The eggshell of insects: differentiation-specific proteins and the control of their synthesis and accumulation during development

Genome-Wide Gene Birth-Death Dynamics Are Associated with Diet Breadth Variation in Lepidoptera

Comparative analyses of gene birth-death dynamics have the potential to reveal gene families that played an important role in the evolution of morphological, behavioral, or physiological variation. Here, we used whole genomes of 30 species of butterflies and moths to identify gene birth-death dynamics among the Lepidoptera that are associated with specialist or generalist feeding strategies. Our work advances this field using a uniform set of annotated proteins for all genomes, investigating associations while correcting for phylogeny, and assessing all gene families rather than a priori subsets. We discovered that the sizes of several important gene families (e.g. those associated with pesticide resistance, xenobiotic detoxification, and/or protein digestion) are significantly correlated with diet breadth. We also found 22 gene families showing significant shifts in gene birth-death dynamics at the butterfly (Papilionoidea) crown node, the most notable of which was a family of pheromone receptors that underwent a contraction potentially linked with a shift to visual-based mate recognition. Our findings highlight the importance of uniform annotations, phylogenetic corrections, and unbiased gene family analyses in generating a list of candidate genes that warrant further exploration.

Aphrophoridae Role in Xylella fastidiosa subsp. pauca ST53 Invasion in Southern Italy

The Philaenus spumarius L. (Hemiptera Aphrophoridae) is a xylem-sap feeder vector that acquires Xylella fastidiosa subsp. pauca ST53 during feeding on infected plants. The bacterium is the plant pathogen responsible for olive quick decline syndrome that has decimated olive trees in Southern Italy. Damage originates mainly from the insect vector attitude that multiplies the pathogen potentialities propagating Xf in time and space. The principal action to manage insect-borne pathogens and to contain the disease spread consists in vector and transmission control. The analysis of an innovative and sustainable integrated pest management quantitative strategy that targets the vector and the infection by combining chemical and physical control means demonstrates that it is possible to stop the Xylella invasion. This review updates the available topics addressing vectors' identification, bionomics, infection management, and induced disease by Xylella invasion to discuss major available tools to mitigate the damage consequent to the disease.

Amyloid-Like Peptide Aggregates

Self-assembly of proteins and peptides into the amyloid fold is a widespread phenomenon in the natural world. The structural hallmark of self-assembly into amyloid fibrillar assemblies is the cross-beta motif, which conveys distinct morphological and mechanical properties. The amyloid fibril formation has contrasting results depending on the organism, in the sense that it can bestow an organism with the advantages of mechanical strength and improved functionality or, on the contrary, could give rise to pathological states. In this chapter we review the existing information on amyloid-like peptide aggregates, which could either be derived from protein sequences, but also could be rationally or de novo designed in order to self-assemble into amyloid fibrils under physiological conditions. Moreover, the development of self-assembled fibrillar biomaterials that are tailored for the desired properties towards applications in biomedical or environmental areas is extensively analyzed. We also review computational studies predicting the amyloid propensity of the natural amino acid sequences and the structure of amyloids, as well as designing novel functional amyloid materials.

Hexapeptide Tandem Repeats Dictate the Formation of Silkmoth Chorion, a Natural Protective Amyloid

Silkmoth chorion is a fibrous structure composed mainly of two major protein classes, families A and B. Both families of silkmoth chorion proteins present a highly conserved, in sequence and in length, central domain, consisting of Gly-rich tandem hexapeptide repetitive segments, flanked by two more variable N-terminal and C-terminal arms. Primary studies identified silkmoth chorion as a functional protective amyloid by unveiling the amyloidogenic properties of the central domain of both protein families. In this work, we attempt to detect the principal source of amyloidogenicity of the central domain by focusing on the role of the tandem hexapeptide sequence repeats. Concurrently, we discuss a possible mechanism for the self-assembly of class A protofilaments, suggesting that the aggregation-prone hexapeptide building blocks may fold into a triangle-shaped β-helical structure.

Identification and functional analysis of a novel chorion protein essential for egg maturation in the brown planthopper

In insect eggs, the chorion has the essential function of protecting the embryo from external agents during development while allowing gas exchange for respiration. In this study, we found a novel gene, Nilaparvata lugens chorion protein (NlChP), that is involved in chorion formation in the brown planthopper, Nilaparvata lugens. NlChP was highly expressed in the follicular cells of female adult brown planthoppers. Knockdown of NlChP resulted in oocyte malformation and the inability to perform oviposition, and electron microscopy showed that the malformed oocytes had thin and rough endochorion layers compared to the control group. Liquid chromatography with tandem mass spectrometry analysis of the eggshell components revealed four unique peptides that were matched to NlChP. Our results demonstrate that NlChP is a novel chorion protein essential for egg maturation in N. lugens, a hemipteran insect with telotrophic meroistic ovaries. NlChP may be a potential target in RNA interference-based insect pest management.

LepChorionDB, a database of Lepidopteran chorion proteins and a set of tools useful for the identification of chorion proteins in Lepidopteran proteomes

Chorion proteins of Lepidoptera have a tripartite structure, which consists of a central domain and two, more variable, flanking arms. The central domain is highly conserved and it is used for the classification of chorion proteins into two major classes, A and B. Annotated and unreviewed Lepidopteran chorion protein sequences are available in various databases. A database, named LepChorionDB, was constructed by searching 5 different protein databases using class A and B central domain-specific profile Hidden Markov Models (pHMMs), developed in this work. A total of 413 Lepidopteran chorion proteins from 9 moths and 1 butterfly species were retrieved. These data were enriched and organised in order to populate LepChorionDB, the first relational database, available on the web, containing Lepidopteran chorion proteins grouped in A and B classes. LepChorionDB may provide insights in future functional and evolutionary studies of Lepidopteran chorion proteins and thus, it will be a useful tool for the Lepidopteran scientific community and Lepidopteran genome annotators, since it also provides access to the two pHMMs developed in this work, which may be used to discriminate A and B class chorion proteins. LepChorionDB is freely available at http://bioinformatics.biol.uoa.gr/LepChorionDB.

Silkmoth chorion gene regulation revisited: promoter architecture as a key player

Regulation of silkmoth chorion genes has long been used as a model system for studying differential gene expression. The large numbers of genes, their overlapping expression patterns and the overall complexity of the system hinted towards an elaborate mechanism for transcriptional control. Recent studies, however, offer evidence of a molecular pathway governed by the interplay between two general transcription factors, CCAAT enhancer binding proteins (C/EBP) and GATA, an architectural protein, high mobility group A and a chromatin remodeller, chromo-helicase/ATPase-DNA binding protein 1. In this review we present a parsimonious model that adequately describes regulation of transcription across all temporally regulated chorion genes, and propose a role for promoter architecture.

Molecular cloning and characterization of hatching enzyme-like gene in the silkworm, Bombyx mori

Hatching is the important process for the life of the metazoan, in which hatching enzyme (HE) plays a key role. In this paper, we cloned the full-length sequence of hatching enzyme-like cDNA from bluish-silkworm-eggs of Bombyx mori (BmHEL) by the method of in silico cloning, SMART cDNA synthesis and RACE-PCR technique. The BmHEL is 974 bp in length, and contains an ORF of 885 bp, encoding 294 amino acids residues. The deduced amino acid sequence of BmHEL has 30.3-47.1% identities to that of HE identified in the other species. Two similar signature sequences of HE gene family harbor in the BmHEL. The BmHEL gene structure is 6-exon-5-intron, and a promoter region with high scores has been predicted, which harbors some basal elements and some embryo-development related transcription factor binding sites. In the silkworm eggs at different developmental stages during incubation, the BmHEL transcripts can be detected and keep at a low level during the early stages, increase dramatically since 7th day of incubation, and reach to the maximum on 9th day. Change of BmHEL transcripts is in accordance with the process of embryo development and hatching, indicated that it plays an important role in these processes. Moreover, BmHEL transcript can be detected in the midgut and testis at larval stage, suggested that BmHEL may have other biological functions. To the best of our knowledge, this is the first report on HE gene in the Lepidoptera insects and will be helpful to provide a molecular basis for understanding the complicated mechanism underlying silkworm hatching.

Amyloid fibril formation propensity is inherent into the hexapeptide tandemly repeating sequence of the central domain of silkmoth chorion proteins of the A-family

Peptide-analogues of the A and B families of silkmoth chorion proteins form amyloid fibrils under a variety of conditions [Iconomidou, V.A., Vriend, G. Hamodrakas, S.J. 2000. Amyloids protect the silkmoth oocyte and embryo. FEBS Lett. 479, 141-145; Iconomidou,V.A., Chryssikos, G.D.,Gionis, V., Vriend, G., Hoenger, A., Hamodrakas, S.J., 2001. Amyloid-like fibrils from an 18-residue peptide-analogue of a part of the central domain of the B-family of silkmoth chorion protein. FEBS Lett. 499, 268-273; Hamodrakas, S.J. Hoenger, A., Iconomidou, V. A., 2004 . Amyloid fibrillogenesis of silkmoth chorion protein peptide-analogues via a liquid crystalline intermediate phase. J. Struct. Biol. 145, 226-235.], which led us to propose that silkmoth chorion is a natural protective amyloid. In this study, we designed and synthesized two mutant peptide-analogues of the central conservative domain of the A family: (a) one, cA_m1, with a length half of that of the central domain of the A family, which folds and self-assembles, in various conditions, into amyloid fibrils very similar in properties and structure to the fibrils formed by the cA peptide, which corresponds to the entire length of the A family central domain [Iconomidou, V.A., Vriend, G. Hamodrakas, S.J. 2000. Amyloids protect the silkmoth oocyte and embryo. FEBS Lett. 479, 141-145.], in full support of our previous proposal, (b) the second, cA_m2, differing from cA_m1 at three positions, where three glutamates have replaced two valines and one alanine residues, does not form amyloid fibrils in any conditions. It appears that (a) the amyloidogenic properties of silkmoth chorion peptides are encoded into the tandemly repeating hexapeptides comprising the central domain of silkmoth chorion proteins, and, that (b) suitable mutations, properly and carefully designed, greatly affect the strong amyloidogenic properties inherent in certain aminoacid sequences and may inhibit amyloid formation.

A novel role for the Bombyx Slbo homologue, BmC/EBP, in insect choriogenesis

One previously unidentified cDNA clone coding for a C/EBP factor, BmC/EBP, was isolated from Bombyx mori follicular cells. This is the first time that a C/EBP factor has been isolated and characterized in Lepidoptera. We provide information concerning structural features and developmental specificity, as well as in vitro interaction properties with chorion gene promoter modules. BmC/EBP was capable of effectively recognizing homologous binding sites from chorion gene promoters derived from flies and other moths, despite significant diversity of chorion structure, gene organization, and gene expression profiles. We propose that the relative concentration of BmC/EBP, in relation to its differential binding affinity for promoter cis-elements, results in activation or repression of silkmoth chorion gene expression.

Comparison of newly isolated cuticular protein genes from six aphid species

This paper reports on the first aphids' cuticular proteins. One gene (Mpcp1) was obtained by screening a cDNA library of Myzus persicae with antibodies to a lepidopteran cuticle protein. MpCP1 presents a putative signal peptide, a central extended R&R domain, flanked by N- and C-terminal repeats of alanine, tyrosine and proline. The mRNA of Mpcp1 could be detected in a larval and in adult stages. Primers based on Mpcp1 allowed isolating and comparing cuticle protein genes from five aphid species, but not from whitefly or thrips. Comparison revealed a high degree of similarity. Data from this paper suggest that this cuticle protein family is typical and predominant to aphids. The conformation of these cuticle proteins and the significance on particular properties of aphid cuticle is discussed.

The orphan nuclear receptors BmE75A and BmE75C of the silkmoth Bombyx mori: hornmonal control and ovarian expression

The steroid hormone 20-hydroxyecdysone (20E) plays a key role in the stimulation of ovarian follicle development in the silkmoth, Bombyx mori. To understand better the mechanism by which 20E regulates silkmoth oogenesis, Bombyx homologs of the ecdysone-inducible orphan nuclear receptor E75 (BmE75) were cloned and their expression was analyzed in developing ovaries and staged follicles during metamorphosis. Of the two BmE75 isoforms isolated, only the A-isoform (BmE75A) has been identified previously in lepidopteran insects. BmE75C, on the other hand, shows significant sequence homology in its N-terminus to the Drosophila E75C isoform. Northern blot analysis shows unique expression patterns for each isoform mRNA during ovarian development. While the A-isoform seems to be mainly implicated in the earlier stages of the ecdysone response during previtellogenesis and vitellogenesis, expression of the C-isoform becomes strongly induced in an ecdysteroid-independent fashion at the transition from vitellogenesis to choriogenesis. Our data indicate a complex regulation of the expression of the BmE75 gene during oogenesis and postulate a new role for the BmE75C receptor at the end of vitellogenesis and the beginning of choriogenesis.

Is beta-pleated sheet the molecular conformation which dictates formation of helicoidal cuticle?

Over 100 sequences for cuticular proteins are now available, but there have been no formal analyses of how these sequences might contribute to the helicoidal architecture of cuticle or to the interaction of these proteins with chitin. A secondary structure prediction scheme (Hamodrakas, S.J., 1988. A protein secondary structure prediction scheme for the IBM PC and compatibles. CABIOS 4, 473-477) that combines six different algorithms predicting alpha-helix, beta-strands and beta-turn/loops/coil has been used to predict the secondary structure of chorion proteins and experimental confirmation has established its utility (Hamodrakas, S.J., 1992. Molecular architecture of helicoidal proteinaceous eggshells. In: Case, S.T. (Ed.), Results and Problems in Cell Differentiation, Vol. 19, Berlin-Heidelberg, Springer Verlag, pp. 116-186 and references therein). We have used this same scheme with eight cuticular protein sequences associated with hard cuticles and nineteen from soft cuticles. Secondary structure predictions were restricted to a conserved 68 amino acid region that begins with a preponderance of hydrophilic residues and ends with a 33 amino acid consensus region, first identified by Rebers and Riddiford (Rebers, J.F., Riddiford, L.M., 1988. Structure and expression of a Manduca sexta larval cuticle gene homologous to Drosophila cuticle genes. J. Mol. Biol. 203, 411-423). Both classes of sequences showed a preponderance of beta-pleated sheet, with four distinct strands in the proteins from 'hard' cuticles and three from 'soft'. In both cases, tyrosine and phenylalanine were found on one face within a sheet, an optimal location for interaction with chitin. We propose that this beta-sheet dictates formation of helicoidal cuticle.

Linkage and evolutionary diversification of developmentally regulated multigene families: tandem arrays of the 401/18 chorion gene pair in silkmoths

The coordinately expressed silkmoth chorion genes, 401 and 18, are closely linked as a pair, in divergent orientation. Analysis of overlapping clones (chromosomal "walk") demonstrated that each of the multiple copies of this gene pair is embedded within a larger deoxyribonucleic acid unit, which is tandemly repeated in a few arrays or possibly a single array. Southern analysis and examination of clones from a single individual moth demonstrated that the repeat units are extensively polymorphic in restriction sites, length, and possibly number, no differential amplification was evident during choriogenesis. Intron and 5'-flanking sequences were shown to be specific for the 401/18 gene pair and not to be present elsewhere in the genome. The spatial distribution of variations in the genes and their flanking sequences were examined.

The eggshell of Drosophila melanogaster: IX. Synthesis and morphogenesis of the innermost chorionic layer

Synthesis and morphogenesis of the innermost chorionic layer (ICL) was investigated by conventional EM methods, freeze-fracturing, tissue culture in Robb's medium, and EM autoradiography. Both autoradiography and fine structure results have shown that ICL-components are secreted prior to other chorion proteins. Their secretion starts on stage 12a but the first layer of ICL molecules is visible at stage 12b. Its thickness is gradually increased during the next stages, taking first, a bilaminar form along with the inner endochorion. Later, at the end of choriogenesis, ICL is detached from the endochorion and takes its final thickness and configuration, consisting of a 3-dimensional crystal, about 40 nm thick. The isolated ICL in conditions of air water interface is a monolayer crystal 10 nm thick. Studies on chorion mutants showed that the amount of protein secreted by the follicle cells is independent to the process of crystallisation. These data show how a proteinaceous extracellular substance is gradually assembled to form a 3-D crystal and how it can be organised to perform functions such as the physiological resistance of the insect eggs against water loss or water uptake, whenever they are laid on substrates with extreme environmental conditions. These functions are performed by ICL in conjunction with the underlying wax layer.

Pattern of ecological shifts in the diversification of Hawaiian Drosophila inferred from a molecular phylogeny

BACKGROUND

The endemic Hawaiian drosophilids, a unique group that are remarkable for their diversity and rapid proliferation, provide a model for analysis of the process of insular speciation. Founder events and accompanying random drift, together with shifts in sexual selection, appear to explain the dramatic divergence in male morphology and mating behaviour among these flies, but these forces do not account for their spectacular ecological diversification into a wide array of breeding niches. Although recognized as contributing to the success of this group, the precise role of adaptive shifts has not been well defined.

RESULTS

To delineate the pattern of ecological diversification in the evolution of Hawaiian Drosophila, we generated a molecular phylogeny, using nucleotide sequences from the yolk protein gene Yp1, of 42 endemic Hawaiian and 5 continental species. By mapping ecological characters onto this phylogeny, we demonstrate that monophagy is the primitive condition, and that decaying leaves were the initial substrate for oviposition and larval development. Shifts to decaying stems, bark and tree fluxes followed in more derived species. By plotting female reproductive strategies, as reflected in ovarian developmental type, on the molecular tree, we also demonstrate a phylogenetic trend toward increasing fecundity. We find some statistical support for correlations between ecological shifts and shifts in female reproductive strategies.

CONCLUSIONS

Because of the short branches at the base of the phylogram, which lead to ecologically diverse lineages, we conclude that much of the adaptive radiation into alternate breeding substrates occurred rapidly, early in the group's evolution in Hawaii. Furthermore, we conclude that this ecological divergence and the correlated changes in ovarian patterns that adapt species to their ecological habitats were contributing factors in the major phyletic branching within the Hawaiian drosophilid fauna.

The eggshell of Drosophila melanogaster. VIII. Morphogenesis of the wax layer during oogenesis

Utilizing freeze-fracturing and conventional electron microscopy methods, we have studied the details of morphogenesis and construction of the wax layer envelope from Oregon R and mutants of Drosophila melanogaster eggs during oogenesis. The wax layer is synthesized and secreted by the follicular cells in the form of lipid vesicles during stage 10b. During secretion (stages 10b, 11 and 12) the lipid vesicles are accumulated on the vitelline membrane surface and become flat. At the late stages of choriogenesis (stages 13, 14) the lipid vesicles are compressed tightly between the vitelline membrane and the other already constructed eggshell layers, so the wax layer becomes very thin and is hardly seen in cross-fractured views.

Silk moth chorion pseudogenes: hallmarks of genomic evolution by sequence duplication and gene conversion

The part of the genetic locus of the domesticated silk moth, Bombyx mori, in which high cysteine (Hc) chorion genes of late developmental specificity reside contains regions encompassing gene-like sequences which exhibit properties distinct from those of functional Hc genes. One of these regions has been characterized and shown to contain a chorion pseudogene, psi HcB.15, which shares pronounced similarities with a transcribed chorion pseudogene, psi HcB.12/13, which was characterized previously. Both pseudogenes are homologous to HcB chorion genes but bear multiple single nucleotide substitutions and short segmental mutations (insertions and deletions) which introduce translational frame shifts and termination codons in the coding regions. Structural characteristics unique to the two pseudogenes suggest that psi HcB.15 was generated first from a functional HcB gene and gave rise subsequently to psi HcB 12/13 as a result of a sequence duplication event. The two pseudogenes can be distinguished from each other by the presence of distinct regions of similarity to the consensus sequence of functional HcB genes which appear to have arisen from gene-conversion-mediated correctional events. These findings lend support to the hypothesis that chorion pseudogene sequences represent reservoirs of genetic information that participates in the evolution of the chorion locus rather than relics of inactivated genes passively awaiting extinction.

Tissue-specific expression of silkmoth chorion genes in vivo using Bombyx mori nuclear polyhedrosis virus as a transducing vector

A pair of silkmoth chorion chromosomal genes, HcA.12-HcB.12, was inserted into a baculovirus transfer vector, pBmp2, derived from the nuclear polyhedrosis virus of Bombyx mori. This vector, which permits the insertion of foreign genetic material in the vicinity of a mutationally inactivated polyhedrin gene, was used to acquire the corresponding recombinant virus. Injection of mutant silkmoth pupae that lack all Hc chorion genes with the recombinant virus resulted in the infection of all internal organs including follicular tissue. Analysis of RNA from infected tissues has demonstrated that the two chorion genes present in the viral genome are correctly transcribed under the control of their own promoter in follicular cells, the tissue in which chorion genes are normally expressed. The chorion primary transcripts are also correctly processed in the infected follicular cells and yield mature mRNAs indistinguishable from authentic chorion mRNAs present in wild-type follicles. These results demonstrate that recombinant nuclear polyhedrosis viruses can be used as transducing vectors for introducing genetic material of host origin into the cells of the organism and that the transduced genes are transiently expressed in a tissue-specific manner under the control of their resident regulatory sequences. Thus we show the in vivo expression of cloned genes under cellular promoter control in an insect other than Drosophila melanogaster. The approach should be applicable to all insect systems that are subject to nuclear polyhedrosis virus infection.

Evolutionary changes in the developmental expression of silkmoth chorion genes and their morphological consequences

Discrete changes in silkmoth choriogenesis have occurred during evolution, as exemplified in the present report in Antheraea polyphemus and Hyalophora cecropia. At the level of morphology, the chorion of A. polyphemus has surface structures, called aeropyle crowns, that are absent from H. cecropia. Aeropyle crowns form during the very late period of choriogenesis and consist of two substructures--lamellae and filler. Filler is present in H. cecropia in greatly reduced amounts. At the level of protein synthesis, overall similarities in the two species are maintained until the very late period of choriogenesis, when synthesis of aeropyle crown components is maximal. In H. cecropia, very late period-specific proteins are reduced in number and abundance. Several of these minor proteins are candidates for E1 and E2, the components of filler. E1 and E2 RNAs are about 35 times more abundant in A. polyphemus, despite very similar gene copy numbers and times of expression in the two species. These results support the hypothesis that evolutionary changes in chorion morphology have resulted from regulatory changes in the expression of chorion genes, either at the level of transcription or mRNA decay. The hypothesis that evolutionary changes in chorion morphology are based on terminal addition onto a preexisting developmental program is discussed.

Organization of regionally expressed silkmoth chorion genes

We described the organization of two silkmoth chorion genes, called E1 and E2, whose expression is largely restricted in time to the very late period of choriogenesis and in space to one of two major subpopulations of follicle cells. Using E1 and E2 clone cDNAs as probes, we showed that gene copy numbers per haploid genome remain constant throughout silkmoth development despite major changes in total DNA content per nucleus. Furthermore, gene copy numbers are the same in both cellular regions of the choriogenic follicle despite differences in nuclear size and levels of E gene expression. Southern analysis indicated between two and four copies each for E1 and E2 genes. Analysis of chromosomal clones showed that single copies of E1 and E2 are separated by about 7.5 kilobases and are transcribed from the same DNA strand. Two distinct pairs of cloned E1 and E2 genes were characterized. No other chorion genes were in their immediate vicinity.

Organization of regionally expressed silkmoth chorion genes

We described the organization of two silkmoth chorion genes, called E1 and E2, whose expression is largely restricted in time to the very late period of choriogenesis and in space to one of two major subpopulations of follicle cells. Using E1 and E2 clone cDNAs as probes, we showed that gene copy numbers per haploid genome remain constant throughout silkmoth development despite major changes in total DNA content per nucleus. Furthermore, gene copy numbers are the same in both cellular regions of the choriogenic follicle despite differences in nuclear size and levels of E gene expression. Southern analysis indicated between two and four copies each for E1 and E2 genes. Analysis of chromosomal clones showed that single copies of E1 and E2 are separated by about 7.5 kilobases and are transcribed from the same DNA strand. Two distinct pairs of cloned E1 and E2 genes were characterized. No other chorion genes were in their immediate vicinity.

Diversity in a chorion multigene family created by tandem duplications and a putative gene-conversion event

Two families of high-cysteine chorion proteins in Bombyx mori are encoded in 15 tandemly arranged nonidentical gene pairs. It is assumed that this locus arose by duplication with subsequent sequence divergence. We have compared DNA sequences from two such neighboring pairs of genes in an attempt to understand the manner in which diversity has been generated and/or removed. A high level of sequence identity (91%-99%) was found between the repeats throughout the transcribed and flanking regions, with two significant exceptions. First, in the DNA segment encoding a conserved region of the chorion proteins, ten substitutions were detected in a 39-base-pair region. This localized region of high variability would suggest an intergene conversion-like event. Second, a length difference of 141 base pairs was detected in a region encoding the carboxy-terminal arm of the protein. This difference can be explained by three separate reiterations of single codons (3 base pairs) separated in time by duplication or triplication events.

DNA sequence transfer between two high-cysteine chorion gene families in the silkmoth Bombyx mori

We have previously shown that one type of high-cysteine silkmoth chorion protein (Hc-A) has evolved from the A family of chorion proteins by radical modifications of the NH2-terminal and COOH-terminal polypeptide arms: most of the arm sequences have been deleted, while short cysteine- and glycine-containing repeats have expanded into long arrays. Strikingly similar modifications of the arms have led to the evolution of a second type of high-cysteine protein (Hc-B) from the B family of chorion proteins. It appears that the parallel evolution of these high-cysteine-encoding gene families has not been entirely independent: examination of 3' untranslated regions shows evidence of information transfer between the two families.

Molecular and cell isoforms during development

Development proceeds by way of a discrete yet overlapping series of biosynthetic and restructuring events that result in the continued molding of tissues and organs into highly restricted and specialized states required for adult function. Individual molecules and cells are replaced by molecular and cellular variants, called isoforms; these arise and function during embryonic development or later life. Isoforms, whether molecular or cellular, have been identified by their structural differences, which allow separation and characterization of each variant. These isoforms play a central and controlling role in the continued and dynamic remodeling that takes place during development. Descriptions of the individual phases of the orderly replacement of one isoform for another provides an experimental context in which the process of development can be better understood.

Developmental and evolutionary comparisons of proteins from purified ribosomal subunits in two silkmoths

Developmental comparisons of silkmoth ribosomal proteins were performed by parallel two-dimensional electrophoretic analyses of proteins derived from developing adult wings and from ovarian follicles at the prechoriogenic and choriogenic stages of development (before and during secretion of the chorion, respectively). Proteins of 40S and 60S subunits were prepared and analyzed separately. A single major developmental difference was observed, exclusively in choriogenic follicles: the majority of a 40S subunit protein, S6, was shifted to a more acidic form, possibly as a result of phosphorylation. A less prominent change was apparently due to quantitative variation between two forms of one large subunit protein. The developmental comparisons were performed in two species of the genus Antheraea, with consistent results. Comparisons between the two species revealed remarkable evolutionary conservation of the ribosomal protein patterns, sharply contrasting with the evolutionary diversification of chorion structural proteins in the same two species. The only detectable interspecies differences in ribosomal components were (a) slightly more acidic behaviour of one small-subunit protein and more basic behaviour of one large-subunit protein in Antheraea polyphemus as compared to Antheraea pernyi, and (b) presence of an apparently species-specific 60 S subunit protein in A. pernyi, which in A. polyphemus probably co-migrates with a neighbouring protein.

Novel amplification and transcriptional activity of chorion genes in Drosophila melanogaster follicle cells

Single-copy chorion genes coding for egg shell proteins are amplified in the follicle cells of Drosophila melanogaster egg chambers. Chromatin spreads of appropriately staged follicle cells reveal complex, multi-forked chromosomal structures in which one chromatin strand branches into two, which themselves branch out, and so on. In one micrograph, 13 strands originating from a single strand were observed. These structures can account for the maximal amplification occurring in the center of the domain, where the major chorion genes are located, and the decreasing amplification of flanking sequences to both sides. The amplification, high transcriptional rate, stage-specific expression, and correlation with known molecular sizes have allowed the putative identification of the single-copy, major chorion genes on the X chromosome and on chromosome III.

The eggshell of hawaiianDrosophila: structural and biochemical studies inD. grimshawi and comparison toD. melanogaster

The eggshell of the Hawaiian speciesDrosophila grimshawi was analyzed in terms of its structure and biochemical composition. It is generally similar toD. melanogaster in radial structure except that the inner endochorion is relatively compact and the outer endochorion is thick (1.9 μm) and traversed by numerous 50-nm-wide aeropyles (not present inD. melanogaster). Regionally the eggshell consists of the various specialized regions, which are: the holey posterior pole; the dorsal ridge; the operculum with very tall imprint borders; the collar with discontinuous outer endochorion; the micropyle with an external eccentric hole and an internal eccentric paracrystalline region; and the four very long respiratory appendages which exhibit tightly covered surface features for underwater plastron respiration. On the basis of the above structural data we suggest that the entireD. grimshawi eggshell (various layers and specialized regions) is the product of at least eight follicle cell subpopulations. In addition, in vitro culturing with³H-proline indicates that the eggshell is made according to a strict developmental program (analogous toD. melanogaster) and consists of several proteins ranging in molecular weight from 14,000 to 37,000 daltons.

Accepted mutations in a gene family: evolutionary diversification of duplicated DNA

We report and compare the DNA sequences of 14 silkmoth (Antheraea polyphemus) chorion genes, derived from either cDNA or chromosomal DNA clones. Seven of these genes are members of the A multigene family, and seven are members of the B family. Where available, the previously reported (Jones and Kafatos 1980) intronic and extragenic flanking DNA sequences are also considered. Closely related sequences are compared, revealing the types of spontaneous mutations that were fixed during paralogous evolution. Segmental mutations (i.e. mutations other than substitutions) are nearly always interpretable as small duplications or deletions, related to small direct repeats. Segmental mutations are strongly constrained in the coding regions, although they do occur. Nucleotide substitutions also appear to be under selective constraints: relatively few substitutions leading to amino acid replacements are accepted, silent substitutions leading to some codons (especially purine-terminated ones) are disfavored, and different compositional biases are maintained in different parts of the sequences. Other sequence differences can be interpreted as indicative of neutral drift, including most differences in non-coding regions and most T/C transitions in third-base positions. In the non-coding regions, which are thought to be only loosely constrained by selection, transitions are observed more frequently than might be expected: they account for 52% of all substitutions, and they appear to be favored two to threefold over transversions when allowance is made for the skewed base composition of these regions.

Evolution of a multigene family of chorion proteins in silkmoths

The evolution of the A family of chorion genes was examined by comparing new protein and DNA sequences from the silkmoths Antheraea pernyi and Bombyx mori with previously known sequences from Antheraea polyphemus. The comparisons indicated that the A family and its major subfamilies are ancient and revealed how parts of the genes corresponding to distinct regions of the protein structure have evolved, both by base substitutions and by segmental reduplications and deletions.

Origin of evolutionary novelty in proteins: how a high-cysteine chorion protein has evolved

The structure of unusual high-cysteine (Hc) proteins (ca. 30 mol %), which are characteristic of the chorion of the silkmoth Bombyx mori, has been determined by determining the sequence of a corresponding cDNA clone. The Hc protein sequence has evolved from a family of more ordinary chorion genes, in large part through fixation of mutations leading to enhanced cysteine content. Mutations of different types are differentially distributed in different parts of the sequence. In two conservative parts, those encoding the amino-terminal signal peptide and the highly structured central region of the protein, only base substitutions have been accepted. By contrast, in two alternating parts, which encode variable arms flanking the central region, deletions and duplications of tandemly repetitive sequences are prominent. Both base substitutions and expansions or deletions of tandemly repetitive elements are important in the evolution of this type of protein; functional constraints of the various protein domains dictate which class of mutations can be accepted.

Evolution of a multigene family of chorion proteins in silkmoths

The evolution of the A family of chorion genes was examined by comparing new protein and DNA sequences from the silkmoths Antheraea pernyi and Bombyx mori with previously known sequences from Antheraea polyphemus. The comparisons indicated that the A family and its major subfamilies are ancient and revealed how parts of the genes corresponding to distinct regions of the protein structure have evolved, both by base substitutions and by segmental reduplications and deletions.

Hybridization-selected translation of Bombyx mori high-cysteine chorion proteins in Xenopus laevis oocytes

Xenopus laevis oocytes were injected with poly(A)+-mRNA isolated from chorionating follicular epithelium of the domesticated silk moth (Bombyx mori). On two-dimensional gel electrophoresis, the resultant translation products comigrated with authentic, secreted, chorion standards, demonstrating that the frog oocyte system synthesizes and correctly process virtually all major chorion components. A cDNA clone has been shown to contain sequences complementary to those of mRNAs encoding B mori high-cysteine (Hc) chorion proteins Hc6-Hc11. mRNAs were selected by hybridization to plasmid m5000 DNA bound to diazobenzyloxymethyl-cellulose and subsequently translated in X. laevis oocytes into forms that comigrated with authentic chorion standards. The selection of a distinct subset of Hc mRNAs under stringent hybridization conditions (70% formamide/0.2 M NaCl, 60 degrees C) suggests that they are encoded by related genes. This is consistent with the pattern obtained by hybridizing radioactive m5000 DNA to Southern blots prepared from EcoRI-cleaved B. mori chromosomal DNA.

Linkage and evolutionary diversification of developmentally regulated multigene families: tandem arrays of the 401/18 chorion gene pair in silkmoths

The coordinately expressed silkmoth chorion genes, 401 and 18, are closely linked as a pair, in divergent orientation. Analysis of overlapping clones (chromosomal "walk") demonstrated that each of the multiple copies of this gene pair is embedded within a larger deoxyribonucleic acid unit, which is tandemly repeated in a few arrays or possibly a single array. Southern analysis and examination of clones from a single individual moth demonstrated that the repeat units are extensively polymorphic in restriction sites, length, and possibly number, no differential amplification was evident during choriogenesis. Intron and 5'-flanking sequences were shown to be specific for the 401/18 gene pair and not to be present elsewhere in the genome. The spatial distribution of variations in the genes and their flanking sequences were examined.

Selection and sequence analysis of a cDNA clone encoding a known chorion protein of the A family

Using as criteria the size, abundance and developmental specificity of hybridizing mRNA sequences, we have selected from our chorion cDNA library a clone corresponding to a specific chorion protein, A4--cl. Comparison between the clone sequence and the largely known sequence of A4--cl validates the use of the cDNA library for sequence analysis of the chorion multigene families. The two major chorion protein families, A and B, share certain structural similarities.

Amplification of genes for chorion proteins during oogenesis in Drosophila melanogaster

The endochorion and exochorion of Drosophila eggs are synthesized by the ovarian follicle cells during a brief period of about 5 hr. In this terminal phase of egg chamber development, the structural genes for several abundant chorion proteins are expressed at high levels according to a temporally regulated program. The female-sterile mutation ocelliless maps at the site of the genes for two of these proteins, the 36,000- and 38,000-dalton chorion proteins (c36 and c38), which are closely linked. The mutation results in a cis-acting reduction in the amounts of c36 and c38 that accumulate in late-stage egg chambers. We have investigated the mechanism that underlies this decreased production by using cDNA clones complementary to these gene sequences. Unexpectedly, it was found that, in normal females, the genes for c36, c38, and at least one other chorion protein are specifically amplified more than 10-fold in the DNA of late-stage egg chambers. The extra replication involves at least some adjacent chromosomal sequences and begins prior to the onset of mRNA and protein synthesis. The additional DNA remains stable after gene expression has ceased. The behavior of these genes is thus reminiscent of the properties of the DNA puffs that have been described in several groups of Diptera. The extent of amplification of c36 and c38, but not of the 18,000-dalton chorion protein c18 (which is unlinked), was decreased in the egg chambers of flies homozygous for ocelliless, suggesting that altered gene dosage may be responsible for the decreased synthesis of chorion proteins in the mutant.

Secretory kinetics in the follicular cells of silkmoths during eggshell formation

Procedures for quantitative autoradiography were used for studying the process of secretion of eggshell (chorion) proteins in the follicular epithelium of silkmoths. The method was based on photometric measurements of the reflectance of vertically illuminated autoradiographic silver grains. Results were analyzed and plotted by computer. Secretory kinetics were also determined by analysis of labeled proteins in physically separated epithelium and chorion. Rapid accumulation of radioactivity into "clumps" visualized by light microscope autoradiography and evidence from preliminary electron microscope autoradiography indicate that, within 2 min from the time of synthesis, labeled chorion proteins move to Golgi regions scattered throughout the cytoplasm. The proteins begin to accumulate in the apical area 10-20 min later and to be discharged from the cell. The time for half-secretion is 20-25 min, and discharge is essentially complete 30-50 min after labeling. At the developmental stages examined, the kinetics of secretion appear to be similar for all proteins. Within the chorion the proteins rapidly assume a characteristic distribution, which varies for different developmental stages. Two relatively slow steps have been identified in secretion, associated with residence in Golgi regions and in the cell apex, respectively. By contrast, translocation of proteins across the cell and deposition of discharged proteins in the chorion are rapid steps.