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 ofangiosperms. These reductions have often been attributed to changes in reproductive biology. In theangiosperm plant family Annonaceae, most species have flowers with six petals, and many stamens andcarpels. In the genus Monanthotaxis several deviations from this pattern have been observed, includingflowers that contain three petals and three stamens only. New DNA sequences were generated for42 specimens of Monanthotaxis. Five chloroplast markers and two nuclear markers for 72 out of 94species of Monanthotaxis were used to reconstruct a phylogeny of the genus, which revealed severalwell-supported, morphologically distinct clades. The evolution of four quantitative and two qualitativefloral characters was mapped onto this phylogeny, demonstrating a reduction in flower size and numberof flower parts in Monanthotaxis. A large variation in stamen forms and numbers, strong correlationsbetween petal size, stamen and carpel number, combined with a non-gradual mode of evolution andthe sympatric co-occurrence of Monanthotaxis species from different clades suggest that the highdiversity in the African rainforest of this genus is caused by switches in pollination systems