The Origin of Vertebrate Gills

Abstract

Pharyngeal gills are a fundamental feature of the vertebrate body plan [1]. However, the evolutionary history of vertebrate gills has been the subject of a long-standing controversy [2-8]. It is thought that gills evolved independently in cyclostomes (jawless vertebrates-lampreys and hagfish) and gnathostomes (jawed vertebrates-cartilaginous and bony fishes), based on their distinct embryonic origins: the gills of cyclostomes derive from endoderm [9-12], while gnathostome gills were classically thought to derive from ectoderm [10, 13]. Here, we demonstrate by cell lineage tracing that the gills of a cartilaginous fish, the little skate (Leucoraja erinacea), are in fact endodermally derived. This finding supports the homology of gills in cyclostomes and gnathostomes, and a single origin of pharyngeal gills prior to the divergence of these two ancient vertebrate lineages.

Keywords: chondrichthyan; development; endoderm; evolution; gills; homology; pharyngeal arch; skate; vertebrate.

Figure 1

The Ecto-endobranchiate Hypothesis The independent evolution of gills in cyclostomes and gnathostomes (from a gill-less common ancestor), based on their distinct embryonic origins from endoderm and ectoderm, respectively. Redrawn after Jarvik [3] and Jollie [5].

Figure 2

Skate Gill Filaments Arise within an Endodermal Shh Expression Domain (A) At stage 22, the sequence of pharyngeal arch formation may be captured along the rostro-caudal axis of a single embryo (rostral to the left). Endodermally derived pharyngeal pouches (pp) contact surface ectoderm and ultimately fuse with this ectoderm (black arrow), giving rise to a gill slit (gs). The columns of tissue that are isolated by adjacent gill slits are pharyngeal arches (pharyngeal arches 3, 4, and 5 shown here). (B) Shh is expressed along the anterior wall of each pharyngeal pouch (pp) and, eventually, along the posterior wall of each pharyngeal arch (pharyngeal arches 2, 3, 4, and 5 shown here). The red dashed line at the interface between Shh-expressing and non-expressing epithelia indicates the predicted interface between endoderm (en) and ectoderm (ec). Note that early gill filaments (gf) arise within Shh-expressing epithelium. The black dashed lines delineate caudal pharyngeal pouch endoderm, and the black arrow indicates a pharyngeal pouch fusing with overlaying surface ectoderm. (C) Schematic illustrating predicted tissue contributions to skate pharyngeal arches. Based on histological and gene expression analyses, we predict that gill filaments (gf) derive from endodermal epithelium. Scale bars represent 40 μm.

Figure 3

Skate Gills Derive from Pharyngeal Endoderm (A and B) Microinjection of the pharyngeal cavity (pc) of skate embryos with CM-DiI at stage 18 (A) allows us to focally label cells (B) within the pharyngeal endoderm. (C and D) CM-DiI-labeling of endodermal epithelium allows visualization of endodermal contributions to the pharyngeal arches prior to (C) and immediately following (D) the perforation of pharyngeal pouches with overlying ectoderm. Endodermally derived epithelium encircles approximately 3/4 of the circumference of the pharyngeal arches. (E and F) By stage 27, CM-DiI-labeled external gill filaments can be observed in whole-mount (E) and in histological section (F), indicating their endodermal origin. (G and G′) By stage 28, CM-DiI-positive internal gill filaments are also observed in histological section, indicating their endodermal origin. Image in (G′) is the same section as in (G), stained with hematoxylin and eosin. (H–H″) Gills of a stage 28 skate embryo (dashed inset box in G and G′), showing CM-DiI-positive gill filaments. Images in (H′) and (H″) are the same section as in (H), stained with CM-DiI only (H′) and hematoxylin and eosin (H″).