Speciation and chromosomal evolution of the Baikalian endemic chironomids of the genus Sergentia Kief. (Diptera, Chironomidae): karyotype divergence and chromosomal polymorphism in the populations of eurybathic species Sergentia flavodentata Tshern. and littoral species Sergentia baicalensis Tshern

Specific karyotype structure and chromosomal polymorphism was investigated in the populations of the Baikalian endemic chironomid species, eurybathic Sergentia flavodentata Tshernovskij, 1949 and littoral Sergentia baicalensis Tshernovskij, 1949. In addition to two fluctuating homozygous inversions in arms A and E, both species were characterized by species-specific karyotype features, namely, nucleolar organizer in the region 1 of chromosome IV in S. flavodentata, and inversion in arm F in S. baicalensis. Moreover, S. baicalensis demonstrated the presence of intraspecific population polymorphism. The populations differing in the presence of secondary overlapping inversion in arm A were found. The highest number of chromosomal rearrangements (7) was detected in S. flavodentata. Most of these rearrangements (six) were found in the population from the underwater thermal spring at a depth of 420 to 430 m (Northern Baikal, Frolikha Bay). In the populations from Middle and Southern Baikal, a rare pericentric inversion in chromosome I was described. In S. baicalensis, in addition to two common heterozygous inversions in arms C and F, disturbance of homologous pairing in different regions of the remaining arms were often detected. Stable chromosomal polymorphism preserved during 13 years in the population of S. flavodentata from the region of hydrothermal venting, serves as an evidence of high genetic plasticity of the species, which favors successful colonization of different Baikal depths and biotopes.     

Speciation and chromosomal evolution of the Baikalian endemic chironomids of the genus Sergentia Kief. (Diptera, Chironomidae): Karyotype divergence and chromosomal polymorphism in the populations of deep-water species Sergentia nebulosa Linevitsh et al. and Sergentia assimilis Proviz V. et Proviz L.

Specific karyotype structure and chromosomal polymorphism was investigated in the populations of Sergentia nebulosa Linevitsh et al., 1984 and Sergentia assimilis Proviz V. et Proviz L., 1999, the deep-water endemic chironomid species (Diptera, Chironomidae) from the Baikal Lake. The distinguishing feature of the karyotypes of these species, compared to the other Baikalian Sergentia, is well-developed nucleolus in region 6 of arm C. Both species display the presence of interspecific population polymorphism, determined by the structure of this arm. In some populations, chromosome regions from 4 to 6 contain a homozygous inversion, which is absent in the other populations. The distinguishing karyotype feature of S. assimilis, which shares fluctuating homozygous inversions with the other species, is the presence of two species-specific homozygous inversions. These are the secondary overlapping inversion in arm A, regions 2 to 7, and the inversion in regions 4 to 10 of arm G. Both species of interest contain nucleolus organizer in region 10 of arm G. In populations of S. nebulosa, six heterozygous inversions localized in arms A, B, C, F, and G were discovered. The highest number of heterozygotes for inversions (71%) was observed in the population from Southern Baikal. In arm B of S. assimilis, one heterozygous inversion and heterozygosity for nucleolus organizer in the chromosome region 16 was detected. Chromosomal evolution of Baikalian Sergentia, and the role of inversion polymorphism in the population adaptation is discussed. Original Russian Text ? V.I. Proviz, 2008, published in Genetika, 2008, Vol. 44, No. 12, pp. 1627–1637.     

Speciation and chromosomal evolution of the palaearctic species of chironomids of the genus Sergentia Kieffer (Diptera, Chironomidae): Chromosome polymorphism in the populations of S. baueri Wülker et al., 1999, S. prima Proviz et al., 1997, and S. electa Proviz et al., 1999 from the Irkutsk reservoir

Chromosome polymorphism was studied in populations of three Palaearctic species of chironomids of the genus Sergentia Kieffer (Diptera, Chironomidae) from the Irkutsk reservoir: S. baueri Wülker et al., 1999, S. prima Proviz et al., 1997, and S. electa Proviz et al., 1999. It was found that in S. baueri heterozygous individuals constitute 72% of the population with 0.8 of inversion per individual. In total, three inversion sequences were detected. The most widespread was the inversion in regions 7–12 of the arm IIIR with the borders similar to those of the inversion in the Far East S. baueri populations and in populations of other species of the genus. In S. prima, heterozygotes constituted 60%, with the number of inversions per individual was 1.3. Seven inversion sequences were revealed, of which three were found in chromosome IV. In the S. electa population, only one inversion sequence was observed in chromosome III in 36% of larvae (0.4 of inversion per individual). The level of chromosome polymorphism in the populations of the Palaearctic species is comparable with that in the populations of endemic Sergentia from the Baikal Lake. Ten rearrangements were revealed in each of the two groups, but similar borders of chromosome regions were established only for two inversions. In the studied Sergentia species, chromosome IV has the most variable structure.     

Chromosomal polymorphism of the unique Baikal deep-water population of <Emphasis Type="Italic">Sergentia flavodentata</Emphasis> (Diptera,Chironomidae) from the region of hydrothermal venting

The subject of this study is chromosomal polymorphism of a population of Baikal endemic species Sergentia flavodentata (Diptera, Chironomidae) from Frolikha Bay region of hydrothermal venting at a depth of 420–430 m. In 35–67% of larvae, six heterozygous inversions were found to be preserved for 13 years (materials of 1991, 1994, 1996, 1999, 2003). The number of zygotic combinations reached 14; 0.5–0.7 inversions per individual was observed. It is suggested that stable chromosomal polymorphism characteristic of this species is nonrandom, being a mechanism of the population adaptation to the specific conditions of hydrothermal venting community based on bacterial biochemosynthesis.Translated from Genetika, Vol. 41, No. 3, 2005, pp. 366–373.Original Russian Text Copyright © 2005 by Proviz.     

Chromosome arm‐specific patterns of polymorphism associated with chromosomal inversions in the major African malaria vector,Anopheles funestus

Chromosomal inversions facilitate local adaptation of beneficial mutations and modulate genetic polymorphism, but the extent of their effects within the genome is still insufficiently understood. The genome of Anopheles funestus, a malaria mosquito endemic to sub‐Saharan Africa, contains an impressive number of paracentric polymorphic inversions, which are unevenly distributed among chromosomes and provide an excellent framework for investigating the genomic impacts of chromosomal rearrangements. Here, we present results of a fine‐scale analysis of genetic variation within the genome of two weakly differentiated populations of Anopheles funestus inhabiting contrasting moisture conditions in Cameroon. Using population genomic analyses, we found that genetic divergence between the two populations is centred on regions of the genome corresponding to three inversions, which are characterized by high values of F_ST, absolute sequence divergence and fixed differences. Importantly, in contrast to the 2L chromosome arm, which is collinear, nucleotide diversity is significantly reduced along the entire length of three autosome arms bearing multiple overlapping chromosomal rearrangements. These findings support the idea that interactions between reduced recombination and natural selection within inversions contribute to sculpt nucleotide polymorphism across chromosomes in An. funestus.     

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.     

Chromosomal polymorphism of the unique Baikal deep-water population of Sergentia flavodentata (Diptera, Chironomidae) from the region of hydrothermal venting

The subject of this study is chromosomal polymorphism of a population of Baikal endemic species Sergentia flavodentata (Diptera, Chironomidae) from the Frolikh Bay region of hydrothermal venting at a depth of 420-430 m. In 35-67% of larvae, six heterozygous inversions were found to be preserved for 13 years (materials of 1991, 1994, 1996, 1999, 2003). The number of zygotic combinations reached 14; 0.5-0.7 inversions per individual was observed. It is suggested that stable chromosomal polymorphism characteristic of this species is nonrandom, being a mechanism of the population adaptation to the specific conditions of hydrothermal venting community based on bacterial biochemosynthesis.     

Phylogenetic relationships of the Baikal endemic Paratanytarsus baicalensis (Tshern.) with representatives of genera Paratanytarsus Thien. et Bause and Micropsectra Kieff. (Diptera, Chironomidae)

The phylogenetic relationships of the Baikal endemic chironomid Paratanytarsus baicalensis (Tshern.) and other representatives of the tribe Tanytarsini were studied using 117 partial sequences of the mitochondrial COI gene from 33 species of the genera Paratanytarsus and Micropsectra. The results show that the Baikal species is close in origin to two geographically distant European species, P. austriacus and P. hyperboreus, and itself splits into two genetically distinct clades.     

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.     

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.     

Chromosomal analysis of twelve species of Microhylidae (Anura) from Madagascar

The karyotypes of four species of Dyscophinae and eight species of Cophylinae were analyzed. The chromosome number was 2n=26 in all cases. Between the two subfamilies a difference in the form of the karyotype was observed; the chromosomes show a gradual decrease in length in the Dyscophinae, whereas in the Cophylinae the karyotype demonstrates a clear discontinuity of size between pairs 5 and 6.Chromosomal polymorphism was found in Plethodontohyla tuberata, the chromosomes of pair 4 were subtelocentric in the homozygous specimens, whereas this pair showed a subtelocentric and a submetacentric chromosome of equal length in the heterozygous one, suggesting a pericentric inversion. Although in the Cophylinae the chromosome number is constant, the number of chromosome arms is variable. Pericentric inversions seem to play an important role in the chromosomal evolution of the Cophylinae.     

Karyotype of <Emphasis Type="Italic">Glyptotendipes glaucus</Emphasis> (Diptera,Chironomidae) from Lake Kotokel (the Lake Baikal Basin)

The karyotype and chromosomal polymorphism of Glyptotendipes glaucus from Lake Kotokel were examined. The degree of the chromosomal polymorphism was rather low, constituting 2.2% heterozygotes and 0.02 inversions per individual. A single glaA4 inversion banding sequence, new to the species, was found in the 46 larvae studied.     

The adaptive significance of chromosomal inversion polymorphisms in Drosophila melanogaster

Chromosomal inversions, structural mutations that reverse a segment of a chromosome, cause suppression of recombination in the heterozygous state. Several studies have shown that inversion polymorphisms can form clines or fluctuate predictably in frequency over seasonal time spans. These observations prompted the hypothesis that chromosomal rearrangements might be subject to spatially and/or temporally varying selection. Here, we review what has been learned about the adaptive significance of inversion polymorphisms in the vinegar fly Drosophila melanogaster, the species in which they were first discovered by Sturtevant in 1917. A large body of work provides compelling evidence that several inversions in this system are adaptive; however, the precise selective mechanisms that maintain them polymorphic in natural populations remain poorly understood. Recent advances in population genomics, modelling and functional genetics promise to greatly improve our understanding of this long‐standing and fundamental problem in the near future.     

Polytene chromosome relationships of five species of the Anopheles dirus complex in Thailand.

Photographic maps and rearrangements of each salivary gland polytene chromosome arm of Anopheles nemophilous (species F) and of An. dirus species A, B, C, and D of the Dirus group from natural populations in Thailand are presented. Structural conformation of heterokaryotypes and comparison of chromosome banding sequences reveal 10 paracentric inversions. The data on fixed inversion of 3Rb and inversion polymorphism of the X chromosome shared by these species were used to construct a phylogeny of the five members of the An. dirus complex, thereby outlining their patterns of speciation through chromosomal rearrangements.     

Patterns of inversion polymorphism in three species of the Drosophila melanogaster species group

In Drosophila, chromosomal polymorphism due to paracentric inversions is very common and constitutes an adaptive character. The degree of chromosomal variability varies in different species and also in different populations of the same species. Chromosomal polymorphism in Indian natural populations of three species, D. melaonogaster, D. ananassae and D. bipectinata which belong to the melanogaster species group has been studied and the quantitative data on frequency of inversions have been reported. Behaviour of chromosome inversions has also been studied in laboratory conditions. The present review summarises the work done on inversion polymorphism in Indian populations of three species which clearly demonstrates that these three species vary in their patterns of inversion polymorphism and have evolved different mechanisms for adjustment to their environments although they belong to the same species group.     

Chromosomal polymorphism in the populations of malaria vector mosquito Anopheles messeae at the south of Russian Plain

Zonal features of the geographic distribution of chromosomal inversions in the populations of An. messae at the south of the Russian Plain were examined. The An. messae was identified based on morphological characters and cytogenetically. The chromosomal inversions identified in populations of An. messae comprised XL₁, XL₄, 2R₁, 3R₁, and 3L₁. Inversion XL₄ was endemic and found with low frequency (2%) in Penza oblast and Moscow oblast. Based on the population karyotype structure similarity, on the territory of Russian Plain, three zones (southwestern, southeastern, and central) were identified. Central zone was characterized by higher levels of inversion polymorphism in all chromosomes, except arm 3L. In the two southern zones, high frequency of the XL₀ inversion, along with complete absence of homo- and heterozygotes for the 2R₁ inversion, and high proportion of the individuals with inversion 3L₁ was observed. Specific feature of southeastern zone was the increased frequency of hetero- and homozygotes for the 3R₁ and 3L₁ inversions. Zonal differences reflected adaptive character of chromosomal polymorphism and pointed to hierarchic organization of the species population structure.     

Cytogenetics and evolution of Simulium ornatipes skuse (Diptera:Simuliidae) I. Sibling speciation

Larval populations of the morphologically distinct species Simulium ornatipes and S. neornatipes and a subspecies of the former, S. ornatipes norfolkense, were analysed using polytene chromosome cytology. Fixed inversion differences, distinct differentiated sex chromosomes and unique arrays of chromosomal polymorphisms reveal two sibling species, S. ornatipes A and B, within S. ornatipes, and two, S. neornatipes 1 and 2, within S. neornatipes. Chromosomally S. ornatipes norfolkense is similar to S. ornatipes A. An unprecedented level of shared inversion polymorphism is shown by S. ornatipes A and B indicating a very close relationship between them. While it may be postulated that such a situation has arisen by independent origin of the shared inversions within each species, or by introgression of these rearrangements between the two species, it is concluded that the shared polymorphisms in fact originate from a common ancestor. Within the four chromosomally defined species, differentiated sex chromosome systems occur which may involve any of the three chromosome pairs. Simulium ornatipes A is particularly interesting because there are indications that it may be polymorphic for differentiated sex chromosomes on two different elements and that it may also share a sex chromosome marker with S. ornatipes B, a unique situation in the Simuliidae. The evolution of sex chromosome systems in the four species is compared with that of other Simulids and its intimate connection with speciation emphasised. Finally the data on fixed inversions, differentiated sex chromosomes and shared polymorphisms is used to construct a phylogeny of the four species outlining their patterns of speciation.     

Global warming and chromosomal inversion adaptation in isolated islands: Drosophila subobscura populations from Madeira

Global warming is an environmental phenomenon to which species must adapt to survive. Drosophila subobscura presents an adaptive capacity due to its chromosomal inversion polymorphism. Until now, the impact of global warming on this polymorphism has been studied in D. subobscura populations located either on a continental mainland or on islands not far from a continent. In this context, gene flow could be a relevant mechanism allowing the movement of thermally adapted inversions between populations. Our aim was to sample and study the chromosomal polymorphism on Madeira, a small isolated island in the Atlantic Ocean. We compared our findings with those reported in the same location approximately four and five decades ago. Moreover, we studied whether global warming has occurred on this island by analyzing mean, maximum and minimum temperatures over a 55‐year period. All atmospheric parameters have increased significantly, consistent with climate change expectations. Frequencies and chromosomal thermal index values of thermal adapted inversions remained quite stable over years. Furthermore, J, U and O chromosomes are almost fixed for “warm” adapted inversions. Thus, if there is little genetic variability remaining and temperatures continue increasing, island populations of D. subobscura might be on the threshold of endangerment. However, apart from selection, genetic drift and inbreeding, other processes, such as phenotypic plasticity or thermoregulatory behavior, could be involved in the survival of the species’ populations. Finally, although in danger, D. subobscura is a generalist that lives in humanized environments, and this fact could favor its persistence on Madeira Island.     

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.     

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.