Evolution: Mouth to mouth

Globular-shaped variable lymphocyte receptors B antibody multimerized by a hydrophobic clustering in hagfish

In hagfish and lampreys, two representative jawless vertebrates, the humoral immunity is directly mediated by variable lymphocyte receptors B (VLRBs). Both monomeric VLRBs are structurally and functionally similar, but their C-terminal tails differ: lamprey VLRB has a Cys-rich tail that forms disulfide-linked pentamers of dimers, contributing to its multivalency, whereas hagfish VLRB has a superhydrophobic tail of unknown structure. Here, we reveal that VLRBs obtained from hagfish plasma have a globular-shaped multimerized form (approximately 0.6 to 1.7 MDa) that is generated by hydrophobic clustering instead of covalent linkage. Electron microscopy (EM) and single-particle analysis showed that the multimerized VLRBs form globular-shaped clusters with an average diameter of 28.7 ± 2.2 nm. The presence of VLRBs in the complex was confirmed by immune-EM analysis using an anti-VLRB antibody. Furthermore, the hydrophobic hagfish C-terminus (HC) was capable of triggering multimerization and directing the cellular surface localization via a glycophosphatidylinositol linkage. Our results strongly suggest that the hagfish VLRB forms a previously unknown globular-shaped antibody. This novel identification of a structurally unusual VLRB complex may suggest that the adaptive immune system of hagfish differs from that of lamprey.

Non-parsimonious evolution of hagfish Dlx genes

Background

The number of members of the Dlx gene family increased during the two rounds of whole-genome duplication that occurred in the common ancestor of the vertebrates. Because the Dlx genes are involved in the development of the cranial skeleton, brain, and sensory organs, their expression patterns have been analysed in various organisms in the context of evolutionary developmental biology. Six Dlx genes have been isolated in the lampreys, a group of living jawless vertebrates (cyclostomes), and their expression patterns analysed. However, little is known about the Dlx genes in the hagfish, the other cyclostome group, mainly because the embryological analysis of this animal is difficult.

Results

To identify the hagfish Dlx genes and describe their expression patterns, we cloned the cDNA from embryos of the Japanese inshore hagfish Eptatretus burgeri. Our results show that the hagfish has at least six Dlx genes and one pseudogene. In a phylogenetic analysis, the hagfish Dlx genes and those of the lampreys tended to be excluded from the clade of the gnathostome Dlx genes. In several cases, the lamprey Dlx genes clustered with the clade consisting of two hagfish genes, suggesting that independent gene duplications have occurred in the hagfish lineage. Analysis of the expression of these genes showed distinctive overlapping expression patterns in the cranial mesenchymal cells and the inner ear.

Conclusions

Independent duplication, pseudogenization, and loss of the Dlx genes probably occurred in the hagfish lineage after its split from the other vertebrate lineages. This pattern is reminiscent of the non-parsimonious evolution of its morphological traits, including its inner ear and vertebrae, which indicate that this group is an early-branching lineage that diverged before those characters evolved.

Evolutionary crossroads in developmental biology: cyclostomes (lamprey and hagfish)

Lampreys and hagfish, which together are known as the cyclostomes or ‘agnathans’, are the only surviving lineages of jawless fish. They diverged early in vertebrate evolution, before the origin of the hinged jaws that are characteristic of gnathostome (jawed) vertebrates and before the evolution of paired appendages. However, they do share numerous characteristics with jawed vertebrates. Studies of cyclostome development can thus help us to understand when, and how, key aspects of the vertebrate body evolved. Here, we summarise the development of cyclostomes, highlighting the key species studied and experimental methods available. We then discuss how studies of cyclostomes have provided important insight into the evolution of fins, jaws, skeleton and neural crest.

Unresolved orthology and peculiar coding sequence properties of lamprey genes: the KCNA gene family as test case

BACKGROUND

In understanding the evolutionary process of vertebrates, cyclostomes (hagfishes and lamprey) occupy crucial positions. Resolving molecular phylogenetic relationships of cyclostome genes with gnathostomes (jawed vertebrates) genes is indispensable in deciphering both the species tree and gene trees. However, molecular phylogenetic analyses, especially those including lamprey genes, have produced highly discordant results between gene families. To efficiently scrutinize this problem using partial genome assemblies of early vertebrates, we focused on the potassium voltage-gated channel, shaker-related (KCNA) family, whose members are mostly single-exon.

RESULTS

Seven sea lamprey KCNA genes as well as six elephant shark genes were identified, and their orthologies to bony vertebrate subgroups were assessed. In contrast to robustly supported orthology of the elephant shark genes to gnathostome subgroups, clear orthology of any sea lamprey gene could not be established. Notably, sea lamprey KCNA sequences displayed unique codon usage pattern and amino acid composition, probably associated with exceptionally high GC-content in their coding regions. This lamprey-specific property of coding sequences was also observed generally for genes outside this gene family.

CONCLUSIONS

Our results suggest that secondary modifications of sequence properties unique to the lamprey lineage may be one of the factors preventing robust orthology assessments of lamprey genes, which deserves further genome-wide validation. The lamprey lineage-specific alteration of protein-coding sequence properties needs to be taken into consideration in tackling the key questions about early vertebrate evolution.

microRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate

Hagfish and lampreys are the only living representatives of the jawless vertebrates (agnathans), and compared with jawed vertebrates (gnathostomes), they provide insight into the embryology, genomics, and body plan of the ancestral vertebrate. However, this insight has been obscured by controversy over their interrelationships. Morphological cladistic analyses have identified lampreys and gnathostomes as closest relatives, whereas molecular phylogenetic studies recover a monophyletic Cyclostomata (hagfish and lampreys as closest relatives). Here, we show through deep sequencing of small RNA libraries, coupled with genomic surveys, that Cyclostomata is monophyletic: hagfish and lampreys share 4 unique microRNA families, 15 unique paralogues of more primitive microRNA families, and 22 unique substitutions to the mature gene products. Reanalysis of morphological data reveals that support for cyclostome paraphyly was based largely on incorrect character coding, and a revised dataset is not decisive on the mono- vs. paraphyly of cyclostomes. Furthermore, we show fundamental conservation of microRNA expression patterns among lamprey, hagfish, and gnathostome organs, implying that the role of microRNAs within specific organs is coincident with their appearance within the genome and is conserved through time. Together, these data support the monophyly of cyclostomes and suggest that the last common ancestor of all living vertebrates was a more complex organism than conventionally accepted by comparative morphologists and developmental biologists.

Jaw muscularization requires Dlx expression by cranial neural crest cells

The origin of active predation in vertebrates is associated with the rise of three major, uniquely derived developmental characteristics of the head: (i) migratory cranial neural crest cells (CNCCs) giving rise to most skeletal skull elements; (ii) expression of Dlx genes by CNCCs in the Hox-free first pharyngeal arch (PA1); and (iii) muscularization of PA1 derivatives. Here we show that these three innovations are tightly linked. Expression of Dlx genes by CNCCs is not only necessary for head skeletogenesis, but also for the determination, differentiation, and patterning of cephalic myogenic mesoderm leading to masticatory muscle formation. In particular, inactivation of Dlx5 and Dlx6 in the mouse results in loss of jaw muscles. As Dlx5/6 are not expressed by the myogenic mesoderm, our findings imply an instructive role for Dlx5/6-positive CNCCs in muscle formation. The defect in muscularization does not result from the loss of mandibular identity observed in Dlx5/6(-/-) mice because masticatory muscles are still present in EdnRA(-/-) mutants, which display a similar jaw transformation. The genesis of jaws and their muscularization should therefore be seen as an integrated Dlx-dependent developmental process at the origin of the vertebrate head. The role of Dlx genes in defining gnathostome jaw identity could, therefore, be secondary to a more primitive function in the genesis of the oral skeletomuscular system.