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Sex determination occurs across animal species, but most of our knowledge about its mechanisms comes from only a handful of bilaterian taxa. This limits our ability to infer the evolutionary history of sex determination within animals.

In this study, we generated a linkage map of the genome of the colonial cnidarianHydractinia symbiolongicarpusand used it to demonstrate that this species has an XX/XY sex determination system. We demonstrate that the X and Y chromosomes have pseudoautosomal and non-recombining regions. We then use the linkage map and a method based on the depth of sequencing coverage to identify genes encoded in the non-recombining region and show that many of them have male gonad-specific expression. In addition, we demonstrate that recombination rates are enhanced in the female genome and that the haploid chromosome number inHydractiniaisn = 15.

These findings establishHydractiniaas a tractable non-bilaterian model system for the study of sex determination and the evolution of sex chromosomes.

 

Hydractinia is a colonial marine hydroid that exhibits remarkable biological properties, including the capacity to regenerate its entire body throughout its lifetime, a process made possible by its adult migratory stem cells, known as i-cells. Here, we provide an in-depth characterization of the genomic structure and gene content of two Hydractinia species, H. symbiolongicarpus and H. echinata, placing them in a comparative evolutionary framework with other cnidarian genomes. We also generated and annotated a single-cell transcriptomic atlas for adult male H. symbiolongicarpus and identified cell type markers for all major cell types, including key i-cell markers. Orthology analyses based on the markers revealed that Hydractinia's i-cells are highly enriched in genes that are widely shared amongst animals, a striking finding given that Hydractinia has a higher proportion of phylum-specific genes than any of the other 41 animals in our orthology analysis. These results indicate that Hydractinia's stem cells and early progenitor cells may use a toolkit shared with all animals, making it a promising model organism for future exploration of stem cell biology and regenerative medicine. The genomic and transcriptomic resources for Hydractinia presented here will enable further studies of their regenerative capacity, colonial morphology, and ability to distinguish self from non-self. 

Ecologists and evolutionary biologists often use body condition indices (BCIs) to approximate an animal’s energetic state, which is important because energy (typically, lipid) reserves influence an animal’s investment into fitness‐related traits. A BCI’s utility is typically assessed by examining its ability to predict standardized fat content (fat mass in fresh mass after correcting for body size) across a population. Less frequently, a BCI is assessed as a response variable by comparing its sensitivity to a particular treatment or factor (e.g. food availability or age) relative to that of standardized fat content. We assessed five common BCIs (or equivalent) with these two approaches in the wing‐dimorphic sand field cricket (Gryllus firmusScudder 1902). All of the BCI methods significantly predicted standardized fat content in both sexes and wing morphs where the scaled mass index and Que ́telet’s index explained the most and least amount of variation, respectively. The BCIs and fat content were also similarly affected by adult age. However, BCIs were universally better proxies for lean mass and water content than for fat content. The relationships between BCIs and standardized lean mass and water content were stronger than those between BCIs and standardized fat content, and the effect sizes of age on BCIs were closer to that of lean mass relative to that of fat content. Thus, although BCIs reasonably predicted energy (fat) stores inG. firmus, they may be more indicative of lean mass or water content in insects. If BCIs are surrogates for protein and water storage, they may still approximate the storage of resources that fuel fitness‐related traits. Yet, future work is required to understand which body components (e.g. fat, protein or water) are most tightly linked to fitness, and whether traditional BCIs explain variation in fitness‐related body components across animal taxa.