Does the bowfin gas bladder represent an intermediate stage during the lung‐to‐gas bladder evolutionary transition?

Abstract

Whether phenotypic evolution occurs gradually through time has prompted the search for intermediate forms between the ancestral and derived states of morphological features, especially when there appears to be a discontinuous origin. The gas bladder, a derived character of the Actinopteri, is a modification of lungs, which characterize the common ancestor of bony vertebrates. While gas bladders and lungs are similar in many ways, the key morphological difference between these organs is the direction of budding from the foregut during development; essentially, the gas bladder buds dorsally and the lungs bud ventrally from the foregut. Did the shift from ventral lungs to dorsal gas bladder transition through a lateral‐budding stage? To answer this question, the precise location of budding during gas bladder development in bowfin, representing the sister lineage to teleosts, has been debated. In the early 20th‐century, it was suggested that the bowfin gas bladder buds laterally from the right wall of the foregut. We used nano‐CT scanning to visualize the early development of the bowfin gas bladder to verify the historical studies of gas bladder developmental morphology and determine whether the direction of gas bladder budding in bowfin could be intermediate between ventrally budding lungs and dorsally budding gas bladders. We found that the bowfin gas bladder buds dorsally from the anterior foregut; however, during early development, the posterior gas bladder twists right. As development progresses, the posterior, right‐hand twist becomes shallower, and the gas bladder itself shifts toward a mid‐dorsal position. The budding site is definitively dorsal, despite the temporary lateral twist of the posterior gas bladder.

Physical constraints on the positions and dimensions of the zebrafish swim bladder by surrounding bones

“…To explore the regulation of the size and position of organs by the surrounding environments in vertebrates, the swim bladder, which some argue should rather be called the gas bladder in terms of function (Facey et al, 2022), may provide a good model. The swim bladder is a gas-filled sac located in the dorsal portion of the body that teleost fish uniquely acquired from the primitive lung (Facey et al, 2022;Funk et al, 2021;Funk et al, 2020). The primary role of the swim bladder in most teleost fishes is controlling buoyancy to maintain the vertical position in aquatic conditions, although it has specialized other roles in respiration, sound production, and sound reception in some species (Burton and Burton, 2018;Cook et al, 2024;Facey et al, 2022;Popper and Fay, 1993;Seymour et al, 2004).…”

Spawning Behavior in a Non-Teleost Actinopterygian: Genetic Evidence for Both Monogamy and Polygamy in Amia

“…In the decade we have been visiting Oneida Lake, New York to search for nests of bowfins, now considered A. ocellicauda (Wright et al, 2022), to collect eggs for evolutionary developmental work (Cass et al, 2013;Funk et al, 2020aFunk et al, , 2020bFunk et al, , 2021, we have never observed spawning and we have rarely seen a female closely associated with a nest. Even so, the accessibility of males guarding eggs and fry on nests constructed in shallow water provided an opportunity to investigate mating in Amia with genetic data.…”