Maintenance of Embryonic Positional Identity Programs in the Adult Sheep Tail: Evidence from HOXB13 Spatial RNA Expression Gradients

Body axis patterning in vertebrates is controlled by HOX genes during embryogenesis, with their expression gradients defining spatial identity along the anterior–posterior axis. While their developmental roles are well known, less is understood about their persistence and function in adult tissues. Tail length in sheep provides a model for studying the phenotypic effects of HOX gene variants. We used Improved Jezersko–Solcava sheep, which segregate for both tail length and HOXB13 promoter variants, to examine genotype–phenotype associations. The HOXB13 genotype was the main determinant of adult tail length, acting primarily by modulating caudal vertebral number. RNA-seq and qPCR analyses revealed a clear anterior–posterior HOXB13 expression gradient in adult tail skin and bones, with stronger distal expression in short-tailed animals. These results provide in vivo evidence that spatially patterned HOX gene expression persists in adulthood, suggesting that residual developmental programs may maintain regional identity in adult tissues. Overall design: RNA-seq profiling was performed on tail skin samples collected post-mortem from adult Improved Jezersko–Solcava rams with homozygous short-tail (A/A) or long-tail (D/D) HOXB13 genotypes. For each genotype (A/A: N=4; D/D: N=3), tissue was sampled at three defined anatomical positions along the anterior–posterior tail axis (base, mid-tail, tip) to investigate spatial expression gradients and genotype-specific transcriptional differences.