No abstract

The gut microbiome forms at an early stage, yet data on the environmental factors influencing the development of wild avian microbiomes is limited. The early studies with wild gut microbiome have shown that the rearing environment may be of importance in gut microbiome formation, yet the results vary across taxa, and the effects of specific environmental factors have not been characterized. Here, wild great tit (Parus major) broods were manipulated to either reduce or enlarge the original brood soon after hatching. We investigated if brood size was associated with nestling bacterial gut microbiome, and whether gut microbiome diversity predicted survival. Fecal samples were collected at mid-nestling stage and sequenced with the 16S rRNA gene amplicon sequencing, and nestling growth and survival were measured. Gut microbiome diversity showed high variation between individuals, but this variation was not explained by brood size or body mass. Additionally, we did not find a significant effect of brood size on body mass or gut microbiome composition. Furthermore, we found no significant association between gut microbiome diversity and short-term (survival to fledging) or mid-term (apparent juvenile) survival. Early-life environment can lead to variation in offspring condition and gut microbiome and therefore, understanding how and which changes in the rearing environment are associated with offspring development is of importance. However, we did not find an association between brood size, gut microbiome diversity and survival, indicating that future studies should expand into other early-life environmental factors e.g., diet composition and quality, and parental influences.

The evolution of multicellularity paved the way for the origin of complex life on Earth, but little is known about the mechanistic basis of early multicellular evolution. Here, we examine the molecular basis of multicellular adaptation in the Multicellularity Long Term Evolution Experiment (MuLTEE). We demonstrate that cellular elongation, a key adaptation underpinning increased biophysical toughness and organismal size, is convergently driven by downregulation of the chaperone Hsp90. Mechanistically, Hsp90-mediated morphogenesis operates by destabilizing the cyclin-dependent kinase Cdc28, resulting in delayed mitosis and prolonged polarized growth. Reintroduction of Hsp90 expression resulted in shortened cells that formed smaller groups with reduced multicellular fitness. Together, our results show how ancient protein folding systems can be tuned to drive rapid evolution at a new level of biological individuality by revealing novel developmental phenotypes.

Maternal hormones, such as thyroid hormones transferred to embryos and eggs, are key signalling pathways to mediate maternal effects. To be able to respond to maternal cues, embryos must express key molecular "machinery" of the hormone pathways, such as enzymes and receptors. While altricial birds begin thyroid hormone (TH) production only at/after hatching, experimental evidence suggests that their phenotype can be influenced by maternal THs deposited in the egg. However, it is not understood, how and when altricial birds express genes in the TH-pathway. For the first time, we measured the expression of key TH-pathway genes in altricial embryos, using two common altricial ecological model species (pied flycatcher, Ficedula hypoleuca and blue tit Cyanistes caeruleus). Deiodinase DIO1 gene expression could not be reliably confirmed in either species, but deiodinase enzyme DIO2 and DIO3 genes were expressed in both species. Given that DIO2 coverts T4 to biologically active T3, and DIO3 mostly T3 to inactive forms of thyroid hormones, our results suggest that embryos may modulate maternal signals. Thyroid hormone receptor (THRA and THRB) and monocarboxyl membrane transporter gene (SLC15A2) were also expressed, enabling TH-responses. Our results suggest that early altricial embryos may be able to respond and potentially modulate maternal signals conveyed by thyroid hormones.

Offspring phenotype at birth is determined by its genotype and the prenatal environment including exposure to maternal hormones. Variation in both maternal glucocorticoids and thyroid hormones can affect offspring phenotype. However, the underlying molecular mechanisms shaping the offspring phenotype, especially those contributing to long-lasting effects, remain unclear. Epigenetic changes (such as DNA methylation) have been postulated as mediators of long-lasting effects of early-life environment. In this study, we determined the effects of elevated prenatal glucocorticoid and thyroid hormones on handling stress response (breath rate), DNA methylation and gene expression of glucocorticoid receptor (GCR) and thyroid hormone receptor (THR) in great tit (Parus major). Eggs were injected before incubation onset with corticosterone (main avian glucocorticoid) and/or thyroid hormones (thyroxine and triiodothyronine) to simulate variation in maternal hormone deposition. Breath rate during handling and gene expression of GCR and THR were evaluated 14 days after hatching. Methylation status of GCR and THR genes were analyzed from the longitudinal blood samples taken 7 and 14 days after hatching, as well as in the following autumn. Elevated prenatal corticosterone level significantly increased the breath rate during handling, indicating enhanced stress response and/or metabolism. Prenatal corticosterone manipulation had CpG-site-specific effects on DNA methylation at the GCR putative promoter region, while it did not significantly affect GCR gene expression. GCR expression was negatively associated with earlier hatching date and chick size. THR methylation or expression did not exhibit any significant relationship with the hormonal treatments or the examined covariates, suggesting that TH signaling may be more robust due to its crucial role in development. This study supports the view that maternal corticosterone may influence offspring metabolism and stress response via epigenetic alterations, yet their possible adaptive role in optimizing offspring phenotype to the prevailing conditions, context-dependency, and the underlying molecular interplay needs further research.

Background The gut microbiome forms at an early stage, yet data on the environmental factors influencing the development of wild avian microbiomes is limited. As the gut microbiome is a vital part of organismal health, it is important to understand how it may connect to host performance. The early studies with wild gut microbiome have shown that the rearing environment may be of importance in gut microbiome formation, yet the results vary across taxa, and the effects of specific environmental factors have not been characterized. Here, wild great tit (Parus major) broods were manipulated to either reduce or enlarge the original brood soon after hatching. We investigated if brood size was associated with nestling bacterial gut microbiome, and whether gut microbiome diversity predicted survival. Fecal samples were collected at mid-nestling stage and sequenced with the 16S rRNA gene amplicon sequencing, and nestling growth and survival were measured. Results Gut microbiome diversity showed high variation between individuals, but this variation was not significantly explained by brood size or body mass. Additionally, we did not find a significant effect of brood size on body mass or gut microbiome composition. We also demonstrated that early handling had no impact on nestling performance or gut microbiome. Furthermore, we found no significant association between gut microbiome diversity and short-term (survival to fledging) or mid-term (apparent juvenile) survival. Conclusions We found no clear association between early-life environment, offspring condition and gut microbiome. This suggests that brood size is not a significantly contributing factor to great tit nestling condition, and that other environmental and genetic factors may be more strongly linked to offspring condition and gut microbiome. Future studies should expand into other early-life environmental factors e.g., diet composition and quality, and parental influences.

Parental care (including postnatal provisioning) is a major component of the offspring early-life environment. In avian species, the number of chicks in the nest and subsequent sibling competition for food are known to affect chick growth, leading in some cases to long-lasting effects for the offspring. Because of its central role in converting energy, variation in the offspring mitochondrial metabolism could be an important pathway underlying variation in growth patterns. Here, we performed a brood size manipulation in great tits (Parus major) to unravel its impact on offspring mitochondrial metabolism and reactive oxygen species (ROS) production in red blood cells. We investigated the effects of brood size on chicks growth and survival, and tested for long-lasting effects on juvenile mitochondrial metabolism and phenotype. As expected, chicks raised in reduced broods had a higher body mass compared to enlarged and control groups. However, mitochondrial metabolism and ROS production were not significantly affected by the treatment either at chick or juvenile stages. Chicks in very small broods were smaller in size and had higher mitochondrial metabolic rates. The nest of rearing has a significant effect on nestling mitochondrial metabolism, yet variation in mitochondrial metabolism at the early-life stages are not associated with survival chances. The contribution of the rearing environment in determining offspring mitochondrial metabolism emphasizes the plasticity of mitochondrial metabolism in changing environments. Further studies would be needed to closely investigate what are the major environmental cues affecting the offspring mitochondrial metabolism during the growth period.

Methylation at the N6-position of adenosine, m6A, is the most abundant mRNA modification in eukaryotes. It is a highly conserved universal regulatory mechanism controlling gene expression in a myriad of biological processes. The role of m6A methylation in sexual maturation, however, has remained largely unexplored. While the maturation process is known to be affected by many genetic and environmental factors, the molecular mechanisms causing variation in the timing of maturation are still poorly understood. Hence, investigation of whether a widespread mechanism like m6A methylation could be involved in controlling of the maturation timing is warranted. In Atlantic salmon (Salmo salar), two genes associated with the age at maturity in human, vgll3 and six6, have been shown to play an important role in maturation timing. In this study, we investigated the expression of 16 genes involved in the regulation of m6A RNA methylation in the hypothalamus of Atlantic salmon with different homozygous combinations of late (L) and early (E) alleles for vgll3 and six6 genes. We found differential expression of ythdf2.2 which encodes an m6A modification reader and promotes mRNA degradation. Its expression was higher in six6*LL compared to other genotypes as well as immature males compared to matures. In addition, we found that the expression levels of genes coding for an eraser, alkbh5, and for a reader, ythdf1, were higher in the hypothalamus of females than in males across all the different genotypes studied. Our results indicate a potential role of the m6A methylation process in sexual maturation of Atlantic salmon, and therefore, provide the first evidence for such regulatory mechanism in the hypothalamus of any vertebrate. Investigation of additional vertebrate species is essential in order to determine the generality of these findings.