A nuclear phylogenomic study of the angiosperm order Myrtales, exploring the potential and limitations of the universal Angiosperms353 probe set

PREMISE

To further advance the understanding of the species-rich, economically and ecologically important angiosperm order Myrtales in the rosid clade, comprising nine families, approximately 400 genera and almost 14,000 species occurring on all continents (except Antarctica), we tested the Angiosperms353 probe kit.

METHODS

We combined high-throughput sequencing and target enrichment with the Angiosperms353 probe kit to evaluate a sample of 485 species across 305 genera (76% of all genera in the order).

RESULTS

Results provide the most comprehensive phylogenetic hypothesis for the order to date. Relationships at all ranks, such as the relationship of the early-diverging families, often reflect previous studies, but gene conflict is evident, and relationships previously found to be uncertain often remain so. Technical considerations for processing HTS data are also discussed.

CONCLUSIONS

High-throughput sequencing and the Angiosperms353 probe kit are powerful tools for phylogenomic analysis, but better understanding of the genetic data available is required to identify genes and gene trees that account for likely incomplete lineage sorting and/or hybridization events.

Evolutionary diversification of new Caledonian Araucaria

New Caledonia is a global biodiversity hotspot. Hypotheses for its biotic richness suggest either that the island is a 'museum' for an old Gondwana biota or alternatively it has developed following relatively recent long distance dispersal and in situ radiation. The conifer genus Araucaria (Araucariaceae) comprises 19 species globally with 13 endemic to this island. With a typically Gondwanan distribution, Araucaria is particularly well suited to testing alternative biogeographic hypotheses concerning the origins of New Caledonian biota. We derived phylogenetic estimates using 11 plastid and rDNA ITS2 sequence data for a complete sampling of Araucaria (including multiple accessions of each of the 13 New Caledonian Araucaria species). In addition, we developed a dataset comprising 4 plastid regions for a wider taxon sample to facilitate fossil based molecular dating. Following statistical analyses to identify a credible and internally consistent set of fossil constraints, divergence times estimated using a Bayesian relaxed clock approach were contrasted with geological scenarios to explore the biogeographic history of Araucaria. The phylogenetic data resolve relationships within Araucariaceae and among the main lineages in Araucaria, but provide limited resolution within the monophyletic New Caledonian species group. Divergence time estimates suggest a Late Cretaceous-Cenozoic radiation of extant Araucaria and a Neogene radiation of the New Caledonian lineage. A molecular timescale for the evolution of Araucariaceae supports a relatively recent radiation, and suggests that earlier (pre-Cenozoic) fossil types assigned to Araucaria may have affinities elsewhere in Araucariaceae. While additional data will be required to adequately resolve relationships among the New Caledonian species, their recent origin is consistent with overwater dispersal following Eocene emersion of New Caledonia but is too old to support a single dispersal from Australia to Norfolk Island for the radiation of the Pacific Araucaria sect. Eutacta clade.

Structural partitioning, paired-sites models and evolution of the ITS transcript in Syzygium and Myrtaceae

The internal transcribed spacers (ITS) of nuclear ribosomal DNA are widely used for phylogenetic inference. Several characteristics, including the influence of RNA secondary structure on the mutational dynamics of ITS, may impact on the accuracy of phylogenies estimated from these regions. Here, we develop RNA secondary structure predictions for representatives of the angiosperm family Myrtaceae. On this basis, we assess the utility of structural (stem vs. loop) partitioning, and RNA-specific (paired-sites) models for a 76 taxon Syzygium alignment, and for a broader, family-wide Myrtaceae ITS data set. We use a permutation approach to demonstrate that structural partitioning significantly improves the likelihood of the data. Similarly, models that account for the non-independence of stem-pairs in RNA structure have a higher likelihood than those that do not. The best-fit RNA models for ITS are those that exclude simultaneous double substitutions in stem-pairs, which suggests an absence of strong selection against non-canonical (G.U/U.G) base-pairs at a high proportion of stem-paired sites. We apply the RNA-specific models to the phylogeny of Syzygium and Myrtaceae and contrast these with hypotheses derived using standard 4-state models. There is little practical difference amongst relationships inferred for Syzygium although for Myrtaceae, there are several differences. The RNA-specific approach finds topologies that are less resolved but are more consistent with conventional views of myrtaceous relationships, compared with the 4-state models.