Phylogenetic placement of two enigmatic genera, Borthwickia and Stixis , based on molecular and pollen data, and the description of a new family of Brassicales, Borthwickiaceae

Capparaceae (Brassicales) as traditionally circumscribed is heterogeneous, and several genera have been segregated from it based on molecular and/or morphological data. However, Borthwickia and Stixis , two Southeast Asian endemic genera of Capparaceae with controversial positions, have not previously been evaluated in a molecular phylogenetic study. Here, we used four plastid DNA regions ( matK, ndhF, rbcL, trnL–trnF ) and pollen data to determine their phylogenetic relationships within core Brassicales. Our results showed that neither Borthwickia nor Stixis is a member of Capparaceae. The two genera, together with Forchhammeria , Gyrostemonaceae, Resedaceae, and Tirania , formed a clade with strong support. Stixis is closely related to Tirania , a relationship that is also supported by morphological characters, such as six sepals and three–or four–locular ovaries. Most interestingly, Borthwickia was resolved as sister to the Forchhammeria –Resedaceae– Stixis–Tirania clade with moderate to strong support. However, Borthwickia differs markedly from its sister group in having opposite leaves, one indistinct stigma, more than four carpels and locules, a linear ovary with ridges, and pollen grains with perforate exine sculpturing. Thus, we describe a new family, Borthwickiaceae, for the genus.