The Mexican Tetra, Astyanax mexicanus, as a Model System in Cell and Developmental Biology

Differential expression of sex regulatory genes in gonads of Astyanax mexicanus surface fish and cavefish

BACKGROUND

Astyanax mexicanus is a single species of fish that consists of river-dwelling (surface) and cave-dwelling morphotypes. Little is known about how sexual determination, differentiation or reproduction have evolved in the surface morphs or cavefish, though divergence in reproductive strategy is expected as the latter have adapted to the novel cave environment. Evolution of the gonad transcriptome may underlie the differences in gamete morphology, fertility, and fecundity previously reported between morphotypes.

RESULTS

We compared the ovary and testis transcriptome of surface fish and cavefish at juvenile and adult stages. We found that samples clustered by developmental stage, sex, and morphotype identity. Several key genes that are typically associated with the female gonad in other vertebrates showed a reversal in sexual dimorphism or were not differentially expressed between sexes in A. mexicanus. In contrast, gene expression typically associated with male gonads was largely conserved and consistent with vertebrate testicular expression profiles. Transcriptional and physiological differences between surface fish and cavefish morphotypes were observed in gonads from both sexes. Cavefish ovaries exhibited unique upregulation of neuron development and differentiation genes, and extensive innervation of the ovarian epithelium, while cavefish testes showed increased expression of angiogenesis regulating genes, and greater vasculature density compared to surface fish testes.

CONCLUSIONS

These results reveal significant gene expression differences between A. mexicanus surface fish and cavefish morphotypes that may have functional consequences in gonad morphogenesis and fertility. Our findings provide a foundation for investigating the evolution of sex regulatory pathways and reproductive strategies in animals adapting to new and challenging environments in which nutrient availability, temperature, and mate selection are suboptimal.

Genomic and cellular contributions to establish the fish Hyphessobrycon heterorhabdus as an Amazonian model for ecotoxicology

The Amazon has environmental characteristics that make it unique, and the species that inhabit it have physiological features that demonstrate their adaptation to an Amazonian context. Amazonian ecosystems have been undergoing transformations that result from inadequate human actions, which have placed biodiversity at the limits of their biological abilities. Therefore, it is essential to identify organisms that reflect the conditions of the Amazon environment, considering the physiology of these species, and that they are established as bioindicators for this region. The fish Hyphessobrycon heterorhabdus is distributed throughout the lower and middle Amazon basin. Aiming to contribute to the establishment of this species as a bioindicator, we carried out for the first time its karyotyping, Illumina sequencing and assembly of the nuclear genome and mitogenome, in addition to establishing a cell line for this species. Individuals of H. heterorhabdus have 48 chromosomes, a number that is held in the cell line. Sequencing and genomic assembly generated a draft genome that is useful for recovering species-specific coding and regulatory genomic sequences. The established cell line was responsive to environmental variables, reflecting in vivo observations. Thus, it was possible to present a set of resources (standard karyotype, centromeric marker, draft genome, mitogenome, and cell line) and their applications and relevance in ecotoxicology. The toolkit should contribute to further studies on the response of organisms to the natural conditions of the Amazon and the impacts that this region has been receiving.

Hypoxia-sonic hedgehog axis as a driver of primitive hematopoiesis development and evolution in cavefish

The teleost Astyanax mexicanus consists of surface dwelling (surface fish) and cave dwelling (cavefish) forms. Cavefish have evolved in subterranean habitats characterized by reduced oxygen levels (hypoxia) and exhibit a subset of phenotypic traits controlled by increased Sonic hedgehog (Shh) signaling along the embryonic midline. The enhancement of primitive hematopoietic domains, which are formed bilaterally in the anterior and posterior lateral plate mesoderm, are responsible for the development of more larval erythrocytes in cavefish relative to surface fish. In this study, we determine the role of hypoxia and Shh signaling in the development and evolution of primitive hematopoiesis in cavefish. We show that hypoxia treatment during embryogenesis increases primitive hematopoiesis and erythrocyte development in surface fish. We also demonstrate that upregulation of Shh midline signaling by the Smoothened agonist SAG increases primitive hematopoiesis and erythrocyte development in surface fish, whereas Shh downregulation via treatment with the Smoothened inhibitor cyclopamine decreases these traits in cavefish. Together these results suggest that hematopoietic enhancement is regulated by hypoxia and Shh signaling. Lastly, we demonstrate that hypoxia enhances expression of Shh signaling along the midline of surface fish embryos. We conclude that hypoxia-mediated Shh plasticity may be a driving force for the adaptive evolution of primitive hematopoiesis and erythrocyte development in cavefish.

Hypoxia-Sonic Hedgehog Axis as a Driver of Primitive Hematopoiesis Development and Evolution in Cavefish

The teleost Astyanax mexicanus consists of surface dwelling (surface fish) and cave dwelling (cavefish) forms. Cavefish have evolved in subterranean habitats characterized by reduced oxygen levels (hypoxia) and show constructive and regressive phenotypic traits controlled by increased Sonic hedgehog (Shh) signaling along the embryonic midline. The enhancement of primitive hematopoietic domains, which are formed bilaterally in the anterior and posterior lateral plate mesoderm, are responsible for the development of more larval erythrocytes in cavefish relative to surface fish. In this study, we determine the role of hypoxia and Shh signaling in the development and evolution of primitive hematopoiesis in cavefish. We show that hypoxia treatment during embryogenesis increases primitive hematopoiesis and erythrocyte development in surface fish. We also demonstrate that upregulation of Shh midline signaling by treatment with the Smoothened agonist SAG increases primitive hematopoiesis and erythrocyte development in surface fish, whereas Shh downregulation via treatment with the Smoothened inhibitor cyclopamine decreases these traits in cavefish. Together these results suggest that hematopoietic enhancement is regulated by hypoxia and the Shh signaling system. Lastly, we demonstrate that hypoxia treatment enhances expression of Shh signaling along the midline of surface fish embryos. Thus, we conclude that a hypoxia-Shh axis may drive the adaptive evolution of primitive hematopoiesis and erythrocyte development in cavefish.

Highlights

Hypoxia increases hematopoiesis and erythrocytes in surface fishShh upregulation increases hematopoiesis and erythrocytes in surface fishShh inhibition decreases hematopoiesis and erythrocytes in cavefishHypoxia upregulates Shh along the embryonic midline in surface fish.