When Outgroups Fail; Phylogenomics of Rooting the Emerging Pathogen, Coxiella burnetii

Abstract

Rooting phylogenies is critical for understanding evolution, yet the importance, intricacies and difficulties of rooting are often overlooked. For rooting, polymorphic characters among the group of interest (ingroup) must be compared to those of a relative (outgroup) that diverged before the last common ancestor (LCA) of the ingroup. Problems arise if an outgroup does not exist, is unknown, or is so distant that few characters are shared, in which case duplicated genes originating before the LCA can be used as proxy outgroups to root diverse phylogenies. Here, we describe a genome-wide expansion of this technique that can be used to solve problems at the other end of the evolutionary scale: where ingroup individuals are all very closely related to each other, but the next closest relative is very distant. We used shared orthologous single nucleotide polymorphisms (SNPs) from 10 whole genome sequences of Coxiella burnetii, the causative agent of Q fever in humans, to create a robust, but unrooted phylogeny. To maximize the number of characters informative about the rooting, we searched entire genomes for polymorphic duplicated regions where orthologs of each paralog could be identified so that the paralogs could be used to root the tree. Recent radiations, such as those of emerging pathogens, often pose rooting challenges due to a lack of ingroup variation and large genomic differences with known outgroups. Using a phylogenomic approach, we created a robust, rooted phylogeny for C. burnetii. [Coxiella burnetii; paralog SNPs; pathogen evolution; phylogeny; recent radiation; root; rooting using duplicated genes.]

Genotyping and Axenic Growth ofCoxiella burnetiiIsolates Found in the United States Environment

“…2) (Pearson et al 2013). Isolates that grow well in ACCM-2 are on one of the main branches of the tree, whereas the isolates that do not grow well are on the other branch.…”

Coxiella burnetiiin Central Italy: Novel Genotypes Are Circulating in Cattle and Goats

“…2013). The tree was drawn according to Hornsta et al (2011) and rooted according to Pearson et al (2013).…”

High prevalence and two dominant host-specific genotypes of Coxiella burnetii in U.S. milk

“…In such clonal organisms with no evidence of lateral gene transfer [22], a single SNP allele can accurately define a lineage, allowing for a small subset of loci to be used for genotyping [20,23,24]. PCR assays using TaqMan chemistry have been shown to approach the theoretical minimum level of detection [24,25] and for C. burnetii , sensitive detection assays have been developed and used to gauge environmental prevalence.…”