Adaptive Radiation of the Hawaiian Silversword Alliance

Natural Hybridization

Last Modification: April 21, 2006

Biosystematic and cytogenetic analyses indicate that hybrids among the group of Dubautia species with 13 pairs of chromosomes are essentially fully fertile. Thus, where such species are sympatric, it is common to find evidence of hybridization, sometimes resulting in spectacular swarms of recombinant types representing every conceivable intermediate between the parental types. One superb example of this type involves D. arborea and D. ciliolata in a small area of sympatry in Waipahoehoe Gulch on Mauna Kea, Hawaii. Interestingly, a form that is very similar to D. menziesii, otherwise known only from Maui, has apparently become reproductively stabilized in the lower part of this gulch and in at least one other area of sympatry on Mauna Kea. The illustrations at the left show an array of shoots (above) and leaves (below) each taken from from a different individual growing in the narrow zone of sympatry in this gulch. (side by side comparison)
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Dubautia scabra is sympatric and hybridizes with several 13-paired species, including D. ciliolata, D. linearis, D. menziesii (Fig. 1), D. platyphylla, D. reticulata, and D. waianapanapaensis. These hybrids can be readily recognized by their intermediate nature, including pale lemon-yellow flowers and a chromosome number of 2n = 27. The maximum meiotic chromosome pairing configuration is always the same, consisting of 12 pairs and 1 chair of three chromosomes. Pollen stainability in these hybrids often ranges from 70-90%. Dubautia ciliolata and D. scabra (Fig. 2) hybridize at countless sites of sympatry on the island of Hawaii. In the saddle area between Mauna Kea and Mauna Loa is a particularly interesting site where D. ciliolata is restricted to kipuka (islands) of a prehistoric, mostly a'a lava flow and D. scabra is restricted to the surrounding 1935 flow of mostly pahoehoe lava. Fig. 3 shows a large kipuka of older lava with shrubby vegetation and a tree surrounded by lava of the more recent flow with sparse vegetation. A very small kipuka of the older reddish-brown lava with a shrub of Dubautia ciliolata is visible in Fig. 4. Hybrids occur primarily on the the perimeter of the recent lava flow, usually at interfaces with the older substrate. A study of flavonoid compounds in hybrids suggests that recombination beyond the F1 generation is occurring at this site.  (side by side comparison)
A striking example of a natural hybrid between congeneric species with very different growth forms involves the mat-forming species, Dubautia scabra, and the tree, D. reticulata. As different as the parents are, the hybrids are comparatively fertile, being identical in chromosome configuration to the D. scabra/D. ciliolata example discussed above. (side by side comparison)
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The preceding examples involve hybrids with full or high fertility between species that are relatively closely related. Among the more spectacular hybrids that occur naturally is the one between Argyroxiphium sandwicense subsp. macrocephalum (Fig. 1) with 14 pairs of chromosomes and Dubautia menziesii (Fig. 2) with 13 pairs of chromosomes. In addition to the difference in chromosome number, the genomes of these species are further differentiated by at least two reciprocal chromosome translocations. This results in a common meiotic configuration of 9 pairs and 3 chains of three chromosomes, and a concomitant reduction in fertility to approximately 9%, based on pollen stainability in F1 hybrids. Despite the tremendous morphological differences of the parents and the reduced fertility of the F1 (Fig.3), backcross progeny are produced in nature. Some of these have been inadvertently cultivated in Haleakala National park (Fig. 4) and several others have been grown experimentally at the University of Hawaii-Manoa. One of the experimentally grown plants that represented a backcross to Dubautia menziesii flowered (Fig 5, note many heads were removed from this plant for chromosome analysis). It had a simplified chromosome pairing configuration of 12 pairs and a chain of 3 chromosomes, was about 80% fertile, and was used to generate a vigorous second backcross progeny of several individuals with Dubautia menziesii as the recurrent parent. These individuals were remarkably uniform morphologically and one that flowered (Fig. 6) exhibited 13 pairs of chromosomes and 99% pollen stainability (see summary in Fig. 7). Some plants seen in the field closely approximate the cultivated backcross progeny of this intergeneric hybrid combination and probably originated in the same manner. Specifically, the type material of D. dolosa appears to represent such an unstabilized hybrid product and is no longer equated with D. waianapanapaensis which is geographically distinct and clearly a reproductively stabilized taxon. However, the similarity of these plants suggests a possible hybrid origin for D. waianapanapaensis. Indeed, the ease of recombination between such strikingly differentiated plants as Argyroxiphium sandwicense subsp. macrocephalum and Dubautia menziesii underscores the potential of hybridization in the evolution of this and other plants, especially in Hawaii, where hybridization appears to be a way of life. (side by side comparison)
dlax_x_ag_mids.jpg (18550 bytes)
Although less well known, several other natural intergeneric hybrid combinations have been detected, e.g., Dubautia laxa subsp. laxa x Argyroxiphium grayanum (greensword), on the summit of W Maui.  (side by side comparison)
Another natural intergeneric hybrid from the summit of W Maui is Dubautia plantaginea subsp. plantaginea x Argyroxiphium grayanum.  This combination has also been synthesized artificially in cultivation.  The plant illustrated is the natural hybrid but grown in cultivation. (side by side comparison)

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