Revealing the mechanisms of the rostral shift of pelvic fins among teleost fishes

Heterochronic development of pelvic fins in zebrafish: possible involvement of temporal regulation of pitx1 expression

Anterior and posterior paired appendages of vertebrates are notable examples of heterochrony in the relative timing of their development. In teleosts, posterior paired appendages (pelvic fin buds) emerge much later than their anterior paired appendages (pectoral fin buds). Pelvic fin buds of zebrafish (Danio rerio) appear at 3 weeks post-fertilization (wpf) during the larva-to-juvenile transition (metamorphosis), whereas pectoral fin buds arise from the lateral plate mesoderm on the yolk surface at the embryonic stage. Here we explored the mechanism by which presumptive pelvic fin cells maintain their fate, which is determined at the embryonic stage, until the onset of metamorphosis. Expression analysis revealed that transcripts of pitx1, one of the key factors for the development of posterior paired appendages, became briefly detectable in the posterior lateral plate mesoderm at early embryonic stages. Further analysis indicated that the pelvic fin-specific pitx1 enhancer was in the poised state at the larval stage and is activated at the juvenile stage. We discuss the implications of these findings for the heterochronic development of pelvic fin buds.

Hand2 delineates mesothelium progenitors and is reactivated in mesothelioma

The mesothelium lines body cavities and surrounds internal organs, widely contributing to homeostasis and regeneration. Mesothelium disruptions cause visceral anomalies and mesothelioma tumors. Nonetheless, the embryonic emergence of mesothelia remains incompletely understood. Here, we track mesothelial origins in the lateral plate mesoderm (LPM) using zebrafish. Single-cell transcriptomics uncovers a post-gastrulation gene expression signature centered on hand2 in distinct LPM progenitor cells. We map mesothelial progenitors to lateral-most, hand2-expressing LPM and confirm conservation in mouse. Time-lapse imaging of zebrafish hand2 reporter embryos captures mesothelium formation including pericardium, visceral, and parietal peritoneum. We find primordial germ cells migrate with the forming mesothelium as ventral migration boundary. Functionally, hand2 loss disrupts mesothelium formation with reduced progenitor cells and perturbed migration. In mouse and human mesothelioma, we document expression of LPM-associated transcription factors including Hand2, suggesting re-initiation of a developmental program. Our data connects mesothelium development to Hand2, expanding our understanding of mesothelial pathologies.

The skeletal ontogeny of Astatotilapia burtoni - a direct-developing model system for the evolution and development of the teleost body plan

Background

The experimental approach to the evolution and development of the vertebrate skeleton has to a large extent relied on “direct-developing” amniote model organisms, such as the mouse and the chicken. These organisms can however only be partially informative where it concerns secondarily lost features or anatomical novelties not present in their lineages. The widely used anamniotes Xenopus and zebrafish are “indirect-developing” organisms that proceed through an extended time as free-living larvae, before adopting many aspects of their adult morphology, complicating experiments at these stages, and increasing the risk for lethal pleiotropic effects using genetic strategies.

Results

Here, we provide a detailed description of the development of the osteology of the African mouthbrooding cichlid Astatotilapia burtoni, primarily focusing on the trunk (spinal column, ribs and epicentrals) and the appendicular skeleton (pectoral, pelvic, dorsal, anal, caudal fins and scales), and to a lesser extent on the cranium. We show that this species has an extremely “direct” mode of development, attains an adult body plan within 2 weeks after fertilization while living off its yolk supply only, and does not pass through a prolonged larval period.

Conclusions

As husbandry of this species is easy, generation time is short, and the species is amenable to genetic targeting strategies through microinjection, we suggest that the use of this direct-developing cichlid will provide a valuable model system for the study of the vertebrate body plan, particularly where it concerns the evolution and development of fish or teleost specific traits. Based on our results we comment on the development of the homocercal caudal fin, on shared ontogenetic patterns between pectoral and pelvic girdles, and on the evolution of fin spines as novelty in acanthomorph fishes. We discuss the differences between “direct” and “indirect” developing actinopterygians using a comparison between zebrafish and A. burtoni development.

Developmental Mechanism of Limb Field Specification along the Anterior-Posterior Axis during Vertebrate Evolution

In gnathostomes, limb buds arise from the lateral plate mesoderm at discrete positions along the body axis. Specification of these limb-forming fields can be subdivided into several steps. The lateral plate mesoderm is regionalized into the anterior lateral plate mesoderm (ALPM; cardiac mesoderm) and the posterior lateral plate mesoderm (PLPM). Subsequently, Hox genes appear in a nested fashion in the PLPM and provide positional information along the body axis. The lateral plate mesoderm then splits into the somatic and splanchnic layers. In the somatic layer of the PLPM, the expression of limb initiation genes appears in the limb-forming region, leading to limb bud initiation. Furthermore, past and current work in limbless amphioxus and lampreys suggests that evolutionary changes in developmental programs occurred during the acquisition of paired fins during vertebrate evolution. This review presents these recent advances and discusses the mechanisms of limb field specification during development and evolution, with a focus on the role of Hox genes in this process.

Tetrapod axial evolution and developmental constraints; Empirical underpinning by a mouse model

The tetrapod vertebral column has become increasingly complex during evolution as an adaptation to a terrestrial life. At the same time, the evolution of the vertebral formula became subject to developmental constraints acting on the size of the cervical and thoraco-lumbar regions. In the course of our studies concerning the evolution of Hox gene regulation, we produced a transgenic mouse model expressing fish Hox genes, which displayed a reduced number of thoraco-lumbar vertebrae and concurrent sacral homeotic transformations. Here, we analyze this mutant stock and conclude that the ancestral, pre-tetrapodial Hox code already possessed the capacity to induce vertebrae with sacral characteristics. This suggests that alterations in the interpretation of the Hox code may have participated to the evolution of this region in tetrapods, along with potential modifications of the HOX proteins themselves. With its reduced vertebral number, this mouse stock violates a previously described developmental constraint, which applies to the thoraco-lumbar region. The resulting offset between motor neuron morphology, vertebral patterning and the relative positioning of hind limbs illustrates that the precise orchestration of the Hox-clock in parallel with other ontogenetic pathways places constraints on the evolvability of the body plan.

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A transgenic mouse line expressing fish Hox genes has anterior homeotic transformations.

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Fish Hox genes are capable of inducing tetrapod specific vertebral characters.

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A sacral Hox-code influences adult hindlimb position, yet not the position of limb budding.

Anterior migration of lateral plate mesodermal cells during embryogenesis of the pufferfish Takifugu niphobles: insight into the rostral positioning of pelvic fins

In vertebrates, paired appendages (limbs and fins) are derived from the somatic mesoderm subsequent to the separation of the lateral plate mesoderm into somatic and splanchnic layers. This is less clear for teleosts, however, because the developmental processes of separation into two layers and of extension over the yolk have rarely been studied. During teleost evolution, the position of pelvic fins has generally shifted rostrally (Rosen; Nelson, 1982, 1994), although at the early embryonic stage the presumptive pelvic fin cells are initially located near the future anus region - the anterior border of hoxc10a expression in the spinal cord - regardless of their final destination. Our previous studies in zebrafish (abdominal pelvic fins) and Nile tilapia (thoracic pelvic fins) showed that the presumptive pelvic fin cells shift their position with respect to the body trunk after its protrusion from the yolk surface. Furthermore, in Nile tilapia, presumptive pelvic fin cells migrate anteriorly on the yolk surface. Here, we examined the embryonic development of the lateral plate mesoderm at histological levels in the pufferfish Takifugu niphobles, which belongs to the highly derived teleost order Tetraodontiformes, and lacks pelvic fins. Our results show that, in T. niphobles, the lateral plate mesoderm bulges out as two separate layers of cells alongside the body trunk prior to its further extension to cover the yolk sphere. Once the lateral plate mesoderm extends laterally, it rapidly covers the surface of the yolk. Furthermore, cells located near the anterior border of hoxc10a expression in the spinal cord reach the anterior-most region of the yolk surface. In light of our previous and current studies, we propose that anterior migration of presumptive pelvic fin cells might be required for them to reach the thoracic or more anterior positions as is seen in other highly derived teleost groups.

Development of the lateral plate mesoderm in medaka Oryzias latipes and Nile tilapia Oreochromis niloticus: insight into the diversification of pelvic fin position

The position of the pelvic fins among teleost fishes has tended to shift rostrally during evolution. This positional shift seems to have led to the diversification of feeding behavior and allowed adaptation to new environments. To understand the developmental basis of this shift in pelvic fin position among teleosts, we investigated the embryonic development of the lateral plate mesoderm, which gives rise to the pelvic fins, at histological levels in the medaka Oryzias latipes (abdominal pelvic fins) and Nile tilapia Oreochromis niloticus (thoracic pelvic fins). Our histological analyses revealed that the lateral plate mesodermal cells expand not only ventrally but also rostrally to cover the yolk during embryogenesis of both medaka and Nile tilapia. In medaka, we also found that the lateral plate mesoderm completely covered the yolk prior to the initiation of the pelvic fin buds, whereas in Nile tilapia the pelvic fin buds appeared in the body wall from the lateral plate mesoderm at the thoracic level when the lateral plate mesodermal cells only covered one-third of the yolk. We discuss the relevance of such differences in the rate of the lateral plate mesoderm expansion on the yolk surface and the position of the pelvic fins.

Molecular and evolutionary basis of limb field specification and limb initiation

Specification of limb field and initiation of limb development involve multiple steps, each of which is tightly regulated both spatially and temporally. Recent developmental analyses on various vertebrates have provided insights into the molecular mechanisms that specify limb field and have revealed several genetic interactions of signals involved in limb initiation processes. Furthermore, new approaches to the study of the developmental mechanisms of the lateral plate mesoderm of amphioxus and lamprey embryos have given us clues to understand the evolutionary scenarios that led to the acquisition of paired appendages during evolution. This review highlights such recent findings and discusses the mechanisms of limb field specification and limb bud initiation during development and evolution.