Abstract
Tree swallows (Tachycineta bicolor) are one of the most commonly studied wild birds in North America. They have advanced numerous research areas, including life history, physiology, and organismal responses to global change; however, transcriptomic resources are scarce. To further advance the utility of this system for biologists across disciplines, we generated a transcriptome for the tree swallow using six tissues (brain, blood, ovary, spleen, liver, and muscle) collected from breeding females. We de novo assembled 207,739 transcripts, which we aligned to 14,717 high confidence protein-coding genes. We then characterized each tissue with regard to its unique genes and processes and applied this transcriptome to two fundamental questions in evolutionary biology and endocrinology. First, we analyzed 3,015 single-copy orthologs and identified 46 genes under positive selection in the tree swallow lineage, including those with putative links to adaptations in this species. Second, we analyzed tissue-specific expression patterns of genes involved in sex steroidogenesis and processing. Enzymes capable of synthesizing these behaviorally relevant hormones were largely limited to the ovary, whereas steroid binding genes were found in nearly all other tissues, highlighting the potential for local regulation of sex steroid-mediated traits. These analyses provide new insights into potential sources of phenotypic variation in a free-living female bird and advance our understanding of fundamental questions in evolutionary and organismal biology.
Highlights
Tree swallows (Tachycineta bicolor) are one of the most commonly studied wild birds in North America
The rapid increase of available transcriptomes for non-model organisms over the past few years has greatly contributed to ecological and evolutionary advances in natural systems[1,2,3]. This is true for avian species[4], yet one key species used in eco-evolutionary and organismal biology has a surprising absence of molecular resources
A major goal in evolutionary biology is to identify genes that are subject to adaptive evolution[18]
Summary
Tree swallows (Tachycineta bicolor) are one of the most commonly studied wild birds in North America. Enzymes capable of synthesizing these behaviorally relevant hormones were largely limited to the ovary, whereas steroid binding genes were found in most other tissues, highlighting the potential for local regulation of sex steroid-mediated traits These analyses provide new insights into potential sources of phenotypic variation in a free-living female bird and advance our understanding of fundamental questions in evolutionary and organismal biology. Tree swallows have one of the highest rates of extra-pair paternity of any socially monogamous songbird (50–90% of nests contain extra-pair young)[31,32,33] This system can facilitate the discovery of protein-coding genes undergoing positive selection, providing insights into lineage-specific adaptations related to competition and immunity. Future work providing detailed knowledge of tissue-specific regulation of the full suite of steroid processing genes would greatly improve our understanding of the potential for correlated phenotypic evolution across tissues
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