The complicated evolution of methionine adenosyltransferase in euglenids and eukaryotes in general

Many eukaryotic genes do not follow vertical inheritance pattern. In the present work, we have chosen as a model the gene for methionine adenosyltransferase (MAT), in which we have decided to examine in detail the evolutionary history. MAT is a ubiquitous essential enzyme that, in eukaryotes, occurs in two relatively divergent paralogs: MAT and MATX. Both paralogs have punctate distributions across the tree of eukaryotes and, except for a few cases, they are mutually exclusive. This points to the complicated evolutionary history of this gene couple, which may be caused by either differential loss of old paralogs or the spread of one of these paralogs by horizontal gene transfer (HGT). We have focused on the evolution of this enzyme particularly within one of the best-known groups of flagellates, the euglenids, because it was hypothesized that MATX evolved in photosynthetic euglenids before it spread to other lineages. We gained 26 new sequences from 23 euglenid lineages and one prasinophyte alga Pyramimonas parkeae. MATX was found only in photoautotrophic euglenids. Both, mixotroph Rapaza viridis and the prasinophyte alga Pyramimonas parkeae, the closest known relative of the euglenid plastid ancestor, only displayed the MAT paralog. In contrast, both paralogues were found in two euglenid species..