Molecular Evolution of Drosophila Metallothionein Genes

The metallothionein genes of Drosophila melanogaster, Mtn and Mto, may play an important role in heavy metal detoxification. Several different tandem duplications of Mtn have been shown to increase cadmium and copper tolerance, as well as Mtn expression. In order to investigate the possibility of increased selection for duplications of these genes in natural populations exposed to high levels of heavy metals, we compared the frequencies of such duplications among flies collected from metal-contaminated and non-contaminated orchards in Pennsylvania, Tennessee and Georgia. Restriction enzyme analysis was used to screen 1666 wild third chromosomes for Mtn duplications and a subset (327) of these lines for Mto duplications. The frequency of pooled Mtn duplications found ranged from 0% to 20%, and was not significantly higher at the contaminated sites. No Mto duplications were identified. Estimates of sequence diversity at the Mtn locus among a subsample (92) of the duplication survey were obtained using four-cutter analysis. This analysis revealed a low level of polymorphism, consistent with both selection at the Mtn locus, and a fairly recent origin for the duplications. To further examine this hypothesis, we sequenced an Mtn allele of Drosophila simulans and measured the amount of nucleotide sequence divergence between D. simulans and its sibling species D. melanogaster. The levels of silent nucleotide polymorphism and divergence in the Mtn region were compared with those in the Adh region, using the neutrality test of R.R. Hudson, M. Kreitman and M. Aguadé.     

Molecular Evolution of TEPP Protein Genes in Metazoans

TEPP is a gene expressed in human reproductive organs such as testis, prostate, and placenta. Here, identification and molecular evolutionary analysis of TEPP proteins in various metazoan animals including deuterostomes (chordates, hemichordates, and echinoderms), lophotrochozoans (mollusks and annelids), and cnidarians (sea anemone and coral) are reported. A multiple sequence alignment revealed two highly conserved regions in TEPP proteins that had no similarity to any other known domains or proteins. Genomic sequence analysis showed frequent shifting of the splice sites of intron 1 in mammalian TEPP genes. A comparison of the intron positions in the coding region showed that the exon/intron structure of the TEPP gene was established in an early metazoan ancestor and that independent loss of a single intron occurred in echinoderms and in vertebrates. The urochordate tunicate TEPP genes are intronless, possibly due to replacement of the original gene by a retrogene. No homolog was detected in birds, insects, nematodes, and teleost fishes despite the extensive sequence data of these species, implying that the TEPP gene might be lost in these lineages.     

Molecular Evolution of hisB Genes

The sixth and eighth steps of histidine biosynthesis are catalyzed by an imidazole glycerol-phosphate (IGP) dehydratase (EC 4.2.1.19) and by a histidinol-phosphate (HOL-P) phosphatase (EC 3.1.3.15), respectively. In the enterobacteria, in Campylobacter jejuni and in Xylella/Xanthomonas the two activities are associated with a single bifunctional polypeptide encoded by hisB. On the other hand, in Archaea, Eucarya, and most Bacteria the two activities are encoded by two separate genes. In this work we report a comparative analysis of the amino acid sequence of all the available HisB proteins, which allowed us to depict a likely evolutionary pathway leading to the present-day bifunctional hisB gene. According to the model that we propose, the bifunctional hisB gene is the result of a fusion event between two independent cistrons joined by domain-shuffling. The fusion event occurred recently in evolution, very likely in the proteobacterial lineage after the separation of the - and the -subdivisions. Data obtained in this work established that a paralogous duplication event of an ancestral DDDD phosphatase encoding gene originated both the HOL-P phosphatase moiety of the E. coli hisB gene and the gmhB gene coding for a DDDD phosphatase, which is involved in the biosynthesis of a precursor of the inner core of the outer membrane lipopolysaccharides (LPS).     

Variable Rates of Evolution among Drosophila Opsin Genes

DNA sequences and chromosomal locations of four Drosophila pseudoobscura opsin genes were compared with those from Drosophila melanogaster, to determine factors that influence the evolution of multigene families. Although the opsin proteins perform the same primary functions, the comparisons reveal a wide range of evolutionary rates. Amino acid identities for the opsins range from 90% for Rh2 to more than 95% for Rh1 and Rh4. Variation in the rate of synonymous site substitution is especially striking: the major opsin, encoded by the Rh1 locus, differs at only 26.1% of synonymous sites between D. pseudoobscura and D. melanogaster, while the other opsin loci differ by as much as 39.2% at synonymous sites. Rh3 and Rh4 have similar levels of synonymous nucleotide substitution but significantly different amounts of amino acid replacement. This decoupling of nucleotide substitution and amino acid replacement suggests that different selective pressures are acting on these similar genes. There is significant heterogeneity in base composition and codon usage bias among the opsin genes in both species, but there are no consistent relationships between these factors and the rate of evolution of the opsins. In addition to exhibiting variation in evolutionary rates, the opsin loci in these species reveal rearrangements of chromosome elements.     

Molecular Evolution of Vertebrate Goose-Type Lysozyme Genes

We have found that mammalian genomes contain two lysozyme g genes. To better understand the function of the lysozyme g genes we have examined the evolution of this small gene family. The lysozyme g gene structure has been largely conserved during vertebrate evolution, except at the 5' end of the gene, which varies in number of exons. The expression pattern of the lysozyme g gene varies between species. The fish lysozyme g sequences, unlike bird and mammalian lysozyme g sequences, do not predict a signal peptide, suggesting that the encoded proteins are not secreted. The fish sequences also do not conserve cysteine residues that generate disulfide bridges in the secreted bird enzymes, supporting the hypothesis that the fish enzymes have an intracellular function. The signal peptide found in bird and mammalian lysozyme g genes may have been acquired as an exon in the ancestor of birds and mammals, or, alternatively, an exon encoding the signal peptide has been lost in fish. Both explanations account for the change in gene structure between fish and tetrapods. The mammalian lysozyme g sequences were found to have evolved at an accelerated rate, and to have not perfectly conserved the known active site catalytic triad of the bird enzymes. This observation suggests that the mammalian enzymes may have altered their biological function, as well.     

Molecular Population Genetics of Drosophila Immune System Genes

A striking aspect of many vertebrate immune system genes is the exceptionally high level of polymorphism they harbor. A convincing case can be made that this polymorphism is driven by the diversity of pathogens that face selective pressures to evade attack by the host immune system. Different organisms accomplish a defense against diverse pathogens through mechanisms that differ widely in their requirements for specific recognition. It has recently been shown that innate defense mechanisms, which use proteins with broad-spectrum bactericidal properties, are common to both primitive and advanced organisms. In this study we characterize DNA sequence variation in six pathogen defense genes of Drosophila melanogaster and D. mauritiana, including Andropin; cecropin genes CecA1, CecA2, CecB, and CecC; and Diptericin. The necessity for protection against diverse pathogens, which themselves may evolve resistance to insect defenses, motivates a population-level analysis. Estimates of variation levels show that the genes are not exceptionally polymorphic, but Andropin and Diptericin have patterns of variation that differ significantly from neutrality. Patterns of interpopulation and interspecific differentiation also reveal differences among the genes in evolutionary forces.     

Molecular Evolution of VEF-Domain-Containing PcG Genes in Plants

Arabidopsis VERNALIZATION2 （VRN2）, EMBRYONIC FLOWER2 （EMF2）, and FERTILIZATION-INDEPENDENT SEED2 （FIS2） are involved in vernalization-mediated flowering, vegetative development, and seed development, respectively. Together with Arabidopsis VEF-L36, they share a VEF domain that is conserved in plants and animals. To investigate the evolution of VEF-domain-containing genes （VEF genes）, we analyzed sequences related to VEF genes across land plants. To date, 24 full-length sequences from 11 angiosperm families and 54 partial sequences from another nine families were identified. The majority of the full-length sequences identified share greatest sequence similarity with and possess the same major domain structure as Arabidopsis EMF2. EMF2-1ike sequences are not only widespread among angiosperms, but are also found in genomic sequences of gymnosperms, lycophyte, and moss. No FIS2- or VEF-L36-1ike sequences were recovered from plants other than Arabidopsis, including from rice and poplar for which whole genomes have been sequenced. Phylogenetic analysis of the full-length sequences showed a high degree of amino acid sequence conservation in EMF2 homologs of closely related taxa. VRN2 homologs are recovered as a clade nested within the larger EMF2 clade. FIS2 and VEF-L36 are recovered in the VRN2 clade. VRN2 clade may have evolved from an EMF2 duplication event that occurred in the rosids prior to the divergence of the eurosid I and eurosid II lineages. We propose that dynamic changes in genome evolution contribute to the generation of the family of VEF-domain-containing genes, Phylogenetic analysis of the VEF domain alone showed that VEF sequences continue to evolve following EM F2NRN2 divergence in accordance with species relationship. Existence of EMF2-1ike sequences in animals and across land plants suggests that a prototype form of EMF2 was present prior to the divergence of the plant and animal lineages. A proposed sequence of events, based on domain organization and occurrence of intermediate seque     

Protein Evolution in the Context of Drosophila Development

The tempo at which a protein evolves depends not only on the rate at which mutations arise but also on the selective effects that those mutations have at the organismal level. It is intuitive that proteins functioning during different stages of development may be predisposed to having mutations of different selective effects. For example, it has been hypothesized that changes to proteins expressed during early development should have larger phenotypic consequences because later stages depend on them. Conversely, changes to proteins expressed much later in development should have smaller consequences at the organismal level. Here we assess whether proteins expressed at different times during Drosophila development vary systematically in their rates of evolution. We find that proteins expressed early in development and particularly during mid–late embryonic development evolve unusually slowly. In addition, proteins expressed in adult males show an elevated evolutionary rate. These two trends are independent of each other and cannot be explained by peculiar rates of mutation or levels of codon bias. Moreover, the observed patterns appear to hold across several functional classes of genes, although the exact developmental time of the slowest protein evolution differs among each class. We discuss our results in connection with data on the evolution of development.This revised article was published online in June 2005 with the final version of the content.Jerel C. Davis and Onn Brandman contributed equally to this work. Reviewing Editor: ManYuan Long     

A Cluster of Cuticle Protein Genes of Drosophila Melanogaster at 65a: Sequence, Structure and Evolution

A 36-kb genomic DNA segment of the Drosophila melanogaster genome containing 12 clustered cuticle genes has been mapped and partially sequenced. The cluster maps at 65A 5-6 on the left arm of the third chromosome, in agreement with the previously determined location of a putative cluster encompassing the genes for the third instar larval cuticle proteins LCP5, LCP6 and LCP8. This cluster is the largest cuticle gene cluster discovered to date and shows a number of surprising features that explain in part the genetic complexity of the LCP5, LCP6 and LCP8 loci. The genes encoding LCP5 and LCP8 are multiple copy genes and the presence of extensive similarity in their coding regions gives the first evidence for gene conversion in cuticle genes. In addition, five genes in the cluster are intronless. Four of these five have arisen by retroposition. The other genes in the cluster have a single intron located at an unusual location for insect cuticle genes.     

Structure and Evolution of the Adh Genes of Drosophila Mojavensis

The nucleotide sequence of the Adh region of Drosophila mojavensis has been completed and the region found to contain a pseudogene, Adh-2 and Adh-1 arranged in that order. Comparison of the sequence divergence of these genes to one another and to the Adh region of Drosophila mulleri and other species has allowed the development of a model for the evolution of the duplication of the Adh genes. There have been two major events. An initial duplication of an Adh gene whose dual promoter structure was similar to Drosophila melanogaster, resulted in a species with two Adh genes, one of which may have had only a proximal promoter. A second duplication of this gene generated an Adh region containing three genes. It is proposed that one of these is the ancestral gene having dual promoters, while the other two possess only proximal promoters. Subsequent events have resulted in both a change in the regulation of Adh-2 such that it is expressed as if it had a "distal" type promoter and the mutational inactivation of the most upstream gene resulting in the creation of a pseudogene. The sequence of the D. mojavensis Adh region has also revealed the presence of an element which is composed of juxtaposed inverted imperfectly repeated elements. There is a surprising and not fully explainable strong similarity of the nucleotide sequence of the 5' flanking region of the pseudogene in D. mojavensis and D. mulleri.     

植物角质层蜡质基因的研究进展

角质层是覆盖在植物地上部分最表层的保护层,具有降低植物表面的水分散失、防止紫外线辐射伤害和抵抗病虫害侵入等环境胁迫等功能,在植物适应外界环境作用方面起重要作用.作对近年来角质层蜡质基因的研究进展进行综述,同时也对蜡质基因的研究前景提出一些看法.     

Molecular Phylogenies and Evolution of crt Genes in Algae

ABSTRACT

The carotenoids constitute the most widespread class of pigments in nature. Most previous work has concentrated on the identification and characterization of their chemical physical properties and bioavailability. In recent years, significant amounts of research have been conducted in an attempt to analyze the genes and the molecular regulation of the genes involved in the biosynthesis of carotenoids. However, it is important not to lose sight of the early evolution of carotenoid biosynthesis. One of the major obstacles in understanding the evolution of the respective enzymes and their patterns of selection is a lack of a well-supported phylogenic analysis. In the present research, a major long-term objective was to provide a clearer picture of the evolutionary history of genes, together with an evaluation of the patterns of selection in algae. These phylogenies will be important in studies characterizing the evolution of algae. The gene sequences of the enzymes involved in the major steps of the carotenoid biosynthetic pathway in algae (cyanobacteria, rhofophyta, chlorophyta) have been analyzed. Phylogenetic relationships among protein-coding DNA sequences were reconstructed by neighbor-joining (NJ) analysis for the respective carotenoid biosynthetic pathway genes (crt) in algae. The analysis also contains an estimation of the rate of nonsynonymous nucleotide substitutions per nonsynonymous site (d_N), synonymous nucleotide substitution per synonymous site (d_S), and the ratio of nonsynonmous (d_N/d_S) for the test of selection patterns. The phylogenetic trees show that the taxa of some genera have a closer evolutionary relationship with other genera in some gene sequences, which suggests a common ancient origin and that lateral gene transfer has occurred among unrelated genera. The d_N values of crt genes in the early pathway are relatively low, while those of the following steps are slightly higher, while the d_N values of crt genes in chlorophyta are higher than those in cyanobacteria. Most of the d_N/d_S values exceed 1. The phylogenetic analysis revealed that lateral gene transfer may have taken place across algal genomes and the d_N values suggest that most of the early crt genes are well conserved compared to the later crt genes. Furthermore, d_N values also revealed that the crt genes of chlorophyta are more evolutionary than cyanobacteria. The amino acids' changes are mostly adaptive evolution under the influence of positive diversity selection.     

The Evolution of Duplicate Glyceraldehyde-3-Phosphate Dehydrogenase Genes in Drosophila

In Drosophila melanogaster there are two genes which encode the enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH), Gapdh-43E and Gapdh-13F. We have shown that Gapdh-43E codes for the GAPDH subunit with an apparently larger molecular weight while Gapdh-13F encodes the GAPDH subunit having an apparently smaller molecular weight. Immunoblots of sodium dodecyl sulfate gels were used to survey species from throughout the genus and results indicated that two classes of GAPDH subunits are present only in Drosophila species of the melanogaster and takahashi subgroups of the melanogaster group. Only the smaller subunit is found in species of the obscura group while all other species have only a large subunit. Drosophila hydei was analyzed at the DNA level as a representative species of the subgenus Drosophila. The genome of this species has a single Gapdh gene which is localized at a cytogenetic position likely to be homologous to Gapdh-43 E of D. melanogaster. Comparison of its sequence with the sequence of the D. melanogaster Gapdh genes indicates that the two genes of D. melanogaster are more similar to one another than either is to the gene from D. hydei. The Gapdh gene from D. hydei contains an intron following codon 29. Neither Gapdh gene of D. melanogaster has an intron within the coding region. Southern blots of genomic DNA were used to determine which species have duplicate Gapdh genomic sequences. Gene amplification was used to determine which species have a Gapdh gene that is interrupted by an intron. Species of the subgenus Drosophila have a single Gapdh gene with an intron. Species of the willistoni and saltans groups have a single Gapdh gene that does not contain an intron.(ABSTRACT TRUNCATED AT 250 WORDS)     

Episodic Evolution of Protein Hormones: Molecular Evolution of Pituitary Prolactin

Previous studies have shown that pituitary growth hormone displays an episodic pattern of evolution, with a slow underlying evolutionary rate and occasional sustained bursts of rapid change. The present study establishes that pituitary prolactin shows a similar pattern. During much of tetrapod evolution the sequence of prolactin has been strongly conserved, showing a slow basal rate of change (approx 0.27 × 10⁹ substitutions/amino acid site/year). This rate has increased substantially (∼12- to 38-fold) on at least four occasions during eutherian evolution, during the evolution of primates, artiodactyls, rodents, and elephants. That these increases are real and not a consequence of inadvertant comparison of paralogous genes is shown (for at least the first three groups) by the fact that they are confined to mature protein coding sequence and not apparent in sequences coding for signal peptides or when synonymous substitutions are examined. Sequences of teleost prolactins differ markedly from those of tetrapods and lungfish, but during the course of teleost evolution the rate of change of prolactin has been less variable than that of growth hormone. It is concluded that the evolutionary pattern seen for prolactin shows long periods of near-stasis interrupted by occasional bursts of rapid change, resembling the pattern seen for growth hormone in general but not in detail. The most likely basis for these bursts appears to be adaptive evolution though the biological changes involved are relatively small. Received: 31 August 1999 / Accepted: 9 February 2000     

Tissue-Specific Evolution of Protein Coding Genes in Human and Mouse

Protein-coding genes evolve at different rates, and the influence of different parameters, from gene size to expression level, has been extensively studied. While in yeast gene expression level is the major causal factor of gene evolutionary rate, the situation is more complex in animals. Here we investigate these relations further, especially taking in account gene expression in different organs as well as indirect correlations between parameters. We used RNA-seq data from two large datasets, covering 22 mouse tissues and 27 human tissues. Over all tissues, evolutionary rate only correlates weakly with levels and breadth of expression. The strongest explanatory factors of purifying selection are GC content, expression in many developmental stages, and expression in brain tissues. While the main component of evolutionary rate is purifying selection, we also find tissue-specific patterns for sites under neutral evolution and for positive selection. We observe fast evolution of genes expressed in testis, but also in other tissues, notably liver, which are explained by weak purifying selection rather than by positive selection.     

Drosophila Cuticular Hydrocarbons Revisited: Mating Status Alters Cuticular Profiles

Most living organisms use pheromones for inter-individual communication. In Drosophila melanogaster flies, several pheromones perceived either by contact/at a short distance (cuticular hydrocarbons, CHs), or at a longer distance (cis-vaccenyl acetate, cVA), affect courtship and mating behaviours. However, it has not previously been possible to precisely identify all potential pheromonal compounds and simultaneously monitor their variation on a time scale. To overcome this limitation, we combined Solid Phase Micro-Extraction with gas-chromatography coupled with mass-spectrometry. This allowed us (i) to identify 59 cuticular compounds, including 17 new CHs; (ii) to precisely quantify the amount of each compound that could be detected by another fly, and (iii) to measure the variation of these substances as a function of aging and mating. Sex-specific variation appeared with age, while mating affected cuticular compounds in both sexes with three possible patterns: variation was (i) reciprocal in the two sexes, suggesting a passive mechanical transfer during mating, (ii) parallel in both sexes, such as for cVA which strikingly appeared during mating, or (iii) unilateral, presumably as a result of sexual interaction. We provide a complete reassessment of all Drosophila CHs and suggest that the chemical conversation between male and female flies is far more complex than is generally accepted. We conclude that focusing on individual compounds will not provide a satisfactory understanding of the evolution and function of chemical communication in Drosophila.     

Absence of Protein Polymorphism in the Ras Genes of Drosophila melanogaster

Sequence analysis of 27 alleles of each of the three Ras-related genes in Drosophila melanogaster indicates that they all have low levels of polymorphism but may experience slightly different evolutionary pressures. No amino acid replacement substitutions were indicated in any of the sequences, or in the sibling species D. simulans and D. mauritiana. The Dras1 gene, which is the major ras homologue in Drosophila, has less within-species variation in D. melanogaster relative to the amount of divergence from the sibling species than does Dras2, although the contrast was not significant by the HKA test. Dras2 appears to be maintaining two classes of haplotype in D. melanogaster, one of which is closer to the alleles observed in the sibling species, suggesting that this is not likely to be a pseudogene despite the absence of a mutant phenotype. Although differences in level of expression may affect the function of the genes, it is concluded that genetic variation in the Ras signal transduction pathways cannot be attributed to catalytic variation in the Ras proteins. Received: 5 November 1998 / Accepted: 26 March 1999