Retinoic acid (RA) is a terpenoid that is synthesized from Vitamin A/retinol (ROL) and binds to the nuclear receptors retinoic acid receptor (RAR)/retinoid X receptor (RXR) to control multiple developmental processes in vertebrates. The available clinic and experimental data provide uncontested evidence for the pleiotropic roles of RA signalling in development of multiple embryonic structures and organs such eyes, central nervous system, gonads, lungs and heart. The development of any of these above-mentioned embryonic organ systems can be effectively utilized to showcase the many strategies utilized by RA signalling. However, it is very likely that the strategies employed to transfer RA signals during cardiac development comprise the majority of the relevant and sophisticated ways through which retinoid signals can be conveyed in a complex biological system. Here, we provide the reader with arguments indicating that RA signalling is exquisitely regulated according to specific phases of cardiac development and that RA signalling itself is one of the major regulators of the timing of cardiac morphogenesis and differentiation. We will focus on the role of signalling by RA receptors (RARs) in early phases of heart development.

Elucidating cardiac evolution has been frustrated by lack of fossils. One celebrated enigma in cardiac evolution involves the transition from a cardiac outflow tract dominated by a multi-valved conus arteriosus in basal actinopterygians, to an outflow tract commanded by the non-valved, elastic, bulbus arteriosus in higher actinopterygians. We demonstrate that cardiac preservation is possible in the extinct fish Rhacolepis buccalis from the Brazilian Cretaceous. Using X-ray synchrotron microtomography, we show that Rhacolepis fossils display hearts with a conus arteriosus containing at least five valve rows. This represents a transitional morphology between the primitive, multivalvar, conal condition and the derived, monovalvar, bulbar state of the outflow tract in modern actinopterygians. Our data rescue a long-lost cardiac phenotype (119-113 Ma) and suggest that outflow tract simplification in actinopterygians is compatible with a gradual, rather than a drastic saltation event. Overall, our results demonstrate the feasibility of studying cardiac evolution in fossils.DOI: http://dx.doi.org/10.7554/eLife.14698.001

Precambrian cellular remains frequently have simple morphologies, micrometric dimensions and are poorly preserved, imposing severe analytical and interpretational challenges, especially for irrefutable attestations of biogenicity. The 1.88 Ga Gunflint biota is a Precambrian microfossil assemblage with different types and qualities of preservation across its numerous geological localities and provides important insights into the Proterozoic biosphere and taphonomic processes. Here we use synchrotronbased ptychographic X-ray computed tomography to investigate well-preserved carbonaceous microfossils from the Schreiber Beach locality as well as poorly-preserved, iron-replaced fossil filaments from the Mink Mountain locality, Gunflint Formation. 3D nanoscale imaging with contrast based on electron density allowed us to assess the morphology and carbonaceous composition of different specimens and identify the minerals associated with their preservation based on retrieved mass densities. In the Mink Mountain filaments, the identification of mature kerogen and maghemite rather than the ubiquitously described hematite indicates an influence from biogenic organics on the local maturation of iron oxides through diagenesis. This non-destructive 3D approach to microfossil composition at the nanoscale within their geological context represents a powerful approach to assess the taphonomy and biogenicity of challenging or poorly preserved traces of early microbial life, and may be applied effectively to extraterrestrial samples returned from upcoming space missions. Understanding Precambrian fossilized microorganisms, where preserved, can provide critical insights into the earliest records of life on Earth and its paleoenvironment 1-6 , especially in light of controversies surrounding the origin of chemical biosignatures such as isotopic fractionation 7,8 and biomolecules 9. Nonetheless, imaging the morphologies of these micrometric structures demands high spatial resolution, while the composition of

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-The secretions from the spermathecal glands in honeybee queens contribute to maintaining the viability of spermatozoa stored in the spermatheca. This viability is reduced as the queen ages. Here, we investigated whether DNA content changes and chromatin remodeling, that could suggest changes in cellular functions, occur in the spermathecal glands of Apis mellifera queens with aging. Feulgen-stained glands from early-mated and old queens were studied by image analysis. Increased polyploidization, heterogeneously distributed chromatin coarseness, and no cell death signs were verified in the spermathecal glands of aged queens. These results, compared with published data for other cellular systems, are suggestive of alterations in the spermathecal gland cells upon aging, possibly affecting their secretory fluid production. In consequence, sperm cell protection in the spermatheca would be less effective.queen / spermathecal glands / polyploidy / chromatin / aging

Montealtosuchus arrudacamposi, a Peirosauridae from the Upper Cretaceous of the Bauru Basin, was a Crocodyliformes of terrestrial habits. The fossils analyzed in this study belong to the pectoral girdle (scapula and coracoid) and anterior appendicular skeleton (humerus, radius, ulna, carpals, radiale, ulnale, metacarpals and phalanges). In this study we infer the locomotion habits of M. arrudacamposi. A morphometric, morphofunctional and 3D reconstruction of the elements of the pectoral girdle and the anterior limbs of M. arrudacamposi were performed. For a better understanding of the most plausible pectoral girdle and anterior limb posture, the studied bones were virtually disarticulated and articulated on 3D reconstruction. The herein results obtained indicate the structures present a relatively thin and elongated aspect, thus allowing an interpretation that M. arrudacamposi possessed more slender anterior limbs than living crocodyliforms. This condition allowed for an adducted stance and cursorial habits that would enable movement through terrestrial environments for prey searching.

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The biogenicity problem of geological materials is one of the most challenging ones in the field of paleo and astrobiology. As one goes deeper in time, the traces of life become feeble and ambiguous, blending with the surrounding geology. Well-preserved metasedimentary rocks from the Archaean are relatively rare, and in very few cases contain structures resembling biological traces or fossils. These putative biosignatures have been studied for decades and many biogenicity criteria have been developed, but there is still no consensus for many of the proposed structures. Synchrotron-based techniques, especially on new generation sources, have the potential for contributing to this field of research, providing high sensitivity and resolution that can be advantageous for different scientific problems. Exploring the X-ray and matter interactions on a range of geological materials can provide insights on morphology, elemental composition, oxidation states, crystalline structure, magnetic properties, and others, which can measurably contribute to the investigation of biogenicity of putative biosignatures. Here, we provide an overview of selected synchrotron-based techniques that have the potential to be applied in different types of questions on the study of biosignatures preserved in the geological record. The development of 3rd and recently 4th generation synchrotron sources will favor a deeper understanding of the earliest records of life on Earth and also bring up potential analytical approaches to be applied for the search of biosignatures in meteorites and samples returned from Mars in the near future.