Floral evolution by simplification in Monanthotaxis (Annonaceae) and hypotheses for pollination system shifts

Simplification by reduction has occurred many times independently in the floral evolution of
angiosperms. These reductions have often been attributed to changes in reproductive biology. In the
angiosperm plant family Annonaceae, most species have flowers with six petals, and many stamens and
carpels. In the genus Monanthotaxis several deviations from this pattern have been observed, including
flowers that contain three petals and three stamens only. New DNA sequences were generated for
42 specimens of Monanthotaxis. Five chloroplast markers and two nuclear markers for 72 out of 94
species of Monanthotaxis were used to reconstruct a phylogeny of the genus, which revealed several
well-supported, morphologically distinct clades. The evolution of four quantitative and two qualitative
floral characters was mapped onto this phylogeny, demonstrating a reduction in flower size and number
of flower parts in Monanthotaxis. A large variation in stamen forms and numbers, strong correlations
between petal size, stamen and carpel number, combined with a non-gradual mode of evolution and
the sympatric co-occurrence of Monanthotaxis species from different clades suggest that the high
diversity in the African rainforest of this genus is caused by switches in pollination systems.