Occurrence of G-banding in metaphase chromosomes of Encarsia berlesei (Hymenoptera: Aphelinidae)

Chromosome study of the Hymenoptera (Insecta): from cytogenetics to cytogenomics

A brief overview of the current stage of the chromosome study of the insect order Hymenoptera is given. It is demonstrated that, in addition to routine staining and other traditional techniques of chromosome research, karyotypes of an increasing number of hymenopterans are being studied using molecular methods, e.g., staining with base-specific fluorochromes and fluorescence in situ hybridization (FISH), including microdissection and chromosome painting. Due to the advent of whole genome sequencing and other molecular techniques, together with the "big data" approach to the chromosomal data, the current stage of the chromosome research on Hymenoptera represents a transition from Hymenoptera cytogenetics to cytogenomics.

Comparative Karyotype Analysis of Parasitoid Hymenoptera (Insecta): Major Approaches, Techniques, and Results

A comprehensive review of main approaches, techniques and results of the chromosome study of parasitic wasps is given. In this group, the haploid chromosome number ranges from n = 3 to 23. Distribution of parasitic wasp species by the chromosome number is bimodal, with two obvious modes at n = 6 and 11. Karyotype analysis based on routinely stained preparations of mitotic chromosomes can be used to identify members of taxonomically complicated parasitoid taxa and to distinguish between them. Morphometric study effectively reveals subtle differences between similar chromosome sets of parasitic wasps. If combined with meiotic analysis and/or cytometric data, information on mitotic karyotypes can highlight pathways of the genome evolution in certain parasitoid taxa. C- and AgNOR-banding as well as staining with base-specific fluorochromes detected important interspecific differences within several groups of parasitic wasps. Fluorescence in situ hybridization (FISH) is successfully used for physical mapping of various DNA sequences on parasitoid chromosomes. These techniques demonstrate that heterochromatic segments are usually restricted to pericentromeric regions of chromosomes of parasitic wasps. Haploid karyotypes carrying one or two nucleolus organizing regions (NORs) are the most frequent among parasitoid Hymenoptera. In combination with chromosome microdissection, FISH could become a powerful tool exploring the genome evolution of parasitic wasps. Perspectives of the comparative cytogenetic study of parasitoid Hymenoptera are outlined.

Chromosomes of parasitic wasps of the superfamily Chalcidoidea (Hymenoptera): An overview

An overview of the current knowledge of chromosome sets of the parasitoid superfamily Chalcidoidea is given. Karyotypes of approximately 240 members of this group, i.e. just above one percent of described species, are studied up to now. Techniques for obtaining and analyzing preparations of chalcid chromosomes are outlined, including the so-called "traditional" and "modern" methods of differential staining as well as fluorescence in situ hybridization (FISH). Among the Chalcidoidea, the haploid chromosome number can vary from n = 3 to n = 11, with a clear mode at n = 6 and a second local maximum at n = 10. In this group, most chromosomes are either metacentric or submetacentric, but acrocentrics and/or subtelocentrics also can predominate, especially within karyotypes of certain Chalcidoidea with higher chromosome numbers. The following main types of chromosomal mutations are characteristic of chalcid karyotypes: inversions, fusions, translocations, polyploidy, aneuploidy and B chromosome variation. Although karyotype evolution of this superfamily was mainly studied using phylogenetic reconstructions based on morphological and/or molecular characters, chromosomal synapomorphies of certain groups were also revealed. Taxonomic implications of karyotypic features of the Chalcidoidea are apparently the most important at the species level, especially among cryptic taxa.

Karyological characterization of the common chameleon (Chamaeleo chamaeleon) provides insights on the evolution and diversification of sex chromosomes in Chamaeleonidae

Chameleons display high karyological diversity in chromosome number (from 2n = 20 to 62), morphology, heterochromatin distribution and location of specific chromosomal markers, making them unique study models in evolutionary cytogenetics. However, most available cytogenetic data are limited to the description of the chromosome number and morphology. Concerning sex chromosomes, our knowledge is limited to ZZ/ZW and Z1Z1Z2Z2/Z1Z2W systems in the genus Furcifer and the isolation of sex-linked, male-specific, sequences in Chamaeleo calyptratus, but the putative XY chromosomes have still to be identified in Chamaeleo and the conservation of male heterogamety in the genus needs confirmation from other species. In this study we performed a molecular and a cytogenetic analysis on C. chamaeleon, using standard, banding methods and molecular cytogenetics to provide a throughout karyological characterization of the species and to identify and locate the putative XY chromosomes. We confirm that the chromosome formula of the species is 2n = 24, with 12 metacentric macrochromosomes, 12 microchromosomes and NORs on the second chromosome pair. Heterochromatin was detected as weak C-bands on centromeric regions, differently from what was previously reported for C. calyptratus. Fluorescence in situ hybridization (FISH) showed the occurrence of interspersed telomeric signals on most macrochromosomes, suggesting that ancient chromosome fusions may have led to a reduction of the chromosome number. Using a combination of molecular and FISH analyses, we proved that male specific Restriction site-Associated DNA sequences (RADseq) isolated in C. calyptratus are conserved in C. chamaeleon and located the putative XY chromosomes on the second chromosome pair. We also identified different transposable elements in the focal taxa, which are highly interspersed on most chromosome pairs.

Microdissection and whole chromosome painting confirm karyotype transformation in cryptic species of the Lariophagus distinguendus (Förster, 1841) complex (Hymenoptera: Pteromalidae)

Karyotypes of two cryptic species of parasitoid Hymenoptera with n = 5 and 6 belonging to the Lariophagus distinguendus (Förster, 1841) complex, which includes cosmopolitan parasitoids of coleopteran stored-product pests, were studied using glass-needle based microdissection, reverse and cross-species fluorescence in situ hybridisation (FISH). This experiment strongly indicates that the largest metacentric chromosome in the karyotype with n = 5 originated from a particular fusion between the only acrocentric and a smaller metacentric chromosome of the set with n = 6, therefore confirming our previous hypothesis based on the karyotypic analysis using chromosome morphometrics. This study represents the first successful application of both microdissection and whole chromosome painting for the reconstruction of karyotypic rearrangements in closely related species of parasitoids, as well as in the order Hymenoptera in general.

Comparative cytogenetic study on two species of the genus Entedon Dalman, 1820 (Hymenoptera, Eulophidae) using DNA-binding fluorochromes and molecular and immunofluorescent markers

Karyotypes of Entedon cionobius Thomson, 1878 and Entedon cioni Thomson, 1878 (Hymenoptera: Eulophidae) were studied using DNA-binding ligands with different base specificity (propidium iodide, chromomycin A3, methyl green and DAPI; all these ligands, except for the last one, were used for the first time in parasitic wasps), C-banding, fluorescence in situ hybridization (FISH) with a 45S rDNA probe and 5-methylcytosine immunodetection. Female karyotypes of both species contain five pairs of relatively large metacentric chromosomes and a pair of smaller acrocentric chromosomes (2n = 12). As in many other Hymenoptera, males of both Entedon Dalman, 1820 species have haploid chromosome sets (n = 6). Fluorochrome staining revealed chromosome-specific banding patterns that were similar between the different fluorochromes, except for the CMA3- and PI-positive and DAPI-negative band in the pericentromeric regions of the long arms of both acrocentric chromosomes. The obtained banding patterns were virtually identical in both species and allowed for the identification of each individual chromosome. C-banding revealed a pattern similar to DAPI staining, although centromeric and telomeric regions were stained more intensively using the former technique. FISH detected a single rDNA site in the same position on the acrocentric chromosomes as the bright CMA3-positive band. Immunodetection of 5-methylcytosine that was performed for the first time in the order Hymenoptera revealed 5-methylcytosine-rich sites in the telomeric, centromeric and certain interstitial regions of most of the chromosomes.

Cytogenetic characterization of telomeres in the holocentric chromosomes of the lepidopteran Mamestra brassicae

Telomeres of the Mamestra brassica holocentric chromosomes were studied by Southern blotting, in-situ hybridization and Bal31 assay evidencing the presence of the telomeric (TTAGG)n repeat. Successively, molecular analysis of telomeres showed that TRAS1 transposable elements were present at the subtelomeric regions of autosomes but not in the NOR-bearing telomeres of the Z and W sex chromosomes. TRAS1 appeared to be transcriptionally active and non-methylated, as evaluated by RT-PCR and digestion with MspI and HpaII. Finally, dot-blotting experiments showed that the 2.8 +/- 0.5% of the M. brassicae genome consists of TRAS1.

Ceratotoxins: Female-specific X-linked genes from the medfly,Ceratitis capitata

In this paper, we report the chromosomal localization of ceratotoxins, a gene family encoding antibacterial female-specific peptides from the mediterranean fruit fly Ceratitis capitata. The analysis of both polytene and mitotic chromosomes by in situ hybridization shows that ceratotoxins are the first case of female-specific X-linked genes from the medfly C. capitata. Southern blot analysis reveals that the ceratotoxin gene family is not specifically amplified in the female reproductive accessory glands of C. capitata.Key words: ceratotoxins, female-specific genes, Ceratitis capitata, X chromosome, in situ hybridization.

The relationship between DNA methylation and chromosome imprinting in the coccid Planococcus citri

The phenomenon of chromosome, or genomic, imprinting indicates the relevance of parental origin in determining functional differences between alleles, homologous chromosomes, or haploid sets. In mealybug males (Homoptera, Coccoidea), the haploid set of paternal origin undergoes heterochromatization at midcleavage and remains so in most of the tissues. This different behavior of the two haploid sets, which depends on their parental origin, represents one of the most striking examples of chromosome imprinting. In mammals, DNA methylation has been postulated as a possible molecular mechanism to differentially imprint DNA sequences during spermatogenesis or oogenesis. In the present article we addressed the role of DNA methylation in the imprinting of whole haploid sets as it occurs in Coccids. We investigated the DNA methylation patterns at both the molecular and chromosomal level in the mealybug Planococcus citri. We found that in both males and females the paternally derived haploid set is hypomethylated with respect to the maternally derived one. Therefore, in males, it is the paternally derived hypomethylated haploid set that is heterochromatized. Our data suggest that the two haploid sets are imprinted by parent-of-origin-specific DNA methylation with no correlation with the known gene-silencing properties of this base modification.

Heterochromatin heterogeneity in the holocentric X chromatin of Megoura viciae (Homoptera, Aphididae)

Holocentric chromosomes, prepared by spreading embryo cells obtained from Megoura viciae parthenogenetic females, have been C-banded, enzymatically digested in situ using the specific endonucleases DdeI (C↓TNAG), DraI (TTT↓AAA), Tru9I (TT↓AA), and CfoI (GCG↓C), and subsequently stained with Giemsa, DAPI, CMA3, and AgNO3. We observed that the X chromosome had the best defined banding patterns. In the M. viciae X chromosome there is a certain amount of heterogeneity in heterochromatic DNA composition. In fact, the GC-rich NOR-associated heterochromatin differs from other heterochromatic bands that are characterized by AT-rich DNAs. Our data also indicate that, in M. viciae holocentric chromosomes, all heterochromatic blocks are accessible to in situ enzyme attack, the only limit to the digestion being the presence or absence of recognition targets. This is an interesting point, since, in monocentric chromosomes, it is well known that in situ endonuclease digestion is heavily affected not only by DNA base composition but also by chromatin compactness that may limit enzyme accessibility to their specific targets. Key words : heterochromatin, holocentric chromosomes, aphids.

G-banding and chromosome condensation in the ant, Tapinoma nigerrimum

Well-defined G-bands were obtained on metaphase chromosomes from Tapinoma nigerrimum using trypsin and warm 2 x SSC in sequence. The G-banded pattern allowed the identification of all chromosomes. Evidence for asynchronous condensation of the chromosomes of this species is provided. Different banding patterns were obtained when metaphase chromosomes were stained with DA/DAPI alone and with DA/DAPI after a standard G-banding procedure. The G-banding phenomenon is discussed using the result obtained.