Horizontal gene transfer is the movement of genetic material between two organisms that are not parent and offspring. It serves an important evolutionary function because the genetic material transferred can confer new traits or functions that change the recipient’s fitness in a heritable fashion. While the phenomenon is frequent between both mitochondrial and nuclear genomes, there has never been a documented, unequivocal case of horizontal gene transfer between plastid genomes in plants.
In a new article published in Annals of Botany, lead author Lars Hedenäs and colleagues sequenced ITS (nuclear) and rpl16 (plastid) sequences to track DNA movement between two distantly-related but co-occurring moss species, Bryum pseudotriquetrum (the donor) and Scorpidium cossonii (the recipient). In cases where horizontal gene transfer was suspected, further marker sequences were sequenced to exclude the possibility of whole genome transfer.
The authors found 14 individuals that possessed the expected nuclear sequences, but foreign plastid sequences. In all cases, the transfer was ascertained to be partial, rather than consisting of the entire plastid genome. The divergence time of the two moss species, between 165 and 185 million years ago, makes explanations such as hybridization or incomplete lineage sorting unlikely.
The exchanges occurred in acidic mires in Sweden that had recently undergone large-scale liming to counteract the acidification, as a conservation measure. As such, they presented expansive, open surfaces ripe for colonization by wind-dispersed spores and seeds. The two moss species often co-occur in this type of habitat. Individuals growing in chemically similar natural fens did not show evidence of horizontal gene transfer.
Mosses may be more susceptible to horizontal gene transfer because of the way their life cycle works. “[T]he colonization phase in combination with unique features of the moss life cycle are key for interspecific plastid HGT [horizontal gene transfer] to occur,” write the authors. “Following spore germination, mosses form a (usually) filamentous and chlorophyllose protonema, from which the green ‘moss plants’ are produced. Early succession habitats, after disturbance, may be reached first by easily wind-dispersed spores, and developing protonemata filaments of different species can grow intimately mixed during massive colonization events.” The exact mechanism of transfer has yet to be determined.
This phenomenon presents a special concern for conservationists, since liming and peat extraction could inadvertently trigger wide-scale genetic modification of a common moss species.
Update 17:11: Title corrected from plasmid to plastid.