Retroposon Insertions within a Multispecies Coalescent Framework Suggest that Ratite Phylogeny is not in the ‘Anomaly Zone’
Authors: Springer Mark S. , Gatesy John 
Published: May 2019

Summary coalescence methods were developed to address the negative impacts of incomplete lineage sorting on species tree estimation with concatenation. Coalescence methods are statistically consistent if certain requirements are met including no intralocus recombination, neutral evolution, and no gene tree reconstruction error. However, the assumption of no intralocus recombination may not hold for many DNA sequence data sets, and neutral evolution is not the rule for genetic markers that are commonly employed in phylogenomic coalescence analyses. Most importantly, the assumption of no gene tree reconstruction error is routinely violated, especially for rapid radiations that are deep in the Tree of Life. With the sequencing of complete genomes and novel pipelines, phylogenetic analysis of retroposon insertions has emerged as a valuable alternative to sequence-based phylogenetic analysis. Retroposon insertions avoid or reduce several problems that beset analysis of sequence data with summary coalescence methods: 1) intralocus recombination is avoided because retroposon insertions are singular evolutionary events, 2) neutral evolution is approximated in many cases, and 3) gene tree reconstruction errors are rare because retroposons have low rates of homoplasy. However, the analysis of retroposons within a multispecies coalescent framework has not been realized. Here, we propose a simple workaround in which a retroposon insertion matrix is first transformed into a series of incompletely resolved gene trees. Next, the program ASTRAL is used to estimate a species tree in the statistically consistent framework of the multispecies coalescent. The inferred species tree includes support scores at all nodes and internal branch lengths in coalescent units. As a test case, we analyzed a retroposon dataset for palaeognath birds (ratites and tinamous) with ASTRAL and compared the resulting species tree to an MP-EST species tree for the same clade derived from thousands of sequence-based gene trees. The MP-EST species tree suggests an empirical case of the ‘anomaly zone’ with three very short internal branches at the base of Palaeognathae, and as predicted for anomaly zone conditions, the MP-EST species tree differs from the most common gene tree. Although identical in topology to the MP-EST tree, the ASTRAL species tree based on retroposons shows branch lengths that are much longer and incompatible with anomaly zone conditions. Simulation of gene trees from the retroposon-based species tree reveals that the most common gene tree matches the species tree. We contend that the wide discrepancies in branch lengths between sequence-based and retroposon-based species trees are explained by the greater accuracy of retroposon gene trees (bipartitions) relative to sequence-based gene trees. Coalescence analysis of retroposon data provides a promising alternative to the status quo by reducing gene tree reconstruction error that can have large impacts on both branch length estimates and evolutionary interpretations.