Microbiota and Diapause-Induced Neuroprotection Share a Dependency on Calcium But Differ in Their Effects on Mitochondrial Morphology.

The balance between the degeneration and regeneration of damaged neurons depends on intrinsic and environmental variables. In nematodes, neuronal degeneration can be reversed by intestinal GABA and lactate-producing bacteria, or by hibernation driven by food deprivation. However, it is not known whether these neuroprotective interventions share common pathways to drive regenerative outcomes. Using a well established neuronal degeneration model in the touch circuit of the bacterivore nematode Caenorhabditis elegans, we investigate the mechanistic commonalities between neuroprotection offered by the gut microbiota and hunger-induced diapause. Using transcriptomics approaches coupled to reverse genetics, we identify genes that are necessary for neuroprotection conferred by the microbiota. Some of these genes establish links between the microbiota and calcium homeostasis, diapause entry, and neuronal function and development. We find that extracellular calcium as well as mitochondrial MCU-1 and reticular SCA-1 calcium transporters are needed for neuroprotection by bacteria and by diapause entry. While the benefits exerted by neuroprotective bacteria require mitochondrial function, the diet itself does not affect mitochondrial size. In contrast, diapause increases both the number and length of mitochondria. These results suggest that metabolically induced neuronal protection may occur via multiple mechanisms.

How maternal age and environmental cues influence embryonic developmental pathways and diapause dynamics in a North American annual killifish.

BACKGROUND: Temporary pools are variable environments with seasonal drought/flood phases. Annual killifish have adapted to life in temporary pools by producing embryos that undergo diapause to traverse the dry phase. To fill existing knowledge gaps about embryo diapause regulation and evolution in annual killifishes, we test the effect of maternal age, incubation temperature, and incubation medium on diapause induction and length in Millerichthys robustus, the only North American fish species that has evolved an annual life history. RESULTS: All embryos at extreme temperatures follow a defined developmental pathway: skipping diapause at 30°C, and entering diapause at 18°C, both regardless of maternal age, and incubation medium. However, maternal age, and incubation medium influenced whether diapause is entered, and time arrested in diapause for embryos incubated at 25°C. At 25°C, five-week-old, and 52-week-old females produced more embryos that entered diapause than 26-week-old females. Also, embryos incubated in aqueous medium skipped diapause more frequently at this intermediate temperature. CONCLUSIONS: Millerichthys developmental dynamics associated with maternal age under intermediate range of temperatures are likely adapted to the particular patterns of flood/drought in North American temporary pools. Millerichthys also exhibits developmental patterns largely comparable with other annual fishes, probably due to common seasonal patterns in temporary pools.

Persistence of the invasive bird-parasitic fly Philornis downsi over the host interbreeding period in the Galapagos Islands.

Many parasites of seasonally available hosts must persist through times of the year when hosts are unavailable. In tropical environments, host availability is often linked to rainfall, and adaptations of parasites to dry periods remain understudied. The bird-parasitic fly Philornis downsi has invaded the Galapagos Islands and is causing high mortality of Darwin's finches and other bird species, and the mechanisms by which it was able to invade the islands are of great interest to conservationists. In the dry lowlands, this fly persists over a seven-month cool season when availability of hosts is very limited. We tested the hypothesis that adult flies could survive from one bird-breeding season until the next by using a pterin-based age-grading method to estimate the age of P. downsi captured during and between bird-breeding seasons. This study showed that significantly older flies were present towards the end of the cool season, with ~ 5% of captured females exhibiting estimated ages greater than seven months. However, younger flies also occurred during the cool season suggesting that some fly reproduction occurs when host availability is low. We discuss the possible ecological mechanisms that could allow for such a mixed strategy.

Embryonic developmental arrest in the annual killifish Austrolebias charrua: A proteomic approach to diapause III.

Diapause is a reversible developmental arrest faced by many organisms in harsh environments. Annual killifish present this mechanism in three possible stages of development. Killifish are freshwater teleosts from Africa and America that live in ephemeral ponds, which dry up in the dry season. The juvenile and adult populations die, and the embryos remain buried in the bottom mud until the next rainy season. Thus, species survival is entirely embryo-dependent, and they are perhaps the most remarkable extremophile organisms among vertebrates. The aim of the present study was to gather information about embryonic diapauses with the use of a "shotgun" proteomics approach in diapause III and prehatching Austrolebias charrua embryos. Our results provide insight into the molecular mechanisms of diapause III. Data are available via ProteomeXchange with identifier PXD025196. We detected a diapause-dependent change in a large group of proteins involved in different functions, such as metabolic pathways and stress tolerance, as well as proteins related to DNA repair and epigenetic modifications. Furthermore, we observed a diapause-associated switch in cytoskeletal proteins. This first glance into global protein expression differences between prehatching and diapause III could provide clues regarding the induction/maintenance of this developmental arrest in A. charrua embryos. There appears to be no single mechanism underlying diapause and the present data expand our knowledge of the molecular basis of diapause regulation. This information will be useful for future comparative approaches among different diapauses in annual killifish and/or other organisms that experience developmental arrest.

Diapause induces functional axonal regeneration after necrotic insult in C. elegans.

Many neurons are unable to regenerate after damage. The ability to regenerate after an insult depends on life stage, neuronal subtype, intrinsic and extrinsic factors. C. elegans is a powerful model to test the genetic and environmental factors that affect axonal regeneration after damage, since its axons can regenerate after neuronal insult. Here we demonstrate that diapause promotes the complete morphological regeneration of truncated touch receptor neuron (TRN) axons expressing a neurotoxic MEC-4(d) DEG/ENaC channel. Truncated axons of different lengths were repaired during diapause and we observed potent axonal regrowth from somas alone. Complete morphological regeneration depends on DLK-1 but neuronal sprouting and outgrowth is DLK-1 independent. We show that TRN regeneration is fully functional since animals regain their ability to respond to mechanical stimulation. Thus, diapause induced regeneration provides a simple model of complete axonal regeneration which will greatly facilitate the study of environmental and genetic factors affecting the rate at which neurons die.

Polarization Microscopy and Infrared Microspectroscopy of Integument Coverings of Diapausing Larvae in Two Distantly Related Nonsocial Bees.

The larvae of the two distantly related nonsocial bees Ericrocis lata (Apidae) and Hesperapis (Carinapis) rhodocerata (Melittidae), which develop mostly under arid desert areas of North America, and that differ in that they either spin (E. lata) or do not spin (H. rhodocerata) protective cocoons before entering diapause, produce transparent films that cover the larval integument. To understand the nature of these films, their responses to topochemical tests and their characteristics when examined with fluorescence and high-performance polarization microscopy and microspectroscopy were studied. A positive staining by Sudan black B, birefringence of negative sign, and a Fourier transform-infrared (FT-IR) spectrum typical of lipids were detected for the integument covering of both species. The FT-IR signature, particularly, suggests a wax chemical composition for these lipid coverings, resembling the waxes that are used as construction materials in the honey cells produced by social bees. Considering the arid environmental conditions under which these larvae develop, we hypothesize that their covering films may have evolved as protection against water depletion. This hypothesis seems especially appropriate for H. rhodocerata larvae, which are capable of undergoing a long diapause period in the absence of a protective cocoon.

Diapause and quiescence: dormancy mechanisms that contribute to the geographical expansion of mosquitoes and their evolutionary success.

Mosquitoes are insects belonging to the order Diptera and family Culicidae. They are distributed worldwide and include approximately 3500 species, of which about 300 have medical and veterinary importance. The evolutionary success of mosquitoes, in both tropical and temperate regions, is due to the various survival strategies these insects have developed throughout their life histories. Of the many adaptive mechanisms, diapause and quiescence, two different types of dormancy, likely contribute to the establishment, maintenance and spread of natural mosquito populations. This review seeks to objectively and coherently describe the terms diapause and quiescence, which can be confused in the literature because the phenotypic effects of these mechanisms are often similar.

Winter is coming: Diapause in the subtropical swallowtail butterfly Euryades corethrus (Lepidoptera, Papilionidae) is triggered by the shortening of day length and reinforced by low temperatures.

Diapause is modulated by genetic responses to some environmental cues. The most common stimulus to trigger diapause is photoperiod, but temperature and humidity can also be important. Subtropical grasslands insects are overexposed to seasonality and can use diapause as strategy to overcome harsh conditions, avoiding freezing winter temperatures and drought summer conditions. Here, we investigate if photoperiod, temperature, and humidity can induce and terminate dormancy using the model Euryades corethrus, a butterfly from Pampa that diapause as pupae. We hypothesize that photoperiod, temperature, and humidity can induce dormancy; to test the hypothesis, individuals from a stock population were subjected to experiments controlling these three factors. Photoperiod and temperature interactions were also tested. To evaluate if the removal of the harsh factor that induced diapause trigger diapause termination, 50% of dormant pupae in each experiment were exposed to amenable conditions. The results indicated that diapause is mainly induced by short photophases, while temperature and humidity separately do not increase dormancy frequency. Short photoperiods and low temperatures interact with each other, increasing dormancy in experimental populations. The evidences suggest that diapause is trigger by short-day lengths and boosted by low temperatures as winter approaches. The incidence of obligatory summer diapause was not supported, but the occurrence of dormant pupae in high-temperature treatments suggests that high temperatures produce facultative diapause. Regarding diapause termination, the softening of harsh conditions that induced diapause was not sufficient to reverse the dormancy state, suggesting that diapause termination is more complex than previously thought, probably involving internal clocks.

The evolution of an annual life cycle in killifish: adaptation to ephemeral aquatic environments through embryonic diapause.

An annual life cycle is characterized by growth, maturity, and reproduction condensed into a single, short season favourable to development, with production of embryos (seeds, cysts, or eggs) capable of surviving harsh conditions which juveniles or adults cannot tolerate. More typically associated with plants in desert environments, or temperate-zone insects exposed to freezing winters, the evolution of an annual life cycle in vertebrates is fairly novel. Killifish, small sexually dimorphic fishes in the Order Cyprinodontiformes, have adapted to seasonally ephemeral water bodies across much of Africa and South America through the independent evolution of an annual life history. These annual killifish produce hardy desiccation-resistant eggs that undergo diapause (developmental arrest) and remain buried in the soil for long periods when fish have perished due to the drying of their habitat. Killifish are found in aquatic habitats that span a continuum from permanent and stable to seasonal and variable, thus providing a useful system in which to piece together the evolutionary history of this life cycle using natural comparative variation. I first review adaptations for life in ephemeral aquatic environments in killifish, with particular emphasis on the evolution of embryonic diapause. I then bring together available evidence from a variety of approaches and provide a scenario for how this annual life cycle evolved. There are a number of features within Aplocheiloidei killifish including their inhabitation of marginal or edge aquatic habitat, their small size and rapid attainment of maturity, and egg properties that make them particularly well suited to the colonization of ephemeral waters.