Biogeography and evolution of the largest adaptive radiation of woody plants (Melicope, Rutaceae) on the Hawaiian Islands
Final Report Abstract
Previous studies have shown that phylogenetic studies based on Sanger Sequencing have largely failed to resolve species-rich and recent radiations. This has been observed in many studies based on Hawaiian plant lineages including Melicope (Rutaceae). With about 54 species, Melicope ranks among the largest plant radiations in the islands and is the largest radiation of trees and shrubs. In order to untangle phylogenetic relationships at and below the species level, this project focused on the High-throughput sequencing method RADseq. During the library preparation, the DNA is digested with a restriction enzyme and Illumina adapters are ligated to the restriction sites, so that these are the starting point of sequencing, resulting in sequencing of homologous regions in closely related taxa. RADseq was performed based on 101 specimens representing 41 out of the 54 species (of which 5 are considered to be extinct) and the raw sequence reads were assembled to up to 59,041 RAD loci. The resulting phylogenies were largely resolved and supported, and most species sampled from multiple accessions were resolved as monophyletic. The Hawaiian Melicope lineage is subdivided into five main clades, which match only partly with the subsections defined based on morphology. The genus Platydesma was confirmed to belong to Melicope. The seven Melicope species endemic to the remote Marquesas Islands are derived from Hawaiian ancestry and are the result of two independent colonization events from Hawaii to the Marquesas Islands. The Hawaiian Islands are often considered a dead-end for dispersal for most taxa, and Melicope is the only known example of a plant genus, that colonized the Marquesas Islands twice from the Hawaiian Islands. Many Hawaiian plant species are polyploids and/or hybrids. While our Flow Cytometric results show a very uniform pattern, suggesting the same ploidy for the whole lineage, analyses of the RADseq data (partitioned d-statistics, Quartet Sampling) identified a past hybridization event involving the ancestors of two main lineages and maybe between two closely related species for which morphological intermediate specimens are known. Quartet Sampling identified several nodes in one main clade, in which hybridization may have occurred. An increased taxon sampling at the population level is needed to further explore putative hybridization in this clade. We performed molecular dating analyses using BEAST and our estimates suggest a Late Miocene origin of Hawaiian Melicope. The lineage is thus one of only few examples of Hawaiian taxa that are older than the current main islands. The time before the formation of the current main islands have often been regarded as a bottleneck for dispersal. Despite the relatively old age of the lineage, most of the current species diversity evolved in the past 1MA and we hypothesize that substantial extinction of Hawaiian Melicope species happened since the establishment of Melicope on the archipelago and that the current diversity is the result of recent divergence of the few lineages (the five main clades) that prevailed on the islands. The age of the two lineages that colonized the Marquesas Islands was estimated to the Mid Pleistocene and both are presumably derived from a Kauaian ancestor. Due to decreasing prices for Illumina sequencing, we were able to include samples of the Myrsine (Primulaceae), which represent the second largest Hawaiian lineage of trees and shrubs, in a separate study. We performed RADseq for 29 samples of Myrsine, which represent 18 out of 20 currently accepted Hawaiian species, and two samples of Ardisia as outgroups. As in Melicope, RADseq resolved the phylogeny of the lineage with high support. The lineage consists of three clades of which one is largely confined to Kauaʻi and contains the majority of the species diversity. The other two clades consist of three widespread species in which several single island endemics are nested. The widespread and morphologically highly diverse species Myrsine lessertiana was shown to polyphyletic and a species new to science was identified. Like in the Melicope study, test to identify hybridization were carried out (Quartet Sampling, HyDe, Bayes Factors through stepping stone analyses). These analyses revealed strong cases of phylogenetic incongruence, which are likely the result of continuous hybridization in all clades. Almost all species have been identified as parental species and hybrids with high or intermediate levels of admixture.
Publications
- 2017. Reduction of the Hawaiian genus Platydesma into Melicope section Pelea (Rutaceae) and notes on the monophyly of the section. Phytokeys 91: 125-137
Appelhans MS, Wood KR, Wagner WL
(See online at https://doi.org/10.3897/phytokeys.91.21363) - 2018. The odd one out or a hidden generalist: Hawaiian Melicope (Rutaceae) do not share traits associated with successful island colonization. Journal of Systematics and Evolution 56: 621-636
Paetzold C, Kiehn M, Wood KR, Wagner WL, Appelhans MS
(See online at https://doi.org/10.1111/jse.12454) - 2019. Phylogeny of Hawaiian Melicope (Rutaceae): RAD-seq resolves species relationships and reveals ancient introgression. Frontiers in Plant Science 10: 1074
Paetzold C, Wood KR, Eaton DAR, Wagner WL, Appelhans MS
(See online at https://doi.org/10.3389/fpls.2019.01074)
