Inter- and intracytogenetic variability was analyzed in 13 natural Palearctic populations of Chironomus riparius Meigen 1804 (syn. Chironomus thummi) by examining hereditary and somatic aberrations (mainly inversions) of the salivary gland polytene chromosomes. In total, 77 different types of inherited inversion sequences and 184 different types of somatic inversions were found. The median percent frequency of inherited inversions was 1.4% and karyotypic divergence between populations was very low. Most hereditary inversions were endemic and always in a heterozygous state. Only six inversion sequences, each of them shared by two very distant populations, may be considered a relic of very ancient ancestral inversions. Unlike inherited inversions, occurrence of somatic aberrations seems to increase with the overall rise in the level of heavy metal pollution of the sediments from which larvae were sampled. In contrast with what occurs in populations of other chironomid species, populations of C. riparius do not seem to undergo a process of cytogenetic differentiation.

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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.

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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 E 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 looked 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.

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The investigation was carried out on a small pond situated on a recent mine spoil at Bolesław in the Olkusz region with Zn-Pb ore deposits. Water of the pond had pH 7.2-8.5 and low concentrations of heavy metals. Concentrations of Pb (487 μg g( - 1)) and Zn (1,991 μg g( - 1)) in the sediment were very high and potentially could lead to toxicological effects. In the pond, 48 taxa of macroinvertebrates belonging to Oligochaeta and water stages of Ephemeroptera, Odonata, Megaloptera, Trichoptera, Heteroptera, Coleoptera and Diptera (mainly Chironomidae family) were found. The influence of heavy metals on macroinvertebrates diversity was not found. Effect of heavy metal pollution was observed on the appearance of chromosome aberrations in the polytene chromosomes of Chironomidae larvae. It was manifested by two ways: (1) in Kiefferulus tendipediformis and Chironomus sp. chromosome rearrangements in fixed state (tandem fusion and homozygous inversions), indicated intensive process of speciation; (2) in Chironomus sp., K. tendipediformis, Glyptotendipes gripekoveni (Chironomidae) somatic chromosome rearrangements (inversions, deficiencies, specific puffs, polyploidy) affected few cells of every individual. The somatic functional and structural alterations in Chironomidae species are particular suitable as biomarkers-they can be easily identified and used for detecting toxic agents in the environment.

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