Evolution of secondary heads in Nassauviinae (Asteraceae, Mutisieae).
The evolution of the inflorescence head in Asteraceae is important in the diversification of this largest angiosperm family. The aggregation of heads into higher-order capitulescences (secondary heads or syncephalia) is considered evolutionarily advanced. The genera Moscharia, Nassauvia, Polyachyrus, and Triptilion of the subtribe Nassauviinae (Mutisieae) have syncephalia with differing degrees of capitula condensation. ITS and plastid trnL-trnF regions were analyzed separately and together using maximum parsimony and maximum likelihood to examine the evolution of syncephalia in the Nassauviinae. The four genera displaying syncephalia do not form a clade minus taxa without syncephalia, indicating that secondary heads in Nassauviinae have either convergently evolved twice in the subtribe (or, very unlikely) once with multiple reversions. Strong support was obtained for a sister relationship between Leucheria (without syncephalium) and Polyachyrus, and both sister to Moscharia. Nassauvia and Triptilion form a distinct clade but are sister to other genera, Perezia and Panphalea, without syncephalium. Previous hypotheses postulated the evolution from simple to more complex secondary heads. We show that the ancestor of Moscharia, Polyachyrus, and Leucheria, in a more arid habitat, had a complex type of secondary head, and loss of complexity occurred in response to a shift from arid to mesic conditions.