Large captivity effect based on gene expression comparisons between captive and wild shrew brains
Compared to their free-ranging counterparts, wild animals in captivity are subject to different conditions with lasting effects on their physiology and behavior. Alterations in gene expression in response to environmental changes occur upstream of physiological and behavioral phenotypes, but there are no experiments analyzing differential gene expression in captive vs. free-ranging mammals. We assessed gene expression profiles of three brain regions (cortex, olfactory bulb, and hippocampus) of wild juvenile shrews (Sorex araneus) in comparison to shrews kept in captivity for two months. We found hundreds of differentially expressed genes in all three brain regions, suggesting a large and uniform captivity effect. Many of the downregulated genes in captive shrews significantly enrich pathways associated with neurodegenerative disease (p<0.001), oxidative phosphorylation (p<0.001), and genes encoding ribosomal proteins (p<0.001). Transcriptomic changes associated with captivity in the shrew resemble responses identified in several human pathologies, such as major depressive disorder and neurodegeneration. Thus, not only does captivity impact brain function and expression, but captivity effects may also confound analyses of natural physiological processes in wild individuals under captive conditions.
Molecular mechanisms of seasonal brain shrinkage and regrowth in Sorex araneus
Human brains typically grow through development, then remain the same size in adulthood, and often shrink through age-related degeneration that induces cognitive decline and impaired functionality. In most cases, however, the neural and organismal changes that accompany shrinkage, especially early in the process, remain unknown. Paralleling neurodegenerative phenotypes, the Eurasian common shrew Sorex araneus, shrinks its brain in autumn through winter, but then reverses this process by rapidly regrowing the brain come spring. To identify the molecular underpinnings and parallels to human neurodegeneration of this unique brain size change, we analyzed multi-organ, season-specific transcriptomics and metabolomic data. Simultaneous with brain shrinkage, we discovered system-wide metabolic shifts from lipid to glucose metabolism, as well as neuroprotection of brain metabolic homeostasis through reduced cholesterol efflux. These mechanisms rely on a finely tuned brain-liver crosstalk that results in changes in expression of human markers of aging and neurodegeneration in Parkinson’s disease and Huntington’s disease. We propose metabolic shifts with signals that cross the brain blood barrier are central to seasonal brain size changes in S. araneus, with potential implications for therapeutic treatment of human neurodegeneration.
The evolution of antimicrobial peptides in Chiroptera
High viral tolerance coupled with an extraordinary regulation of the immune response makes bats a great model to study host-pathogen evolution. Although many immune-related gene gains and losses have been previously reported in bats, important gene families such as antimicrobial peptides (AMPs) remain understudied. We built an exhaustive bioinformatic pipeline targeting the major gene families of defensins and cathelicidins to explore AMP diversity and analyze their evolution and distribution across six bat families. A combination of manual and automated procedures identified 29 AMP families across queried species, with α-, β-defensins, and cathelicidins representing around 10% of AMP diversity. Gene duplications were inferred in both α-defensins, which were absent in five species, and three β-defensin gene subfamilies, but cathelicidins did not show significant shifts in gene family size and were absent in Anoura caudifer and the pteropodids. Based on lineage-specific gains and losses, we propose diet and diet-related microbiome evolution may determine the evolution of α- and β-defensins gene families and subfamilies. These results highlight the importance of building species-specific libraries for genome annotation in non-model organisms and shed light on possible drivers responsible for the rapid evolution of AMPs. By focusing on these understudied defenses, we provide a robust framework for explaining bat responses to pathogens.
Ecological constraints on highly evolvable olfactory receptor genes and morphology in neotropical bats
While evolvability of genes and traits may promote specialization during species diversification, how ecology subsequently restricts such variation remains unclear. Chemosensation requires animals to decipher a complex chemical background to locate fitness-related resources, and thus the underlying genomic architecture and morphology must cope with constant exposure to a changing odorant landscape; detecting adaptation amidst extensive chemosensory diversity is an open challenge. In phyllostomid bats, an ecologically diverse clade that evolved plant-visiting from an insectivorous ancestor, the evolution of novel food detection mechanisms is suggested to be a key innovation, as plant-visiting species rely strongly on olfaction, supplementarily using echolocation. If this is true, exceptional variation in underlying olfactory genes and phenotypes may have preceded dietary diversification. We compared olfactory receptor (OR) genes sequenced from olfactory epithelium transcriptomes and olfactory epithelium surface area of bats with differing diets. Surprisingly, although OR evolution rates were quite variable and generally high, they are largely independent of diet. Olfactory epithelial surface area, however, is relatively larger in plant-visiting bats and there is an inverse relationship between OR evolution rates and surface area. Relatively larger surface areas suggest greater reliance on olfactory detection and stronger constraint on maintaining an already diverse OR repertoire. Instead of the typical case in which specialization and elaboration are coupled with rapid diversification of associated genes, here the relevant genes are already evolving so quickly that increased reliance on smell has led to stabilizing selection, presumably to maintain the ability to consistently discriminate among specific odorants — a potential ecological constraint on sensory evolution.