Recent sequencing and software enhancements have advanced our understanding of the evolution of genomic structure and function, especially addressing novel evolutionary biology questions. Yet fragmentary turtle genome assemblies remain a challenge to fully decipher the genetic architecture of adaptive evolution. Here, we use optical mapping to improve the contiguity of the painted turtle (Chrysemys picta) genome assembly and use de novo fluorescent in situ hybridization (FISH) of bacterial artificial chromosome (BAC) clones, BAC-FISH, to physically map the genomes of the painted and slider turtles (Trachemys scripta elegans). Optical mapping increased C. picta’s N50 by ~242% compared to the previous assembly. Physical mapping permitted anchoring ~45% of the genome assembly, spanning 5544 genes (including 20 genes related to the sex determination network of turtles and vertebrates). BAC-FISH data revealed assembly errors in C. picta and T. s. elegans assemblies, highlighting the importance of molecular cytogenetic data to complement bioinformatic approaches. We also compared C. picta’s anchored scaffolds to the genomes of other chelonians, chicken, lizards, and snake. Results revealed a mostly one-to-one correspondence between chromosomes of painted and slider turtles, and high homology among large syntenic blocks shared with other turtles and sauropsids. Yet, numerous chromosomal rearrangements were also evident across chelonians, between turtles and squamates, and between avian and non-avian reptiles.

Understanding genome‐wide responses to environmental conditions during embryogenesis is essential for discerning the evolution of developmental plasticity and canalization, two processes generating phenotypic variation targeted by natural selection. Here, we present the first comparative trajectory analysis of matched transcriptomic developmental time series from two reptiles incubated under identical conditions, a turtle with a ZZ/ZW system of genotypic sex determination (GSD), Apalone spinifera, and a turtle with temperature‐dependent sex determination (TSD), Chrysemys picta. Results from our genome‐wide, hypervariate gene expression analysis of sexed embryos across five developmental stages revealed that substantial transcriptional plasticity in the developing gonads can persist for >145 Myr, long after the canalization of sex determination via the evolution of sex chromosomes, while some gene‐specific thermal sensitivity drifts or evolves anew. Such standing thermosensitivity represents an underappreciated evolutionary potential harbored by GSD species that may be co‐opted during future adaptive shifts in developmental programing, such as a GSD to TSD reversal, if favored by ecological conditions. Additionally, we identified novel candidate regulators of vertebrate sexual development in GSD reptiles, including sex‐determining candidate genes in a ZZ/ZW turtle.

Painted turtles are remarkable for their well-developed freeze tolerance and associated resilience to hypoxia/anoxia, oxidative stress, and ability to supercool. They are, therefore, are an ideal model for biomedical research on hypoxia-induced injuries (such as strokes), tissue cooling during extensive surgeries, and organ cryopreservation. Yet, the seasonal reproduction and slow maturation of turtles hinder basic and applied biomedical research. To overcome these limitations, we have developed for the first time, adult stem cell-derived turtle hepatic organoids, which provide 3D self-assembled structures that closely mimic their original tissue and allow for in vitro testing and experimentation without constantly harvesting donor tissue and screening offspring. Our pioneering work with turtles represents the first for this vertebrate Order and complements the only other organoid lines from non-avian reptiles, which were produced from snake venom glands. Here we report the isolation and characterization of hepatic organoids derived from painted, snapping, and spiny softshell turtles spanning ~175 million years of evolution, with a subset being preserved in a biobank. Morphological and transcriptomics revealed organoid cells resembling cholangiocytes, which was then compared to the tissue of origin. Deriving organoids from multiple turtle species and life stages demonstrates that our organoid culturing techniques are broadly applicable to chelonians, permitting the development of functional genomic tools currently missing in most herpetological research. When combined with genetic editing, this platform will further support studies of genome-to-phenome mapping, gene function, genome architecture, and adaptive responses to climate change, among others. We discuss the unique abilities of painted turtles, including their overwintering potential, an adaptation with important implications for ecological, evolutionary, and biomedical research.