On July 26, the research team headed by Prof. GUO Guoji from the Zhejiang University School of Medicine published an open-access article entitled “Cell landscape of larval and adult Xenopus laevis at single-cell resolution” in the journal Nature Communications. They constructed the Xenopus cell landscape (XCL) using Microwell-seq, a self-developed high-throughput single-cell RNA sequencing platform. This comprehensive XCL contained over 500,000 single cells isolated from four Xenopus laevis stages and 17 adult tissues. Common cell lineage-specific transcription factors were thus identified in vertebrates, including fish, amphibians and mammals.

With the rapid development of high-throughput single-cell RNA sequencing technology, a large number of organism-wide single-cell atlases have been constructed for various species, such as humans, zebrafish, mice, and fruit flies. From an evolutionary perspective, the amphibian is an intermediate taxon that links mammals to vertebrates of more ancient origin. However, this intermediate taxon is still lacking, thereby rendering it hard to explain the cross-species evolutionary hierarchy.

Fig. 1: Constructing an XCL using Microwell-seq.

In their study, GUO Guoji et al. compared cell types across vertebrates using the constructed organism-wide cell atlases for humans, mice, zebrafish and Xenopus laevis. Their results demonstrated that endothelial cells were highly conserved in the evolutionary process of vertebrates. By comparing lineage-specific transcription factors (TFs) among these four species, they identified conserved regulatory TFs in the vertebrate lineage. Moreover, their data revealed that Xenopus alveolar epithelial (AE) cells were strongly correlated to with zebrafish swim bladder epithelial cells and mouse alveolar type 2 (AT2) cells, AT1 cells. Meanwhile, Xenopus pulmonary ionocytes exhibited a strong relationship with zebrafish gill ionocytes, zebrafish swim bladder epithelial cells, and mouse AT2 cells. These findings shed light on the evolutionary relationship in the respiratory systems of aquatic, amphibian and terrestrial animals at the single-cell level.

Fig. 2: Cell landscape of larval Xenopus during metamorphosis.

Metamorphosis is a process of self-renewal and organ remodeling in which juvenile cells are replaced by adult ones. To describe the cellular mechanisms regulating amphibian metamorphosis at the single-cell level, the researchers analyzed four key stages during Xenopus metamorphosis: NF48 (premetamorphosis), NF54 (prometamorphosis), NF59 (metamorphosis climax), and NF66 (end of metamorphosis). Notably, hba1 (larval heterogeneity) and hbd (adult heterogeneity) were expressed during premetamorphosis at NF59, confirming that hemoglobin was co-expressed in individual erythrocytes. The existence of this erythrocyte suggested that there was a transient cluster of cells containing both larval and adult hemoglobin. In addition, they analyzed cell lineages from the ectoderm, the mesoderm, and the endoderm and found that antigen processing and presentation might be a shared biological pathway involving tissue remodeling during metamorphosis.

Overall, this study provides a large-scale valuable resource for research on Xenopus metamorphosis and adult organs.