The endoderm cell trajectory of urochordate Styela clava reveals the dual developmental origin and evolution of digestive tract

Reviewer #1 (Public review):

Summary:

The authors employ state-of-the-art single-cell sequencing technologies to map the gene expression profiles of the developing digestive tract in the ascidian Styela clava, a member of the invertebrate sister group to vertebrates. This data has the potential to provide a new perspective on the relationships between the guts of an invertebrate like this ascidian relative to vertebrate systems. Key findings include the elaboration of our understanding that the Styela gut arises from two distinct cellular origins, with this being comparable to the dual embryogenic origin of vertebrate guts (at least, as exemplified by the mouse digestive tract arising from both definitive and visceral endoderm).

Strengths:

The resolution that can be achieved from the series of developmental stages analysed by the authors through the metamorphosis and early gut specification and development is vital to the strength of this new dataset. This new scRNAseq data is likely to provide a useful foundation for future work that delves into the functions of various genes within regions of the ascidian gut.

Weaknesses:

The main weakness of the manuscript as it currently stands is the lack of clarity about the genetic comparisons between ascidian and mouse, and what the precise genetic underpinnings are for any statements of similarity.

Reviewer #2 (Public review):

This manuscript explores endodermal lineage specification during metamorphosis in Styela clava. As biphasic lifestyle organisms, the endoderm exists as a rudiment in the non-feeding larvae that differentiates throughout metamorphosis to build the digestive components of the adult body plan. The authors of this manuscript use scRNA sequencing of individuals throughout the metamorphic process, as well as maturing juveniles, to follow the trajectories of the endodermal precursors. They identify two distinct populations that give rise to the stomach and intestinal lineages, and they suggest that there are homologous relationships between tunicate & vertebrate dual-origin endodermal lineages. Additionally, the authors highlight the role of conserved FGF signal-dependent programs in digestive organ patterning and suggest that endodermal fate restriction occurs earlier in Styela in comparison with the mouse gut.

Overall, the paper is the first in-depth look at tunicate endodermal fate from a single-cell sequencing perspective and provides a robust framework for understanding the evolutionary origins of the deuterostome/chordate gut. The data is substantial and of great interest. However, we find their discussion of evolutionary implications to be highly problematic, and there are also numerous major issues regarding the clarity and cogency of their data presentation. Thus, we consider that substantial revision is required to provide a more accurate analysis of this data and its evolutionary implications. This revision would not require further experimentation.