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PubMed1998

Paternal effects in Drosophila: implications for mechanisms of early development.

The study of paternal effects on development provides a means to identify sperm-supplied products required for fertilization and the initiation of embryogenesis. This review describes paternal effects on animal development and discusses their implications for the role of the sperm in egg activation, centrosome activity, and biparental inheritance in different animal species. Paternal effects observed in Caenorhabditis elegans and in mammals are briefly reviewed. Emphasis is placed on paternal effects in Drosophila melanogaster. Genetic and cytologic evidence for paternal imprinting on chromosome behavior and gene expression in Drosophila are summarized. These effects are compared to chromosome imprinting that leads to paternal chromosome loss in sciarid and coccid insects and mammalian gametic imprinting that results in differential expression of paternal and maternal loci. The phenotypes caused by several early-acting maternal effect mutations identify specific maternal factors that affect the behavior of paternal components during fertilization and the early embryonic mitotic divisions. In addition, maternal effect defects suggest that two types of regulatory mechanisms coordinate parental components and synchronize their progression through mitosis. Some activities are coordinated by independent responses of parental components to shared regulatory factors, while others require communication between paternal and maternal components. Analyses of the paternal effects mutations sneaky, K81, paternal loss, and Horka have identified paternal products that play a role in mediating the initial response of the sperm to the egg cytoplasm, participation of the male pronucleus in the first mitosis, and stable inheritance of the paternal chromosomes in the early embryo.

Animals↗

PubMed1998

Neocentromere activity of structurally acentric mini-chromosomes in Drosophila.

Chromosome fragments that lack centromeric DNA (structurally acentric chromosomes) are usually not inherited in mitosis and meiosis. We previously described the isolation, after irradiation of a Drosophila melanogaster mini-chromosome, of structurally acentric mini-chromosomes that display efficient mitotic and meiotic transmission despite their small size (under 300 kb) and lack of centromeric DNA. Here we report that these acentric mini-chromosomes bind the centromere-specific protein ZW10 and associate with the spindle poles in anaphase. The sequences in these acentric mini-chromosomes were derived from the tip of the X chromosome, which does not display centromere activity or localize ZW10, even when separated from the rest of the X. We conclude that the normally non-centromeric DNAs present in these acentric mini-chromosomes have acquired centromere function, and suggest that this example of 'neocentromere' formation involves appropriation of a self-propagating centromeric chromatin structure. The potential relevance of these observations to the identity, propagation and function of normal centromeres is discussed.

Animals↗

PubMed2006

Karyotypes, constitutive heterochromatin, and genomic DNA values in the blowfly genera Chrysomya, Lucilia, and Protophormia (Diptera: Calliphoridae).

The karyotypes and C-banding patterns of Chrysomya species C. marginalis, C. phaonis, C. pinguis, C. saffranea, C. megacephala (New Guinean strain), Lucilia sericata, and Protophormia terraenovae are described. All species are amphogenic and have similar chromosome complements (2n = 12), including an XY-XX sex-chromosome pair varying in size and morphology between species. Additionally, the C-banding pattern of the monogenic species Chrysomya albiceps is presented. The DNA contents of these and of further species Chrysomya rufifacies, Chrysomya varipes, and Chrysomya putoria were assessed on mitotic metaphases by Feulgen cytophotometry. The average 2C DNA value of the male genomes ranged from 1.04 pg in C. varipes to 2.31 pg in C. pinguis. The DNA content of metaphase X chromosomes varied from 0.013 pg (= 1.23% of the total genome) in C. varipes to 0.277 pg (12.20%) in L. sericata; that of Y chromosomes ranged from 0.003 pg (0.27%) in C. varipes to 0.104 pg (5.59%) in L. sericata. In most species, the corresponding 5 large chromosome pairs showed similar relative DNA contents. The data suggest that the interspecific DNA differences in most species are mainly due to quantitative variation of (repetitive) sequences lying outside the centromeric heterochromatin blocks of the large chromosomes. The results are also discussed with regard to phylogenetic relationships of some species.

Animals↗

PubMed1976

The release of a compound-chromosome stock in a vineyard cellar population of Drosophila melanogaster.

The prospect of autocidal insect control was investigated in a cellar population of D. melanogaster using a compound-chromosome stock. The released stock was synthesized by irradiating virgin female progeny derived from the cellar and crossing to a second-chromosome compound laboratory stock. Incorporation of an appropriate genetic background into the compound stock was tested in laboratory studies. Larval development to adult emergence and adult survival studies indicated the compound release stock to be relatively similar to the wild population, while behavioral tests detected no mating isolation between wild or compound genotypes. An unstable equilibrium point of compound frequency 0.7 was observed in population cage experiments with the two genotypes. Adults were released into the cellar at a 50:1 ratio in favor of the compound. Five hundred newly hatched compound-chromosome larvae were also released. Adult, larval and pupal samples were regularly made. The compound stock successfully bred in the cellar maintaining an adult frequency of at least 90% for 108 days after the release. The rapid decline in compound frequency after this period is thought to be due to the migration of inseminated wild-type females from wine storage areas adjacent to the cellar. The results indicate that a compound stock may limit the rate of population expansion in an area and may be a useful mechanism of autocidal control. It cannot be overemphasized that the probability of a successful release will be related to the level of understanding of the adaptive strategy of the population into which the release is made.

Animals↗

PubMed1993

Localization of tandemly repeated DNA sequences in beetle chromosomes by fluorescent in situ hybridization.

In situ hybridization to chromosomes and nuclei of Tenebrio molitor shows the massive presence of a species-specific satellite DNA in all chromosomes and six sites of rDNA in mitotic chromosomes. These sites are located in two autosomal pairs and in the X and Y chromosomes. In a related species, Misolampus goudoti, in which two different families of highly repetitive DNA have been previously characterized, one family is located in centromeric regions of all chromosomes with the exception of chromosome Y, while the other repeated DNA family is present both in centromeric and distal regions of all chromosomes. rRNA genes in this species are present in a medium-sized autosomal pair only. These results show that molecular cytogenetics can be applied to coleopteran chromosomes and open the way for a physical mapping of DNA sequences in these organisms. The results also provide insights into the type of meiotic association of the X and Y chromosomes in Coleoptera and the distribution of repeated DNAs within the genome of these insects.

Animals↗

PubMed2000

Anopheles culicifacies Y-chromosome dimorphism indicates sibling species (B and E) with different malaria vector potential in Sri Lanka.

In Sri Lanka, malaria is transmitted mainly by Anopheles culicifacies Giles sensu lato (Diptera: Culicidae). In India, this nominal taxon comprises sibling species A, B, C, D and E, distinguished by their chromosome morphology. Species B (identified by polytene chromosome sequence Xab, 2g1 + h1) is not such an efficient vector of malaria as other members of the An. culicifacies complex in India. All specimens of An. culicifacies s.l. examined from Sri Lanka possess Xab, 2g1 + h1 polytenes, previously interpreted as species B, despite their important vector status. Recently, species E was described from Rameshwaram Island (Tamil Nadu, India) between Sri Lanka and the Indian mainland, where both species B and E are sympatric. Species B and E share polytene sequence Xab, 2g1 + h1 but differ by the mitotic Y-chromosome being acrocentric in species B, submetacentric in species E, the latter implicated as vector of vivax malaria. From May 1999 to January 2000, we surveyed Y-chromosomes of male progeny from An. culicifacies Xab, 2g1 + h1 females collected from cattle bait in diverse malarious districts of Sri Lanka: Badulla, Monaragala, Puttalam and Trincomalee. Karyotypes of readable quality were obtained from 42/83 families examined, with overall proportions 24% acrocentric and 76% submetacentric Y-chromosome carriers, both types being sympatric in at least 3/4 localities sampled. By analogy with the situation on Rameshwaram Island, we interpret these observations to demonstrate widespread presence of two members of the An. culicifacies complex in Sri Lanka, their karyotypes being compatible with species B and E, the latter predominant and having greater vector potential.

Animals↗

PubMed2004

Evidence that positive selection drives Y-chromosome degeneration in Drosophila miranda.

Why does the Y chromosome harbor so few functional loci? Evolutionary theory predicts that Y chromosomes degenerate because they lack genetic recombination. Both positive and negative selection models have been invoked to explain this degeneration, as both can result in the recurrent fixation of linked deleterious mutations on a nonrecombining Y chromosome. To distinguish between these models, I investigated patterns of nucleotide variability along 37 kb of the recently formed neo-Y chromosome in Drosophila miranda. Levels of nucleotide variability on this chromosome are 30 times lower than in highly recombining portions of the genome. Both positive and negative selection models can result in reduced variability levels, but their effects on the frequency spectrum of mutations differ. Using coalescent simulations, I show that the patterns of nucleotide variability on the neo-Y chromosome are unlikely under deleterious mutation models (including background selection and Muller's ratchet) but are expected under recent positive selection. These results implicate positive selection as an important force driving the degeneration of Y chromosomes; adaptation at a few loci, possibly increasing male fitness, occurs at the cost of most other genes on this chromosome.

Animals↗

PubMed2001

Meiotic and mitotic instability of two EMS-produced centric fragments in the haplodiploid wasp Nasonia vitripennis.

Terminal deletions that result in chromosomal fragments with centromeres (centric fragments) are relatively easy to generate and study in the haplodiploid insect Nasonia. We investigated the transmission stability of two chromosomal fragments generated by chemical mutagenesis. Visible mutations at the R locus (peach-233 and St-DR) and a linked body-colour mutant (purple) were used to track transmission of the centric fragments (which lack the purple locus and are wild-type at the R locus). Transmission rates in meiotic oogenesis were low (medians 0.15-0.18) and comparable to previous data on centric fragments in this species. The homologous chromosome genetic background strongly affected meiotic stability of one centric fragment (CF2) but not the other (CF1). Specifically, in peach/scarlet R locus heterozygous females, CF2 showed a normal segregation proportion with the chromosome bearing the scarlet allele (0.16), but near complete failure to segregate with peach (0.0002). Data show that this is due to loss of CF2 in eggs receiving peach, rather than to preferential segregation of CF2 with scarlet or mortality of CF2-bearing males. CF1 shows typical segregation ratios with both chromosomes. We hypothesize that deletions (or rearrangements) associated with the peach-233 mutant inhibit proper pairing and segregation of CF2. Consistent with the model, CF2 did segregate with chromosomes that had undergone recombination between peach and purple (a body-colour mutation 10 cM from peach), indicating that the domains inhibiting segregation are closely linked to peach. Mitotic instability also differed between the two fragments; reduced mitotic stability may relate to absence of telomeres on these centric fragments. Given the relative ease of generating and tracking terminal deletions in Nasonia, we propose this as a good system for studying mitotic and meiotic stability of centric fragments. Finally, results are discussed in relation to the evolution of B chromosomes from centric fragments.

Animals↗

PubMed2003

Downregulation of Trypanosoma brucei VSG expression site promoters on circular bacterial artificial chromosomes.

Trypanosoma brucei has about 20 telomeric variant surface glycoprotein (VSG) gene expression sites (ESs), which are downregulated in the insect form. We investigated the transcriptional behaviour of ES promoters on bacterial artificial chromosomes (BACs) containing two different ESs and their flanking regions on fragments of about 140kb. Four different BACs containing either the 221 or the VO2 ES were introduced into insect form T. brucei. The BACs replicated as circular episomes as shown using pulsed field gel (PFG) analysis of DNA exposed to increasing doses of gamma radiation, and digestion with Dam methylation-sensitive restriction enzymes. BAC copy number per cell varied from about 3 for the 221 ES BACs to about 15 for the VO2 ES BACs. Increasing drug selection pressure on the VO2 BAC T. brucei transformants resulted in amplification to about 80 BACs per cell. Although BACs were maintained in the absence of drug selection for at least 56 days, copy number fell and there was no evidence for centromere activity. ES promoters on small plasmid episomes introduced into insect form T. brucei in transient transfections are derepressed. In contrast, ES promoters on large BAC episomes are downregulated both on the original ES BACs, and on ES BACs selected for a drug marker driven by a rDNA promoter fused to the BAC vector. This indicates that downregulation of ES promoters in insect form T. brucei is influenced by genomic context, but does not necessitate proximity to a chromosome end.

Animals↗

PubMed2006

The dot chromosome of Drosophila: insights into chromatin states and their change over evolutionary time.

Historically, chromatin has been subdivided into heterochromatin, transcriptionally inactive regions that remain densely packaged throughout the cell cycle, and euchromatin, transcriptionally active regions that take on a diffuse appearance as the cell enters interphase. The banded portion of the small fourth chromosome (dot chromosome) of Drosophila melanogaster is unusual in exhibiting many characteristics of heterochromatic domains, and at the same time maintaining a gene density typical of euchromatin. Similar to genes embedded in pericentric heterochromatin, many of the dot chromosome genes have adapted to a heterochromatic environment. Little is known about the regulation of these genes and less about their evolution in a chromatin context. Interestingly, most of the genes from the D. melanogaster fourth chromosome remain clustered on a small chromosome throughout the genus Drosophila; yet the dot chromosome appears euchromatic in some species, such as D. virilis. Existing genomic sequence data allow an exploration of the underlying differences in DNA sequence organization between species. Here we review the available data describing the dot chromosome, which derives primarily from D. melanogaster. With its unusual and changing nature, the dot chromosome in the genus Drosophila provides a unique opportunity for the examination of transitions between chromatin states during evolution.

Animals↗

PubMed2004

Chromosomal inversion polymorphisms and adaptation.

Chromosomal inversion polymorphisms continue to be identified from an increasing number of populations of insects, plants, bacteria and humans. In the fruit fly Drosophila, chromosomal polymorphisms were used in classic studies of natural selection. Recent molecular genetic studies suggest that inversion polymorphisms are dynamical systems. These studies also indicate patterns of disequilibrium and variation that are consistent with co-adapted gene complexes. Although these complexes have yet to be identified, recent studies have identified traits, such as body size, that are linked to inversion polymorphisms. Selection acting on these polymorphisms is strong because latitudinal clines in inversion frequency become re-established rapidly after a new continent is colonized. A combined molecular and phenotypic approach is helping to identify the role of inversion polymorphisms in adaptive divergence, but the genes responsible for associations between traits and inversion polymorphisms remain to be identified.

Journal Article↗

PubMed1999

Mosquito genomes: structure, organization, and evolution.

A great deal of information has been accumulated on chromosome numbers and heterochromatin distribution as well as on genome size and organization in the mosquito family Culicidae. A number of trends in genome evolution emerge when these data are reviewed in light of recent cladistic phylogenies of Culicidae and its sister families. Anophelinae have heteromorphic sex chromosomes and a small genome size, and repetitive elements are distributed in a long-period interspersion pattern. In contrast, Culicinae have homomorphic sex chromosomes, and repetitive DNA is organized in a short-period interspersion pattern. There has been a general increase in genome size during the evolution of culicine tribes. The organization of the ancestral culicid genome remains uncertain awaiting studies on genome organization in Chaoboridae-Corethrellidae taxa. The most parsimonious hypothesis for the evolution of sex chromosomes and genome organization in Culicidae would be that homomorphic sex chromosomes and a long-period interspersion pattern was ancestral in lineages leading to Toxorhynchitinae and Culcinae. Larger genomes developed in subsequent culicine lineages through accumulation of short-period interspersed repetitive elements. Heteromorphic sex chromosomes evolved early in the evolution of Anophelinae, and a long-period interspersion pattern was retained. The alternative scenario proposed by Rao and Rai (1987a) is that Culicidae arose from a chaoborid Mochlonyx-like ancestor with heteromorphic sex chromosomes and possibly short-period interspersion. This scenario would require the loss of heteromorphic sex chromosomes in the lineage leading to Toxorhynchitinae and Culicinae and the "shedding" of repetitive elements in the lineage leading to Anophelinae. Several interesting patterns have emerged from studies of C-banding, and the distribution of heterochromatin in Culicidae and phylogenies derived from these studies are supported by the modern cladistic analyses. Recent intensive multipoint linkage map studies suggest that recombination frequencies per genome have remained relatively constant over the course of culicid evolution such that Anophelinae, with a relatively small genome size, has a linkage map of similar size to Aedini. As a consequence, taxa in Anophelinae have higher amounts of recombination per haploid genome size than Culicinae. Although several key questions have yet to be addressed, the Culicidae remain one of the best-studied systems of genome evolution in animals.

Animals↗

PubMed2001

The teflon gene is required for maintenance of autosomal homolog pairing at meiosis I in male Drosophila melanogaster.

In recombination-proficient organisms, chiasmata appear to mediate associations between homologs at metaphase of meiosis I. It is less clear how homolog associations are maintained in organisms that lack recombination, such as male Drosophila. In lieu of chiasmata and synaptonemal complexes, there must be molecules that balance poleward forces exerted across homologous centromeres. Here we describe the genetic and cytological characterization of four EMS-induced mutations in teflon (tef), a gene involved in this process in Drosophila melanogaster. All four alleles are male specific and cause meiosis I-specific nondisjunction of the autosomes. They do not measurably perturb sex chromosome segregation, suggesting that there are differences in the genetic control of autosome and sex chromosome segregation in males. Meiotic transmission of univalent chromosomes is unaffected in tef mutants, implicating the tef product in a pairing-dependent process. The segregation of translocations between sex chromosomes and autosomes is altered in tef mutants in a manner that supports this hypothesis. Consistent with these genetic observations, cytological examination of meiotic chromosomes suggests a role of tef in regulating or mediating pairing of autosomal bivalents at meiosis I. We discuss implications of this finding in regard to the evolution of heteromorphic sex chromosomes and the mechanisms that ensure chromosome disjunction in the absence of recombination.

Alleles↗

PubMed2000

Chromosome homogeneity in populations of Triatoma brasiliensis Neiva 1911 (Hemiptera - reduviidae - triatominae).

Triatoma brasiliensis is the most important vector of Chagas disease in the semiarid zone of the Northeast of Brazil. Several authors have reported the occurrence of four chromatic patterns with morphological, ecological, and genetic differences. In order to determine the existence of cytogenetic differentiation between these chromatic forms, we analyzed their karyotypes and the chromosome behavior during the male meiotic process. Triatoma brasiliensis shows distinct and specific chromosome characteristics, which differ from those observed in all other triatomine species. However, no cytogenetic differences were observed between the four chromatic forms of T. brasiliensis. The lack of chromosome differentiation among them could indicate that the populations of this species are in a process of differentiation that does not involve their chromosomal organization.

Animals↗

PubMed2005

Development of genetic sexing strains in Lepidoptera: from traditional to transgenic approaches.

The sterile insect technique (SIT) is currently being used for the control of many agricultural pests, including some lepidopteran species. The SIT relies on the rearing and release of large numbers of genetically sterile insects into a wild population. The holokinetic chromosomes of Lepidoptera respond differently to radiation than do species where there is a localized centromere. This difference has enabled a variation of the SIT to be developed for Lepidoptera where a substerilizing dose of radiation is given to the insects before their release with the result that a certain level of sterility is inherited by the F1 offspring. The development of genetic sexing strains for fruit flies, enabling the release of males only, has resulted in enormous economic benefits in the mass rearing and has increased the efficiency of the field operations severalfold. This article outlines Mendelian approaches that are currently available to separate large numbers of males and females efficiently for different lepidopteran species and describes their difficulties and constraints. Successful transgenesis in several lepidopteran species opens up new possibilities to develop genetic sexing strains. The proposal to develop genetic sexing strains described in this article takes advantage of the fact that in Lepidoptera, the female is the heterogametic sex, with most species having aWZ sex chromosome pair, whereas the males are ZZ. This means that if a conditional lethal gene can be inserted into the W chromosome, then all females should die after the application of the restrictive condition. The assumptions made to accommodate this model are discussed, and the advantages to be gained for control programs are elucidated.

Animals↗

PubMed1990

Cloning of cDNA and chromosomal location of genes encoding the three types of subunits of the wheat tetrameric inhibitor of insect alpha-amylase.

We have characterized three cDNA clones corresponding to proteins CM1, CM3 and CM16, which represent the three types of subunits of the wheat tetrameric inhibitor of insect alpha-amylases. The deduced amino acid sequences of the mature polypeptides are homologous to those of the dimeric and monomeric alpha-amylase inhibitors and of the trypsin inhibitors. The mature polypeptides are preceded by typical signal peptides. Southern blot analysis of appropriate aneuploids, using the cloned cDNAs as probes, has revealed the location of genes for subunits of the CM3 and of the CM16 type within a few kb of each other in chromosomes 4A, 4B and 4D, and those for the CM 1 type of subunit in chromosomes 7A, 7B and 7D. Known subunits of the tetrameric inhibitor corresponding to genes from the B and D genomes have been previously characterized. No proteins of this class have been found to be encoded by the A genome in hexaploid wheat (genomes AA, BB, DD) or in diploid wheats (AA) and no anti alpha-amylase activity has been detected in the latter, so that the A-genome genes must be either silent (pseudogenes) or expressed at a much lower level.

Amino Acid Sequence↗

PubMed1998

A polymer model for the structural organization of chromatin loops and minibands in interphase chromosomes.

A quantitative model of interphase chromosome higher-order structure is presented based on the isochore model of the genome and results obtained in the field of copolymer research. G1 chromosomes are approximated in the model as multiblock copolymers of the 30-nm chromatin fiber, which alternately contain two types of 0.5- to 1-Mbp blocks (R and G minibands) differing in GC content and DNA-bound proteins. A G1 chromosome forms a single-chain string of loop clusters (micelles), with each loop approximately 1-2 Mbp in size. The number of approximately 20 loops per micelle was estimated from the dependence of geometrical versus genomic distances between two points on a G1 chromosome. The greater degree of chromatin extension in R versus G minibands and a difference in the replication time for these minibands (early S phase for R versus late S phase for G) are explained in this model as a result of the location of R minibands at micelle cores and G minibands at loop apices. The estimated number of micelles per nucleus is close to the observed number of replication clusters at the onset of S phase. A relationship between chromosomal and nuclear sizes for several types of higher eukaryotic cells (insects, plants, and mammals) is well described through the micelle structure of interphase chromosomes. For yeast cells, this relationship is described by a linear coil configuration of chromosomes.

Animals↗

PubMed1976

The inheritance of radiation induced semi-sterility in Rhodnius prolixus.

Eggs from crosses of 40 adult male R. prolixus irradiated with 6K rad gamma-rays with normal females had a mean fertility of 23.9%, only 2 crosses being completely sterile. The 86 F1 progeny of both sexes, when outcrossed with normal mates, had a mean egg fertility of 12.6;, and 43 of these matings were completely sterile. Twenty-eight "F2" bugs reared from F1 X normal crosses were mated with normal partners and had a mean fertility of 44.6%, 6 of them being fully fertile, a reversal towards normal fertility. Cytogenetic examination of F1, "F2" and "F3" males showed that these changes in fertility correlated well with the degree of chromosomal abnormality found. The very high recovery rate of translocations in F1 generation males can be related to the holocentric chromosomes of these bugs which precludes the formation of dicentric chromosomes which are inviable in monocentric species. In F1 and "F2" males the majority of one bug was found with a ring of IV chromosome association and it is suggested that chromosome morphology, combined with a low chiasma frequency, favours chain association. Most chain multivalents showed linear orientation which may lead to duplication deficiencies and zygotic death. However, "parellel", "indifferent" and the more stable "convergent" modes of chain orientation were also all observed indicating that survival of some translocations in this species may be possible. The survival to the "F2" generation of chromosomal fragments confirmed the holocentric nature of triatomine chromosomes. It is suggested that semi-sterile males would prove more effective than releases of completely sterile males for reducing wild populations of R. prolixus, because of the delayed effects of sterilizing radiation consequent upon the holocentric structure of triatomine chromosomes.

Animals↗