Potential horizontal gene transfer (HGT) of Desiccation Related Proteins in the lichen photobiont Trebouxia gelatinosa

Desiccation Related Proteins (DRPs), firstly described in desiccation tolerant plants, are present also in several unrelated bacterial and algal groups. Although their role is still unknown, their presence seems to confer an increased desiccation tolerance in some extremophile bacteria, including the Deinococcus/Thermus phylum. In this study we describe our findings on the DRPs present in the transcriptome of Trebouxia gelatinosa, a member of the most common lichen-forming genus of green algae. Thirteen sequences were classified as DRPs: they are characterized by a c.170 aa long ferritin-like domain (PF13668), followed by one or two domains of unknown function. This gene family has undergone relevant expansion in T. gelatinosa: in the majority of green algae DRPs are present in few copies or they are completely absent. Further, their diversification finds no parallelism in other desiccation tolerant organisms investigated so far. Bayesian phylogenetic inference pointed out that DRPs of green algae are unlikely to be orthologous to those found in Embryophyta. Conversely, they share an unexpected sequence similarity to DRPs found in bacteria. This result led us to consider a bacterial origin for Trebouxia DRP genes, which may have ancestrally been acquired by horizontal gene transfer (HGT) from lichen-associated bacteria.