Cross-species analysis and comparison of the inner ear between chickens and mice.

The inner ear of mammals includes the cochlea and vestibule, which house specialized hair cells that are responsible for hearing and balance, respectively. While cochlear hair cells fail to regenerate following damage, those of the utricle, which is part of the vestibular apparatus, show partial regeneration. In birds, the macula lagena, a unique ear structure in this clade, has the ability to regenerate hair cells similarly to the utricle. Many studies have sought to explain regeneration in terms of evolution and species differences. However, it remains unclear what the cellular and molecular basis is behind the differences in inner ear structures and between avians and mammals. In the present study, we first investigated the anatomical structures of the inner ear of both chickens and rodents. We then performed RNA sequencing (RNA-Seq) and made cross-species analyses of the expression of homologous genes obtained from the inner ear tissue from both chickens and mice. Finally, we focused on the lagena, the basilar papilla, and the utricle in chickens and identified differentially expressed genes between tissues and determined the expression patterns of genes involved in inner ear structure formation by single-cell RNA sequencing and bulk RNA-Seq. We concluded that the cellular and molecular composition of the lagena is more similar to that of the utricle than the cochlea. Taken together, our study provides a valuable resource for the study of inner ear evolution and development.