The karyotypes of birds, turtles and snakes are characterized by two distinct chromosomal components, macrochromosomes and microchromosomes. This close karyological relationship between birds and reptiles has long been a topic of speculation among cytogenetists and evolutionary biologists; however, there is scarcely any evidence for orthology at the molecular level. To define the conserved chromosome synteny among humans, chickens and reptiles and the process of genome evolution in the amniotes, we constructed comparative cytogenetic maps of the Chinese soft-shelled turtle (Pelodiscus sinensis) and the Japanese four-striped rat snake (Elaphe quadrivirgata) using cDNA clones of reptile functional genes. Homology between the turtle and chicken chromosomes is highly conserved, with the six largest chromosomes being almost equivalent to each other. On the other hand, homology to chicken chromosomes is lower in the snake than in the turtle. Turtle chromosome 6q and snake chromosome 2p represent conserved synteny with the chicken Z chromosome. These results suggest that the avian and turtle genomes have been well conserved during the evolution of the Arcosauria. The avian and snake sex Z chromosomes were derived from different autosomes in a common ancestor, indicating that the causative genes of sex determination may be different between birds and snakes.3
In the development of most arthropods, the caudal region of the elongating germ band (the growth zone) sequentially produces new segments. Previous work with the spider Cupiennius salei suggested involvement of Delta-Notch signaling in segmentation. Here, we report that, in the spider Achaearanea tepidariorum, the same signaling pathway exerts a different function in the presumptive caudal region before initiation of segmentation. In the developing spider embryo, the growth zone becomes morphologically apparent as a caudal lobe around the closed blastopore. We found that, preceding caudal lobe formation, transcripts of a Delta homolog, At-Delta, are expressed in evenly spaced cells in a small area covering the closing blastopore and then in a progressively wider area of the germ disc epithelium. Cells with high At-Delta expression are likely to be prospective mesoderm cells, which later express a twist homolog, At-twist, and individually internalize. Cells remaining at the surface begin to express a caudal homolog, At-caudal, to differentiate as caudal ectoderm. Knockdown of At-Delta by parental RNA interference results in overproduction of At-twist-expressing mesoderm cells at the expense of At-caudal-expressing ectoderm cells. This condition gives rise to a disorganized caudal region that fails to pattern the opisthosoma. In addition, knockdown of Notch and Suppressor of Hairless homologs produces similar phenotypes. We suggest that, in the spider, progressive activation of Delta-Notch signaling from around the blastopore leads to stochastic cell fate decisions between mesoderm and caudal ectoderm through a process of lateral inhibition to set up a functional caudal lobe.
BackgroundPlanarians are considered to be among the extant animals close to one of the earliest groups of organisms that acquired a central nervous system (CNS) during evolution. Planarians have a bilobed brain with nine lateral branches from which a variety of external signals are projected into different portions of the main lobes. Various interneurons process different signals to regulate behavior and learning/memory. Furthermore, planarians have robust regenerative ability and are attracting attention as a new model organism for the study of regeneration. Here we conducted large-scale EST analysis of the head region of the planarian Dugesia japonica to construct a database of the head-region transcriptome, and then performed comparative analyses among related species.ResultsA total of 54,752 high-quality EST reads were obtained from a head library of the planarian Dugesia japonica, and 13,167 unigene sequences were produced by de novo assembly. A new method devised here revealed that proteins related to metabolism and defense mechanisms have high flexibility of amino-acid substitutions within the planarian family. Eight-two CNS-development genes were found in the planarian (cf. C. elegans 3; chicken 129). Comparative analysis revealed that 91% of the planarian CNS-development genes could be mapped onto the schistosome genome, but one-third of these shared genes were not expressed in the schistosome.ConclusionsWe constructed a database that is a useful resource for comparative planarian transcriptome studies. Analysis comparing homologous genes between two planarian species showed that the potential of genes is important for accumulation of amino-acid substitutions. The presence of many CNS-development genes in our database supports the notion that the planarian has a fundamental brain with regard to evolution and development at not only the morphological/functional, but also the genomic, level. In addition, our results indicate that the planarian CNS-development genes already existed before the divergence of planarians and schistosomes from their common ancestor.
We have previously shown that mammalian follicle-stimulating hormone (FSH) promotes the proliferation of spermatogonia and their differentiation into primary spermatocytes in organ culture of newt testis. In the current study, we performed microarray analysis to isolate local factors secreted from somatic cells upon FSH treatment and acting on the germ cells. We identified neuregulin 1 (NRG1) as a novel FSH-upregulated clone homologous to mouse NRG1 known to control cell proliferation, differentiation and survival in various tissues. We further isolated cDNAs encoding two different clones. Amino acid sequences of the two clones were 75% and 94% identical to Xenopus leavis immunoglobulin (Ig)-type and cysteine-rich domain (CRD)-type NRG1, respectively, which had distinct sequences in their N-terminal region but identical in their epidermal growth factor (EGF)-like domain. Semi-quantitative and quantitative PCR analyses indicated that both clones were highly expressed at spermatogonial stage than at spermatocyte stage. In vitro FSH treatment increased newt Ig-NRG1 (nIg-NRG1) mRNA expression markedly in somatic cells, whereas newt CRD-NRG1 (nCRD-NRG1) mRNA was only slightly increased by FSH. To elucidate the function of newt NRG1 (nNRG1) in spermatogenesis, recombinant EGF domain of nNRG1 (nNRG1-EGF) was added to organ and reaggregated cultures with or without somatic cells: it promoted spermatogonial proliferation in all cases. Treatment of the cultures with the antibody against nNRG1-EGF caused remarkable suppression of spermatogonial proliferation activated by FSH. These results indicated that nNRG1 plays a pivotal role in promoting spermatogonial proliferation by both direct effect on spermatogonia and indirect effect via somatic cells in newt testes.
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