Chloroplast DNA restriction-site comparisons were made among 24 species of the Hawaiian silversword alliance (Argyroxiphium, Dubautia, and Wilkesia) and 7 species of North American perennial tarweeds in Adenothamnus, Madia, Raillardella, and Railkrdiopsis (Asteraceae-Madiinae).These data and results from intergeneric hybridization indicated surprisingly close genetic affinity of the monophyletic Hawaiian group to two diploid species of montane perennial herbs in California, Madia bolanderi and Raillardiopsis muiri.Of 117 restriction-site mutations shared among a subset of two or more accessions, more than one-fifth (25 mutations) separated the silversword alliance, M. bolanderi, and Raillardiopsis from Adenothamnus and Railardella. An additional 10 mutations distinguished the silversword alliance, M. bolanderi, and The Hawaiian silversword alliance (Asteraceae) has been regarded "the most outstanding example of adaptive radiation among Hawaiian angiosperms" (2). This woody group comprises 28 species in three genera (Argyroxiphium, Dubautia, and Wilkesia), including trees, shrubs, subshrubs, rosette plants, cushion plants, and a vine (3). These taxa display great diversity in morphological, anatomical, and ecophysiological traits and collectively span habitats ranging from some of the wettest recorded on earth to extreme desert-like environments (3). Despite this enormous diversity, structural (4, 5), biosystematic (6), allozymic (7), and chloroplast DNA (cpDNA) (8) data indicate the silversword alliance originated from a single colonizing species.Carlquist (1) presented convincing anatomical evidence indicating taxonomic alignment of the Hawaiian silversword alliance with the almost exclusively herbaceous American Madiinae or tarweeds. Gray (9) earlier suggested such affinity for Argyroxiphium, which was disputed by Keck (10) based on presumed morphological dissimilarities and the magnitude of the oceanic barrier to migration. Herein, we compare cpDNA restriction sites between the silversword alliance and several North American perennial tarweeds to further evaluate this relationship and its implications for adaptive radiation and long-distance dispersal. Our focus on cpDNA was based primarily on its utility for assessing phylogeny in higher plants, including Asteraceae (11), and its ease of evolutionary analysis compared with other plant DNA. MATERIALS AND METHODSWe examined one or two populations of 24 silversword alliance species, seven perennial North American tarweed species (Fig. 1), and four Heliantheae sensu lato species outside both groups (ref. 12; Fig. 2). American tarweed taxa were chosen from among 99 Madiinae species based on perenniality, other phenotypic similarities to the Hawaiian assemblage (1,12), and a preliminary cpDNA restriction site analysis of all Madiinae genera (B.G.B., unpublished results).Total DNA was isolated from fresh leaves by CsCl centrifugation (12,13). DNAs were digested with each of 16 restriction endonucleases (Fig. 1) tTo whom reprint requests should be addressed ...

Abstract. -The Hawaiian silversword alliance consists of the three genera Dubautia, Argyroxiphium, and Wilkesia, and is a classic example ofadaptive radiation in an insular setting. Genetic variation and interspecific genetic differentiation based on ten enzyme loci are described for Dubautia and Wilkesia. Genetic identities among species span the range of values expected from interpopulation comparisons within a single species (l = 0.90-1.00) to those typical of interspecific comparisons (1 = 0.67). Genetic-identity values correspond to biogeographic distribution and morphological distinctiveness, supporting a correlation of increasing genetic distance associated with the time of separation among lineages. It may be inferred that the high genetic identities observed within the Hawaiian Madiinae and other island plant groups are due to limited time spans available for taxa to accumulate new genetic variation through mutation. It appears that species may remain genetically similar (l > 0.90) even after time spans on the order of magnitude of 1,000,000 years.

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The Hawaiian silversword alliance of Argyroxiphium, Dubautia, and Wilkesia, in spite of exhibiting spectacular morphological, ecological, physiological, and chromosomal diversity, is remarkably cohesive, genetically. This is attested to by the ease of production of artificial hybrids and by the high frequency of spontaneous hybridization among such life forms as mat-forming subshrub, monocarpic rosette shrub, polycarpic shrub, cushion plant, tree, and vine. Even the least fertile of these hybrids is capable of producing backcross progeny. Moreover, first generation interspecific and intergeneric hybrids have been successfully used to produce trispecific hybrids in a number of instances. In general, the widest hybrid combinations have been as readily produced as crosses within a species. At present eight genomes or chromosome races distinguished by reciprocal translocations are recognized on the basis of meiotic analysis of artificial and spontaneous hybrids. Seven of these races are found among those species with 14 pairs of chromosomes. The eighth genome very likely characterizes all nine species of this alliance that have 13 pairs of chromosomes. The cytogenetic data indicate that redundancy of translocations involving the same chromosomes has been a recurrent theme in the chromosomal differentiation of these taxa. There appears to be little, if any, correlation between chromosomal evolution and adaptive radiation as assessed by gross habital differentiation in this group. However, within Dubautia, a novel ecophysiological trait associated with colonization of xeric habitats is restricted to species with n = 13. In contrast to the bulk of the Hawaiian flora, which is characterized by self-compatibility and chromosomal stability, it is suggested that the occurrence of self-incompatibility in the Hawaiian Madiinae may have favored selection of supergenes via chromosomal repatterning, and this may account for the diversity of chromosome structure seen in this group.

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