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

Identifying the phylogenomic underpinnings of specialized phenotypes that
fueled evolutionary transitions into new adaptive zones is central to
evolutionary biology. The order Eulipotyphla (e.g., moles, shrews, and
hedgehogs) is ideally suited to address this question as semi-fossorial,
fossorial, and semi-aquatic forms have repeatedly arisen from terrestrial
forbearers. However, our understanding of the ecomorphological pathways
leading to these diverse lifestyles has been confounded by a fragmentary
fossil record and potential morphological convergence. The net surface
charge of myoglobin (ZMb) is readily determined from its primary structure
and provides an objective target to map ancient evolutionary transitions
due to mechanistic linkages of ZMb with myoglobin concentration. Myoglobin
facilitates O2 storage and transport in muscle and its concentration is
sharply elevated in breath-hold divers relative to terrestrial mammals,
with fossorial and high-elevation species only showing minor increases.
Here we trace the evolution of ZMb to unravel the history of lifestyle
transitions in the clade containing the world’s smallest endothermic
divers. We first constructed a comprehensive phylogeny that resolved
previously intractable intra-family relationships, and confirmed that ZMb
accurately predicts aquatic habits within Eulipotyphla. Ancestral
reconstructions of ZMb, which included representatives from all seven
recognized semi-aquatic genera, provide key insights into the timing and
mode of adaptations that underpin the evolution of the diverse
ecomorphotypes within Eulipotyphla, and unambiguously revealed that
semi-aquatic lifestyles evolved twice in moles, and three times in shrews.
Our phylogenetically informed analysis supports ZMb as an effective tool
to trace ancient secondary aquatic transitions of mammals based on protein
sequence alone.