Intercontinental karyotypic differentiation of Chironomus entis Shobanov, a Holarctic member of the C. plumosus group (Diptera, Chironomidae).

Analysis of banding sequences of polytene chromosomes in Palearctic (Russian) and Nearctic (North American) Chironomus entis shows strong karyotype divergence between populations on the two continents. Four out of seven chromosomal arms in the North American C. entis karyotype are characterized by sequences found only in the Nearctic. In total, 44 banding sequences are now known for this species across the Holarctic, including 22 exclusively Palearctic, 6 Holarctic, and 16 exclusively Nearctic sequences. The degree of cytogenetic differentiation between Palearctic and Nearctic C. entis populations is an order of magnitude greater than differentiation among populations within either continent, but is only one third as great as the cytogenetic distance between the sibling species C. entis and C. plumosus. C. entis is the only sibling species of C. plumosus uncovered during cytological identification of Chironomus species from more than 50 North American lakes, indicating that the plumosus sibling-species group is much smaller in the Nearctic than in the Palearctic, where a dozen sibling species are known. Cytogenetic distance values calculated between Nearctic and Palearctic representatives of both C. entis and its sibling species C. plumosus are similar, but result from different patterns of karyotype divergence. New World C. entis is distinguished from Old World populations by the 16 uniquely Nearctic sequences, four of which occur in the homozygous state. In contrast, North American C. plumosus has fewer uniquely Nearctic sequences, and only one that occurs as a homozygote. However, four chromosomal arms in C. plumosus that are polymorphic in the Palearctic show fixation, or near fixation, of Holarctic sequences in the Nearctic C. plumosus karyotype. Thus, both the fixation of Holarctic sequences, and the occurrence or fixation of distinctly Nearctic sequences, contribute significantly to karyotype divergence. Patterns of karyotype divergence in Palearctic and Nearctic populations of different Holarctic chironomid species are discussed relative to intercontinental cytogenetic differentiation in other dipterans.     

Karyotype structure and polymorphism peculiarities of Chironomus nuditarsis Keyl, 1961 (Diptera,Chironomidae) from natural populations of North Caucasus

The study presents data on the karyotype characteristics and features of chromosomal polymorphism of Chironomus nuditarsis Keyl, 1961 (Diptera, Chironomidae) from seven natural populations of Caucasus (Northwest, Central and East Caucasus). We found 16 banding sequences in the Caucasian populations. We observed inversion polymorphism almost in all chromosome arms except for arms C and E. The genetic distances between all the studied populations of Ch. nuditarsis were calculated using Nei criteria (1972). In spite of relative geographic proximity, the genetic distances between populations of the Caucasus are quite big, and they do not form a single cluster of Caucasian populations. The population of the Northwest Caucasus goes to European cluster; the populations of Central and East Caucasus form their own separate clusters. The principal component analysis (PCA) shows the similar picture. We suggest that such a clear separation of Caucasian populations in distinct clusters is a result of differences of collection sites in terms of geography and climate (complex diverse terrain and microclimate conditions). Four of the Caucasian populations do not follow Hardy-Weinberg expectation. In two populations, there being a marked deficiency of heterozygotes in arms B, F and G. In two other populations, there being a marked excess of heterozygotes in arms B and G. One can suggest that observed pictures could be a reflection of multi-directional selection of heterozygotes in different populations. The populations of Ch. nuditarsis from different parts of the Caucasus possibly diverged from each other at the level of subspecies. All the obtained data are indicative of the complex genetic structure of Caucasian populations of Ch. nuditarsis and total complexity of microevolution processes occurring in the Caucasus region.     

The geographic varyability of the polytene chromosome banding sequence of non-biting midje Chironomus pseudothummi str. (Diptera, Chironomidae)

The karyotypes and chromosomal polymorphism of Chironomus pseudothummi were investigated in different parts of its range. It was established that chromosomal variability in the natural populations of this species was represented mainly by the inversion polymorphism of arm G. Only rare and unique inversions were found as heterozygous in arms C, D, and E. In total, the 14 banding sequences of polytene chromosomes form the banding sequences pool of Ch. pseudothummi. Geographic differences in distribution of chromosomal banding sequences throughout the range were established. The presence of banding sequences pstG1 and pstG2 is characteristic of European populations. Banding sequence pstG1 was completely vanished with simultaneous increase in frequency of pstG2 and appearance of new inversion banding sequence pstG3 in Siberian populations. The differences in the set of the rare and unique inversions in arms C, D, and E between west-European and west-Siberian populations were revealed.     

Chromosomal restructuring of three species of chiromonads from the Chernobyl region (Diptera, Chironomidae)

Chromosomal polymorphism of three species--Chironomus plumosus, Ch. balatonicus and Glyptotendipes glaucus collected from the Chernobyl Zone demonstrated following characteristics: lack of standard karyotype, the presence of hetero- and homozygotic inversions (seven para- and one pericentric), increase in centromeric heterochromatin (55% larvae in homo- and heterozygotic state), the presence of B chromosomes (21%)--in Ch. plumosus; only two larvae had a standard karyotype, the rest demonstrating hetero- and homozygotic inversions (eleven paracentrics), reciprocal translocations of the IVF and IA arms, B chromosomes (5.4%), increase in telomeric (43.6%) and centromeric (1.8%) heterochromatin--in Ch. balatonicus; two types of hetero- and homozygotic inversions, replacement of standard sequences in C and D for inversional homozygotic ones--in Gl. glaucus.     

Divergence of karyofunds in sibling species of the plumosus group (Diptera: Chironomidae)

Cytogenetic differentiation of eight sibling species of the plumosus group was examined. The karyofunds of these sibling species were shown to diverge incompletely. In each species karyofund, the banding sequences homologous to those of the remaining species of this group were revealed. The number of banding sequences that displayed interspecific homology varied from 3 to 13 per species karyofund. In a species karyotype, the homologous sequences were localized to chromosome arms 1-6. Both similar and contrasting frequencies of homologous banding sequences were observed in karyofunds of different sibling species. The average cytogenetic distance between sibling species of the plumosus group was 2.618 +/- 0.400. The presence of species-specific banding sequences, the absence of homologous banding sequences in some chromosome arms of the karyotype, and different frequencies of the homologous banding sequences determined the cytogenetic divergence of the sibling species.     

Karyotype and morphology of Chironomus wulkeri (Diptera, Chironomidae) from Lake Baikal region

The karyotype and morphology of the Palaearctic chironomid, Chironomus wulkeri Filinkova et Belyanina 1993, inhabiting the Baikal coast were studied. The banding patterns of chromosomal arms B, C, and D are presented for the first time. The karyotype and chromosome polymorphism of Ch. wulkeri from the Baikal population have no significant difference from those in northern Sweden and the Polar Ural populations. One heterozygous inversion in arm A was found in 38% of larvae. A comparative morphological analysis of 46 quantitative characters of larvae from Baikal region and the Polar Urals showed that they did not differ from each other. However, there are some differences in the width of the ventromental plates and height of the mentum.     

Molecular-cytogenetic characteristics of B-chromosomes in chironomid (Diptera, Chironomidae)

Morphological and molecular study of B-chromosomes of three Chironomus species (siblings Ch. borokensis and Ch. phumosus from plumosus group, and Ch. heterodentatus from obtusidens group) was carried out. Morphological similarity of B-chromosome banding pattern and telomer-centromeric region banding pattern of chromosome IV in Ch. borokensis was shown. Polytene B-chromosomes of Ch. borokensis and Ch. heterodentatus were microdissected, and their DNA was amplified using degenerate oligonucleotide primer polymerase chain reaction. Comparative analysis of the localization of homologous B-chromosome DNA sequences of A- and B-polytene chromosomes was made using in situ fluorescence hybridization. It has been shown that B-chromosomes in the studied species are composed mainly of repetitive DNA sequences homologous to sequences of centromeric and telomeric DNA of A-chromosomes, and also these of the mobile element NLRCthl. The B-chromosome DNA, homologous to sequences of DNA mobile element, was scattered on A-chromosomes (more than 100 sites). No ribosomal DNA repeats were identified in B-chromosome. Heterologous FISH of B-chromosome DNA to polytene A-chromosomes of Ch. thummi, a species lacking B-chromosomes, enabled us to reveal the presence of numerous sites homologous to DNA of B-chromosomes. These are mainly mobile element sites. An origin of B-chromosomes and peculiarities of their organization in chironomids are discussed.     

Karyotype of the midge Chironomus heterodentatus Konstantinov from the group Obtusidens (Diptera, Chironomidae)

The karyotype of Chironomus heterodentatus, belonging to the obtusidens-group described by Konstantinov from the Volga in 1956, was studied in detail. Combinations of chromosomal arms are AB, CD, EF and G (cytocomplex thummi). The Ch. heterodentatus karyotype cleary differs from those of other members of the obtusidens-group with a species-specific banding pattern in arms A, B, C and D. Chromosomal polymorphism on homo- and heterozygous inversions was found in arms, A, B, D, E and G. 19 inversion banding sequences and their 27 genotypic combinations have been recorded. The shortest arm G is highly polymorphic. Heterozygotes on the Balbiani ring activity were found in arm G along with homo- and heterozygotes on inversions. B-chromosomes with a frequency equal to 2.7-25.0% were recorded in some Volga populations.     

Geographic variability of the polytene chromosome banding sequence of non-biting midge <Emphasis Type="Italic">Chironomus pseudothummi</Emphasis> Str. (Diptera,Chironomidae)

The karyotypes and chromosomal polymorphism of Chironomus pseudothummi were investigated in different parts of its areal. It was established that chromosomal variability in the natural populations of this species was represented mainly by the inversion polymorphism of arm G. Only rare and unique inversions were found to be heterozygous in arms C, D, and E. In total, 14 banding sequences of polytene chromosomes form the banding sequence pool of C. pseudothummi. Geographic differences in the distribution of chromosomal banding sequences throughout the areal were established. The presence of banding sequences pstG1 and pstG2 is characteristic of European populations. The banding sequence pstG1 disappeared completely with a simultaneous increase in the frequency of pstG2 and with the appearance of a new inversion banding sequence pstG3 in Siberian populations. Differences in the set of rare and unique inversions in arms C, D, and E between the West-European and West-Siberian populations have been revealed.     

Chromosomal variability in natural populations of Chironomus cingulatus meigen (Diptera, Chironomidae)

Chromosomal polymorphism has been studied in seven natural populations of Chironomus cingulatus from Western Europe, Western Siberia, and the Republic of Sakha (Yakutia). The banding sequences pool of the species includes 15 banding sequences. Chromosomal polymorphism was revealed in five out of seven chromosomal arms. Arm B is the most polymorphic with four banding sequences. There are three banding sequences in arm A. Arms D, E, and G have two banding sequences. None of the chromosome rearrangements were revealed in arms C and F. The populations of C. cingulatus differ clearly in their number and frequency of banding sequences, which indicates that different gene sequences are adaptive in different populations.     

Individual variability in the karytype of Chironomus plumosus: atypical puffs in larva from a natural population from the Chita region

Atypical puffing of polytene chromosomes of Chironomus plumosus (1 larvae, IV stage) from the Ivan lake in Chita region, southern part of Siberia, has been described. At the sites of ordinary localization of interdisks and puff-patterns typical of Ch. plumosus the puffs in our material displayed different levels of activity: from a light vacuolous spot to puffs of class 5. Most of these puffs were revealed in Ch. plumosus for the first time, namely, puffs IA10a-r, IB12v-y + 13a-d, IIC14p-z + 15a-z + 16a-e, IIC14p-z + 15a-h, IID14a-m + 13s-w, IID11-2a-d, IID1p-x + 2a-d, IIIE3g-a and IIIF13h-p + 14a-e. Some other puffs, such as IB16a-k, IB15m-r + 16a-m, IB21a-o, IIC20, IVG6 and IVG7, were described earlier (Maksimova, 1979, 1983). The majority of observed puffs turned out to be heterozygous. Only one puff-knob, IIIE3g-a of class 5 activity, was found in all cells of the studied salivary glands. Its origin may be due to the appearance of heterozygoous inversion pluE1.2. All other puffs were observed in some part of cells. It is supposed that the appearance of larvae with unusually high functional activity of chromosomes may be presumably induced by stress influence of certain environmental factors.     

Cytogenetic characteristics of species of the Chironomus plumosus group (Chironomidae, Diptera) in Finland

The cytogenetic characteristics of species of the Chironomus plumosus group in Finland were examined. The species included C. balatonicus Devai, Wülker, Scholl, C. entis Shobanov, C. plumosus L., C. muratensis Ryser, Scholl, Wülker, and two karyotypes of unknown species. All belonged to the thummi complex having 2n = 8 chromosomes, with arm combinations of AB, CD, EF, G. In C. balatonicus arms A to G the band sequences corresponded with those of other populations. A new band sequence was found in arm B. In C. entis an arm A had a band sequence similar to those in C. agilis. A large pericentric inversion was observed in chromosome AB. The arms B to G had band sequences typical for C. entis. The chromosome arms A and B in C. plumosus from Lake Marsjon had band sequences corresponding to those of C. agilis and C. entis, respectively. C. plumosus from Helsinki may be a more divergent population with a large amount of centromere heterochromatin. C. muratensis was not distinguishable by band sequences from those of other Palaearctic populations. Two new karyotypes similar to those of species of the plumosus group have been described.     

The chromosomal polymorphism and divergence of populations in Chironomus nuditarsis Str. (Diptera, Chironomidae)

The karyotype structure and chromosomal polymorphisms were investigated in 6 natural and 2 laboratory populations of Chironomus nuditarsis from Europe and Asia. The pool of rearranged polytene chromosome banding sequences of this species was determined that includes 16 inversion banding sequences and sequences with giant DNA-knobs (ndtG1k, ndtG2k). Obvious differences were demonstrated in the level of chromosomal polymorphism between European and Asian (Siberian) populations: the former were highly polymorphic, while the latter were practically monomorphic. It was suggested to consider the Siberian populations as marginal one. Cytogenetic distances between populations of C. nuditarsis as well between C. nuditarsis and the related species C. plumosus were estimated. The data obtained show that chromosomal rearrangements play a very important role in cytogenetic divergence of populations.     

Homoeology of banding sequences and centromeres of Glyptotendipes paripes Edwards, 1929 and G. glaucus meigen, 1818 (Diptera, Chironomidae)

The karyotype analysis of the Glyptotendipes paripes Edwards, 1929 from water reservoirs of Saratov region has been conducted. In the studied population, the following combination of banding sequences predominated: A1.1, B1.1 and G1.1. The frequency of larvae with heterozygous inversions was 72.9 %, the average number of heterozygous inversions per specimen was 0.667, and the most frequent heterozygous inversions were B1.2 (39.5%) and D1.2 (29.2%). Localization of centromeres in the polytene chromosomes G. paripes was defined with the help of C-banding. Homoeology of polytene chromosomes of two chironomidae species (G. paripes and G. glaucus Mg.) has been analyzed and redesignation of chromosomes arms G. paripes has been offered: instead of par D--par A, instead of par C--par B, instead of par A--par D and instead of par B--par C.     

The role of chromosomal polymorphism in divergence of populations and species of the genus Chironomus (Diptera)

The data on the structure and level of chromosomal polymorphism in natural populations of species of the genus Chironomus are summarized. A very high level of chromosomal polymorphism was noted for most species. Paracentric inversions prevailed among the chromosomal rearrangements found in natural populations. Changes in the set and frequency of inversion sequences are the most important factor of cytogenetic divergence of populations. Several cytogenetic types of populations were distinguished. The Palaearctic and Nearctic populations of Holarctic species diverged to a greater extent due to the formation of endemic Palearctic and Nearctic inversion sequences. The sequences common for both regions indicated a common ancestry of the populations. The cytogenetic distances between the Palearctic and Nearctic populations are greater by an order of magnitude than those between populations within each zoogeographic region. Divergence of species karyotypes was found to result from fixation of different inversion sequences in the course of evolution. The karyotypes of Palearctic and Nearctic species mainly differ by the presence of endemic Palearctic and Nearctic banding sequences. Several basic sequences common for some species allow the cytogenetic history of their origin to be revealed. A NJ phylogenetic tree was built for the genus Chironomus, demonstrating chromosomal evolution of its species.     

Inherited and somatic cytogenetic variability in Palearctic populations of Chironomus riparius Meigen 1804 (Diptera, Chironomidae).

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.     

About the possibility of hybridogenesis in the species origin of the midge Chironomus usenicus Loginova et Beljanina (Chironomidae,Diptera)

Hybridogenesis as a possible way of speciation in Chironomidae was considered with special reference to the species Chironomus usenicus resulting from hybridization between C. plumosus and C. behningi. The three species had 2n = 8 and belonged to the thummi cytocomplex with chromosome arm combinations AB, CD, EF, and G. Arm G had a marker chromosome disk sequence (CDS) and was used to demonstrate the hybrid origin of C. usenicus. Most C. usenicus larvae were heterozygous in CDS of arm G. CDS use G2 proved to be identical to CDS beh G1 of C. behningi and CDS use G1, to CDS plu G1 of C. plumosus. It was assumed that C. usenicus results from hybridization between eurybiont C. plumosus and stenobiont C. behnigni at the boundary of their species areas, in freshwater or brackish water bodies of the southern Saratov oblast and northern Kazakhstan. Morphologically and karyotypically, the hybrid was probably similar to C. plumosus. Crosses with C. plumosus eliminated virtually all C. behningi chromosome sequences from the karyotype of the hybrid. Further chromosome divergence resulting in C. usenicus involved a number of chromosome rearrangements, including duplication of pericentric heterochromatin and other chromosome regions; inversion, which occurred in arm F (regions 13-16) and was fixed in the karyotype; and other paracentric inversions and deletions accumulated in heterozygote in the karyotype pool of the species. Since C. behningi was eliminated from the introgression zone and its species are reduced, the assimilation character was assumed for introgressive hybridization of C. behningi and C. plumosus.     

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.     

Comparative cytogenetic variability of polytene chromosomes from salivary glands of Chironomus riparius Mg., 1804 (Diptera, Chironomidae) from two polluted biotopes in Bulgaria and Russia

The structural-functional variation of Chironomus 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.     

Differentiation of chromosome banding sequence pools and genomic dna in Holarctic natural populations of <Emphasis Type="Italic">Chironomus entis</Emphasis> Shobanov (Diptera,Chironomidae)

Polymorphism and differentiation of the chromosome banding sequence pools and genomic DNA were studied in three natural populations of Chironomus entis from Europe and North America. These populations showed a moderate level of chromosomal polymorphism and high RAPD polymorphism of genomic DNA. The Palearctic and Nearctic populations of this species did not differ significantly in the levels of chromosome and genomic DNA polymorphism. Estimation of the cytogenetic (GD_cg) and genetic (GD_DNA) distances between these C. entis populations showed that their chromosome banding sequence pools and cytogenetic structures are differentiated to a greater extent than genomic DNA. The values of cytogenetic and genetic distances between the Palearctic and Nearctic populations of C. entis are higher than the values of the corresponding distances between the Nearctic populations, but they do not reach the level of divergence between species.