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Commentary: Plastid establishment did not require a chlamydial partner

S. Ball,D. Bhattacharya,H. Qiu,A. Weber

Published 2016 in Frontiers in Cellular and Infection Microbiology

ABSTRACT

Several groups have independently proposed an active role for Chlamydiales in primary plastid establishment in Archaeplastida (Huang and Gogarten, 2007; Becker et al., 2008; Moustafa et al., 2008). We relied on a combination of biochemical and phylogenetic evidence to erect the MAT (Menage a Trois) hypothesis (Ball et al., 2013; Facchinelli et al., 2013). Under this scenario, Chlamydiales sheltered the once free-living cyanobacterial plastid ancestor from host defenses and provided critical components such as carbohydrate transporters and protein effectors that allowed the storage of exported carbohydrates into host glycogen pools. A recent paper by Domman et al. (2015) reassessed the phylogenies published by us and others on these components. These authors applied evolutionary models that better account for across-site and across-branch sequence compositional variation (i.e., Bayesian approaches with the CAT family of evolutionary models Lartillot and Philippe, 2004) to reanalyze proteins involved in glycogen metabolism. These are either chlamydial effectors (GlgC, ADP-glucose pyrophosphorylase; GlgP, glycogen phosphorylase; GlgX, glycogen debranching enzyme; and GlgA, glycogen synthase) or chlamydial transporters (UhpC, G6P import protein). Previous trees often used automated phylogenomic pipelines that relied on single-matrix (usually best-fit) substitution models (e.g., LG, WAG) that could potentially provide incorrect inference due to rate heterogeneity across sites (Morgan et al., 2013). Based on their results, the authors (Domman et al., 2015) argued that GlgC and GlgP now show evidence of being of cyanobacterial and host origin, respectively, that Chlamydiales and Archaeplastida are united by the LGT (Lateral Gene Transfer) of GlgX but that the direction of transfer from chlamydiales to Archaeplastida is no longer clear. Furthermore, our hypothesis that chlamydiales have provided GlgA and UhpC to the Archaeplastida is now in question. Below, we inspect these issues in detail.

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