Numerical Analysis of Generic Relationships in Thelypodieae (Brassicaceae)

Abstract

Numerical methods were used to evaluate phenetic similarities and hypotheses of phylogenetic relationships among the ten genera of Thelypodieae, considered by some to be the most primitive tribe in Brassicaceae. The complemen- tary phenetic methods of cluster analysis and ordination with a nearest neighbor directed network indicated why the genera are sometimes difficult to delimit. Several of these genera are close and nearly equidistant from each other. Three phylogenetic hypotheses were tested by assuming different hypothetical ancestors for the tribe or for the nine genera that occur in North America. Given data available at present, we suggest that the phylogenetic tree that uses a Cleome-like ancestor most reliably rep- resents the evolutionary relationships among the nine genera restricted to North America. The tenth genus, Macropodium, remains an enigma because of its restriction to temperate eastern Asia. Thelypodieae, as circumscribed here, was first recognized as a natural assemblage by Robinson (1895) who used the name Stanleyeae. Hayek (1911) published a worldwide classification of the family and adopted the earlier name Thelypodieae (table 1). Hayek included all members of the present tribe, except for the exclusion of Stanfordia, now a part of Caulanthus. He also included Schoenocrambe, which is no longer included in Thelypodieae. In addition, Hayek probably overlooked the work of Rydberg (1907); he did not include segregates of Thelypodium. In an excellent monographic study, Payson (1923) defined generic lines within the tribe more precisely.