Myoglobin primary structure reveals multiple convergent transitions to semi-aquatic life in the world's smallest mammalian divers

The speciose mammalian order Eulipotyphla (moles, shrews, hedgehogs, solenodons) combines an unusual diversity of semi-aquatic, semi-fossorial, and fossorial forms that arose from terrestrial forbearers. However, our understanding of the ecomorphological pathways leading to these lifestyles has been confounded by a fragmentary fossil record, unresolved phylogenetic relationships, and potential morphological convergence, calling for novel approaches. The net surface charge of the oxygen-storing muscle protein myoglobin (ZMb), which can be readily determined from its primary structure, provides an objective target to address this question due to mechanistic linkages with myoglobin concentration. Here, we generate a comprehensive 71 species molecular phylogeny that resolves previously intractable intra-family relationships and then ancestrally reconstruct ZMb evolution to identify ancient lifestyle transitions based on protein sequence alone. Our phylogenetically informed analyses confidently resolve fossorial habits having evolved twice in talpid moles and reveal five independent secondary aquatic transitions in the order housing the world’s smallest endothermic divers.
eLife digest The shrews, moles and hedgehogs that surround us all belong to the same large group of insect-eating mammals. While most members in this ‘Eulipotyphla order’ trot on land, some, like moles, have evolved to hunt their prey underground. A few species, such as the water shrews, have even ventured to adopt a semi-aquatic lifestyle, diving into ponds and streams to retrieve insects. These underwater foragers share unique challenges, burning a lot of energy and losing heat at a high rate while not being able to store much oxygen. It is still unclear how these semi-aquatic habits have come to be: the fossil record is fragmented and several species tend to display the same adaptations even though they have evolved separately. This makes it difficult to identify when and how many times the Eulipotyphla species started to inhabit water. The protein myoglobin, which gives muscles their red color, could help in this effort. This molecule helps muscles to capture oxygen from blood, a necessary step for cells to obtain energy. Penguins, seals and whales, which dive to get their food, often have much higher concentration of myoglobin so they can spend extended amount of time without having to surface for air. In addition, previous work has shown that eight groups of mammalian divers carry genetic changes that help newly synthetized myoglobin proteins to not stick to each other. This means that these animals can store more of the molecule in their muscles, increasing their oxygen intake and delivery. He et al. therefore speculated that all semi-aquatic Eulipotyphla species would carry genetic changes that made their myoglobin less likely to clump together; underground species, which also benefit from absorbing more oxygen, would display intermediate alterations. In addition, reconstructing the myoglobin sequences from the ancestors of living species would help to spot when the transition to aquatic life took place. A variety of approaches were harnessed to obtain myoglobin and other sequences from 55 eulipotyphlan mammals, which then were used to construct a strongly supported family tree for this group. The myoglobin results revealed that from terrestrial to subterranean to semi-aquatic species, genetic changes took place that would diminish the ability for the proteins to stick to each other. This pattern also showed that semi-aquatic lifestyles have independently evolved five separate times – twice in moles, three times in shrews. By retracing the evolutionary history of specific myoglobin properties, He et al. shed light on how one of the largest orders of mammals has come to be fantastically diverse.