Allopolyploid origin and diversification of the Hawaiian endemic mints

Island systems provide important contexts for studying processes
underlying lineage migration, species diversification, and organismal
extinction. The Hawaiian endemic mints (Lamiaceae family) are the second
largest plant radiation on the isolated Hawaiian Islands. We generated a
chromosome-scale reference genome for one Hawaiian species, Stenogyne
calaminthoides, and resequenced 45 relatives, representing 34 species, to
uncover the continental origins of this group and their subsequent
diversification. We further resequenced 109 individuals of two Stenogyne
species, and their purported hybrids, found high on the Mauna Kea volcano
on the island of Hawai’i. The three distinct Hawaiian genera,
Haplostachys, Phyllostegia, and Stenogyne, are nested inside a fourth
genus, Stachys. We uncovered four independent polyploidy events within
Stachys, including one allopolyploidy event underlying the Hawaiian mints
and their direct western North American ancestors. While the Hawaiian taxa
may have principally diversified by parapatry and drift in small and
fragmented populations, localized admixture may have played an important
role early in lineage diversification. Our genomic analyses provide a view
into how organisms may have radiated on isolated island chains, settings
that provided one of the principal natural laboratories for Darwin’s
thinking about the evolutionary process.