Development of the Pectoral Lobed Fin in the Australian Lungfish Neoceratodus forsteri

The evolutionary transition from paired fins to limbs involved the establishment of a set of limb muscles as an evolutionary novelty. In parallel, there was a change in the topography of the spinal nerves innervating appendicular muscles, so that distinct plexuses were formed at the bases of limbs. However, the key developmental changes that brought about this evolutionary novelty have remained elusive due to a lack of data on the development of lobed fins in sarcopterygian fishes. Here, we observed the development of the pectoral fin in the Australian lungfish Neoceratodus forsteri (Sarcopterygii) through synchrotron radiation X-ray microtomography. Neoceratodus forsteri is a key taxon for understanding the fin-to-limb transition due to its close phylogenetic relationships to tetrapods and well-developed lobed fins. At the onset of the fin bud in N. forsteri, there is no mesenchyme at the junction between the axial body wall and the fin bud, which corresponds to the embryonic position of the brachial plexus formed in the mesenchyme in tetrapods. Later, concurrent with the cartilage formation in the fin skeleton, the fin adductor and abductor muscles become differentiated within the surface ectoderm of the fin bud. Subsequently, the girdle muscle, which is homologous to the tetrapod serratus muscle, newly develops at the junction between the axial body wall and the fin. Our study suggests that the acquisition of embryonic mesenchyme at the junction between the axial body wall and the appendicular bud opened the door to the formation of the brachial plexus and the specialization of individual muscles in the lineage that gave rise to tetrapods.

Topological Analysis of the Brainstem of the Australian Lungfish Neoceratodus forsteri

This paper presents a survey of the cell masses in the brainstem of the Australian lungfish <i>Neoceratodus forsteri</i>, based on<i></i>transversely cut Bodian-stained serial sections, supplemented by immunohistochemical data from the recent literature. This study is intended to serve a double purpose. First it concludes and completes a series of publications on the structure of the brainstem in representative species of all groups of anamniote vertebrates. Within the framework of this comparative program the cell masses in the brainstem and their positional relations are analyzed in the light of the Herrick-Johnston concept, according to which the brainstem nuclei are arranged in four longitudinal, functional zones or columns, the boundaries of which are marked by ventricular sulci. The procedure employed in this analysis essentially involves two steps: first, the cell masses and large individual cells are projected upon the ventricular surface, and next, the ventricular surface is flattened out, that is, subjected to a one-to-one continuous topological transformation [J Comp Neurol. 1974;156:255–267]. The second purpose of the present paper is to complement our mapping of the longitudinal zonal arrangement of the cell masses in the brainstem of <i>Neoceratodus</i>with a subdivision in transversely oriented neural segments. Five longitudinal rhombencephalic sulci – the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, the sulcus intermedius dorsalis and the sulcus medianus superior – and four longitudinal mesencephalic sulci – the sulcus tegmentalis medialis, the sulcus tegmentalis lateralis, the sulcus subtectalis and the sulcus lateralis mesencephali – could be distinguished. Two obliquely oriented grooves, present in the isthmic region – the sulcus isthmi dorsalis and ventralis – deviate from the overall longitudinal pattern of the other sulci. Although in <i>Neoceratodus</i> most neuronal perikarya are situated within a diffuse periventricular gray, 45 cell masses could be delineated. Ten of these are primary efferent or motor nuclei, eight are primary afferent or sensory centers, six are considered to be components of the reticular formation and the remaining 21 may be interpreted as “relay” nuclei. The topological analysis showed that in most of the rhombencephalon the gray matter is arranged in four longitudinal zones or areas, termed area ventralis, area intermedioventralis, area intermediodorsalis and area dorsalis. The sulcus intermedius ventralis, the sulcus limitans, and the sulcus intermedius dorsalis mark the boundaries between these morphological entities. These longitudinal zones coincide largely, but not entirely, with the functional columns of Herrick and Johnston. The most obvious incongruity is that the area intermediodorsalis contains, in addition to the viscerosensory nucleus of the solitary tract, several general somatosensory and special somatosensory centers. The isthmus region does not exhibit a clear morphological zonal pattern. The mesencephalon is divisible into a ventral, primarily motor zone and a dorsal somatosensory zone. The boundary between these zones is marked by the sulcus tegmentalis lateralis, which may be considered as an isolated rostral extremity of the sulcus limitans. The results of this study are summarized in a “classical” topological map, as well as in a “modernized” version of this map, in which neuromere borders are indicated.

Sarcopterygian fin ontogeny elucidates the origin of hands with digits

How the hand and digits originated from fish fins during the Devonian fin-to-limb transition remains unsolved. Controversy in this conundrum stems from the scarcity of ontogenetic data from extant lobe-finned fishes. We report the patterning of an autopod-like domain by hoxa13 during fin development of the Australian lungfish, the most closely related extant fish relative of tetrapods. Differences from tetrapod limbs include the absence of digit-specific expansion of hoxd13 and hand2 and distal limitation of alx4 and pax9, which potentially evolved through an enhanced response to shh signaling in limbs. These developmental patterns indicate that the digit program originated in postaxial fin radials and later expanded anteriorly inside of a preexisting autopod-like domain during the evolution of limbs. Our findings provide a genetic framework for the transition of fins into limbs that supports the significance of classical models proposing a bending of the tetrapod metapterygial axis.

Recruitment failure in the Australian lungfish, Neoceratodus forsteri (Osteichthyes:Dipnoi), in south-east Queensland

Changes to the environment of the Australian lungfish, Neoceratodus forsteri, are associated with loss of recruitment of young lungfish to the adult population. Eggs laid by poorly fed adults lack volatile fatty acids and fail to develop normally. Problems in young fish first appeared in some specimens collected at Fernvale on the Brisbane River during a drought, when food supplies in the river began to fail. In 2016, after poor recruitment for several years, hatchlings from Lake Samsonvale were able to feed, and reached advanced stages in the laboratory, after a moderate amount of food for parent lungfish appeared in the lake during the summer before the 2016 spawning season. However, all died after 14 months. Lungs, intestines and nervous systems in the juveniles were anomalous, and would have precluded continued development in the wild. Survival of several young to juvenile stages in the laboratory does not mean that survival and recruitment to the adult population in the wild will follow.