Evolution of neural crest and placodes: amphioxus as a model for the ancestral vertebrate?
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Recent studies of protochordates (ascidian tunicates and amphioxus) have given insights into possible ancestors of 2 of the characteristic features of the vertebrate head: neural crest and placodes. The neural crest probably evolved from cells on either side of the neural plate-epidermis boundary in a protochordate ancestral to the vertebrates. In amphioxus, homologues of several vertebrate neural crest marker genes (BMP2/4, Pax3/7, Msx, Dll and Snail) are expressed at the edges of the neural plate and/or adjacent nonneural ectoderm. Some of these markers are also similarly expressed in tunicates. In protochordates, however, these cells, unlike vertebrate neural crest, neither migrate as individuals through embryonic tissues nor differentiate into a wide spectrum of cell types. Therefore, while the protochordate ancestor of the vertebrates probably had the beginnings of a genetic programme for neural crest formation, this programme was augmented in the earliest vertebrates to attain definitive neural crest. Clear homologues of vertebrate placodes are lacking in protochordates. However, both amphioxus and tunicates have ectodermal sensory cells. In tunicates these are all primary neurons, sending axons to the central nervous system, while in amphioxus, the ectodermal sensory cells include both primary neurons and secondary neurons lacking axons. Comparisons of developmental gene expression suggest that the anterior ectoderm in amphioxus may be homologous to the vertebrate olfactory placode, the only vertebrate placode with primary, not secondary, neurons. Similarly, biochemical, morphological and gene expression data suggest that amphioxus and tunicates also have homologues of the adenohypophysis, one of the few vertebrate structures derived from nonneurogenic placodes. In contrast, the origin of the other vertebrate placodes is very uncertain. (+info)
Perturbation of developmental gene expression in rat liver by fibric acid derivatives: lipoprotein lipase and alpha-fetoprotein as models.
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Liver lipoprotein lipase (LPL) and alpha-fetoprotein (AFP) gene expression show similar developmental patterns. Both mRNAs are abundantly expressed in neonatal rat liver and gradually disappear upon ageing. Treatment with fibric acid derivatives, such as fenofibrate, not only delays the developmental extinction of the LPL gene, but also increases LPL mRNA levels in neonatal rat liver. Similarly, the developmental extinction of the AFP gene in the liver is clearly delayed after fenofibrate. In adult rat liver, fibric acid derivatives transcriptionally reinduce a mRNA with similar size as LPL, but no effect on AFP mRNA was detected. Sequence comparison of clones isolated from a fenofibrate-induced cDNA library demonstrates that the fenofibrate-(re)induced mRNA in adult rat liver is encoding for LPL. The induction of LPL after fenofibrate is tissue-specific, since heart and adipose tissue LPL mRNA levels remain unchanged. In conclusion, fibric acid derivatives modulate developmental expression patterns in rat liver, and may selectively reinduce the expression of extinct genes in adult rat liver. (+info)
Differential distribution patterns of CRABP I and CRABP II transcripts during mouse embryogenesis.
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We have compared the transcript distribution of cellular retinoic acid binding protein (CRABP) I and II genes in mouse embryos at various stages of development. Both CRABP transcripts are present in embryonic structures from the earliest stages studied and exhibit specific patterns of distribution, suggesting that the two retinoic acid (RA) binding proteins perform different functions during mouse embryogenesis. The CRABP I transcript distribution correlates well with structures known to be targets of excess retinoid-induced teratogenesis (e.g. neural crest cells and hindbrain), suggesting that cells expressing CRABP I are those that cannot tolerate high levels of RA for their normal developmental function. The embryonic structures expressing CRABP II transcripts include those structures that have been shown to be adversely affected by excess of retinoids, such as limbs and hindbrain, but CRABP II transcripts are also found in structures not known to be specifically vulnerable to raised RA levels. The CRABP II gene is coexpressed with retinoic acid receptor (RAR)-beta and cellular retinol binding protein (CRBP) I genes in a number of tissues such as the gut endoderm, hypophysis and interdigital mesenchyme, all of which are devoid of CRABP I transcripts. Interestingly, the expression of the three genes, RAR-beta, CRABP II and CRBP I, is induced by retinoic acid, which suggests a link between the synthesis of RA from retinol and the control of expression of subsets of RA-responsive genes. The transcript distribution of CRABP I and II is discussed in relation to the teratogenic effects of RA, and compared to the RA-sensitive pattern of expression of other important developmental genes. (+info)
The product of a developmental gene, crgA, that coordinates reproductive growth in Streptomyces belongs to a novel family of small actinomycete-specific proteins.
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On solid media, the reproductive growth of Streptomyces involves antibiotic biosynthesis coincident with the erection of filamentous aerial hyphae. Following cessation of growth of an aerial hypha, multiple septation occurs at the tip to form a chain of unigenomic spores. A gene, crgA, that coordinates several aspects of this reproductive growth is described. The gene product is representative of a well-conserved family of small actinomycete proteins with two C-terminal hydrophobic-potential membrane-spanning segments. In Streptomyces avermitilis, crgA is required for sporulation, and inactivation of the gene abolished most sporulation septation in aerial hyphae. Disruption of the orthologous gene in Streptomyces coelicolor indicates that whereas CrgA is not essential for sporulation in this species, during growth on glucose-containing media, it influences the timing of the onset of reproductive growth, with precocious erection of aerial hyphae and antibiotic production by the mutant. Moreover, CrgA subsequently acts to inhibit sporulation septation prior to growth arrest of aerial hyphae. Overexpression of CrgA in S. coelicolor, uncoupling any nutritional and growth phase-dependent regulation, results in growth of nonseptated aerial hyphae on all media tested, consistent with a role for the protein in inhibiting sporulation septation. (+info)
Arrays of ultraconserved non-coding regions span the loci of key developmental genes in vertebrate genomes.
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BACKGROUND: Evolutionarily conserved sequences within or adjoining orthologous genes often serve as critical cis-regulatory regions. Recent studies have identified long, non-coding genomic regions that are perfectly conserved between human and mouse, termed ultra-conserved regions (UCRs). Here, we focus on UCRs that cluster around genes involved in early vertebrate development; genes conserved over 450 million years of vertebrate evolution. RESULTS: Based on a high resolution detection procedure, our UCR set enables novel insights into vertebrate genome organization and regulation of developmentally important genes. We find that the genomic positions of deeply conserved UCRs are strongly associated with the locations of genes encoding key regulators of development, with particularly strong positional correlation to transcription factor-encoding genes. Of particular importance is the observation that most UCRs are clustered into arrays that span hundreds of kilobases around their presumptive target genes. Such a hallmark signature is present around several uncharacterized human genes predicted to encode developmentally important DNA-binding proteins. CONCLUSION: The genomic organization of UCRs, combined with previous findings, suggests that UCRs act as essential long-range modulators of gene expression. The exceptional sequence conservation and clustered structure suggests that UCR-mediated molecular events involve greater complexity than traditional DNA binding by transcription factors. The high-resolution UCR collection presented here provides a wealth of target sequences for future experimental studies to determine the nature of the biochemical mechanisms involved in the preservation of arrays of nearly identical non-coding sequences over the course of vertebrate evolution. (+info)
Possible role of eclosion rhythm in mediating the effects of light-dark environments on pre-adult development in Drosophila melanogaster.
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BACKGROUND: In insects, circadian clocks have been implicated in affecting life history traits such as pre-adult development time and adult lifespan. Studies on the period (per) mutants of Drosophila melanogaster, and laboratory-selected lines of Bactrocera cucurbitae suggested a close link between circadian clocks and development time. There is a possibility of clock genes having pleiotropic effects on clock period and pre-adult development time. In order to avoid such pleiotropic effects we have used wild type flies of same genotype under environments of different periodicities, which phenotypically either speeded up or slowed down the eclosion clock of D. melanogaster. RESULTS: We assayed pre-adult development time and pre-adult survivorship of four laboratory populations of D. melanogaster, under five different light regimes, continuous light (LL), continuous darkness (DD), and light-dark (LD) cycles of 10:10 h (T20), 12:12 h (T24), and 14:14 h (T28). Although the development time was significantly different in most light regimes, except for females under T24 &T28, pre-adult survivorship remained largely unaffected. The development time was shortest under LL, followed by T20, DD, T24 and T28 regimes, in that order. Interestingly the development time showed a positive correlation with the period of eclosion rhythm, i.e., faster oscillations were associated with faster development, and slower oscillations with slower development. CONCLUSION: Based on these results we conclude that periodicity of imposed LD cycles, and/or of eclosion rhythm plays a key role in regulating the duration of pre-adult development in D. melanogaster in a manner that does not involve direct pleiotropic effects of clock genes on both clock period and development time. (+info)
Delta activity independent of its activity as a ligand of Notch.
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BACKGROUND: Delta, Notch, and Scabrous often function together to make different cell types and refine tissue patterns during Drosophila development. Delta is known as the ligand that triggers Notch receptor activity. Scabrous is known to bind Notch and promote Notch activity in response to Delta. It is not known if Scabrous binds Delta or Delta has activity other than its activity as a ligand of Notch. It is very difficult to clearly determine this binding or activity in vivo as all Notch, Delta, and Scabrous activities are required simultaneously or successively in an inter-dependent manner. RESULTS: Using Drosophila cultured cells we show that the full length Delta promotes accumulation of Daughterless protein, fringe RNA, and pangolin RNA in the absence of Scabrous or Notch. Scabrous binds Delta and suppresses this activity even though it increases the level of the Delta intracellular domain. We also show that Scabrous can promote Notch receptor activity, in the absence of Delta. CONCLUSION: Delta has activity that is independent of its activity as a ligand of Notch. Scabrous suppresses this Delta activity. Scabrous also promotes Notch activity that is dependent on Delta's ligand activity. Thus, Notch, Delta, and Scabrous might function in complex combinatorial or mutually exclusive interactions during development. The data reported here will be of significant help in understanding these interactions in vivo. (+info)
The identification and functional characterisation of conserved regulatory elements in developmental genes.
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Understanding the mechanisms that govern the expression of genomes is one of the major challenges of the post-genomic era. Phylogenetic footprinting, which identifies genomic regions under evolutionary constraints, has proven helpful in finding cis-regulatory elements of transcription; however, this method may not be applicable across all evolutionary distances and for all types of genes. Recent results from vertebrate comparisons indicate that strong conservation of cis-regulatory regions may occur more frequently in developmental regulator genes. This paper reviews methods of identifying conserved regulatory elements of developmental genes by comparative genomics, including new attempts to detect conserved features beyond simple sequence similarities. The results obtained are outlined and the authors comment on their functional and evolutionary implications. Finally, an evaluation of currently available methods of characterising the function of presumed conserved regulatory regions is presented, and problems such as promoter compatibility, assigning distant elements to their cognate genes and multifunctionality of elements, discussed. (+info)