Annual killifish adaptations to ephemeral environments: diapause I in two Austrolebias species

Background: Many organisms are able to survive in extreme environments by entering a state of dormancy. In dormancy, vital activities are reduced until environmental conditions are compatible with active life. Annual killifishes show a special developmental pattern characterized by a phase of dispersion-reaggregation of the blastomeres that separates epiboly from organogenesis, and the capability to enter dormancy in diapause. High tolerance to environmental stress confers annual killifish embryos the condition of extremophiles. At present, the questions of our research group are focused on the understanding of the mechanisms involved in diapause regulation through an interdisciplinary approach. As a first step, it is necessary to characterize diapauses at orphological and physiological levels and to evaluate induction cues under laboratory conditions. In this context, we characterized diapause I in two Austrolebias species. Results: Our experimental approach to induce apause I was successful and revealed the co-existence of two diapause I phenotypes named A and B instead of one. These phenotypes showed a tendency for lower total extractable RNA content compared with active developmental stages (80–100% epiboly and early reaggregate). Conclusions: These phenotypes are alternative diapause I stages and may have ecological relevance because both were found in embryos in natural ponds. Developmental Dynamics 246:848–857, 2017. VC 2017 Wiley Periodicals, Inc.CSIC: 147_C61034

Sex differentiation pattern in the annual fish Austrolebias charrua (Cyprinodontiformes: Rivulidae)

Sex differentiation process, determination of sexual strategy, and gametogenesis of the annual fish Austrolebias charrua are established. Evidence of histological sex differentiation in an antero-posterior gradient was observed in pre-hatching stages. Sexual strategy corresponds to the “differentiated gonochoric” pattern. Histological analyses of adult gonads showed an asynchronous spawning mode for females and continuous spawning for males. Mature oocytes presented fluid yolk. Testis organization corresponded to a restricted spermatogonial model. Herein, we report the ultrastructural organization of the vitelline envelope and the main features of the sperm of A. charrua. Taking together these results also contribute to phylogenetic studies and provide base line data to propose A. charrua as a biomonitor of contamination in a protected are

Plasticity of cell proliferation in the retina of Austrolebias charrua fish under light and darkness conditions

Austrolebias annual fishes exhibit cell proliferation and neurogenesis throughout life. They withstand extreme environmental changes as their habitat dries out, pressuring nervous system to adapt. Their visual system is challenged to adjust as the water becomes turbid. Therefore, this study focused on how change in photic envi- ronment can lead to an increased cell proliferation in the retina. We administered 5-chloro-2′- deoxyuridine (CldU) and 5-iodo-2′-deoxyuridine (IdU) at different temporal windows to detect cell proliferation in natural light and permanent darkness. Stem/progenitor cells were recognized as IdU+/CldU+ nuclei co-labeled with Sox2, Pax6 or BLBP found in the ciliary marginal zone (CMZ). The expression pattern of BLBP + glial cells and ultrastructural analysis indicates that CMZ has different cell progenitors. In darkness, the number of dividing cells significantly increased, compared to light conditions. Surprisingly, CMZ IdU+/CldU + cell number was similar under light and darkness, suggesting a stable pool of stem/progenitor cells possibly responsible for retinal growth. Therefore, darkness stimulated cell progenitors outside the CMZ, where Müller glia play a crucial role to generate rod precursors and other cell types that might integrate rod-dependent circuits to allow darkness adaptation. Thus, the Austrolebias fish retina shows great plasticity, with cell proliferation rates significantly higher than that of brain visual areas

First complete mitochondrial genome of the South American annual fish Austrolebias charrua (Cyprinodontiformes: Rivulidae): peculiar features among cyprinodontiforms mitogenomes

Selected nucleotide substitution models after the third codon positions were removed from the codon alignments. (PDF 7 kb

The Neotropical Genus \u3cem\u3eAustrolebias\u3c/em\u3e: An Emerging Model of Annual Killifishes

Annual fishes are found in both Africa and South America occupying ephemeral ponds that dried seasonally. Neotropical annual fishes are members of the family Rivulidae that consist of both annual and non-annual fishes. Annual species are characterized by a prolonged embryonic development and a relatively short adult life. Males and females show striking sexual dimorphisms, complex courtship, and mating behaviors. The prolonged embryonic stage has several traits including embryos that are resistant to desiccation and undergo up to three reversible developmental arrests until hatching. These unique developmental adaptations are closely related to the annual fish life cycle and are the key to the survival of the species. Most of the available data on Neotropical annual fishes derive from studies on the genus Austrolebias. Herein, we review and summarize the available data on the evolution, reproduction strategy, developmental biology and conservation status of these Neotropical fishes

Caracterización histólogica de adaptaciones de la retina de peces anuales Austrolebias charrúa, en relaciones a cambios en la luminosidad en su ciclo de vida

Si bien la retina posee una citoarquitectura conservada en los vertebrados, es sabido que sus características dependen de la especie y de las condiciones lumínicas del entorno. Austrolebias charrua es un pez anual que vive en charcos donde la luminosidad varía drásticamente en su corto ciclo de vida. En este trabajo se aplicaron las técnicas de: Cajal de Castro,Inmunohistoquímica, marcado con un trazador neuronal (DiI) y Microscopía Electrónica de Transmisión (MET) con el fin de identificar posibles características de la retina que le permitan adaptarse a un medio lumínico cambiante. La técnica Cajal de Castro y la Inmunohistoquímica, revelaron la composición celular y los grosores de las capas de la retina, característicos de peces con hábitos diurnos. Sin embargo, la capa de células ganglionares mostró una organización en grupos celulares separados por haces axonales, disposición descrita en la retina de peces nocturnos. La MET nos proporcionó detalles ultraestructurales de los fotorreceptores. En particular se evidenció la distribución de los melanosomas del epitelio pigmentario, que se encuentran distribuidos tanto a lo largo de los segmentos externos e internos de los fotorreceptores como a nivel perinuclear de las células del epitelio pigmentario. Los animales adaptados a la luz presentan una distribución de melanosomas a lo largo de los segmentos de los fotorreceptores, mientras que en los adaptados a la oscuridad los pigmentos se ubican en la zona perinuclear. Este resultado nos permite proponer que A. charrua posee características histomorfológicas que le permiten adaptarse a un medio de luz variable

Transcriptome-based SNP discovery and validation in the hybrid zone of the neotropical annual fish genus Austrolebias

The genus Austrolebias (Cyprinodontiformes: Rivulidae) represents a specious group of taxa following annual life cycles in the neotropical ichthyofauna. They live in temporary ponds and each generation must be completed in a few months, depending on environmental stochasticity. Annual fish survive the dry season through diapausing eggs buried in the substrate of these ponds. A hypothesized bimodal hybrid zone between two taxa of the genus, A. charrua and A. reicherti from Dos Patos Merin lagoon system, was recently proposed based on genetics and morphological analyses. However, hundreds of additional nuclear molecular markers should be used to strongly support this hypothesized bimodal pattern. In the present paper, we conducted RNA-seq-based sequencing of the transcriptomes from pools of individuals of A. charrua, A. reicherti and their putative natural hybrids from the previously characterized hybrid zone. As a result, we identified a set of 111,725 SNP (single nucleotide polymorphism) markers, representing presumably fixed allelic di erences among the two species. The present study provided the first panel of 106 SNP markers as a single diagnostic multiplex assay and validated their capacity to reconstruct the patterns of the hybrid zone between both taxa. These nuclear markers combined with Cytb gene and morphological analyses detected a population structure in which some groups among the hybrid swarms showed di erent level of introgression towards one or the other parental species according to their geographic distribution. High-quality transcriptomes and a large set of gene-linked SNPs should greatly facilitate functional and population genomics studies in the hybrid zone of these endangered species

Embryonic Development of Natural Annual Killifish populations of the genus Austrolebias: Evolutionary parallelism and the role of environment

Repeated, independent emergence of the same trait within different phylogenetic lineages is termed parallel evolution. It typically occurs as a result of similar selective pressures. Annual killifish have adapted to survive in the extreme habitat of temporary pools on three continents and present an especially amenable system for studying fundamental principles of evolutionary parallelism. When the pools dry, annual killifish embryos survive through the dry phase in the bottom substrate in a stage of dormancy—a diapause. The diapause is a complex set of three different developmental stages, none of which is obligate, thus leading to a multitude of potential developmental trajectories. While the intricacy of the killifishes\u27 embryonic development has been thoroughly studied in the laboratory, information on their natural development is virtually absent. We hypothesised that the natural development of annual killifishes is largely synchronised and governed by ambient conditions as shown in the lineage of the African genus Nothobranchius. We sampled wild embryo banks of the South American genus Austrolebias, which evolved its diapause independently of the African lineage. We sampled during two consecutive dry seasons, using both longitudinal and snapshot monitoring, and conducted transplant experiments to determine the extent of the evolutionary parallelism and role of the environment in Austrolebias spp. embryo development. Main habitat phases were characterised by largely synchronised embryo banks. Different inter-seasonal or local environmental conditions were reflected in a different developmental profile of the embryo banks, suggesting a high degree of environmental control. We found striking similarity in the habitat phase–embryo stage associations between the two lineages. The diapause in the two annual killifish lineages represents a unique example of evolutionary parallelism, with the analogy manifested in very close detail. We highlight the similarity of the selective forces in the two genera despite the different geographic origins, climate zones and reversed seasonality. The repeatedly occurring strict association of the same developmental stages with the same habitat conditions suggests a limited array of developmental settings that can be applied to cope with the given environmental challenges

Viviparity Stimulates Diversification in an Order of Fish

International audienceSpecies richness is distributed unevenly across the tree of life and this may be influenced by the evolution of novel phenotypes that promote diversification. Viviparity has originated B150 times in vertebrates and is considered to be an adaptation to highly variable environments. Likewise, possessing an annual life cycle is common in plants and insects, where it enables the colonization of seasonal environments, but rare in vertebrates. The extent to which these reproductive life-history traits have enhanced diversification and their relative importance in the process remains unknown. We show that convergent evolution of viviparity causes bursts of diversification in fish. We built a phylogenetic tree for Cyprino-dontiformes, an order in which both annualism and viviparity have arisen, and reveal that while both traits have evolved multiple times, only viviparity played a major role in shaping the patterns of diversity. These results demonstrate that changes in reproductive life-history strategy can stimulate diversification