Acquisition of oxygenic photosynthesis by eukaryotes occurred through a unique primary endosymbiosis of a cyanobacterium within a heterotrophic ancestor that resulted in the emergence of the three primary lineages known as the Archaeplastida consisting of the Rhodophyceae (red algae), Chloroplastida (green algae) and Glaucophyta (glaucophytes) Recently, a paradigm shift in the acquisition of photosynthesis was proposed: the implication of an intracellular obligate pathogen in plastid establishment. This hypothesis, dubbed the Ménage-à-trois Hypothesis (MATH), specifically addresses the central issue of disconnected supply and demand of carbon at the time of plastid endosymbiosis, suggesting an active and direct role of Chlamydiales in the success of primary endosymbiosis, which would have provided many critical genes to the cyanobiont hosted in a common vesicle known as the chlamydial inclusion. The expression and efficient localization of specific genes, such as key transporters and glucan transferases, would have initiated the biochemical fluxes of symbiosis. MATH is supported by molecular, biochemical and phylogenetic evidence but remains highly controversial. The major criticism concerns both the interpretation of single gene phylogenetic trees and the existence of other contributions assigned to different groups of bacteria thereby questioning a specific role for Chlamydiales in the endosymbiotic process. This work aims to test the Ménage à Trois Hypothesis by first evaluating the chlamydial footprint in the evolution of Archaeplastida and then comparing this signal to analogous contributions from other lineages of the bacterial domain. A bioinformatic pipeline was designed to identify all lateral gene transfer (LGT) events between Chlamydia and Archaeplastida, for which a manual analysis of the trees confirmed the occurrence very early during the endosymbiotic process. We then compared this chlamydial signal in the Archaeplastida to control signals, to ensure the specificity of both the bacterial donors and the eukaryotic acceptors involved in these LGT.