Salamander-like tail regeneration in the West African lungfish.

Salamanders, frog tadpoles and diverse lizards have the remarkable ability to regenerate tails. Palaeontological data suggest that this capacity is plesiomorphic, yet when the developmental and genetic architecture of tail regeneration arose is poorly understood. Here, we show morphological and molecular hallmarks of tetrapod tail regeneration in the West African lungfish Protopterus annectens, a living representative of the sister group of tetrapods. As in salamanders, lungfish tail regeneration occurs via the formation of a proliferative blastema and restores original structures, including muscle, skeleton and spinal cord. In contrast with lizards and similar to salamanders and frogs, lungfish regenerate spinal cord neurons and reconstitute dorsoventral patterning of the tail. Similar to salamander and frog tadpoles, Shh is required for lungfish tail regeneration. Through RNA-seq analysis of uninjured and regenerating tail blastema, we show that the genetic programme deployed during lungfish tail regeneration maintains extensive overlap with that of tetrapods, with the upregulation of genes and signalling pathways previously implicated in amphibian and lizard tail regeneration. Furthermore, the lungfish tail blastema showed marked upregulation of genes encoding post-transcriptional RNA processing components and transposon-derived genes. Our results show that the developmental processes and genetic programme of tetrapod tail regeneration were present at least near the base of the sarcopterygian clade and establish the lungfish as a valuable research system for regenerative biology.

Globin E is a myoglobin-related, respiratory protein highly expressed in lungfish oocytes.

Globins are a classical model system for the studies of protein evolution and function. Recent studies have shown that - besides the well-known haemoglobin and myoglobin - additional globin-types occur in vertebrates that serve different functions. Globin E (GbE) was originally identified as an eye-specific protein of birds that is distantly related to myoglobin. GbE is also present in turtles and the coelacanth but appeared to have been lost in other vertebrates. Here, we show that GbE additionally occurs in lungfish, the closest living relatives of the tetrapods. Each lungfish species harbours multiple (≥5) GbE gene copies. Surprisingly, GbE is exclusively and highly expressed in oocytes, with mRNA levels that exceed that of myoglobin in the heart. Thus, GbE is the first known oocyte-specific globin in vertebrates. No GbE transcripts were found in the ovary or egg transcriptomes of other vertebrates, suggesting a lungfish-specific function. Spectroscopic analysis and kinetic studies of recombinant GbE1 of the South American lungfish Lepidosiren paradoxa revealed a typical pentacoordinate globin with myoglobin-like O2-binding kinetics, indicating similar functions. Our findings suggest that the multiple copies of GbE evolved to enhance O2-supply in the developing embryo of lungfish, analogous to the embryonic and fetal haemoglobins of other vertebrates. In evolution, GbE must have changed its expression site from oocytes to eyes, or vice versa.