Additional file 3: Table S1. of Palaeospondylus as a primitive hagfish

Comparison of craniofacial primordia in cyclostomes and crown gnathostomes. Based on [22]. (DOC 29 kb

Additional file 2: Figure S2. of Palaeospondylus as a primitive hagfish

Referred Palaeospondylus specimens illustrated in previous studies. (a,b) A specimen in dorsal (a) and ventral (b) views, from [26]. (c,d) three-dimensional model reconstructed from sections in dorsal (c) and ventral (d) views, from [5]. (e) NHM (Natural History Museum, London) P 16123 in dorsal view, from [8]. (f), NHM P 16125 in ventral view, from [8]. amp, ampyx; cp, caudal plate; gam, gammation; hem, hemidome; otc, otic capsule; ros, rostralia; ve, V-shaped element; vert, vertebra. (JPG 860 kb

Additional file 4: Table S2. of Palaeospondylus as a primitive hagfish

Comparison of cranial skeletal elements among extant cyclostomes and Palaeospondylus. Names of skeletal elements in Palaeospondylus are shown by abbreviations employed in the present paper. For the nomenclature of the lamprey and hagfish chondrocrania, see [23]. (DOC 38 kb

Additional file 1: Figure S1. of Palaeospondylus as a primitive hagfish

Fossils of Palaeospondylus from the Middle Devonian of the Scotland. (a) entire specimen of Palaeospondylus gunni (AMNH FF 10743) in ventral view. (b) cranial skeleton of P. gunni (AMNH FF 7586) in dorsal view. (c) Line drawing of B. (d) cranial skeleton of P. gunni (AMNH FF 10742) in ventral view. (e) Line drawing of D. amp, ampyx; cp, caudal plate; gam, gammation; hem, hemidome; otc, otic capsule; ros, rostralia; rp, rostral plate; ve, V-shaped element; vert, vertebra. Scale bar, 1 mm. (JPG 3 mb

Non-parsimonious evolution of hagfish <it>Dlx</it> genes

Abstract 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.</p

The evolutionary origins of chordate hematopoeisis and vertebrate endothelia

AbstractThe vertebrate circulatory system is the most complex vascular system among those of metazoans, with key innovations including a multi-chambered heart and highly specialized blood cells. Invertebrate vessels, on the other hand, consist of hemal spaces between the basal laminae of epithelia. How the evolutionary transition from an invertebrate-type system to the complex vertebrate one occurred is, however, poorly understood. We investigate here the development of the cardiovascular system of the cephalochordate amphioxus Branchiostoma lanceolatum in order to gain insight into the origin of the vertebrate cardiovascular system. The cardiac markers Hand, Csx (Nkx2-5) and Tbx4/5 reveal a broad cardiac-like domain in amphioxus; such a decentralized organization during development parallels that seen in the adult anatomy. Our data therefore support the hypothesis that amphioxus never possessed a proper heart, even transiently during development. We also define a putative hematopoietic domain, supported by the expression of the hematopoietic markers Scl and Pdvegfr. We show that this area is closed to the dorsal aorta anlages, partially linked to excretory tissues, and that its development is regulated by retinoic acid, thus recalling the aorta-gonads-mesonephros (AGM) area of vertebrates. This region probably produces Pdvegfr+ hemal cells, with an important role in amphioxus vessel formation, since treatments with an inhibitor of PDGFR/VEGFR lead to a decrease of Laminin in the basal laminae of developing vessels. Our results point to a chordate origin of hematopoiesis in an AGM-like area from where hemal Pdvegfr+ cells are produced. These Pdvegfr+ cells probably resemble the ancestral chordate blood cells from which the vertebrate endothelium later originated

Thyroid and endostyle development in cyclostomes provides new insights into the evolutionary history of vertebrates.

The endostyle is an epithelial exocrine gland found in non-vertebrate chordates (amphioxi and tunicates) and the larvae of modern lampreys. It is generally considered to be an evolutionary precursor of the thyroid gland of vertebrates. Transformation of the endostyle into the thyroid gland during the metamorphosis of lampreys is thus deemed to be a recapitulation of a past event in vertebrate evolution. In 1906, Stockard reported that the thyroid gland in hagfish, the sister cyclostome group of lampreys, develops through an endostyle-like primordium, strongly supporting the plesiomorphy of the lamprey endostyle. However, the findings in hagfish thyroid development were solely based on this single study, and these have not been confirmed by modern molecular, genetic, and morphological data pertaining to hagfish thyroid development over the last century. Here, we showed that the thyroid gland of hagfish undergoes direct development from the ventrorostral pharyngeal endoderm, where the previously described endostyle-like primordium was not found. The developmental pattern of the hagfish thyroid, including histological features and regulatory gene expression profiles, closely resembles that found in modern jawed vertebrates (gnathostomes). Meanwhile, as opposed to gnathostomes but similar to non-vertebrate chordates, lamprey and hagfish share a broad expression domain of Nkx2-1/2-4, a key regulatory gene, in the pharyngeal epithelium during early developmental stages. Based on the direct development of the thyroid gland both in hagfish and gnathostomes, and the shared expression profile of thyroid-related transcription factors in the cyclostomes, we challenge the plesiomorphic status of the lamprey endostyle and propose an alternative hypothesis where the lamprey endostyle could be obtained secondarily in crown lampreys