Early Stage Of Vertebrate Evolution Research Articles

Comparative genomics of medaka: the major histocompatibility complex (MHC).

The major histocompatibility complex (MHC) is one of the best characterized regions of the vertebrate genome. The human MHC has three subregions, classes I, II, and III. The MHC of chicken and Xenopus contain all three subregions. In contrast, in all teleost species so far analyzed, the class I and II genes are not linked to each other, suggesting that there was extensive genomic reorganization of the MHC region during an early stage of vertebrate evolution. To elucidate the details of this reorganization, we carried out genetic and physical analyses of the medaka MHC genes. We isolated the medaka counterparts of human MHC genes, class I A, class II B, LMP2, LMP7, TAP2, complement Bf and C4, and subjected them to linkage analysis. Except for the linkage of class IA, LMP2, LMP7, and TAP2 on linkage group (LG)11, all other genes were assigned to separate linkage groups. Thus, the class IA gene and the genes involved in class I antigen presentation seem to form the evolutionary stable core of the MHC. A physical analysis of medaka MHC class I region is in progress.

Complement system of bony and cartilaginous fish

Accumulating evidence indicates that the complement system experienced a discontinuous development at an early stage of vertebrate evolution. Invertebrates such as echinoderms and ascidians, and the most primitive extant vertebrates, the cyclostomes, seem to have a primitive complement system equipped only with the alternative and lectin pathways. In contrast, cartilaginous fish and higher vertebrates seem to have a modern complement system which has two additional pathways, namely the classical and lytic pathways. Recent molecular analyses of the complement system of bony and cartilaginous fish have not only confirmed the above conclusion, but also revealed a unique characteristic of the complement system of fish, where certain key component genes are duplicated. The complement system seems to play a more pivotal role in body defence in fish, whose adaptive immunity is considered to be at a relatively undeveloped state.

Origin and evolution of the complement system.

The vertebrate immune system is composed of two parts, innate immunity which recognizes the invading microbes using germline-encoded molecules, and adaptive immunity, which depends on recognition molecules generated by somatic mechanisms during the ontogeny of each individual organism (Medzhitov and Janeway 1997). All data available to date indicate that adaptive immunity became established at the early stage of vertebrate evolution around the time of cartilaginous fish emergence. Thus the genes which encode the pivotal elements of adaptive immunity, such as immunoglobulin (Litman et al. 1993), T-cell receptor (Rast et al. 1997), major histocompatibility complex (MHC) class I (Hasulmoto et al. 1992) and class II molecules (Kasahara et al. 1992; Bartle and Weissman 1994) and recombination activating gene (Greenhalgh and Steiner 1995) have been identified in cartilaginous fish and higher vertebrates. None of the attempts to isolate these genes from the most primitive extant vertebrates, cyclostomes, has yet succeeded. In contrast, vertebrate innate immunity is believed to have a more ancient origin, and an apparently primitive complement system has been found in lamprey (Nonaka et al. 1983). However, it was not clear until recently whether the origin of the complement system can be traced back to invertebrate. Identification of C3/C4/C5-like expressed sequence tag (Est) from sea urchin coelomocytes (Smith et al. 1996) and molecular studies of the complement system in sea urchin and ascidian established the presence of the multicomponent, opsonic complement system in invertebrates.

Monoalleleic transcription of the insulin-like growth factor-II gene (Igf2) in chick embryos.

A polymorphism in the igf2 gene of chickens was identified using NlaIII (GenBank accession number AF218827). In some embryos, the igf2 alleles were expressed monoallelically from either maternal or paternal alleles. These data demonstrate that genomic imprinting is not confined to mammalian vertebrates and suggest that genomic imprinting evolved at an early stage of vertebrate evolution. The observations that the igf2 gene is imprinted in a minority of embryos suggest that the imprinting in birds is unrelated to embryonic growth. Genome imprinting may provide opportunities for evolution of genes in a nonexpressed state. In poultry breeding, the presence of imprinted genes may make a major contribution to unequal performance in reciprocal matings between commercial lines.

Immunohistochemical Distribution of Neuropeptide Y-Related Substance in the Brain and Hypophysis of the Arctic Lamprey, Lethenteron japonica

The distribution of neuropeptide Y (NPY)-like immunoreactivity in the brain and hypophysis of the arctic lamprey, Lethenteron japonica, was studied by the use of polyclonal anti-synthetic porcine NPY antibody. Immunoreactivity was found throughout the brain, with varied densities among different areas. NPY-positive cell bodies were located in the dorsal pallium of the telencephalon; in the preoptic area, the thalamus, and the hypothalamus of the diencephalon; in the tegmentum of the mesencephalon; and in the octavolateralis nucleus, the middle reticular nucleus, and the vagal motor nucleus of the rhombencephalon. The majority of the NPY-positive cells in the thalamus and in the hypothalamus appeared as cerebrospinal fluid-contacting neurons. NPY-positive fibers were widely distributed in the brain and locally dense in the ventromedial part of the lateral pallium, in the posterior part of the interpeduncular nucleus, and in the raphe region. Conversely, they were less dense or well scattered in the olfactory bulb, the lateral pallium except for its ventromedial part, and the optic tectum. NPY-positive innervation in the neurohypophysis was evident, suggesting that involvement of NPY-related substance in the hypothalamo-hypophysial system had occurred in an early stage of vertebrate evolution.

Insulin-family peptide–receptor interaction at the early stage of vertebrate evolution

This is an overview of our studies on insulin and insulin-like growth factor-I (IGF-I) interactions with their own and each other’s receptors in the lamprey ( Lampetra fluviatilis L.), an extant representative of the ancient vertebrate group of Agnathans as compared to mammal (rat). Lamprey insulin receptor shows species specificity, namely, it binds its own insulin with higher affinity than mammalian hormone. Nevertheless, and unlike mammalian insulin receptor, lamprey receptor discriminates relatively poorly between insulin and IGF-I. Autophosphorylation patterns are identical for both receptors. In contrast, IGF-I receptors in lamprey tissues are very similar to mammalin IGF-I receptors confirming known evolutionary conservatism of IGF receptor system. Presumed common evolutionary origin of insulin and IGF-I receptors and poor ability of lamprey insulin receptor to discriminate between two ligands, implies that lamprey insulin receptor is closer to putative ancestral protein than IGF-I receptor. Contrary to the common belief, ambient temperatures for lampreys (4–15°C) put no constraints on either downregulation of receptors or the endocytosis of hormone–receptor complexes.

Complete sequence of a sea lamprey (Petromyzon marinus) mitochondrial genome: early establishment of the vertebrate genome organization.

The complete nucleotide sequence of a sea lamprey (Petromyzon marinus) mitochondrial genome has been determined. The lamprey genome is 16,201 bp in length and contains genes for 13 proteins, two rRNAs, 22 tRNAs and two major noncoding regions. The order and transcriptional polarities of protein-coding genes are basically identical to those of other chordate mtDNAs, demonstrating that the common mitochondrial gene organization of vertebrates was established at an early stage of vertebrate evolution. The two major noncoding regions are separated by two tRNA genes. The first region probably functions as the control region because it contains distinctive conserved sequence blocks (CSB-II and III) common to other vertebrate control regions. The central conserved domain observed in other vertebrate control regions is not found in the lamprey, suggesting that it is a recently evolved functional domain in vertebrates. Noncoding segments are not found in the expected position of the origin of replication for the second strand, suggesting either that one of the tRNA genes has a dual function or that the second noncoding region may function as the second-strand origin. The base composition at the wobble positions of fourfold degenerate codon families is highly biased toward thymine (32.7%). Values of GC- and AT-skew are typical of vertebrate mitochondrial genomes.

Nucleoside uptake by red blood cells from a primitive vertebrate, the pacific hagfish ( Eptatretus stouti), is mediated by a nitrobenzylthioinosine-insensitive transport system

Red blood cells from the Pacific hagfish ( Eptatretus stouti) were found to possess a facilitated diffusion nucleoside transport system insensitive to inhibition by the nucleoside transport inhibitor nitrobenzylthioinosine (NBMPR). Uridine uptake by this route was saturable (apparent K m 0.14 mM; V max 2 mmol/l cells per h at 10°C), inhibited by inosine and adenosine, and blocked both by the vasodilator dipyridamole and by the thiol-reactive agent p-chloromercuriphenylsulphonate. The properties of this carrier resemble closely those of NBMPR-insensitive nucleoside transport systems in some mammalian neoplastic cell lines and in rat red cells. The presence of this type of carrier in a primitive vertebrate suggests that such transporters have a broad biological distribution and that they pre-date or arose at an early stage of vertebrate evolution.

An isthmo-optic system in a bony fish.

This investigation reveals the existence of an isthmo-optic system in a bony fish for the first time. Two cell aggregates of the isthmic region project bilaterally to each eye in Polypterus. The crossed connections are significantly more developed than the uncrossed ones. These findings provide further evidence for the presence of bilaterally projecting isthmo-optic systems in early stages of vertebrate evolution. Furthermore, they suggest that a loss of one or all of these connections during evolutionary or ontogenetic development reflects a parcellation process as proposed by the parcellation theory.

Immunohistochemical study of the pancreatic endocrine cells of the ray, Dasyatis akajei.

The pancreatic endocrine cells of the ray, Dasyatis akajei was studied by aldehyde-fuchsin-Masson-Goldner's staining and by immunohistochemical methods. The pancreatic endocrine cells of the ray do not form islets by represent the most primitive distribution among elasmobranchs, occupying the outer layer of the double-layered duct epithelium of the pancreas. The endocrine cells showed no evidence of reaching the duct lumen, i. e., they are closed in type. By the use of immunohistochemical techniques, insulin-, glucagon-, somatostatin-and pancreatic polypeptide (PP)-like immunoreactivities were detected in the endocrine cells. The PP-positive cells could not be differentiated from the somatostatin-immunoreactive cells, although the former were smaller in number. The possible reasons for this result were discussed. The present finding supports the view that those four peptides are essential regulatory substances of the endocrine system of the pancreas at a fairy early stage of vertebrate evolution.

Corticalization of two divisions of the visual system during vertebrate evolution

Data on the evolution of the visual system in vertebrate phylogeny are described. Visual projections are demonstrated in the telencephalon of cyclostomata (lampreys). The existence of a retino-thalamo-telencephalic pathway is demonstrated in elasmobranchs (skates). Two visual pathways are present in amphibians (frogs) and reptiles (turtles): retino-thalamo-telencephalic and retino-tecto-thalamo-telencephalic, and these overlap partly at the thalamic level in the lateral geniculate nucleus and completely in the telencephalon. In turtles the earliest visual and tectal impulses relay on their way to the telencephalon in the lateral geniculate body, and later impulses relay in the nucleus rotundus. In mammals (rats) visual tecto-cortical connections are seen; judging from the latent period of potentials arising in the visual cortex in response to stimulation of the superior colliculi these connections have one synaptic relay in the thalamus. The much shorter latent periods of visual evoked potentials recorded in the tectum of the monkey than in turtles (under identical chronic experimental conditions) confirm the views of morphologists on the progressive development of the tectal division of the visual system in vertebrate phylogeny. It is concluded that corticalization of both divisions of the visual system, i.e., the existence of telencephalic representation, appears in the early stages of vertebrate evolution.

Formation of the structural and functional organization of the paleo-, archi- and neocortex in premammalian phylogeny.

The following general conclusions are drawn from the results of electrophysiological and neuromorphologieal investigations conducted on cyclostomes, amphibians and r ep tiles. 1. Three formations of the endbrain (paleophallium, archipallium and neopallium), starting from the early stages of vertebrate evolution, appear and develop under the p r e s sure of three sources of afferent supply: the olfactory system, the paleoand neohypothalamus and the nuclear structures of the dorsal thalamus. These relationships are established in accordance with a definite principle: the phylogenetically old structures of the endbrain are elaborated mainly upon old formations and the new brain structures upon new. 2. Corticalization of the central nervous system follows the principle of development from diffuse structural and fimctional organization to more discrete, specialized forms of nervous activity. Orbeli (1, 2), a founder of evolutionary physiology, emphasized that the main task of this new discipline is to explain not only the pattern of formation of part icular functions or functional systems in the course of evolution of the animal world, but also to discover causal relationships and the significance of factors which determine the course of the evolutionary process. At the same time, it should predict the pathways and trends of future evolutionary transformations in structure and function of the brain. These fundamental ideas expounded by Orbeli led to electrophysiological, conditioned-reflex and neuromorphological investigations or representat ives of the Acrania, Cyclosto~ and Plagiostom fishes, amphibians, repti les and lower mammals (3, 4). A number of generalizations were made on the functional evolution both of the brain as a whole and on its individual systems of integration: cerebellocortical , thalamocortical, and hypothalamocortical. The following conclusions have been drawn: first , the principle of development by stages of the nervous system, according to which a shift of functions is observed in the central nervous system at ascending stages of vertebrate phylogeny; second, in the process of corticalization, and encephalization there is a tendency for development from diffuse, less, specialized forms of neural function to discrete , specialized forms. My subsequent researches into the structural and functional organization of the cerebel lotelencephalic and diencephalo-telencephalic levels of integration led logically to the problem of origin and the development of the highest level of integration, namely the cerebral cortex and, in particular, the neocortex. Currently, this problem is of great strategic importance and since the second half of the last century, contradictory views have been expressed about it (5-7). Since the endbrain in premammals receives p re dominantly olfactory afferents, the g~nesis of the neocortex has been correlated with the