Interspecific hybrid ancestry of a plant adaptive radiation: Allopolyploidy of the Hawaiian silversword alliance (Asteraceae) inferred from floral homeotic gene duplications

The polyploid Hawaiian silversword alliance (Asteraceae), a spectacular exa mple of adaptive radiation in plants, was shown previously to have descende d from North American tarweeds of the Madia/Raillardiopsis group; a primari ly diploid assemblage. The origin of the polyploid condition in the silvers word alliance was not resolved in earlier biosystematic, cytogenetic, and m olecular studies, apart from the determination that polyploidy in modern sp ecies of Madia/Raillardiopsis arose independent of that of the Hawaiian gro up. We determined that two floral homeotic genes, ASAP3/TM6 and ASAP1, are found in duplicate copies within members of the Hawaiian silversword allian ce and appear to have arisen as a result Of interspecific hybridization bet ween two North American tarweed species. Our molecular phylogenetic analyse s of the 4SAP3/TM6 loci suggest that the interspecific hybridization event in the ancestry of the Hawaiian silversword alliance involved members of li neages that include Raillardiopsis muirii (and perhaps Madia nutans) and Ra illardiopsis scabrida. The ASAP1 analysis also indicates that the two speci es of Raillardiopsis are among the closest North American relatives of the Hawaiian silversword alliance. Previous biosystematic evidence demonstrates the potential for allopolyploid formation between members of the two North American tarweed lineages; a vigorous hybrid between ii. muirii and R. sca brida has been produced that formed viable, mostly tetraporate (diploid) po llen, in keeping with observed meiotic failure. Various genetic consequence s of allopolyploidy may help to explain the phenomenal evolutionary diversi fication of the silversword alliance.