Understanding the distribution of Alocasia: Historical biogeography and climate niche separation in sister species pairs
Final Report Abstract
This project centered on the biogeography of a clade of Araceae in the Malay Archipelago, a region that has received much attention because of its complex geological history and richness in endemic plants and animals. The original idea was to use the genus "Alocasia", with 113 species, to understand the time frame of species formation in the Archipelago and the role of geographic isolation and dispersal. "Alocasia" is bird-dispersed, and its range extends from India and Sri Lanka through Indochina to China and southern Japan, the Malay Archipelago and Oceania; a single species is indigenous in Australia. The ethno-botanically most important species is A. "macrorrhizos" (giant taro), a tropical ornamental that is cultivated throughout tropical Asia for its edible tubers and leaves. Although the fieldwork for the climate niche modelling was carried out as planned, we could not obtain a dense enough sampling of geo-referenced individuals for multiple independent sister species pairs. One of the problems was that plastid and nuclear sequence data yielded partially incongruous phylogenies and that nuclear phytochrome C (PhyC) gene sequences proved difficult to obtain from herbarium specimens. I therefore re-oriented the project to focus on the historical biogeography of "Alocasia" and the biogeography of the entire Araceae for which Munich has an exceptionally large collection of cultivated plants and herbarium specimens. We generated over 400 sequences of PhyC and four plastid loci and calibrated the resulting phylogeny with the age of leaf fossils from the Messel Formation near Frankfurt, Germany, that resemble species of "Alocasia" and "Colocasia" in their colocasioid leaf venation. We also used alternative calibration approaches. The chronogram implied that “Alocasia” diverged from its closest surviving relatives at the transition of the Oligocene to the Miocene (c. 24 Ma ago) and that most "Alocasia" lineage splits fall into the Pleistocene, apparently connected with sea-level changes. Thus, of the 54 divergence events in the tree, 18 or 19 appear to have involved trans-oceanic isolation events. For the Araceae family, we inferred a "water associated" origin, which matches several lines of evidence. Firstly, an origin in wet habitats in Laurasia fits the ecology of, and fossils associated with, early-diverging clades in the family, such a gymnostachyoids and orontioids, all of which are restricted to wet habitats. Secondly, the next-oldest divergence involves the entirely aquatic Lemnoideae, dating to c. 104 Ma. Considering the near-basal position of these wet habitat-adapted lineages and the large number of aquatic lineages in the Araceae sister group, namely the other 13 Alismatales families, an origin of aroids in water-associated swampy habitats is plausible. Thirdly, Late Cretaceous and Paleocene fossils of free-floating Araceae ("Limnobiophyllum scutatum" and "Cobbania corrugate") indicate that transitions from a terrestrial growth habit to an aquatic one had occurred early during the evolution of the family. Methodologically, our study was innovative in including extinct ranges (from fossil Araceae) in the biogeographic models, instead of only relying on the ranges occupied by the surviving (sequenced) lineages as is done in most computer-based biogeographic reconstructions.
Publications
- 2012. Giant taro and its relatives: A phylogeny of the large genus Alocasia (Araceae) sheds light on Miocene floristic exchange in the Malesian region. Molecular Phylogenetics and Evolution 63: 43-51
Nauheimer, L., P. C. Boyce, and S. S. Renner
(See online at https://doi.org/10.1016/j.ympev.2011.12.011) - 2012. Global history of the ancient monocot family Araceae inferred with models accounting for past continental positions and previous ranges based on fossils. New Phytologist 195: 938-950
Nauheimer, L., D. Metzler, and S. S. Renner
(See online at https://doi.org/10.1111/j.1469-8137.2012.04220.x)
