Adaptive radiation in the Hawaiian silversword alliance (Compositae: Madiinae) : I. Cytogenics of spontaneous hybrids

Carr, Gerald; Kyhos, D

The Hawaiian endemic genera Argyroxiphium, Dubautia, and Wilkesia comprise one of the most remarkable examples of adaptive radiation known to science. Although the alliance includes rosette, subherbaceous, shrubby, arborescent and lianous forms occupying areas of less than 45 cm of annual precipitation to perhaps the wettest spot on earth, the occurrence of numerous interspecific and intergeneric spontaneous hybrids attests to their relatively recent divergence, probably from a single colonizing species. In addition to the spectacular morphological and ecological diversity, this group may be unique among the Hawaiian flora in its cytological diversity which includes differentiation through aneuploidy and reciprocal chromosome translocations. This cytological diversity has allowed positive identification of many spontaneous F1 hybrids, several of which have previously been given formal Latin names as self-reproducing taxa. An analysis of these hybrids has revealed the presence of at least five distinct genomes that can be arranged to form an emerging phylogenetic scheme. These include two aneuploid cohorts, one with n = 14 and a second with n = 13. The cytological data, the amount of morphological and ecological diversity and discontinuity within each aneuploid cohort, the geographical distribution and numbers of species of each cohort and the range of individual species are all consistent with the hypothesis that the 14-paired group is ancestral and has given rise to the 13-paired group through aneuploid reduction. Field observations of the Hawaiian silversword alliance indicate that hybridization between species is very common and that the number of hybrids produced at a given site is likely to be highest in instances where internal isolation is lowest. Apparent interspecific recombinants are frequent at sites of sympatry involving species with little or no internal isolation and the potential for gene flow even between well-marked genera has been retained. Thus, even among members of the Hawaiian silversword alliance that possess chromosomal isolation, hybridization has very likely been an important mechanism for the generation of recombinants capable of exploiting the myriad of highly diversified, yet closely spaced habitats in the Hawaiian environment. There can be little doubt that hybridization has been at least equally important in other Hawaiian groups where except for the extremely low incidence of polyploidy, significant internal isolation is unreported.

Adaptive radiation in the Hawaiian silversword alliance (Compositae: Madiinae) : I. Cytogenics of spontaneous hybrids

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