Comparison of the Single-Cell Immune Landscape of Testudines from Different Habitats

Abstract

Testudines, also known as living fossils, have evolved diversely and comprise many species that occupy a variety of ecological niches. However, the immune adaptation of testudines to the different ecological niches remains poorly understood. This study compared the composition, function, and differentiation trajectories of peripheral immune cells in testudines (Chelonia mydas, Trachemys scripta elegans, Chelonoidis carbonaria, and Pelodiscus sinensis) from different habitats using the single-cell RNA sequencing (scRNA-seq) technique. The results showed that T. scripta elegans, which inhabits freshwater and brackish environments, had the most complex composition of peripheral immune cells, with 11 distinct immune cell subsets identified in total. The sea turtle C. mydas, had the simplest composition of peripheral immune cells, with only 5 distinct immune cell clusters. Surprisingly, neither basophils were found in C. mydas nor T cells in C. carbonaria. Basophil subsets in peripheral blood were identified for the first time; two basophil subtypes (GATA2-high-basophils and GATA2-low-basophils) were observed in the peripheral blood of T. scripta elegans. In addition, ACKR4 cells, CD4 T cells, CD7 T cells, serotriflin cells, and ficolin cells were specifically identified in the peripheral blood of T. scripta elegans. Furthermore, LY6G6C cells, SPC24 cells, and NKT cells were specifically observed in C. carbonaria. Moreover, there were differences in the functional status and developmental trajectory of peripheral immune cells among the testudine species. The identification of specific features of peripheral immune cells in testudines from different habitats may enable elucidation of the adaptation mechanism of testudines to various ecological niches.

Single‐cell transcriptomic landscape of peripheral blood cells provides insights into adaptation of red‐eared sliders (Trachemys scripta elegans)

“…Moreover, compared with those in other turtles, the peripheral blood cell composition of red‐eared sliders is the most complicated (Guo et al. 2022). This suggests that red‐eared sliders may have evolved different hemocytes to adapt to their environment.…”

“…Compared with other turtles, red‐eared sliders have more complex peripheral immune cells (Guo et al. 2022). The composition of peripheral immune cells differed among testudines from different habitats.…”

“…Red‐eared sliders from freshwater and brackish environment had the most complex composition of peripheral immune cells, compared with other habitats turtles (shallow tropical and subtropical waters, terrestrial, and sedimentary fresh‐water areas), meanwhile the more complex the habitat environment of testudines, the more abundant the peripheral immune cell population (Guo et al. 2022). Compared with other animals, the composition and function of peripheral immune cells in red‐eared sliders are more similar to that of mammals (Dong et al.…”

“…Compared with those in mammals such as humans and mice, the peripheral blood cell composition of red-eared sliders is simpler (Dong et al 2020;Xie et al 2021); however, it is more complicated than those of oysters (Meng & Wang 2022), shrimp (Koiwai et al 2021), and drosophilae (Tattikota et al 2020). Moreover, compared with those in other turtles, the peripheral blood cell composition of redeared sliders is the most complicated (Guo et al 2022). This suggests that red-eared sliders may have evolved different hemocytes to adapt to their environment.…”