Unusual Diversity of Myoglobin Genes in the Lungfish

Abstract

Myoglobin is a respiratory protein that serves as a model system in a variety of biological fields. Its main function is to deliver and store O in the heart and skeletal muscles, but myoglobin is also instrumental in homeostasis of nitric oxide (NO) and detoxification of reactive oxygen species (ROS). Almost every vertebrate harbors a single myoglobin gene; only some cyprinid fishes have two recently duplicated myoglobin genes. Here we show that the West African lungfish Protopterus annectens has at least seven distinct myoglobin genes (PanMb1-7), which diverged early in the evolution of lungfish and showed an enhanced evolutionary rate. These myoglobins are lungfish specific, and no other globin gene was found amplified. The myoglobins are differentially expressed in various lungfish tissues, and the brain is the main site of myoglobin expression. The typical myoglobin-containing tissues, the skeletal muscle and the heart, have much lower myoglobin mRNA levels. Muscle and heart express distinct myoglobins (PanMb1 and PanMb3, respectively). In cell culture, lungfish myoglobins improved cellular survival under hypoxia albeit with different efficiencies and reduced the production of reactive oxygen species. Only Mb2 and Mb6 enhanced the energy status of the cells. The unexpected diversity of myoglobin hints to a functional diversification of this gene: some myoglobins may have adapted to the O requirements of the specific tissue and help the lungfish to survive hypoxic periods; other myoglobins may have taken over the roles of neuroglobin and cytoglobin, which appear to be missing in the West African lungfish.

Discovery and evolution of novel hemerythrin genes in annelid worms

“…The presence of multiple copies of myoHr genes across the annelid phylogeny implies these proteins have undergone several instances of gene duplication during their evolution, as previously reported for other bacterial, archeal, and eukaryotic taxa [26]. Moreover, the unexpected diversity of myoHrs could be associated with functional diversification of this gene, as observed for myoglobins [44] and also for Hrs involved in heavy metal detoxification and aspects of innate immunity [45]. …”

Genetic and functional diversity of the multiple lungfish myoglobins

“…The transcriptomes of each lungfish species were assembled using the CLC Genomics Workbench, version 11.0.1 (Qiagen, Hilden, Germany) using default parameters. Mb cDNA sequences were identified employing BLAST searches, using the Mb cDNA sequences from P. annectens as queries. When required, the putative Mb sequences were re‐assembled from the Illumina reads.…”

“…The Mb amino acid sequences were included in an alignment of vertebrate Mbs used before (Table S2). A multiple sequence alignment was obtained with MAFFT using the L‐INS‐i method .…”

“…Moreover, lungfish harbour various specific morphological, physiological and molecular adaptations that allow them to tolerate hypoxic environments . Recent studies identified eight distinct myoglobin ( Mb ) genes in the West African lungfish ( Protopterus annectens ) . This diversity is unusual because most vertebrates harbour a single Mb in the haploid genomes .…”

“…Using quantitative real‐time reverse transcription PCR and RNA sequencing (RNA‐Seq), we found that the multiple Mb genes of the West African lungfish (PanMbs) are differentially expressed in tissues . Surprisingly, most of the PanMbs have the highest expression levels in the brain.…”