In fish, the hypothalamus-pituitary-interrenal axis (HPIaxis), the equivalent of the hypothalamus-pituitary-adrenal axis (HPA-axis) in mammals, is activated during stress and leads to production and release of cortisol by the interregnal cells in the head kidney. In mammals, the cytokine interleukin-1b (IL-1b) takes a key position in the innate immune and inflammatory responses and influences the HPA-axis. In fish, studies that address the effects of cytokines on HPI-axis activation are limited. We quantitatively assessed expression of IL-1b and its receptor, IL-1RI (the latter was cloned and sequenced), in an acute restraint stress paradigm in common carp, Cyprinus carpio. We also considered expression of the pituitary hormones prolactin (PRL) and GH that have been shown to be structurally related to cytokines and have immunomodulatory actions. Pituitary PRL expression increased fourfold during stress; GH mRNA levels were unaffected. Following restraint, hypothalamic IL-1b expression was upregulated; in head kidney and pituitary pars intermedia, IL-1RI expression significantly increased. We suggest that during acute stress IL-1b signalling in the HPI-axis becomes more sensitive, since both ligand and receptor expressions are enhanced. In vitro, recombinant carp IL-1b stimulates release of a-MSH and N-Ac b-endorphin from the pituitary gland. This observation concurs with increased in vivo plasma levels of a-MSH and N-Ac b-endorphin following restraint. Our findings combined lead us to conclude that IL-1b affects the activity of the HPI-axis and, in turn, expression profiles of genes encoding IL-1b and its receptor are modified during acute stress. Our study provides convincing evidence for bi-directional communication of the HPI-axis and the immune system in fish.

In order to investigate the mechanisms creating and maintaining variability at the major histocompatibility (MH) class II alpha (DAA) locus we examined patterns of polymorphism in two isolated Atlantic salmon populations which share a common post-glacial origin. As expected from their common origin, but contrary to the observation at the MH class I locus, these populations shared the majority of DAA alleles: out of 17 sequences observed, 11 were common to both populations. Recombination seems to play a more important role in the origin of new alleles at the class II alpha locus than at the class I locus. A greater than expected proportion of sites inferred to be positively selected (potentially peptide binding residues, PBRs) were found to be involved in recombination events, suggesting a mechanism for increasing MH variability through an interaction between recombination and natural selection. Thus it appears that although selection and recombination are important mechanisms for the evolution of both class II alpha and class I loci in the Atlantic salmon, the pattern of variability differs markedly between these classes of MH loci.

We compared major histocompatibility class I allelic diversity in two currently reproductively isolated Atlantic salmon (Salmo salar) populations (Irish and Norwegian) with a common postglacial origin in order to test for among-population differences in allelic composition and patterns of recombination and point mutation. We also examined the evidence for adaptive molecular divergence at this locus by analyzing the rate of amino acid replacement in relation to a neutral expectation. Contrary to our prediction, and in contrast to the situation for other genetic markers, the two populations have almost nonoverlapping sets of major histocompatibility class I alleles. Although there is a strong signal of point mutation that predates population divergence, recent recombination, acting in similar, but not identical, ways in both populations appears to be a significant force in creating new alleles. Moreover, selection acting on peptide-binding residues seems to favor new recombinant alleles and is likely to be responsible for the rapid divergence between populations.

The early stages of ovarian folliculogenesis generally progress independent of gonadotropins, whereas later stages require signaling initiated by FSH. In Siberian hamsters, cycles of folliculogenesis are mediated by changes in photoperiod which depress the hypothalamic pituitary gonadal axis. Reduced gonadotropins lead to decreases in mature follicle development and ovulation; however, early stages of folliculogenesis have not been explored in regressed ovaries. We hypothesized that intraovarian factors that contribute predominantly to later stages of folliculogenesis would react to changes in photoperiod, whereas factors contributing to earlier stages would not change. To probe if the early stages of folliculogenesis continue in the photoinhibited ovary while late stages decline, we measured the mRNA abundance of factors that interact with FSH signaling (Fshr, Igf1, Cox2) and factors that can function independently of FSH (c-Kit, Kitl, Foxo3, Figla, Nobox, Sohlh1, Lhx8). While plasma FSH, antral follicles, and corpora lutea numbers declined with exposure to inhibitory photoperiod, the numbers of primordial, primary, and secondary follicles did not change. Expression of factors that interact with FSH signaling changed with changes in photoperiod; however, expression of factors that do not interact with FSH were not significantly altered. These results suggest that the photoinhibited ovary is not completely quiescent, as factors important for follicle selection and early follicle growth are still expressed in regressed ovaries. Instead, the lack of gonadotropin support that characterizes the non-breeding season appears to inhibit only final stages of folliculogenesis in Siberian hamsters.

SummaryIn adult vertebrates, endurance training leads to physiological, metabolical and molecular adaptations which improve endurance performance. Only very few studies have focused on adaptive responses to endurance training during early vertebrate development, and molecular data is limited. Here, we explored the effect of swim-training on the transcriptome of the zebrafish during early development on a quantitative and spatial gene expression level. We subjected larval zebrafish from 5 to 14 dpf (days post fertilization) to swim-training and performed a whole genome microarray analysis of trained and control fish sampled at 10 dpf. In addition, we investigated if swim-training affected the expression of genes involved in muscle growth and structure with quantitative real-time PCR in trained and control fish sampled at 5 and 14 dpf. To obtain a general overview of the effects of swimtraining on the transcriptome, we selected 52 genes from the whole genome microarray analysis based on a number of criteria. In situ hybridization demonstrated that 15 genes were specifically expressed in the brain, muscle, kidneys, liver, pancreas or intestines. Thus, swim-training led to molecular changes already after 6 days of swim-training and in a variety of organ systems. In addition, the expression of slow fiber markers was increased after 10 days of swim-training, indicating that muscle can already shift towards a slow aerobic phenotype during zebrafish larval development. Taken together, this study demonstrates that significant changes occur, even at early stages, as an adaptive response to endurance training during early vertebrate development.