A case of mitochondrial capture in jacamars.
I guess you have probably heard the phrase ‘Mitochondria are the powerhouse of the cell.’ And indeed they are, these small organelles generate the energy (in the form of ATP) that keeps our cells going. Inside these sausage-shaped structures lies a circular piece of DNA – mitochondrial DNA or mtDNA – that became the most popular molecular marker of the 1990s (thanks to John Avise).
When you construct a phylogenetic tree based on mtDNA, chances are that it will differ from tree based on nuclear DNA. This phenomenon – known as mitonuclear discordance – has troubled biologists for decades, because several biological processes can explain this pattern. You can check out this recent paper by Timothée Bonnet and colleagues for an overview, but I would like to focus on one of these processes: mitochondrial capture.
When two species are hybridizing , mtDNA can be exchanged. In the simplest case you have two variants – or haplotypes – of mtDNA, one for each species. If one variant is superior over the other (i.e. better adapted to local conditions), it will increase in frequency until all individuals of both species share the same haplotype. In technical terms, we say that the variant went to fixation. At this point, one species has captured the mtDNA of the other: mitochondrial capture!
This process probably occurred between species of jacamar (genus Galbula) in the Amazon region. Mateus Ferreira and his colleagues collected samples of two species: bronzy jacamar (G. leucogastra) and purplish jacamar (G. chalcothorax). Next, they sequenced the DNA of these birds and constructed phylogenetic trees for mtDNA and nuclear DNA. As you might have guessed, the trees looked different. The nuclear genes nicely separated the species into two distinct groups, while the mtDNA mixed the two species. Specifically, G. leucogastra specimens from the Madeira clade clustered together with their geographical neighbours G. chalcothorax. The authors state:
Our hypothesis for this incongruence is that an ancient event of hybridization between G. chalcothorax and the Madeira lineage of G. leucogastra caused the introgression of the Madeira lineage mtDNA into the G. chalcothorax lineage, replacing its “original” mtDNA lineage. This mitochondrial capture may have been influenced by the populational and ecological context of differentiation within WSE.
What is this WSE they mention? WSE stands for white-sand ecosystems, a unique type of habitat within Amazonia which consists of patches of ‘differentiated habitats scattered across the landscape and isolated by the forest matrix.’ Surprisingly, the different patches have distinct geological origins. In the northeast, these patches are the result of podzolization, in which nutrients are leached away from the top layers of soil, leaving only sand. In other regions, the white sands are the result of river deposits.
The distribution of these patches within a forest matrix resembles an island setting. And researchers have been curious how this peculiar environment has shaped the animals that live in it. The geological and climatic history of the white sands might have facilitated contact between the jacamar species at some point, leading to the capture of mtDNA. The authors could not reconstruct the exact scenario, but I bet it will be a ‘captiviting’ story when they figure it out.
Ferreira, M.; Fernandes, A.M., Aleixo, A., Antonelli, A., Olsson, U., Bates, J.M., Cracraft, J. & Riba, C.C. (2018) Evidence for mtDNA capture in the jacamar Galbula leucogastra / chalcothor-ax species-complex and insights on the evolution of white-sand ecosystems in the Amazon basin. Molecular Phylogenetics and Evolution
The paper has been added to the Piciformes page.