Disentangling lianas: The systematics of Monanthotaxis (Annonaceae)

The family Annonaceae consists of woody plants and are mostly restricted to tropical rain forests. In these forests it is one of the most speciose and abundant plant families, making it an important family for understanding the diversity, ecology and evolution of tropical plants.

The two most important subfamilies of the Annonaceae are the Annonoideae and the Malmeoideae. These subfamilies are sister groups and together contain over 95% of Annonaceae species. Already in the first phylogenies published a remarkable difference between the branch lengths of these subfamilies became apparent. This suggests a difference in the rate of evolution between these subfamilies. Lineage-specific rate heterogeneity could mislead phylogenetic inferences with currently available models. In chapter 2 these differences in the molecular rate of evolution within the family Annonaceae were studied. Using high-throughput sequencing data 72 genes, 99 spacers and 16 introns from 24 chloroplast genomes and nuclear ribosomal DNA of 23 species were used to compare the rate of evolution between the two major sister subfamilies of the Annonaceae, the Annonoideae and Malmeoideae. In all analyses we found higher substitution rates for the Annonoideae compared to the Malmeoideae. Also the size of the chloroplast genome varied widely, with larger sizes for the Annonoideae compared to the Malmeoideae. Furthermore, values for omega (dN/dS)-ratios were higher for Annonoideae than the Malmeoideae. This could indicate less stringent purifying (negative) selection in the Annonoideae, a pattern that occurs in groups with small population sizes.

The genus Monanthotaxis belongs to the faster evolving subfamily Annonoideae. It is the second most species rich genus of Annonaceae in Africa. Before studying the evolution of the genus a firm taxonomic foundation is needed. This is realised in chapter 3 and 4. 

The generic boundaries and phylogenetic relationships of Monanthotaxis and related genera were not clear with the genus Friesodielsia even being polyphyletic. In chapter 3 this was studied using two nuclear and five chloroplast DNA regions of 101 taxa. Bayesian, maximum likelihood and maximum parsimony analyses revealed that the species of Friesodielsia formed five distinct clades. To ensure strict monophyly of the genera, the name Friesodielsia was restricted to the Asian clade, while one of the African species formerly assigned to Friesodielsia was transferred to the genus Afroguatteria, one other to the genus Sphaerocoryne and the other 8 African species to the genus Monanthotaxis. Further, two monospecific genera, Exellia and Gilbertiella, were synonymised with Monanthotaxis, rendering the genus Monanthotaxis monophyletic.

In chapter 4 the continental African species of Monanthotaxis were revised. This includes 79 species and one variety of which 13 species were newly described. Ten additional species were described previously in this PhD project. In this chapter a key for flowering material was provided and a synoptic key including 45 characters. Topics included in the revision are the history of the taxonomy of Monanthotaxis, morphology, leaf anatomy, floral biology, distribution and habitat, phylogeny and finally ethnobotany and phytochemistry. Each species was fully described including synonymy, notes on distribution, habitat & ecology, vernacular names, uses and a preliminary IUCN conservation status.

Based on this taxonomic foundation, the floral evolution of Monanthotaxis was studied in chapter 5. In the plant family Annonaceae, flowers generally have six petals and many stamens and carpels. In the genus Monanthotaxis several deviations from this pattern have been observed, some flowers only contain three petals and three stamens. Five chloroplast and two nuclear DNA regions were used of 72 out of 94 Monanthotaxis species to reconstruct a phylogeny of the genus. Four quantitative and two qualitative floral characters were mapped onto this phylogeny. A strong correlation between petal size, stamen and carpel number showed a reduction in flower size and number of floral parts within the genus Monanthotaxis. The large variation in stamen shape and numbers combined with a non-gradual mode of evolution and 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.