphium sandwicense, the Hawaiʻi silversword, or hinahina is a species of silversword. It is endemic to Hawaii. The two subspecies are separated by geography. Both subspecies are rare, threatened and federally protected.

<i>Argyroxiphium kauense</i> Species of plant

The Mauna Loa silversword, Argyroxiphium kauense, or Kaʻū silversword, is a rare species of flowering plant in the aster family. It is endemic to the eastern and southern slopes of Mauna Loa on the island of Hawaiʻi. A. kauense occurs in mountainous shrublands, bogs, and open mesic forest. The species is managed by the National Park Service and Hawaiʻi State Department of Forestry and Wildlife. It is a federally listed endangered species of the United States. There are three known populations remaining, for a total of fewer than 1000 individuals.

Argyroxiphium virescens was a species of flowering plant in the family Asteraceae that was last seen in 1945. It was found only in the Hawaiian Islands where it was endemic to the eastern part of Maui. Its natural habitats were subtropical or tropical moist montane forests and subtropical or tropical high-altitude shrubland. It is officially declared extinct, but in 1989 plants were discovered that appear to be hybrids between it and the Haleakalā silversword. This hybrid is known as the Pu'u 'Alaea greensword.

<i>Dubautia arborea</i> Species of plant

Dubautia arborea, the tree dubautia or Mauna Kea dubautia, is a species of flowering plant in the family Asteraceae. A member of the silversword alliance, it is endemic to the island of Hawaiʻi. It is a shrub or small tree. It is an endangered species that is threatened by feral grazing animals.

<i>Dubautia</i> Genus of plants

Dubautia or na'ena'e is a genus of flowering plant in the family Asteraceae. The genus was named after Joseph Eugène DuBaut (1796–1832), an officer in the French Navy who participated in Freycinet's expedition.

<span class="mw-page-title-main">Madieae</span> Tribe of flowering plants

Madieae is a tribe of flowering plants in the family Asteraceae. It is sometimes considered a subtribe of Heliantheae. Notable species include the tarweeds of the Western United States as well as the silverswords of Hawaii.

<i>Wilkesia</i> Genus of flowering plants

Wilkesia is a genus of Hawaiian plants in the tribe Madieae within the family Asteraceae. It contains two perennials, both of which are endemic to the island of Kauaʻi in Hawaiʻi. Wilkesia is a component of the silversword alliance and is named after Captain Charles Wilkes.

<i>Wilkesia hobdyi</i> Species of plant

Wilkesia hobdyi, the dwarf iliau, is a species of flowering plant in the family Asteraceae that is endemic to the island of Kauaʻi in Hawaii. It was first collected in 1968, and was not formally described until 1971. The number of living plants is estimated at fewer than 300. It is considered Critically Imperiled in global rank which means it is at very high risk of extinction. It has been a federally protected species since 1992. The genus Wilkesia is one of three genera constituting the silversword alliance, a group of highly diverse yet genetically exceedingly closely related species, all thought to be descended from a colonizing ancestor related to the tarweed of North America.

<i>Argyroxiphium grayanum</i> Species of plant

Argyroxiphium grayanum, commonly known as the greensword, is a species of flowering plant in the family Asteraceae, and a member of the silversword alliance, a group of over 50 species which are diverse in morphology and habitat but are genetically closely related.

<span class="mw-page-title-main">Mauna Kea silversword</span> Subspecies of flowering plant

Argyroxiphium sandwicense subsp. sandwicense, the Mauna Kea silversword, is a highly endangered flowering plant endemic to the island of Hawaiʻi of Hawaii. It is the "crown jewel" of the volcanic mountain Mauna Kea, from which it derives its English name. The Hawaiian name is ʻahinahina; it applies to silverswords more broadly. The Mauna Kea silversword was once common on the volcano, and extraordinary conservation efforts are being made to preserve the species.

Dubautia kalalauensis is a rare species of flowering plant in the family Asteraceae. It is endemic to Hawaii where it is known only from the island of Kauai. There is only a single known population composed of 26 plants. It was federally listed as an endangered species of the United States in 2010. Like other Dubautia this plant is known as na`ena`e.

Dubautia kenwoodii, the Kalalau rim dubautia, is an "extremely rare" species of flowering plant in the family Asteraceae. It is endemic to Hawaii where it is known only from the island of Kauai. Only one plant has ever been seen: the type specimen. A part of this plant was collected in 1991 and the individual was described as a new species in 1998. It was federally listed as an endangered species of the United States in 2010. Like other Dubautia this plant is known as naʻenaʻe.

Dubautia syndetica, the Wahiawa dubautia, is a plant species endemic to the island of Kauai in the state of Hawaii. It is found only in the upper portions of the Wahiawa drainage basin at elevations of 680–950 m.

Argyroxiphium × kai, commonly known as the Kai silversword, is a hybrid species of silversword plant in the family Asteraceae, and is a part of the silversword alliance. The name derived from the Hawaiian word "kai", which translates to "sea". It was described by David D. Keck in 1936. Argyroxiphium × kai is a natural hybrid between Argyroxiphium caliginis and Argyroxiphium grayanum, so the name Argyroxiphium calignis × grayanum can also apply, but is generally less common. It is endemic to Maui, Hawaii, where it grows between other Argyroxiphium species, primarily the two species that hybridized between each other. The population of the hybrid species is unknown, although it is likely that it may be endangered, or unstable due to invasive feral goats and pigs in its natural habitat. No conservation status has been assigned to Argyroxiphium × kai by the IUCN Red List, for it is a hybrid species, so a conservation status does not apply.

Argyroxiphium virescens × sandwicense subsp. macrocephalum, commonly known as the Pu'u 'Alaea greensword is a hybrid species of silversword plant in the family Asteraceae, and is a part of the silversword alliance. It is a hybrid between two taxons of silversword plant in the genus Argyroxiphium, being the presumably extinct Argyroxiphium virescens, and a subspecies, Argyroxiphium sandwicense subsp. macrocephalum. The hybrid species was discovered in 1989, when an individual flowered, which revealed hybridization features of two species of silversword. It is endemic to East Maui, primarily near the town of Pu’u Alaea, where it is seen growing beside other Argyroxiphium species. The population of the hybrid species is unknown, but is likely under threat of feral goats, and pigs that plague silversword plants.

<i>Argyroxiphium sandwicense</i> subsp. <i>macrocephalum</i> x <i>Dubautia menziesii</i> Hybrid species of plant

Argyroxiphium sandwicense subsp. macrocephalum x Dubautia menziesii, commonly known as the Dubautia-silversword, is a hybrid species of silversword plant in the family Asteraceae, and is a part of the silversword alliance. The hybrid was formed between Argyroxiphium sandwicense subsp. macrocephalum and Dubautia menziesii, which are both species of plant native to the Haleakalā Shield Volcano, Maui, primarily in the Haleakalā Crater, that grow in alpine and subalpine zones.

<i>Dubautia laevigata</i> Species of plant

Dubautia laevigata, commonly known as the smooth dubautia, is a species of Dubautia endemic to moderate elevations on the island of Kaua'i in the state of Hawaii, United States. A member of the Silversword alliance, it only grows in the understory of mesic Hawaiian tropical rainforests. It is declining in population and is threatened by the spread of invasive species, with approximately 5500 individuals remaining.

References

  1. ↑ "Hawaiian silversword alliance, UH Botany". www.botany.hawaii.edu. Retrieved 2019-03-02.
  2. ↑ Carlquist, S., B. G. Baldwin, and G. D. Carr, eds. 2003. Tarweeds & Silverswords: Evolution of the Madiinae (Asteraceae). Missouri Botanical Garden Press.
  3. ↑ "Hawaiian silversword alliance, UH Botany". www.botany.hawaii.edu. Retrieved 2019-03-08.
  4. ↑ Vickie Caraway, Gerald D. Carr and Clifford W. Morden (2001), "Assessment of hybridization and introgression in lava-colonizing Hawaiian Dubautia (Asteraceae: Madiinae) using RAPD markers", American Journal of Botany, 88 (9): 1688–1694, doi: 10.2307/3558414 , JSTOR   3558414, PMID   21669703
  5. ↑ Blonder, Benjamin; Baldwin, Bruce G.; Enquist, Brian J.; Robichaux, Robert H. (2016). "Variation and macroevolution in leaf functional traits in the Hawaiian silversword alliance (Asteraceae)" (PDF). Journal of Ecology. 104 (1): 219–228. doi: 10.1111/1365-2745.12497 . ISSN   1365-2745.
  6. ↑ 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. ISSN   1365-294X. PMID   17784920. S2CID   18623958.
  7. ↑ Friar, Elizabeth A.; McGlaughlin, Mitchell E. (2011-03-01). "Evolutionary diversification and geographical isolation in Dubautia laxa (Asteraceae), a widespread member of the Hawaiian silversword alliance". Annals of Botany. 107 (3): 357–370. doi:10.1093/aob/mcq252. ISSN   0305-7364. PMC   3043929 . PMID   21193480.
  8. ↑ 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. ISSN   0363-6445. JSTOR   40211922. S2CID   86257628.
  9. ↑ Sanderson, Michael J.; Baldwin, Bruce G. (1998-08-04). "Age and rate of diversification of the Hawaiian silversword alliance (Compositae)". Proceedings of the National Academy of Sciences. 95 (16): 9402–9406. Bibcode:1998PNAS...95.9402B. doi: 10.1073/pnas.95.16.9402 . ISSN   0027-8424. PMC   21350 . PMID   9689092.
  10. ↑ For instance, Blonder and Baldwin 2016 cite 33 species, Friar 2011 cites 33, Lawton‐Rauh 2007 cites 30, Sanderson and Baldwin cite 28.
  11. ↑ "Hawaiian silversword alliance, UH Botany". www.botany.hawaii.edu. Retrieved 2019-03-08.
  12. ↑ "Hawaiian silversword alliance, UH Botany". www.botany.hawaii.edu. Retrieved 2019-03-08.
  13. ↑ "Hawaiian silversword alliance, UH Botany". www.botany.hawaii.edu. Retrieved 2019-03-08.


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