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

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 Raillardiopsis (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 muirii. 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 Raillardella. An additional 10 mutations distinguished the silversword alliance, M. bolanderi, and R. muirii from Adenothamnus, Raillardella, and Raillardiopsis scabrida. Phylogenetic analyses of these data and production of vigorous hybrids of the combinations Dubautia laevigata x R. muirii and (Dubautia knudsenii x Dubautia laxa) x M. bolanderi reinforce and refine Carlquist's hypothesis [Carlquist, S. (1959) Aliso 4, 171-236] that the Hawaiian silversword alliance arose from American tarweeds. Ultimate origin of silversword alliance chloroplast DNA from within the Californian-endemic paraphyletic genus Raillardiopsis was supported with high bootstrap confidence. Geologic considerations and the distribution of sporophytic self-incompatibility among these species demonstrate that the tarweed ancestor of the silverswords overcame (i) a dispersal barrier of at least 3900 km of open ocean and (ii) the breeding barrier of self-incompatibility.

本研究针对夏威夷银剑菊联盟（Hawaiian silversword alliance）的24个物种（隶属于Argyroxiphium、Dubautia及Wilkesia属），以及隶属于Adenothamnus、Madia、Raillardella和Raillardiopsis属的7个北美多年生松脂菊类物种（菊科-松脂菊亚族Asteraceae-Madiinae），开展了叶绿体DNA（chloroplast DNA）限制性酶切位点（restriction-site）比较分析。本研究获得的数据与属间杂交（intergeneric hybridization）结果均表明，这支单系（monophyletic）的夏威夷类群与加州的两种山地多年生二倍体草本植物Madia bolanderi和Raillardiopsis muirii具有出人意料的紧密遗传亲缘关系。在117个至少在两份种质样品（accession）中共有的限制性酶切位点突变中，超过五分之一（25个突变）可将银剑菊联盟、M. bolanderi及Raillardiopsis与Adenothamnus和Raillardella区分开来。另有10个突变可将银剑菊联盟、M. bolanderi和R. muirii与Adenothamnus、Raillardella以及Raillardiopsis scabrida区分开。对上述数据开展的系统发育分析（phylogenetic analyses），以及组合Dubautia laevigata × R. muirii与(Dubautia knudsenii × Dubautia laxa) × M. bolanderi所获得的健壮杂交种的创制，进一步支持并完善了Carlquist提出的假说[Carlquist, S. (1959) Aliso 4, 171-236]：夏威夷银剑菊联盟起源于北美松脂菊类植物。银剑菊联盟的叶绿体DNA最终起源于加州特有并系属Raillardiopsis内部，该结论得到了较高的自展置信度（bootstrap confidence）支持。结合地质学考量与这些物种间孢子体自交不亲和性（sporophytic self-incompatibility）的分布特征可知，银剑菊的松脂菊祖先先后克服了两大障碍：一是至少3900公里的开阔大洋扩散屏障，二是自交不亲和带来的生殖隔离障碍。