RESUMO
The genus Manihot, with around 120 known species, is native to a wide range of habitats and regions in the tropical and subtropical Americas. Its high species richness and recent diversification only c. 6 million years ago have significantly complicated previous phylogenetic analyses. Several basic elements of Manihot evolutionary history therefore remain unresolved. Here, we conduct a comprehensive phylogenomic analysis of Manihot, focusing on exhaustive sampling of South American taxa. We find that two recently described species from northeast Brazil's Atlantic Forest were the earliest to diverge, strongly suggesting a South American common ancestor of Manihot. Ancestral state reconstruction indicates early Manihot diversification in dry forests, with numerous independent episodes of new habitat colonization, including into savannas and rainforests within South America. We identify the closest wild relatives to Manihot esculenta, including the crop cassava, and we quantify extensive wild introgression into the cassava gene pool from at least five wild species, including Manihot glaziovii, a species used widely in breeding programs. Finally, we show that this wild-to-crop introgression substantially shapes the mutation load in cassava. Our findings provide a detailed case study for neotropical evolutionary history in a diverse and widespread group, and a robust phylogenomic framework for future Manihot and cassava research.
Assuntos
Manihot , Evolução Biológica , Pool Gênico , Manihot/genética , Filogenia , América do SulRESUMO
Evolutionary radiations of colonists on archipelagos provide valuable insight into mechanisms and modes of speciation. The apparent diversification of Galapagos mockingbirds (Nesomimus) provoked Darwin's initial conception of adaptive radiation, but the monophyly of this historically important exemplar has not been evaluated with molecular data. Additionally, as with most Galapagos organisms, we have a poor understanding of the temporal pattern of diversification of the mockingbirds following colonization(s) from source populations. Here we present a molecular phylogeny of Galapagos and other mockingbird populations based on mitochondrial sequence data. Monophyly of Galapagos mockingbirds was supported, suggesting a single colonization of the archipelago followed by diversification. Our analyses also indicate that Nesomimus is nested within the traditional genus Mimus, making the latter paraphyletic, and that the closest living relatives of Galapagos mockingbirds appear to be those currently found in North America, northern South America, and the Caribbean, rather than the geographically nearest species in continental Ecuador. Thus, propensity for over-water dispersal may have played a more important role than geographic proximity in the colonization of Galapagos by mockingbirds. Within Galapagos, four distinct mitochondrial DNA clades were identified. These four clades differ from current taxonomy in several important respects. In particular, mockingbirds in the eastern islands of the archipelago (Española, San Cristóbal, and Genovesa) have very similar mitochondrial DNA sequences, despite belonging to three different nominal species, and mockingbirds from Isabela, in the west of the archipelago, are more phylogenetically divergent than previously recognized. Consistent with current taxonomy is the phylogenetic distinctiveness of the Floreana mockingbird (N. trifasciatus) and close relationships among most mockingbirds from the central and northern region of the archipelago (currently considered conspecific populations of N. parvulus). Overall, phylogeographic patterns are consistent with a model of wind-based dispersal within Galapagos, with colonization of more northerly islands by birds from more southern populations, but not the reverse. Further radiation in Galapagos would require coexistence of multiple species on individual islands, but this may be prevented by relatively limited morphological divergence among mockingbirds and by lack of sufficient habitat diversity in the archipelago to support more than one omnivorous mimid.