Chloroplast DNA evidence for a North American origin of the Hawaiian silversword alliance (Asteraceae).

Abstract

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 ...

Accelerated regulatory gene evolution in an adaptive radiation

“…Molecular phylogenetic analyses using chloroplast DNA and rDNA internal transcribed spacer (ITS) sequences (6,10,12) have confirmed an early hypothesis that the closest relatives of the Hawaiian silversword alliance can be found among North American tarweeds (Asteraceae: Heliantheae-Madiinae) in the ''Madia'' lineage (13). Both cytogenetic (14) and allozyme (15) data indicate that the Hawaiian species are tetraploids (n ϭ [13][14], in contrast to the basally diploid condition (n ϭ 6-9) in the most closely related North American species within the ''Madia'' lineage (13,16).…”

Rapid parallel adaptive radiations from a single hybridogenic ancestral population

“…[4,5]). This was interpreted as evidence for high rates of diversification from a single ancestral lineage.…”

Chloroplast DNA evidence of colonization, adaptive radiation, and hybridization in the evolution of the Macaronesian flora.

“…Oceanic islands are relatively simple systems where both patterns of dispersal and natural selection can be more easily assessed than in continental systems. Most studies of island biota have been restricted to the Pacific Ocean archipelagos of Hawaii (3)(4)(5)(6), Galapagos (7,8), and Juan Fernandez (9)(10)(11). Few studies (12,13) have focused on the Atlantic Ocean archipelagos that constitute the biogeographical region known as Macaronesia (Fig.…”