Weber, C. J. | Search bioRxiv

bioRxiv2026

Flexibility of cell fates and functions across sex determination systems revealed by comparative single-cell analyses

Sex determination in vertebrates can be initiated by a wide range of genetic or environmental triggers. Yet, the degree to which gonadal cell types and genetic programs are conserved remains unresolved. Here we employed single-cell transcriptomics to characterize the temperature-dependent sex determination (TSD) program in gonads from the turtle species Trachemys scripta. Comparative analyses against species with genetic sex determination, like mouse (XY) and chicken (ZW), revealed a marked divergence in cell type repertoires and functions during vertebrate evolution. Unlike mammals, fetal Leydig cells are absent from the early gonads of T. scripta, where the supporting lineage expresses genes required for androgen synthesis. Evolutionary reconstructions show that this lineage derives from a Pax2-positive mesenchymal population, suggesting an ancestral condition in Archelosauria that differs from the primarily coelomic epithelium origin in the mammalian clade. Transcriptional dynamics and co-expression analyses revealed the recruitment of lineage-specific transcription factors, including Twist1 or Runx1, into the genetic programs of vertebrate clades. Our findings reveal extensive plasticity of the cellular and genetic mechanisms of vertebrate sex determination and suggest that this flexibility is a key feature of gonadal evolution.

evolutionary biology↗

bioRxiv2024

Cellular and molecular mechanisms that shape the development and evolution of tail vertebral proportion in mice and jerboas

Despite the functional importance of the vertebral skeleton, little is known about how individual vertebrae elongate or achieve disproportionate lengths as in the giraffe neck. Rodent tails are an abundantly diverse and more tractable system to understand mechanisms of vertebral growth and proportion. In many rodents, disproportionately long mid-tail vertebrae form a crescendo-decrescendo of lengths in the tail series. In bipedal jerboas, these vertebrae grow exceptionally long such that the adult tail is 1.5x the length of a mouse tail, relative to body length, with four fewer vertebrae. How do vertebrae with the same regional identity elongate differently from their neighbors to establish and diversify adult proportion? Here, we find that vertebral lengths are largely determined by differences in growth cartilage height and the number of cells progressing through endochondral ossification. Hypertrophic chondrocyte size, a major contributor to differential elongation in mammal limb bones, differs only in the longest jerboa mid-tail vertebrae where they are exceptionally large. To uncover candidate molecular mechanisms of disproportionate vertebral growth, we performed intersectional RNA-Seq of mouse and jerboa tail vertebrae with similar and disproportionate elongation rates. Many regulators of posterior axial identity and endochondral elongation are disproportionately differentially expressed in jerboa vertebrae. Among these, the inhibitory natriuretic peptide receptor C (NPR3) appears in multiple studies of rodent and human skeletal proportion suggesting it refines local growth rates broadly in the skeleton and broadly in mammals. Consistent with this hypothesis, NPR3 loss of function mice have abnormal tail and limb proportions. Therefore, in addition to genetic components of the complex process of vertebral evolution, these studies reveal fundamental mechanisms of skeletal growth and proportion.

developmental biology↗