Karen E. Sears

Wildlife surveillance is critical for tracking disease emergence, characterizing pathogen diversity, and assessing spillover risks. Blood‐borne pathogens are of particular interest for such efforts due to their global distribution, broad host taxa, and zoonotic potential. Despite the need to monitor blood‐borne pathogens, blood collection is costly for biologists and sampled wildlife (i.e., time‐consuming and stressful), hindering our ability to expand surveillance efforts. Using vascular tissues to detect infections can minimize sampling effort and stress on wildlife, but it is unclear how pathogen detection and characterization from these tissues compared to blood. To evaluate the reliability of using vascular tissues for detecting blood‐borne pathogens in wildlife, we collected paired samples of blood and wing biopsies from common vampire bats ( Desmodus rotundus ) and molecularly screened for bartonellae, hemotropic mycoplasmas (hemoplasmas), and trypanosomes. Probabilities of detection were lower in wing tissues than in blood for all pathogens, possibly due to blood vessel avoidance when collecting the former. However, we detected infection in wing tissues of at least two individual bats for each pathogen. Paired‐positive individuals mostly showed high sequence concordance between tissues, and sample sizes needed to detect a single infection and observed prevalences were similar (i.e., n = 10–39). Due to lower probabilities of infection in wing tissues compared to blood, we suggest that using these samples to estimate infection prevalence of blood‐borne pathogens is not ideal. However, our results demonstrate that vascular tissues can be used for initial pathogen assessment and discovery to help target surveillance efforts in the future.

Marsupial embryos develop under intense functional constraints: neonates are born after a short gestation but must immediately crawl to the teat using precociously developed forelimbs. To meet this demand, marsupials have evolved extreme acceleration of limb morphogenesis, yet the cellular and molecular mechanisms underlying this shift remain unresolved. Using two distantly related marsupials, the fat-tailed dunnart (Sminthopsis crassicaudata) and the gray short-tailed opossum (Monodelphis domestica), we show that this acceleration extends upstream to the earliest stages of lateral plate mesoderm (LPM) formation. The forelimb field is specified in relative isolation from other axial structures, marked by accelerated activation of limb genes prior to neural tube and somite formation. Forelimb outgrowth begins before overt LPM subdivision and epithelial-mesenchymal transitions, with limb mesenchyme arising from an undifferentiated LPM. These findings reveal unexpected flexibility in the temporal relationships between axial morphogenesis and limb initiation, highlighting evolutionary plasticity in the processes that govern vertebrate limb patterning.

Mammalian limb development is a complex system involving several signaling centers and coordinated cell behaviors to sculpt a functioning limb capable of the diverse locomotory strategies that mammals exhibit. To investigate the changes in development that facilitate the generation of the wide array of limb phenotypes across mammals, we take a correlation network approach to investigate the developing limbs of mice, bats, and opossums, which represent typical limb development, a novel limb phenotype, and a shift in developmental timing, respectively. Using transcriptomic data of early limb development across these taxa, we build module correlation networks and identify a difference in network connectivity and the distribution of limb development genes across bat limb development. We identify a unique signature of increased modularity in the bat forelimb that is not detected in mouse or opossum. This modularity is not associated with increased specialization of limb development modules, but rather is marked by target limb development genes being spread evenly across several modules. The opossum, with its standard phenotype but altered developmental timing, does not show a difference in modularity relative to mouse. This work points toward the benefit of a network-minded approach to transcriptomic networks, which reveals developmental modularity and potential gene targets for exploration of developmental system evolution.

Mammalian brains vary in size, structure and function, but the extent to which evolutionarily novel cell types contribute to this variation remains unresolved^1,2,3,4. Previous studies suggest that there is a primate-specific population of striatal inhibitory interneurons—the TAC3 interneurons⁵. However, broader taxonomic and developmental characterization is required to address novelty in cell-type evolution. Here we examine gene expression in inhibitory neurons across 10 mammalian species, spanning 160 million years of divergence from primates. We find that the initial class of newborn TAC3 interneurons specified during development represents an ancestral, medial ganglionic eminence (MGE)-derived striatal population that is also present in pig and ferret cortex. This discovery prompted a re-examination of Glires, including mice, which are thought to lack the TAC3 type^5,6. Targeted enrichment of MGE precursors in mice revealed conservation of the TAC3 initial class, camouflaged by reduced expression of Tac2 (the mouse orthologue of TAC3) and a gain of Th expression. Extending our analysis to the adult striatum further supported the homology of primate TAC3 and mouse Th striatal interneurons, and also uncovered a rare Tac2 subpopulation in the mouse ventromedial striatum. This study suggests that initial classes of telencephalic inhibitory neurons are largely conserved, and that during evolution, neuronal types in the mammalian brain change through redistribution and fate refinement, rather than by derivation of novel precursors early in development.

Many developmental biologists seldom leave the lab for research, relying instead on establishing colonies of traditional and emerging model systems. However, to fully understand the mechanisms and principles of development and evolution, including the role of ecology and the environment, it is important to study a diverse range of organisms in context. In this Perspective, we hear from five research teams from around the world about the importance and challenges of going into the field to study their organisms of interest. We also invite you to share your own fieldwork stories on the Node.

Sensory organs must develop alongside the skull within which they are largely encased, and this relationship can manifest as the skull constraining the organs, organs constraining the skull, or organs constraining one another in relative size. How this interplay between sensory organs and the developing skull plays out during the evolution of sensory diversity; however, remains unknown. Here, we examine the developmental sequence of the cochlea, the organ responsible for hearing and echolocation, in species with distinct diet and echolocation types within the ecologically diverse bat super‐family Noctilionoidea. We found the size and shape of the cochlea largely correlates with skull size, with exceptions of Pteronotus parnellii , whose high duty cycle echolocation (nearly constant emission of sound pulses during their echolocation process allowing for detailed information gathering, also called constant frequency echolocation) corresponds to a larger cochlear and basal turn, and Monophyllus redmani , a small‐bodied nectarivorous bat, for which interactions with other sensory organs restrict cochlea size. Our findings support the existence of developmental constraints, suggesting that both developmental and anatomical factors may act synergistically during the development of sensory systems in noctilionoid bats.

Sugrue VJ, Zoller JA, Narayan P, Lu AT, Ortega-Recalde OJ, Grant MJ, Bawden CS, Rudiger SR, Haghani A, Bond DM, Hore RR, Garratt M, Sears KE, Wang N, Yang XW, Snell RG, Hore TA, Horvath S. 2021. Castration delays epigenetic aging and feminizes DNA methylation at androgen-regulated loci. eLife 10:e64932. doi: 10.7554/eLife.64932.