Adaptive Radiation of the Hawaiian Silversword Alliance

Artificial Hybridization

Last Modification: June 23, 2000

Among the more interesting intergeneric artificial hybrids produced is that between Dubautia scabra and Wilkesia hobdyi. Although both parents have 2n = 28 chromosomes their genomes are differentiated by at least 2 reciprocal chromosome translocations, resulting in 11 pairs and a chain of 6 chromosomes at meiosis and a pollen stainability of about 28%.  (side by side comparison)
Another artificial hybrid involves Dubautia knudsenii and Wilkesia hobdyi. It has a pollen stainability of about 23 % and at meiosis exhibits 11 pairs and 2 chains of 3 or a chain of 6 chromosomes.  (side by side comparison)
Great effort was taken to produce the artificial hybrid between Argyroxiphium sandwicense subsp. macrocephalum and Wilkesia gymnoxiphium. The species are found at opposite ends of the island chain and having them flower synchronously in cultivation is an extremely unlikely event. In fact, the flowering times of the species in the field are usually mutually exclusive. However, during very favorable years there is a slight overlap in flowering and one such year afforded the opportunity to produce a hybrid by collecting pollen from Wilkesia on Kauai and flying it to Maui on the same day to pollinate stigmas of an early-flowering individual of Argyroxiphium. This pollination "by jet aircraft" resulted in several hybrid fruits. Though it was successfully cultivated for some time, unfortunately this hybrid did not reach flowering maturity.  (side by side comparison)
Although the silversword/iliau hybrid died before flowering, the hybrid between Argyroxiphium grayanum (greensword) and Wilkesia hobdyi (dwarf iliau) was more successful. Meiosis in this hybrid indicated that a minimum of 2 reciprocal chromosome translocations distinguish the parental genomes. Pollen stainability was about 20%. Note the very conspicuous glandular hairs with droplets of "tar," a condition similar to many of the "tarweeds" of North America.  (side by side comparison)
The artificial intergeneric hybrid between Dubautia plantaginea ssp. humilis and the silversword, Argyroxiphium sandwicense ssp. macrocephalum bears a striking similarity to the greensword, A. grayanum. Silverswords and Dubautia plantaginea are sympatric on both E Maui and W Maui where greenswords occur, suggesting the possibility of this sort of origin for A. grayanum.  (side by side comparison)
This remarkable hybrid combines the genomes of the Mauna Kea silversword (Argyroxiphium sandwicense ssp. sandwicense), the greensword (A. grayanum), and Dubautia linearis. It was produced artificially by first crossing the two Argyroxiphium species. This hybrid had about 63% pollen stainability and exhibited 12 pairs and a ring of 4 chromosomes during meiosis. This primary hybrid was then crossed with Dubautia linearis to produce the trispecific hybrid which had about 12% stainable pollen and exhibited 9 pairs, a chain of 6, and a chain of 3 chromosomes.  (side by side comparison)
This final example combines the genomes of 4 species of Dubautia. It was produced by crossing two highly fertile primary hybrids that exhibited normal meiosis (Dubautia knudsenii x D. laxa and D. microcephala x D. plantaginea ssp. humilis). The secondary hybrid also had normal meiosis and about 98 % pollen stainability.  (side by side comparison)

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