N. A. Petrova

CYTOGENETIC CHARACTERISTICS OF A POPULATION OF CHIRONOMUS RIPARIUS MEIGEN 1804 (DIPTERA, CHIRONOMIDAE) FROM A POLLUTED PO RIVER STATION

A population of Chironomus riparius from a Po river station near Moncalieri (a trace-metal polluted station) was studied. In this population was established a great variability of band structure of polytene chromosomes as well as paracentric heterozygous inversions, deletions, deficiencies, partial breaks, diploid chromosome fragments, and changes in functional activity and appearance of heterochromatin. In arms A through F, some bands had an increased size compared to the standard chromosomic map. Some bands appeared in a heterozygous or normal homozygous state or were amplified. In all arms, many condensed stable bands appeared in the decondensed state when compared to the standard map. Asynaptic zones in arms E and G as well as heterozygous Balbiani rings and NORs were established. Very often the 4th chromosome was almost completely heteropycnotic and looded like a ‘pompon’ chromosome. For the first time in this species, a high frequency of ectopic pairings of different arms was observed. Telomeric regions involved in ectopic pairings had a granular appearance, as did some centromeres. The hypothesis is advanced that such a high frequency of structural rearrangements could be correlated with genomic distribution of specific mobile elements.

Structural-functional rearrangements in chromosome G in Chironomus riparius (Diptera, Chironomidae) collected from a heavy metal-polluted area near Turin, Italy

A natural population of Chironomus riparius from the heavy metal-polluted Piedmont station (Italy) was analysed. Chromosome G, on which the Balbiani rings BRa, BRb and BRc are located, was very sensitive to the pollution. The BR system was considered to be an interesting model for studying the response of the genome to the heavy metal pollution. Together, with a standard activity of BRs (BRc>BRb), we observed a clear reversed level of activity of BRb and BRc: BRc was repressed while BRb became activated. Also, there were a number of cells in which BRb and BRc were collapsed. A change in the activity of the nucleolar organizing region (N) was found, from a very high activity till completely collapsed. For the first time it was established that there was an activation at the telomere region of chromosome G. Additional active sites in sections Dc and E2de were observed. The conjugation of both homologues was often disturbed. A high frequency of ectopic pairings was also observed. In addition, structural rearrangements as inversions and deletions were found. Some deletions led to the formation of pompon-like chromosome G. Chromosome G of C. riparius is suggested to be used to test for the presence of genotoxic concentrations of polluting agents in an aquatic ecosystem.

Genotoxic effects of chromium onpolytene chromosomes of Chironomus riparius Meigen 1804 (Diptera, Chironomidae)

Abstract Genotoxic effect as tested of chronic exposure to three different concentrations of Chromium (III) on polytene chromosomes of larvae of Chironomus riparius (syn. Chironomus thummi) from the embryonic stage to the IV larval instar for two successive generations. In chromosomes AB, CD, EF and G significant differences of chromosome aberrations were found between exposed and control larvae as well as changes in functional activity (induction of novel puffs not corresponding to those induced during normal larval development in arms A, B, C and E, telomeric and centromeric decondensations especially at telomeres of chromosome G and arm C.). No significant differences were found between the effects of the three treatments nor between the two generations. In chromosome G the Balbiani Ring system (where some permanently active regions are involved in transcription of salivary proteins) appeared as a model for studying the response of the genome to Cr (III) treatment. In approximately one-third of the cells of the exposed larvae, the activity of the Balbiani rings BRc and BRb was reversed. In 10% of the cells of both generations of treated larvae, deletion or collapse of BRc was observed. Pomponlike G chromosomes were present in 6% of the cells. This type of chromoosme appeared either in a very decondesed or in a highly condensed state. In 5 % of the treated larvae the apical region of the chromosome G folded back so that the nucleolar organizer region appeared as if it was at the end of the chromosome. These structural and functional chromosomal changes are interpreted as a reaction of the genome to stressful rearing conditions.

Rearrangements in the salivary gland chromosomes of Chironomus riparius Mg. (Diptera, Chironomidae) following exposure to lead

Abstract The effect of lead (Pb) nitrate on the structural and functional organization of the polytene chromosomes of Chironomus riparius larvae was examined. No specimens with standard polytene chromosomes were found in the material treated with different concentrations of Pb ions. The polytene chromosomes of all individuals exposed to Pb showed various somatic chromosome rearrangements (heterozygous inversions, deletions, duplications and deficiencies), which were not detected in the studied controls. Deletions in chromosome G occurred in a high frequency resulting in the formation of so called “pompons”. The activity of Balbiani rings (BRs) and nucleolar organizer (NOR) significantly decreased with increasing the concentrations of Pb ions. The number of BRc/BRb showing little or no activity (-/-) increased following exposure of the F2 generation to low and high concentrations of Pb ions. Sections of the polytenechromosomes where somatic aberrations were concentrated are considered as “weak” sites. It was shown that in these sites were distributed repetitive DNA elements (Alu and Hinf) that might be activated by stress agents and generated many chromosome rearrangements. It is evident that lead nitrate has a marked genotoxic effect on the salivary polytene chromosomes of Chironomus riparius.

Chironomids (Diptera) and their salivary gland chromosomes as indicators of trace-metal genotoxicity

Abstract The long-term impact of pollution in freshwater aquatic environments can be assessed realistically only if the mutagenic effects of pollutants on organisms inhabiting aquatic biotopes are evaluated. To do this, the larvae of Chironomids (Diptera) can serve as suitable indicators of potential environmental genotoxicity. These larvae exhibit aberrations in their polytene chromosomes, which, when analyzed, indicate an early response to toxicity more efficiently than morphological analysis, up to now the usual technique. Chironomids have excellent salivary gland chromosomes with well-defined bands and permanent somatic pairing, where there are two important regions of active transcription—Balbiani rings (BRs) and Nucleolar organizers (NOR). We have described the responses of different Chironomid species in two situations: in the field and in the laboratory, where we performed Cr, Al, Pb and Cu trace-metal-exposure experiments with Chironomus riparius. In both situations, the genome response consisted in a statistically significant increase in somatic chromosome aberrations and decrease in BR and NOR activity to levels lower than those of larvae under standard conditions. The main chromosome aberrations consisted in inversions, amplifications, deletions and deficiencies. Deletions in the chromosome G in C. riparius transformed it into a pompon-like structure. Chromosome aberrations were not randomly distributed: they occurred mainly in sites with repetitive DNA elements that have a highly interspersed location in the C. riparius genome. Hence C. riparius is the species most likely to produce aberrations and can be used as a model for cost-effective monitoring of the early genomic response to trace metals and other stress agents.

Comparative Cytogenetic Variation of the Salivary Gland Polytene Chromosomes in Chironomus riparius Mg., 1804 (Diptera, Chironomidae) from Two Polluted Biotopes of Bulgaria and Russia

The structural–functional variation ofChironomus riparius salivary gland polytene chromosomes was studied in two geographically isolated Palearctic regions, Bulgaria (village Pancharevo) and Russia (St. Petersburg). The two biotopes, where larvae were collected, were polluted with various heavy metals from anthropogenic sources. Hereditary paracentric heterozygous inversions were characteristic of the Russian population, whereas somatic paracentric or pericentric heterozygous inversions were more common in the Bulgarian one. All inversions occurred at low frequencies. Other aberrations found in the two populations included somatic deletions resulting in a pompon structure of chromosome IVG, heterozygous translocation between chromosomes IVG and IIIF, enlargement of individual disks, and the appearance of a heterozygous block close to the centromere of chromosome IVG. In addition, changes in functional activity of the nucleolus organizer and Balbiani rings (BRc/BRb) were observed. Several aberration breakpoints proved to coincide with satellites of the Alu and Hinf families.

[Inversion polymorphism of Glyptotendipes glaucus Mg. (Diptera: Chironomidae) from the reservoirs of Kaliningrad].

The karyotypes of larvae Glyptotendipes glaucus Mg. 1804 from three reservoirs of Kaliningrad city (Pen’kovoe and Karasevka lakes and Chistyi pond) were studied. The levels of the natural inversion polymorphism for the three populations were determined. Ten new inversion sequences (gla B4, gla B5, gla B6, gla D5, gla E4, gla E5, gla E6, gla E7, gla F6, and gla F7) were detected in the species studied. Inversion including the centromeric region in IIIEF (gla E6 + gla F7) was pericentric. Several cases of gla B5 and gla B6 combination with gla B2 resulting in genotypic combination gla B2.5 and gla B2.6 were found. The dependence of the number of inversion sequences and genotype combinations from the content of heavy metal ions in sediments of the reservoirs was revealed. All populations were characterized by the prevalence of inversion sequence gla B2 in chromosome I.

Initial Stage of Sympatric Divergency in Species of the Genus Glyptotendipes Kieff. (Diptera, Chironomidae)

SUMMARYCarrying out a comparative karyological analysis of species of the genus Glyptotendipes Kieff. the authors established polymorphism with regard to the Ist chromosome. The data obtained allow the authors to consider the genome of the species Gl. gripekoveni and Gl. paripes as consisting of two parts: conservative (Und, IIIrd and IVth chromosomes) and unstable (Ist chromosome). As to the unstable part—the Ist chromosome—two forms were found in Gl. gripekoveni. In one of the forms the Ist chromosome has changed considerably in result of multiple homozygous inversions. The same chromosome in the other form has a hybrid origin (Gl. gripekoveni X Gl. paripes). In Gl. paripes there are three forms established which have been produced by the step-like development of one form into another via homozygous inversions. The inter-species differences in the unstable part of the genome (in the structure of the 1st chromosome) of both species are interpreted as an initial stage of inter-species divergency.

A new anhydrobiotic midge from Malawi, Polypedilum pembai sp.n. (Diptera: Chironomidae), closely related to the desiccation tolerant midge, Polypedilum vanderplanki Hinton

The sleeping chironomid (Polypedilum vanderplanki Hinton) lives on temporary rock pools in the semi‐arid tropical regions of Africa. Its larvae are able to survive the dry season in a completely desiccated ametabolic state known as anhydrobiosis. So far, P. vanderplanki was the only species among all insects showing demonstrated anhydrobiotic ability. Here, we show that a new related species originating from Malawi, Polypedilum pembai sp.n., is also anhydrobiotic and that its desiccation tolerance mechanism is probably similar to what is observed in P. vanderplanki. The new species, P. pembai sp.n., is described with special attention to the common and different morphological features, compared with P. vanderplanki. Phylogenetic analysis showed that both species are closely related, suggesting that anhydrobiosis evolved only once in their common ancestor about 49 Ma somewhere in Africa, before the divergence of two species, one in the sub‐Saharan area and another in southeastern Africa.

Cytogenetic Features of Blackfly Wilhelmia paraequina Puri (Diptera: Simuliidae) from Armenia

Three blackfly Wilhelmia paraequina populations of Armenia (rivers Debet and Megriget, channel Megri) were studied. 2n = 6: IS + IIL, IIS + IL, IIIS + IIIL. Cytological maps of polytene chromosomes were constructed. High inversion polymorphism (95.63%) was observed, with 2.6 inversions per individual. Three inversions proved to be associated with male development: Y1 was characterized by a combination of two heterozygous inversions, IIL-3 + IIL-1, while Y2 had another combination of heterozygous inversions, IIL-5 + IIL-3. The X chromosome had a standard homozygous IIL band pattern. The populations were shown to be similar in autosomal polymorphism. A tendency for differentiation was observed with respect to frequencies and types of sex-linked inversions: the sex determination system was Y1X–XX in the Debet and Megriget populations and Y2X–XX in the channel (Megri) population. On the strength of these findings, one W. paraequina morphotype was assumed to involve two cytotypes, A (Debet and Megriget populations) and B (Megri population).