Oviductal fluid counterbalances the negative effect of high temperature on sperm in an ectotherm model.

Global warming is affecting biodiversity; however, the extent to which animal reproductive processes respond to predicted temperature increments remains largely unexplored. The thermal environment has a pronounced impact on metabolic rates of ectotherms; therefore, an interesting question to assess is whether temperature increase might affect specific reproductive mechanisms like sperm performance in ectotherms. Moreover, in many species, oviductal fluid (OF) is known to regulate and maintain sperm quality; however, the role of OF in relation to the effects of high temperature on sperm remains unclear. Our aim was to experimentally test the effect of increased temperature on sperm velocity, swimming path and percentage of motility in neutral conditions at ejaculation (without OF) and in female's reproductive tract fluid (with OF), in a social ectotherm lizard model, Tropidurus spinulosus, which has specific thermal requirements for reproduction. Our results suggest that a rising temperature associated with global warming (+4°C) affects negatively sperm dynamics and survival. However, OF ameliorated the harmful effects of high temperature. This is an important point, as this study is the first to have tested the role of OF in preserving sperm from a warmer pre-fertilization environment. These results contribute to our understanding of how thermal environment changes might affect post-copulatory reproductive mechanisms. This article has an associated First Person interview with the first author of the paper.

Developing a sense of scents: plasticity in olfactory placode formation.

The sense organs of the vertebrate head arise predominantly from sensory placodes. The sensory placodes have traditionally been grouped as structures that share common developmental and evolutionary characteristics. In attempts to build a coherent model for development of all placodes, the fascinating differences that make placodes unique are often overlooked. Here I review olfactory placode development with special attention to the origin and cell movements that generate the olfactory placode, the derivatives of this sensory placode, and the degree to which it shows plasticity during development. Next, through comparison with adenohypophyseal, and lens placodes I suggest we revise our thinking and terminology for these anterior placodes, specifically by: (1) referring to the peripheral olfactory sensory system as neural ectoderm because it expresses the same series of genes involved in neural differentiation and differentiates in tandem with the olfactory bulb, and (2) grouping the anterior placodes with their corresponding central nervous system structures and emphasizing patterning mechanisms shared between placodes and these targets. Sensory systems did not arise independent of the central nervous system; they are part of a functional unit composed of peripheral sensory structures and their targets. By expanding our analyses of sensory system development to also include cell movements, gene expression and morphological changes observed in this functional unit, we will better understand the evolution of sensory structures.

A novel function for the Xslug gene: control of dorsal mesendoderm development by repressing BMP-4.

The Snail family of genes comprise a group of transcription factors with characteristic zinc finger motifs. One of the members of this family is the Slug gene. Slug has been implicated in the development of neural crest in chick and Xenopus by antisense loss of function experiments. Here, we have generated functional derivatives of Xslug by constructing cDNAs that encode the Xslug protein fused with the transactivation domain of the virus-derived VP16 activator or with the repressor domain of the Drosophila Engrailed protein. Our results suggest that Xslug normally functions as a transcriptional repressor and that Xslug-VP16 behaves as a dominant negative of Xslug. In the present work, we confirm and extend previous results that suggest that Xslug has an important function in neural crest development, by controlling its own transcription. In addition we have uncovered a new function for Xslug. We show that Xslug is expressed in the dorsal mesendoderm at the beginning of gastrulation, where is it able to upregulate the expression of dorsal genes. On the other hand when Xslug is expressed outside of the organizer it represses the expression of ventral genes. Our results indicate that this effect on mesodermal patterning depends on BMP activity, showing that Xslug can directly control the transcription of BMP-4.

Neural crest formation in Xenopus laevis: mechanisms of Xslug induction.

A study of the induction of the prospective neural crest in Xenopus laevis embryos has been carried out, using the expression of Xslug as a specific marker for the neural crest. We have analyzed the competence and the specification of the neural crest. The competence to express Xslug was analyzed using two different approaches: (1) in vitro culture of conjugates of dorsal mesoderm and ectoderm taken from embryos at different ages and (2) grafts of equivalent pieces of ectoderm in the neural fold region of a gastrula or a neurula. Similar results were obtained with both methods: the ectoderm loses the competence to respond to neural fold induction at the end of gastrulation. Neural crest specification was analyzed by culturing a region of the ectoderm that contained the prospective neural crest and analyzing Xslug expression. Our results show that neural folds are specified autonomously to express Xslug by the end of gastrulation. By grafting labeled neural plate into lateral epidermis we have shown that neural crest can be induced by an interaction between neural plate and epidermis. Furthermore, neural crest cells come from both tissues. We have discarded the possibility that these neural crest cells are induced by a signal coming from the underlying lateral plate, by a homeogenetic signal coming from the host neural plate, or by regeneration of crest cells from the dissected neural plate. We propose a model to explain how the neural crest cells are induced at the border of the neural plate in X. laevis.

Ectoderm promotes differentiation of the presumptive ventral mesoderm in early gastrulae of Bufo arenarum.

Cultures of both isolated and conjugated explants from early gastrulae of Bufo arenarum were prepared for a study of the development of ventral mesoderm. Only combinations including components of the deep ventral marginal zone and the animal pole successfully differentiated into blood cells (erythrocytes). Histological studies indicated that, while prospective mesodermal cells constituted the only source of such cells, prospective ectodermal cells provided the necessary stimulus for this kind of differentiation. Differentiated cultures, in which the tracer of cell-lineage fluorescein dextran amine was used to label these components, confirmed the above conclusions. These findings are discussed in the context of current concepts about the formation of mesoderm.

Mechanisms underlying the organizer formation in Bufo arenarum embryos.

In the early gastrula of Bufo arenarum the prospective mesoderm was previously identified as a marginal belt of grey cells. To analyze their differentiation capacity explants of these cells were cultured within ectodermal vesicles, in isolation and in combination with vegetal components. When cultured in isolation, dorsal and ventral fragments from the deep marginal zone behaved differently. Whilst ventral explants produced blood cells, dorsal explants failed to differentiate, remaining as masses of yolk-laden cells. On the other hand, both cultures were drastically modified when associated with superficial cells from the blastoporal zone, which caused the following effects: a) Promotion of differentiation in dorsal marginal explants, able now to produce notochordal and somitic structures, in addition to mesenchymatic cells. b) Promotion of dorsalization in ventral marginal explants, which changed their expected destiny developing axial components, similar to those furnished by "activated" dorso marginal explants. On the contrary, combined cultures of animal and vegetal pieces were unable to generate mesodermal structures. These studies suggest that the axial mesoderm, identified as the "organizer", develops from a marginal substrate of genuine mesodermal cells through a dorsalizing inductive stimulus originated in superficial periblastoporal cells.