I. I. Kiknadze

PALEARCTIC AND NEARCTIC CHIRONOMUS (CAMPTOCHIRONOMUS) TENTANS (FABRICIUS) ARE DIFFERENT SPECIES (DIPTERA: CHIRONOMIDAE)

Morphological comparison of populations of Chironomus (Camptochironomus) tentans (Fabricius) from Europe, Asia and North America has confirmed earlier cytogenetic evidence that two distinct species inhabit the Palearctic and the Nearctic under this name. The Palearctic species is the true C. tentans, whereas Nearctic populations constitute a new species described here under the name Chironomus (Camptochironomus) dilutus. Descriptions of the larva, pupa and adult male of both species are presented, and the taxonomic structure of the subgenus Camptochironomus is examined.

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.

Chromosomes and Continents

The high level of inversion polymorphism and, correspondingly, the abundance of inversion banding sequences (BSs) of polytene chromosomes in the banding sequence pool of Chironomus species permit scientists to reconstruct the cytogenetic evolution of the genus and to evaluate the role of structural rearrangements in the genome during population divergence and speciation. We performed a quantitative assessment of the important role of inversion polymorphism in the differentiation of natural populations and demonstrated the adaptive significance of different gene orders in populations of species occurring in different regions. For the first time, it has been shown that the BS pools of populations of the same species on different continents differed much in the sets and frequencies of gene inversion orders. BS pools of populations on each continent were found to contain continent-specific BSs in addition to sequences occurring on several continents. This intraspecies diversity of the linear organization of the genome is one of the major factors maintaining the evolutionary stability of species in dramatically different environments. In addition to endemic species-specific sequences, the BS pool of the genus Chironomus contains sequences common for different species, cytocomplexes, and continents. These sequences, termed basic sequences, are very important for reconstruction of genome divergence in the course of evolution. It is suggested that they are close to the initial primitive sequences existing on ancient supercontinents, whereas continent-specific BSs were formed after continent separation. Comparison of all currently known BSs in the sequence pool of the genus Chironomus showed that the genomes of the most distant species differed by more than 90 inversion breaks, causing changes of their linear structure. In such cases, conserved genome regions span about 10 bands.

Divergence of the polytene chromosome banding sequences as a reflection of evolutionary rearrangements of the genome linear structure

Banding sequences of five chromosomal arms (A, C, D, E, and F), accounting for about 70% of the total genome size in 63 Chironomus species, were used as markers to analyze divergence patterns of the linear genome structure during the evolution. The number of chromosomal breakpoints between the pairs of banding sequences compared served as a measure of divergence. It was demonstrated that the greater the divergence between the species compared, the higher the number of chromosomal breakpoints and the smaller the size of the conserved chromosomal segments. A banding sequences comparison in sibling species demonstrated a lower number of chromosomal breakpoints; the breakpoint number was maximum in a comparison of the banding sequences in the subgenera Chironomus and Camptochironomus. The use of the number of chromosomal breakpoints as a genome divergence measure provided establishment of phylogenetic relationships between 63 Chironomus species and discrimination of sibling species groups and cytocomplexes on a phylogenetic tree.

Geographic differentiation of genomic DNA of Chironomus plumosus (Diptera, Chironomidae) in natural holarctic populations

Using RAPD markers, polymorphism and differentiation of genomic DNA was examined in seven natural populations of Chironomus plumosus from Europe, Siberia, and North America. All these populations showed high polymorphism of genomic DNA. The Palearctic and Nearctic populations of this species were not statistically significantly different in the genomic DNA polymorphism level. The genetic distance (GD), which characterizes the extent of intraspecific differentiation of population genetic structure, was determined among the natural populations of C. plumosus. The genetic distance was on average 0.245. It was demonstrated that genetic structures of the Palearctic and Nearctic populations of C. plumosus was differentiated to a higher extent than in Palearctic. However, the genetic distances between the populations from different zoogeographical zones (0.313) did not exceed the level characteristic of the among-population differences, which do not disturb the species genetic integrity.

A new Chironomus species of the plumosus sibling-group (Diptera, Chironomidae) from China

Abstract Chironomus sinicus sp. n. is described from China. The new species was referred to previously as C. plumosus (L.). Study of the karyotype has shown strong karyological difference between C. sinicus and the true C. plumosus. Five of the seven chromosomal arms (B, C, D, F, G) have different species-specific banding sequences in C. sinicus in comparison with C. plumosus. Two chromosomal arms (A and E) are identical in both species. Despite of the karyological differences indicated, the general pattern of banding sequences in C. sinicus is related closely to C. plumosus and other members of the plumosus sibling group. Larva, pupa and adult morphology of C. sinicus are very similar to C. plumosus. The larva, however, have ventral tubules of the semireductus type. The pupa has shagreen on sternite III – IV.

Chironomus suwai, a New Species of the plumosus Group (Diptera, Chironomidae) from Japan

Cytological examination of the larvae of the species previously referred to as Chironomus plumosus in Japan, reveals that it is actually a new member of the plumosus group. This species is described as C. suwai on the basis of the karyotype, since this is the most reliable method for recognizing the species. It belongs to the thummi cytocomplex, and is polymorphic in chromosome arms A, B and E. A B-chromosome was present in one of the two populations (Tsukuba) studied. The banding pattern of the polytene chromosomes indicates that C. suwai is most closely related to C. borokensis, from which it differs by the smaller centromeric bands, and the presence of three unique inversion polymorphisms. The difference in centromeric size is an important indicator that these are separate species, since such differences are common between species of Chironomus. Some notes are included on the larvae, particularly in relation to their use in lake typology, and the later life history stages.

Chironomus albimaculatus sp. n. and C. trabicola sp. n. (Diptera, Chironomidae) from Polar Russia

The paper describes larval, pupal and adult morphology as well as karyotypes of two new species, C. albimaculatus and C. trabicola, from the Lena-delta in Russia (74°N latitude). Both species have polar or circumpolar distribution and show apparently basic characters.

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.

Cytogenetic differentiation of natural populations of Chironomus obtusidens (Diptera, Chironomidae)

The cytogenetic structure of Siberian and European populations of Chironomus obtusidens Goetgh. was studied. The clear differentiation of these populations was revealed using quantitative analysis of sets and frequencies of inversion banding sequences. The Siberian populations were found to have six new inversion sequences, and the alternative obtG2 sequence dominated instead of the main obtG1 sequence typical of European populations. In general, chromosomal polymorphism was 3–4 times higher in the Siberian populations. Therefore, the cytogenetic distances (Dcg) between the Siberian and European populations were higher (Dcg 0.059–0.097) than Dcg between the populations in each of the geographic zones investigated (Dcg 0–0.015). The NJ phylogenetic tree constructed for the chromosomal evolution of the genus Chironomus demonstrates that Chironomus obtusidens is the most divergent member among Palaearctic species of the obtusidens group. In addition, it is closer related to the Nearctic Ch. decorus group.