Differential sex chromosome replication and dosage compensation in polytene trichogen cells of Lucilia cuprina (Diptera: Calliphoridae)

The genomes of a monogenic fly: views of primitive sex chromosomes

The production of male and female offspring is often determined by the presence of specific sex chromosomes which control sex-specific expression, and sex chromosomes evolve through reduced recombination and specialized gene content. Here we present the genomes of Chrysomya rufifacies, a monogenic blow fly (females produce female or male offspring, exclusively) by separately sequencing and assembling each type of female and the male. The genomes (> 25X coverage) do not appear to have any sex-linked Muller F elements (typical for many Diptera) and exhibit little differentiation between groups supporting the morphological assessments of C. rufifacies homomorphic chromosomes. Males in this species are associated with a unimodal coverage distribution while females exhibit bimodal coverage distributions, suggesting a potential difference in genomic architecture. The presence of the individual-sex draft genomes herein provides new clues regarding the origination and evolution of the diverse sex-determining mechanisms observed within Diptera. Additional genomic analysis of sex chromosomes and sex-determining genes of other blow flies will allow a refined evolutionary understanding of how flies with a typical X/Y heterogametic amphogeny (male and female offspring in similar ratios) sex determination systems evolved into one with a dominant factor that results in single sex progeny in a chromosomally monomorphic system.

Dosage Compensation of X-Linked Muller Element F Genes but Not X-Linked Transgenes in the Australian Sheep Blowfly

In most animals that have X and Y sex chromosomes, chromosome-wide mechanisms are used to balance X-linked gene expression in males and females. In the fly Drosophila melanogaster, the dosage compensation mechanism also generally extends to X-linked transgenes. Over 70 transgenic lines of the Australian sheep blowfly Lucilia cuprina have been made as part of an effort to develop male-only strains for a genetic control program of this major pest of sheep. All lines carry a constitutively expressed fluorescent protein marker gene. In all 12 X-linked lines, female larvae show brighter fluorescence than male larvae, suggesting the marker gene is not dosage compensated. This has been confirmed by quantitative RT-PCR for selected lines. To determine if endogenous X-linked genes are dosage compensated, we isolated 8 genes that are orthologs of genes that are on the fourth chromosome in D. melanogaster. Recent evidence suggests that the D. melanogaster fourth chromosome, or Muller element F, is the ancestral X chromosome in Diptera that has reverted to an autosome in Drosophila species. We show by quantitative PCR of male and female DNA that 6 of the 8 linkage group F genes reside on the X chromosome in L. cuprina. The other two Muller element F genes were found to be autosomal in L. cuprina, whereas two Muller element B genes were found on the same region of the X chromosome as the L. cuprina orthologs of the D. melanogaster Ephrin and gawky genes. We find that the L. cuprina X chromosome genes are equally expressed in males and females (i.e., fully dosage compensated). Thus, unlike in Drosophila, it appears that the Lucilia dosage compensation system is specific for genes endogenous to the X chromosome and cannot be co-opted by recently arrived transgenes.

Sex chromosomes and associated rDNA form a heterochromatic network in the polytene nuclei of Bactrocera oleae (Diptera: Tephritidae)

The olive fruit fly, Bactrocera oleae, has a diploid set of 2n = 12 chromosomes including a pair of sex chromosomes, XX in females and XY in males, but polytene nuclei show only five polytene chromosomes, obviously formed by five autosome pairs. Here we examined the fate of the sex chromosomes in the polytene complements of this species using fluorescence in situ hybridization (FISH) with the X and Y chromosome-derived probes, prepared by laser microdissection of the respective chromosomes from mitotic metaphases. Specificity of the probes was verified by FISH in preparations of mitotic chromosomes. In polytene nuclei, both probes hybridized strongly to a granular heterochromatic network, indicating thus underreplication of the sex chromosomes. The X chromosome probe (in both female and male nuclei) highlighted most of the granular mass, whereas the Y chromosome probe (in male nuclei) identified a small compact body of this heterochromatic network. Additional hybridization signals of the X probe were observed in the centromeric region of polytene chromosome II and in the telomeres of six polytene arms. We also examined distribution of the major ribosomal DNA (rDNA) using FISH with an 18S rDNA probe in both mitotic and polytene chromosome complements of B. oleae. In mitotic metaphases, the probe hybridized exclusively to the sex chromosomes. The probe signals localized a discrete rDNA site at the end of the short arm of the X chromosome, whereas they appeared dispersed over the entire dot-like Y chromosome. In polytene nuclei, the rDNA was found associated with the heterochromatic network representing the sex chromosomes. Only in nuclei with preserved nucleolar structure, the probe signals were scattered in the restricted area of the nucleolus. Thus, our study clearly shows that the granular heterochromatic network of polytene nuclei in B. oleae is formed by the underreplicated sex chromosomes and associated rDNA.

Cytogenetic analysis of the Ethiopian fruit fly Dacus ciliatus (Diptera: Tephritidae)

The Ethiopian fruit fly, Dacus ciliatus, is an important pest of cucurbits, which recently invaded the Middle East. The genetics and cytogenetics of D. ciliatus have been scarcely studied. Such information is, however, an essential basis for understanding the biology of insect pests, as well as for the design of modern control strategies. We report here the mitotic karyotype and detailed photographic maps of the salivary gland polytene chromosomes of this species. The mitotic metaphase complement consists of six pairs of chromosomes, including one pair of heteromorphic sex (XX/XY) chromosomes. The heterogametic sex is ascribed to the male. The analysis of the salivary gland polytene complement shows a total number of five long chromosomes (10 polytene arms), which correspond to the five autosomes of the mitotic nuclei, and a heterochromatic mass corresponding to the sex chromosomes. Banding patterns, as well as the most characteristic features and prominent landmarks of each polytene chromosome are presented and discussed. Chromosomal homologies between D. ciliatus and Bactrocera oleae are proposed by comparing chromosome banding patterns and by in situ hybridization of the hsp70 gene.

Genetic and cytogenetic analysis of the fruit fly Rhagoletis cerasi (Diptera: Tephritidae)

The European cherry fruit fly, Rhagoletis cerasi, is a major agricultural pest for which biological, genetic, and cytogenetic information is limited. We report here a cytogenetic analysis of 4 natural Greek populations of R. cerasi, all of them infected with the endosymbiotic bacterium Wolbachia pipientis. The mitotic karyotype and detailed photographic maps of the salivary gland polytene chromosomes of this pest species are presented here. The mitotic metaphase complement consists of 6 pairs of chromosomes, including one pair of heteromorphic sex chromosomes, with the male being the heterogametic sex. The analysis of the salivary gland polytene complement has shown a total of 5 long chromosomes (10 polytene arms) that correspond to the 5 autosomes of the mitotic nuclei and a heterochromatic mass corresponding to the sex chromosomes. The most prominent landmarks of each polytene chromosome, the "weak points", and the unusual asynapsis of homologous pairs of polytene chromosomes at certain regions of the polytene elements are also presented and discussed.

Polytene chromosome maps of the melon fly Bactrocera cucurbitae (Diptera: Tephritidae)

Standard photographic maps of the polytene chromosomes are presented for the melon fly Bactrocera cucurbitae, a serious pest of fleshy fruits and vegetables. Five larval salivary gland polytene chromosomes (10 polytene arms) were isolated, and their characteristic features and landmarks have been recognized. Banding patterns of each of the polytene arms are presented, where variation in band intensity and puffs appear to reflect fundamental differences in chromosomes. The whole polytene genome has been typically mapped by dividing it into 100 sections and the subsections were lettered. The mitotic chromosomes of larval brain ganglia are also examined, five pairs of autosomes and an XX/XY sex chromosome pair. In addition, a heterochromatic mass corresponding to the sex chromosomes are observed in the polytene nuclei of salivary gland tissue. This investigation showed that B. cucurbitae has excellent cytological material for polytene chromosome analysis and proved to be very useful for obtaining more detailed genetic information on the pest's natural populations.

Electron microscope investigation of polytene chromosomes in the Mediterranean fruit fly Ceratitis capitata

Ultrastructural analyses of polytene chromosomes from male pupal orbital bristle cells and from larval salivary glands of Ceratitis capitata were carried out. It was shown that chromatin complexes corresponding to the X chromosome heterochromatic network are surrounded by material containing ribonucleoprotein (RNP) granules 250-300 A (1 A = 0.1 nm) in diameter. RNP granules of similar size surround the spherical Y chromosome. These data point out the presence of transcriptional activity in both of these chromosomes. The absence of clear structure in chromosomal regions situated between large bands in both types of tissues was observed. These results support the hypothesis of weak synapsis between chromatids or small chromomeres of polytene chromosomes in this species. In addition, we describe a specific puff revealed in both orbital trichogen cells and salivary glands that is morphologically similar to the 93D puff of Drosophila melanogaster.

Differential replication of heterochromatin in polytene chromosomes of the Old World screw-worm fly, Chrysomya bezziana (Diptera: Calliphoridae), analysed by fluorescence staining

The distribution and replication of heterochromatin in polytene trichogen chromosomes of the Old World screw-worm fly, Chrysomya bezziana, were studied using fluorescent staining techniques. Quinacrine and distamycin-DAPI, which selectively stain AT-rich DNA, and chromomycin, specific for GC-rich sequences, were used. Bright quinacrine and DA-DAPI fluorescence was found in the sex chromosome body and in all autosomal centromere regions. Chromomycin (CMA) staining results in very little bright fluorescence of the sex chromosome body and autosomal centromeric regions, but many bright bands of varying morphology are distributed in autosomal arms. The expected negative CMA staining of quinacrine and DA-DAPI bright regions was not found. The lack of reciprocal staining patterns may result from changes in the higher order chromatin structure of polytene chromosomes, or intercalation of divergent heterochromatic sequences. Comparison of the different staining techniques in mitotic and polytene cells shows that heterochromatin is differentially under-replicated, so that the proportions of the distinct fluorescent-specific chromatin changes during polytenization. CMA staining within autosomal arms suggests that repeated sequences intercalated in euchromatin are co-replicated during polytenization. The numerous fluorescent markers described also provide further morphological features for use in comparative cytological analysis of C. bezziana.

Mitotic and salivary gland chromosome analyses in the Musca domestica L. (house fly) (Diptera: Muscidae)

The mitotic chromosomes in Musca domestica consist of five pairs of autosomes and an X, Y sex chromosome pair. They respond to C-banding with procentric bands on all autosomes and deep staining over most of the X and Y chromosomes. Polytene chromosomes were previously found in several larval and pupal tissue of Musca domestica. Polytene chromosome reference maps of the two sexual and the five autosomal chromosomes of Musca domestica from salivary gland cells are shown. Characteristic features of each chromosome are described identifying areas that are difficult to analyse.

The genome of the Mediterranean fruitfly Ceratitis capitata: localization of molecular markers by in situ hybridization to salivary gland polytene chromosomes

We hybridized cloned DNA segments to salivary gland polytene chromosomes of the medfly, Ceratitis capitata, and thus established molecular markers for 24 sites on 6 out of 10 autosomal arms. An additional marker identified a medfly repetitive element that hybridizes to approximately 100 autosomal sites as well as a granular network that is thought to represent the X chromosome. Some of the markers correspond to 9 characterized transcription units, while 17 remain anonymous; at least 3 of the latter are restriction fragment length polymorphism (RFLP) markers. The characterized transcription units document that chromosomal arm 5L of C. capitata is homologous to the Drosophila melanogaster X chromosome, in agreement with previous inferences based on the extensive conservation of linkage groups in Diptera.

Characterization and chromosomal distribution of a tandemly repeated DNA sequence from the Australian sheep blowfly, Lucilia cuprina

In the course of making a Lucilia cuprina genomic DNA library, a ladder of bands was seen in partial Sau3A digests. Complete digestion reduced this ladder to predominantly monomer units of approximately 190 bp. Nine independently isolated copies of this repeat were cloned and sequenced. Only two of these isolates are identical in sequence, the most divergent being 71% homologous. This satellite DNA occurs in all three wild-type strains tested, and, for the single case examined, in the embryonic, larval, pupal, and adult DNA. It represents approximately 3%-4% of the genome. Data obtained from in situ chromosome hybridizations indicate that this sequence is concentrated around the centromeric regions of the autosomes and over most of the sex chromosomes. Labelling is much stronger in mitotic compared with polytene chromosomes showing directly that this centromeric satellite DNA is grossly under-replicated during polytenization. This under-replication is even more pronounced on the sex chromosomes compared with the autosomes.

Male crossing over and genetic sexing systems in the Australian sheep blowfly Lucilia cuprina

Field-female killing (FK) systems based on deleterious mutations and Y-autosome translocations are being evaluated for genetic control of the Australian sheep blowfly, Lucilia cuprina. Experience during field trials has shown that mass-reared colonies of FK strains are subject to genetic deterioration, caused mainly by genetic recombination in males. A previous study found higher male recombination frequencies in two Y-linked translocation strains than in chromosomally normal males. However, the results of the present study indicate that breakage of the Y chromosome is neither sufficient nor necessary for increased levels of male recombination. The frequency of male recombination appears to be unrelated to the presence of specific chromosome rearrangements.

Homozygous-viable pericentric inversions for genetic control of Lucilia Cuprina

The isolation of homozygous-viable pericentric inversions for inclusion in field-female killing (FK) systems in Lucilia cuprina is described. From 7,236 irradiated chromosomes screened, 16 pericentric inversions were isolated. Four of these were viable as homozygotes. One of these, In (3LR) 14, possesses the properties required for inclusion in FK systems (tight linkage of one inversion break-point to the white-eye gene and substantial genetic exchange within the inversion in heterozygous females).

Correlation of compound autosome segregation and sex-chromosome disjunction in femaleLucilia cuprina(Wied.) (Diptera: Calliphoridae)

InLucilia cuprina(Wiedemann) females, non-segregation of compound fifth-chromosomes is correlated with non-disjunction of theXchromosomes. Approximately two-thirds of eggs which inherit both maternal compound elements are nullo-X, suggesting that meiotic pairing betweenXchromosomes and compound autosomes has occurred. There was no evidence for pairing of the compounds and theXorYchromosome in males. A limited amount of data suggests that high non-segregation frequencies may occur in the offspring of putativeX/X/XandX/Ofemales. These results suggest the existence of non-homologous (distributive) pairing inL. cuprinafemales.