Identification of a unique lamprey gene with tandemly repeated sequences and pharyngeal chondrocyte-specific expression

Development of the vertebra and fin skeleton in the lamprey and its implications for the homology of vertebrate vertebrae

BACKGROUND

Although vertebrae are the defining character of vertebrates, they are found only in rudimentary form in extant agnathans. In addition, the vertebrae of agnathans possess several unique features, such as elastin-like molecules as the main matrix component and late (post-metamorphosis) differentiation of lamprey vertebrae. In this study, by tracing the developmental process of vertebrae in lamprey, we examined the homology of vertebrae between lampreys and gnathostomes.

RESULTS

We found that the lamprey somite is first subdivided mediolaterally, with myotome cells differentiating medially and non-myotome cells emerging laterally. Subsequently, collagen-positive non-myotome cells surround the myotome. This pattern of somitogenesis is rather similar to that in amphioxi and sheds doubt on the presence of a sclerotome, in terms of mesenchyme cells induced by a signal from the notochord, in lamprey. Further tracing of non-myotome cell development revealed that fin cartilage develops in ammocoete larvae approximately 35 mm in body length. The development of the fin cartilage occurs much earlier than that of the vertebra whose development proceeds during metamorphosis.

CONCLUSION

We propose that the homology of vertebrae between agnathans and gnathostomes should be discussed carefully, because the developmental process of the lamprey vertebra is different from that of gnathostomes.

Cartilage diversification and modularity drove the evolution of the ancestral vertebrate head skeleton

The vertebrate head skeleton has evolved a myriad of forms since their divergence from invertebrate chordates. The connection between novel gene expression and cell types is therefore of importance in this process. The transformation of the jawed vertebrate (gnathostome) head skeleton from oral cirri to jointed jaw elements required a diversity of cartilages as well as changes in the patterning of these tissues. Although lampreys are a sister clade to gnathostomes, they display skeletal diversity with distinct gene expression and histologies, a useful model for addressing joint evolution. Specifically, the lamprey tissue known as mucocartilage has noted similarities with the jointed elements of the mandibular arch in jawed vertebrates. We thus asked whether the cells in lamprey mucocartilage and gnathostome joint tissue could be considered homologous. To do this, we characterized new genes that are involved in gnathostome joint formation and characterized the histochemical properties of lamprey skeletal types. We find that most of these genes are minimally found in mucocartilage and are likely later innovations, but we do identify new activity for gdf5/6/7b in both hyaline and mucocartilage, supporting its role as a chondrogenic regulator. Contrary to previous works, our histological assays do not find any perichondrial fibroblasts surrounding mucocartilage, suggesting that mucocartilage is non-skeletogenic tissue that is partially chondrified. Interestingly, we also identify new histochemical features of the lamprey otic capsule that diverge from normal hyaline. Paired with our new insights into lamprey mucocartilage, we propose a broader framework for skeletal evolution in which an ancestral soxD/E and gdf5/6/7 network directs mesenchyme along a spectrum of cartilage-like features.

Cranial cartilages: Players in the evolution of the cranium during evolution of the chordates in general and of the vertebrates in particular

The present contribution is chiefly a review, augmented by some new results on amphioxus and lamprey anatomy, that draws on paleontological and developmental data to suggest a scenario for cranial cartilage evolution in the phylum chordata. Consideration is given to the cartilage-related tissues of invertebrate chordates (amphioxus and some fossil groups like vetulicolians) as well as in the two major divisions of the subphylum Vertebrata (namely, agnathans, and gnathostomes). In the invertebrate chordates, which can be considered plausible proxy ancestors of the vertebrates, only a viscerocranium is present, whereas a neurocranium is absent. For this situation, we examine how cartilage-related tissues of this head region prefigure the cellular cartilage types in the vertebrates. We then focus on the vertebrate neurocranium, where cyclostomes evidently lack neural-crest derived trabecular cartilage (although this point needs to be established more firmly). In the more complex gnathostome, several neural-crest derived cartilage types are present: namely, the trabecular cartilages of the prechordal region and the parachordal cartilage the chordal region. In sum, we present an evolutionary framework for cranial cartilage evolution in chordates and suggest aspects of the subject that should profit from additional study.

A Comprehensive Analysis of Fibrillar Collagens in Lamprey Suggests a Conserved Role in Vertebrate Musculoskeletal Evolution

Vertebrates have distinct tissues which are not present in invertebrate chordates nor other metazoans. The rise of these tissues also coincided with at least one round of whole-genome duplication as well as a suite of lineage-specific segmental duplications. Understanding whether novel genes lead to the origin and diversification of novel cell types, therefore, is of great importance in vertebrate evolution. Here we were particularly interested in the evolution of the vertebrate musculoskeletal system, the muscles and connective tissues that support a diversity of body plans. A major component of the musculoskeletal extracellular matrix (ECM) is fibrillar collagens, a gene family which has been greatly expanded upon in vertebrates. We thus asked whether the repertoire of fibrillar collagens in vertebrates reflects differences in the musculoskeletal system. To test this, we explored the diversity of fibrillar collagens in lamprey, a jawless vertebrate which diverged from jawed vertebrates (gnathostomes) more than five hundred million years ago and has undergone its own gene duplications. Some of the principal components of vertebrate hyaline cartilage are the fibrillar collagens type II and XI, but their presence in cartilage development across all vertebrate taxa has been disputed. We particularly emphasized the characterization of genes in the lamprey hyaline cartilage, testing if its collagen repertoire was similar to that in gnathostomes. Overall, we discovered thirteen fibrillar collagens from all known gene subfamilies in lamprey and were able to identify several lineage-specific duplications. We found that, while the collagen loci have undergone rearrangement, the Clade A genes have remained linked with the hox clusters, a phenomenon also seen in gnathostomes. While the lamprey muscular tissue was largely similar to that seen in gnathostomes, we saw considerable differences in the larval lamprey skeletal tissue, with distinct collagen combinations pertaining to different cartilage types. Our gene expression analyses were unable to identify type II collagen in the sea lamprey hyaline cartilage nor any other fibrillar collagen during chondrogenesis at the stages observed, meaning that sea lamprey likely no longer require these genes during early cartilage development. Our findings suggest that fibrillar collagens were multifunctional across the musculoskeletal system in the last common ancestor of vertebrates and have been largely conserved, but these genes alone cannot explain the origin of novel cell types.

Generation of knock-in lampreys by CRISPR-Cas9-mediated genome engineering

The lamprey represents the oldest group of living vertebrates and has been a key organism in various research fields such as evolutionary developmental biology and neuroscience. However, no knock-in technique for this animal has been established yet, preventing application of advanced genetic techniques. Here, we report efficient generation of F₀ knock-in lampreys by CRISPR-Cas9-mediated genome editing. A donor plasmid containing a heat-shock promoter was co-injected with a short guide RNA (sgRNA) for genome digestion, a sgRNA for donor plasmid digestion, and Cas9 mRNA. Targeting different genetic loci, we succeeded in generating knock-in lampreys expressing photoconvertible protein Dendra2 as well as those expressing EGFP. With its simplicity, design flexibility, and high efficiency, we propose that the present method has great versatility for various experimental uses in lamprey research and that it can also be applied to other “non-model” organisms.