Biology subjects

Criswell, K. E.

Publications and source records attributed to Criswell, K. E..

5 recordsLinked to original sources

bioRxiv2026

Whole body elongation drives coordinated vertebral shape evolution in Lake Malawi cichlid fishes

Understanding how anatomical structures evolve requires disentangling the roles of integration and modularity in shaping morphological variation. The vertebral column, a serially repeated and regionally differentiated structure, provides a powerful system for investigating these processes. Here, we examine how vertebral morphology evolves in relation to whole-body elongation across the adaptive radiation of Lake Malawi cichlid fishes. We tested for evolutionary integration between the precaudal and caudal domains, as well as assessed the contributions of vertebral count, centrum shape, and intervertebral spacing on body elongation. We find strong evolutionary integration between the shapes of precaudal and caudal vertebrae, with both vertebral shapes varying along similar axes. Despite this, precaudal and caudal vertebral counts evolve independently, indicating a decoupling between the specification of identity and the development of their respective shapes. Whole-body elongation is significantly associated with coordinated changes in vertebral and rib morphology, including proportional increases in centrum size, posterior displacement of neural and haemal spines, and increased rib curvature. In contrast, centrum elongation and intervertebral spacing do not contribute to body elongation across the radiation. These results demonstrate that body elongation in cichlids necessitates integrated, multivariate changes in axial morphology. Our findings highlight the importance of morphological integration in facilitating coordinated evolutionary responses in anatomical systems.

evolutionary biology↗

bioRxiv2023

The skate spiracular organ develops from a unique neurogenic placode, distinct from lateral line placodes

The spiracular organ is an epithelial pouch or tube lined with mechanosensory hair cells, found embedded in the wall of the spiracle/spiracular chamber (remnant of the first pharyngeal cleft) in many non-teleost jawed fishes. It is innervated via a branch of the anterior lateral line nerve and usually considered a specialised lateral line organ, although it most likely functions as a proprioceptor for jaw movement. It is homologous to the paratympanic organ: a hair cell-lined epithelial pouch embedded in the wall of the middle ear (which evolved from the spiracular chamber) of birds, alligators and Sphenodon. A previous fate-mapping study showed that the chicken paratympanic organ and its afferent neurons originate from a molecularly distinct placode immediately dorsal to the geniculate (first epibranchial) placode. Here, DiI fate-mapping in a cartilaginous fish (little skate, Leucoraja erinacea) shows that the spiracular organ derives from a previously unrecognised neurogenic placode immediately dorsal to the geniculate placode and molecularly distinct from lateral line placodes. These findings further support the developmental and evolutionary independence of this unique jawed-vertebrate mechanosensory organ from the lateral line system. Summary statementFate-mapping in a cartilaginous fish supports the developmental independence from the lateral line system of the mechanosensory spiracular organ, consistent with its homology with the amniote paratympanic organ.

developmental biology↗

bioRxiv2021

Conserved and unique transcriptional features of pharyngeal arches in the skate (Leucoraja erinacea) and evolution of the jaw

The origin of the jaw is a long-standing problem in vertebrate evolutionary biology. Classical hypotheses of serial homology propose that the upper and lower jaw evolved through modifications of dorsal and ventral gill arch skeletal elements, respectively. If the jaw and gill arches are derived members of a primitive branchial series, we predict that they would share common developmental patterning mechanisms. Using candidate and RNAseq/differential gene expression analyses, we find broad conservation of dorsoventral patterning mechanisms within the developing mandibular, hyoid and gill arches of a cartilaginous fish, the skate (Leucoraja erinacea). Shared features include expression of genes encoding members of the ventralising BMP and endothelin signalling pathways and their effectors, the joint markers bapx1 and gdf5 and pro-chondrogenic transcription factors barx1 and gsc, and the dorsalising transcription factor pou3f3. Additionally, we find that mesenchymal expression of eya1/six1 is an ancestral feature of the mandibular arch of jawed vertebrates, while differences in notch signalling distinguish the mandibular and gill arches in skate. Comparative transcriptomic analyses of mandibular and gill arch tissues reveal additional genes differentially expressed along the dorsoventral axis of the pharyngeal arches, including scamp5 as a novel marker of the dorsal mandibular arch, as well as distinct transcriptional features of mandibular and gill arch muscle progenitors and developing gill buds. Taken together, our findings reveal conserved patterning mechanisms in the pharyngeal arches of jawed vertebrates, consistent with serial homology of their skeletal derivatives, as well as unique transcriptional features that may underpin distinct jaw and gill arch morphologies.

developmental biology↗

bioRxiv2019

Resegmentation is an ancestral feature of the gnathostome vertebral skeleton

The vertebral skeleton is a defining feature of vertebrate animals. However, the mode of vertebral segmentation varies considerably between major lineages. In tetrapods, adjacent somite halves recombine to form a single vertebra through the process of \"resegmentation\". However, in teleost fishes, there is considerable mixing between cells of the anterior and posterior somite halves, without clear resegmentation. To determine whether resegmentation is a tetrapod novelty, or an ancestral feature of jawed vertebrates, we tested the relationship between somites and vertebrae in a cartilaginous fish, the skate (Leucoraja erinacea). Using cell lineage tracing, we show that skate trunk vertebrae arise through tetrapod-like resegmentation, with anterior and posterior halves of each vertebra deriving from adjacent somites. We further show that tail vertebrae also arise through resegmentation, despite a duplication of the number of vertebrae per body segment. These findings resolve axial resegmentation as an ancestral feature of the jawed vertebrate body plan.

developmental biology↗

bioRxiv2019

Extraocular, rod-like photoreceptors in a flatworm express xenopsin photopigment

Animals detect light using opsin photopigments. One recently classified opsin clade, the xenopsins, found in lophotrochozoans, challenges our views on opsin and photoreceptor evolution. Originally thought to belong to the Gi-coupled ciliary opsins, xenopsins are now understood to have diverged from ciliary opsins in pre-bilaterian times, but little is known about the cells that deploy these proteins, or if they form a photopigment and drive phototransduction. We characterized xenopsin in a flatworm, Maritigrella crozieri, and found that it is expressed in a larval eyespot, and in an abundant extraocular cell type around the adult brain. These distinct cells house hundreds of cilia in an intra-cellular vacuole (a phaosome). Cellular assays show Mc xenopsin forms a photopigment and couples to Gi/o in response to light. These findings reveal a novel photoreceptor cell type and opsin/G-protein couple, and highlight the convergent enclosure of photosensitive cilia in flatworm phaosomes and jawed vertebrate rods.

zoology↗