Hox Gene Clusters of Early Vertebrates: Do They Serve as Reliable Markers for Genome Evolution?

AbstractHox genes, responsible for regional specification along the anteroposterior axis in embryogenesis, are found as clusters in most eumetazoan genomes sequenced to date. Invertebrates possess a single Hox gene cluster with some exceptions of secondary cluster breakages, while osteichthyans (bony vertebrates) have multiple Hox clusters. In tetrapods, four Hox clusters, derived from the so-called two-round whole genome duplications (2R-WGDs), are observed. Overall, the number of Hox gene clusters has been regarded as a reliable marker of ploidy levels in animal genomes. In fact, this scheme also fits the situations in teleost fishes that experienced an additional WGD. In this review, I focus on cyclostomes and cartilaginous fishes as lineages that would fill the gap between invertebrates and osteichthyans. A recent study highlighted a possible loss of the HoxC cluster in the galeomorph shark lineage, while other aspects of cartilaginous fish Hox clusters usually mark their conserved nature. In contrast, existing resources suggest that the cyclostomes exhibit a different mode of Hox cluster organization. For this group of species, whose genomes could have differently responded to the 2R-WGDs from jawed vertebrates, therefore the number of Hox clusters may not serve as a good indicator of their ploidy level

Technological and evolutionary challenges for reconstructing our ancestor’s genome from 500 million years ago

It was long known that our genome harbors redundant components, which was introduced by the so-called two-round whole genome duplications (2R-WGD). In 2009, the speaker of this seminar proposed for the first time that the 2R-WGD occurred before the evolutionary lineage of cyclostomes (hagfishes and lampreys) branched off from the future jawed vertebrate lineage (Kuraku et al., 2009. Mol. Evol. Biol. 26:47-; ‘PV4 hypothesis’). Since then, the genome sequence information of cyclostomes (mainly northern hemisphere lampreys) has been repeatedly analyzed, which however has not resulted in synthetic understanding that reconciles different aspects of their genomes including multiplicity of landmark gene clusters (e.g. Hox gene clusters) and phylogenetic properties of paralogous genes. Most recently, the speaker’s research group performed whole genome sequencing and molecular evolutionary analysis of elasmobranch sharks (Hara et al., 2018. Nat. Ecol. Evol. 2:1761-). These genomes exhibit remarkably lesser degrees of sequence divergence and thus highlight a derived nature of cyclostome genomes (reviewed in Manousaki et al., 2016.). This seminar will summarize the history of studies on the 2R-WGD and discuss potential solutions for disentangling it (see Onimaru & Kuraku, 2018. Brief. Funct. Genomics 17:352-)

Evidence for the prepattern/cooption model of vertebrate jaw evolution

The appearance of jaws was a turning point in vertebrate evolution because it allowed primitive vertebrates to capture and process large, motile prey. The vertebrate jaw consists of separate dorsal and ventral skeletal elements connected by a joint. How this structure evolved from the unjointed gill bar of a jawless ancestor is an unresolved question in vertebrate evolution. To understand the developmental bases of this evolutionary transition, we examined the expression of 12 genes involved in vertebrate pharyngeal patterning in the modern jawless fish lamprey. We find nested expression of Dlx genes, as well as combinatorial expression of Msx, Hand and Gsc genes along the dorso-ventral (DV) axis of the lamprey pharynx, indicating gnathostome-type pharyngeal patterning evolved before the appearance of the jaw. In addition, we find that Bapx and Gdf5/6/7, key regulators of joint formation in gnathostomes, are not expressed in the lamprey first arch, whereas Barx, which is absent from the intermediate first arch in gnathostomes, marks this domain in lamprey. Taken together, these data support a new scenario for jaw evolution in which incorporation of Bapx and Gdf5/6/7 into a preexisting DV patterning program drove the evolution of the jaw by altering the identity of intermediate first-arch chondrocytes. We present this “Pre-pattern/Cooption” model as an alternative to current models linking the evolution of the jaw to the de novo appearance of sophisticated pharyngeal DV patterning

The origin of bmp16, a novel Bmp2/4 relative, retained in teleost fish genomes

<p>Abstract</p> <p>Background</p> <p>Whole genome sequences have allowed us to have an overview of the evolution of gene repertoires. The target of the present study, the TGFβ superfamily, contains many genes involved in vertebrate development, and provides an ideal system to explore the relationships between evolution of gene repertoires and that of developmental programs.</p> <p>Results</p> <p>As a result of a bioinformatic survey of sequenced vertebrate genomes, we identified an uncharacterized member of the TGFβ superfamily, designated <it>bmp16</it>, which is confined to teleost fish species. Our molecular phylogenetic study revealed a high affinity of <it>bmp16 </it>to the <it>Bmp2/4 </it>subfamily. Importantly, further analyses based on the maximum-likelihood method unambiguously ruled out the possibility that this teleost-specific gene is a product of teleost-specific genome duplication. This suggests that the absence of a <it>bmp16 </it>ortholog in tetrapods is due to a secondary loss. <it>In situ </it>hybridization showed embryonic expression of the zebrafish <it>bmp16 </it>in the developing swim bladder, heart, tail bud, and ectoderm of pectoral and median fin folds in pharyngula stages, as well as gut-associated expression in 5-day embryos.</p> <p>Conclusion</p> <p>Comparisons of expression patterns revealed (1) the redundancy of <it>bmp16 </it>expression with its homologs in presumably plesiomorphic expression domains, such as the fin fold, heart, and tail bud, which might have permitted its loss in the tetrapod lineage, and (2) the loss of craniofacial expression and gain of swim bladder expression of <it>bmp16 </it>after the gene duplication between <it>Bmp2</it>, <it>-4 </it>and <it>-16</it>. Our findings highlight the importance of documenting secondary changes of gene repertoires and expression patterns in other gene families.</p

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

The evolutionally-conserved function of group B1 Sox family members confers the unique role of Sox2 in mouse ES cells.

BACKGROUND: In mouse ES cells, the function of Sox2 is essential for the maintenance of pluripotency. Since the Sox-family of transcription factors are well conserved in the animal kingdom, addressing the evolutionary origin of Sox2 function in pluripotent stem cells is intriguing from the perspective of understanding the origin of pluripotency. RESULTS: Here we approach this question using a functional complementation assay in inducible Sox2-null ES cells. Assaying mouse Sox proteins from different Groups, we found that only Group B1 and Group G proteins were able to support pluripotency. Interestingly, invertebrate homologs of mammalian Group B1 Sox proteins were able to replace the pluripotency-associated function of mouse Sox2. Moreover, the mouse ES cells rescued by the Drosophila SoxNeuro protein are able to contribute to chimeric embryos. CONCLUSIONS: These data indicate that the function of mouse Sox2 supporting pluripotency is based on an evolutionally conserved activity of the Group B1 Sox family. Since pluripotent stem cell population in developmental process could be regarded as the evolutional novelty in vertebrates, it could be regarded as a co-optional use of their evolutionally conserved function

Ancestral and derived attributes of the dlx gene repertoire, cluster structure and expression patterns in an African cichlid fish

<p>Abstract</p> <p>Background</p> <p>Cichlid fishes have undergone rapid, expansive evolutionary radiations that are manifested in the diversification of their trophic morphologies, tooth patterning and coloration. Understanding the molecular mechanisms that underlie the cichlids' unique patterns of evolution requires a thorough examination of genes that pattern the neural crest, from which these diverse phenotypes are derived. Among those genes, the homeobox-containing <it>Dlx </it>gene family is of particular interest since it is involved in the patterning of the brain, jaws and teeth.</p> <p>Results</p> <p>In this study, we characterized the <it>dlx </it>genes of an African cichlid fish, <it>Astatotilapia burtoni</it>, to provide a baseline to later allow cross-species comparison within Cichlidae. We identified seven <it>dlx </it>paralogs (<it>dlx1a</it>, <it>-2a</it>, <it>-4a</it>, <it>-3b</it>, <it>-4b</it>, <it>-5a </it>and <it>-6a</it>), whose orthologies were validated with molecular phylogenetic trees. The intergenic regions of three <it>dlx </it>gene clusters (<it>dlx1a-2a</it>, <it>dlx3b-4b</it>, and <it>dlx5a-6a</it>) were amplified with long PCR. Intensive cross-species comparison revealed a number of conserved non-coding elements (CNEs) that are shared with other percomorph fishes. This analysis highlighted additional lineage-specific gains/losses of CNEs in different teleost fish lineages and a novel CNE that had previously not been identified. Our gene expression analyses revealed overlapping but distinct expression of <it>dlx </it>orthologs in the developing brain and pharyngeal arches. Notably, four of the seven <it>A. burtoni dlx </it>genes, <it>dlx2a</it>, <it>dlx3b</it>, <it>dlx4a </it>and <it>dlx5a</it>, were expressed in the developing pharyngeal teeth.</p> <p>Conclusion</p> <p>This comparative study of the <it>dlx </it>genes of <it>A. burtoni </it>has deepened our knowledge of the diversity of the <it>Dlx </it>gene family, in terms of gene repertoire, expression patterns and non-coding elements. We have identified possible cichlid lineage-specific changes, including losses of a subset of <it>dlx </it>expression domains in the pharyngeal teeth, which will be the targets of future functional studies.</p

Evolution of oropharyngeal patterning mechanisms involving Dlx and endothelins in vertebrates

AbstractIn jawed vertebrates, the Dlx code, or nested expression patterns of Dlx genes, specify the dorsoventral polarity of pharyngeal arches, downstream of endothelin-1 (Edn-1) and its effectors, Bapx1 (Nkx3.2) and dHand (Hand2). To elucidate the evolution of the specification mechanism of the oropharyngeal skeletal system, lamprey homologs of Dlx, Edn, endothelin receptor (Ednr), Bapx1, and dHand were identified. Our analysis suggested that the Edn gene family emerged at the advent of vertebrates, and that gene duplications leading to the different Edn gnathostome subtypes (Edn1–3) occurred before the cyclostome–gnathostome split. This timing of gene duplications, giving rise to multiple subtypes, was also implied for Dlx, Ednr, Hand, and Bapx. In lamprey embryos, nested expressions of Dlx genes were not observed in pharyngeal arches, nor was any focal expression of Bapx1, known in gnathostomes to specify the jaw joint. The dHand homolog, however, was expressed more intensively ventrally, as in gnathostomes. Lamprey homologs of Edn-1 and EdnrA were also shown to be expressed as described in mice, indicating involvement of this signaling pathway in the craniofacial patterning early in vertebrate evolution. These results suggest that the last common ancestor of all the extant vertebrates would have possessed basic gene repertoires involved in oropharyngeal patterning in gnathostomes, but the elaborate genetic program leading to the Dlx code is likely to have been acquired uniquely in gnathostomes

Monophyly of lampreys and hagfishes supported by nuclear DNA-coded genes

Abstract. The phylogenetic position of hagfishes in vertebrate evolution is currently controversial. The 18S and 28S rRNA trees support the monophyly of hagfishes and lampreys. In contrast, the mitochondrial DNAs suggest the close association of lampreys and gnathostomes. To clarify this controversial issue, we have conducted cloning and sequencing of the four nuclear DNA-coded single-copy genes encoding the triose phosphate isomerase, calreticulin, and the largest subunit of RNA polymerase II and III. Based on these proteins, together with the Mn superoxide dismutase for which hagfish and lamprey sequences are available in database, phylogenetic trees have been inferred by the maximum likelihood (ML) method of protein phylogeny. It was shown that all the five proteins prefer the monophyletic tree of cyclostomes, and the total log-likelihood of the five proteins significantly supports the cyclostome monophyly at the level of ±1 SE. The ML trees of aldolase family comprising three nonallelic isoforms and the complement component group comprising C3, C4, and C5, both of which diverged during vertebrate evolution by gene duplications, also suggest the cyclostome monophyly