Molecular Biology and Evolution, Vol 16, 1105-1113, Copyright © 1999 by Society for Molecular Biology and Evolution
M Barrier, BG Baldwin, RH Robichaux and MD Purugganan
The polyploid Hawaiian silversword alliance (Asteraceae), a spectacular
example of adaptive radiation in plants, was shown previously to have
descended from North American tarweeds of the Madia/Raillardiopsis group, a
primarily diploid assemblage. The origin of the polyploid condition in the
silversword alliance was not resolved in earlier biosystematic,
cytogenetic, and molecular studies, apart from the determination that
polyploidy in modern species of Madia/Raillardiopsis arose independent of
that of the Hawaiian group. We determined that two floral homeotic genes,
ASAP3/TM6 and ASAP1, are found in duplicate copies within members of the
Hawaiian silversword alliance and appear to have arisen as a result of
interspecific hybridization between two North American tarweed species. Our
molecular phylogenetic analyses of the ASAP3/TM6 loci suggest that the
interspecific hybridization event in the ancestry of the Hawaiian
silversword alliance involved members of lineages that include
Raillardiopsis muirii (and perhaps Madia nutans) and Raillardiopsis
scabrida. The ASAP1 analysis also indicates that the two species 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 R. muirii and R.
scabrida has been produced that formed viable, mostly tetraporate (diploid)
pollen, in keeping with observed meiotic failure. Various genetic
consequences of allopolyploidy may help to explain the phenomenal
evolutionary diversification of the silversword alliance.
ORIGINAL ARTICLE
Interspecific hybrid ancestry of a plant adaptive radiation: allopolyploidy of the Hawaiian silversword alliance (Asteraceae) inferred from floral homeotic gene duplications
Department of Genetics, North Carolina State University, Raleigh 27695, USA.
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