Fins into limbs: Recent insights from sarcopterygian fish

Limbs with digits evolved as sarcopterygian fish transitioned to a terrestrial life, giving rise to modern tetrapods. Since the Devonian, most of the sarcopterygian fish diversity became extinct, with the only surviving representatives being two coelacanth and six lungfish species. As the sister group of tetrapods, sarcopterygian fish constitute the ideal models to address questions regarding the transition of vertebrates from water to land. However, distantly related yet experimentally amenable teleost fish species have instead become the organisms of choice for geneticists and developmental biologists. Comparative studies using teleosts, such as zebrafish, have greatly enriched our knowledge on the genetic and regulatory mechanisms underlying fin development. However, their highly derived fin anatomy provides limited insights on the origin of limbs and the teleost-specific whole-genome duplication represents a further complication to comparisons of gene function and regulation. In recent years, refined morphological and behavioral studies and access to lungfish embryos and availability of genetic resources have underscored the value of nontetrapod sarcopterygians as model organisms. Here we review recent studies using lungfishes and coelacanths that shed light on various aspects of the evolution of the tetrapod limb, including the origin of the tetrapod pelvis, limb musculature, digits, locomotion, and regenerative capacity.