Phylogenetics of Cecropieae (Urticaceae) and the evolution of an ant-plant mutualism in Cecropia
2017-07
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Phylogenetics of Cecropieae (Urticaceae) and the evolution of an ant-plant mutualism in Cecropia
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2017-07
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Mutualisms are common in all ecosystems and can influence ecosystem function in aspects including nutrient cycling and community building. Mutually beneficial interactions between species vary in degrees of specificity from facultative and generalized associations to obligate and specialized associations. Ant-plant mutualisms are especially common in the tropics and are popular for ecological studies of mutualism. Investigating the evolution of tropical ant-plant mutualisms requires a phylogenetic framework. The neotropical ant-plant mutualism involving the large genus Cecropia (Urticaceae) and associated ants, including the genus Azteca, has been the subject of extensive ecological study but has not been examined from a phylogenetic perspective. Large woody plant genera have often been difficult to treat taxonomically and resolve phylogenetically. It is unknown whether this is due to variation in traditional DNA markers or if other factors are involved. Next generation sequencing techniques can provide more data than direct methods that improve phylogenetic resolution and provide opportunity to infer historical introgression that could also influence phylogenetic resolution. The goal of this dissertation is to investigate evolutionary relationships in Cecropia and its closest relatives and to examine the evolutionary history of the mutualism. Relationships between genera in the tribe Cecropieae (Urticaceae), including Cecropia, Coussapoa, Musanga, Myrianthus, and Pourouma, were unknown but are necessary to investigate the evolutionary history of the Cecropia-ant mutualism. Chapter 1 used molecular phylogenetics to infer relationships between genera in Cecropieae and investigates the influence of phylogenetic resolution in Cecropia on reconstructing the ancestry of myrmecophytism. Bayesian phylogenetic analysis of the NADH dehydrogenase (ndhF) chloroplast gene region, the 26S region of nuclear ribosomal DNA, and an exon-primed intron-crossing DNA region supported non-myrmecophytic African Musanga within a paraphyletic Cecropia. Neotropical Pourouma and Coussapoa were supported as sister taxa with African Myrianthus as their closest relative. Although it was uncertain whether myrmecophytism was the ancestral condition of Cecropia, a close relationship between non-myrmecophytic Cecropia sciadophylla and Musanga suggests that the loss of ant associations did not accompany African colonization by Musanga. In Chapter 2, restriction site associated DNA (RAD) sequencing was used to infer relationships among myrmecophytic and non-myrmecophytic Cecropia species. RAD sequence data resolved and supported species level relationships beyond what could be inferred from direct sequencing. The D-statistic to test for introgression among Cecropia species was used to examine whether hybridization might account for some of the difficulty associated with diagnosing species in the genus. Most Cecropia species sampled were not deeply diverged genetically but a non-myrmecophytic clade included lineages that could be considerably older than most of the ant-associated species. Results of ABBA BABA tests could be interpreted as evidence of recent introgression among closely related myrmecophytic species. However, test results implying geographically implausible introgression between neotropical C. sciadophylla and afrotropical Musanga suggest that the D-statistic is sensitive to the extent of genetic divergence among clades and may yield type I error in the case of deeply diverged clades. Evidence from geographically widespread and morophologically hetergenous C. obtusifolia and C. angustifolia suggests that current synonymy lumps together genetically dissimilar lineages and that future taxonomic revision should consider splitting. Chapter 3 investigated the origin and evolutionary history of myrmecophytism in Cecropia sensu lato (including Musanga). The most highly supported phylogeny for Cecropia was used in ancestral state reconstructions of ant association and the myrmecophytic traits of domatia (nest cavitiy) and trichilia (food bodies) to investigate the evolutionary history of the mutualism. Although it was unknown whether the common ancestor of Cecropia was myremecophytic, the deepest split in the clade revealed ecological differences between the two oldest lineages of Cecropia sensu lato. The clade including C. sciadophylla and Musanga more likely had a non-myrmecophytic ancestor while myrmecophytism was most likely the ancestral state of the more species-rich Cecropia sensu stricto. Trichilia were associated with the origin and loss of ant associations where as domatia where not. Cecropia is distributed across a broad range of elevation and the absence of ant associations with high montane species was associated with the evolutionary loss of trichilia in two independent cases. A comparative analysis showed that gains and losses of myrmecophytism in Cecropia were correlated with the presence or absence of trichillia and domatia. Ant associations were more dependent on the presence of trichilia than on domatia. A resolved and highly supported phylogeny of over half the genus may be used to inform future ecological and evolutionary studies of the Cecropia-ant mutualism. A phylogenetic framework for Cecropia will allow for studies to take relatedness into account when comparing morphologogical traits of species. RADseq provided the data needed to begin to resolve relationships within Cecropia and may be what is necessary in other unresolved tropical tree genera. This will also allow for mutualism to be studied in a phylogenetic context where it previously was not possible.
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University of Minnesota Ph.D. dissertation. July 2017. Major: Plant Biological Sciences. Advisor: George Weiblen. 1 computer file (PDF); xii, 114 pages.
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Treiber, Erin. (2017). Phylogenetics of Cecropieae (Urticaceae) and the evolution of an ant-plant mutualism in Cecropia. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/206617.
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