Nucleotide Sequence of rbcL and Phylogenetic Relationships of Setchellanthus caeruleus (Setchellanthaceae)

The nucleotide sequence (1462 bases) from the chloroplast gene for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL) was utilised for the monotypic Mexican endemic genus Setchellanthus to assess its phylogenetic affinities. Phylogenetic analyses of rbcL sequence data robustly group Setchellanthus with other mustard oil producing plants included in Dahlgren's expanded order Capparales. Within this assemblage, maximum parsimony places Setchellanthus sister to a clade comprised of the North American family Limnanthaceae, the sister taxa Koeberlinia + Salvadora + Batis, and the core Capparales (Brassicaceae + Capparaceae, Gyrostemonaceae + Resedaceae, Pentadiplandraceae, and Tovariaceae). Maximum likelihood analyses reverse the relationship of Setchellanthus and Limnanthaceae, placing Setchellanthus sister to the Koeberlinia + Salvadora + Batis plus the core Capparales assemblage, with Limnanthaceae sister to these. The rbcL sequence of Setchellanthus possesses a putative stop codon ending at position 1440, twelve bases beyond the 1428 site usual for dicots. This extended 3' terminus is consistent within the expanded Capparales for a clade that comprises the above-named taxa. The position of Setchellanthus in the rbcL phylogram provides support for elevation of this unispecific genus to family rank, postulated in view of its unique morphological characters.