Further evidence for latitudinal inversion clines in natural populations of Drosophila melanogaster from India.

Chromosomal analysis of eight Indian natural populations (six from north India and two from south India) of Drosophila melanogaster revealed the presence of 13 paracentric inversions, including one in the X chromosome. All four types of inversions--common cosmopolitan (4), rare cosmopolitan (3), recurrent endemic (2), and unique endemic (4)--were detected. The frequency of commonly occurring inversions and the level of inversion heterozygosity were found to be higher in the two southern populations. The south Indian populations are genetically more differentiated than those from the north. Also, latitudinal clines in the frequencies of the four common cosmopolitan inversions were detected. These results provide further evidence for the existence of inversion clines in Indian populations of D. melanogaster.     

Genetic differentiation and inversion clines in Indian natural populations of Drosophila melanogaster.

To study the genetic differentiation and inversion clines in Indian natural populations of Drosophila melanogaster, 14 natural populations (6 from the north and 8 from the south) were screened for chromosome inversions. The chromosomal analysis revealed the presence of 23 paracentric inversions, which include 4 common cosmopolitan, 4 rare cosmopolitan, 2 recurrent endemic, and 13 unique endemic (new inversions detected for the first time) inversions. The difference in karyotype frequencies between populations from the north and south were highly significant and the level of inversion heterozygosity was higher in populations from the south. Statistically significant negative correlations were found between each of the four common cosmopolitan inversions and latitude. These findings are in accord with results from other worldwide geographic regions and show that Indian populations of D. melanogaster have undergone considerable genetic differentiation at the level of inversion polymorphism.     

Enzyme and Chromosome Polymorphisms in Japanese Natural Populations of DROSOPHILA MELANOGASTER

Collections of D. melanogaster from Japanese populations were analyzed for enzyme and chromosomal polymorphisms. Allelic frequencies at the Adh and alphaGpd loci were compared with polymorphic inversion (In(2L)B, In(2R)C) frequencies in the second chromosome. There was a significant positive correlation between the frequencies of AdhS and In(2L)B, caused by linkage. On the other hand, inversion-free cage populations maintained in the laboratory for a long time showed considerably larger variation in the frequencies of these enzyme alleles, which seem very likely to be a consequence of random drift. Two fitness components of these enzyme and chromosomal variants were measured in two different environmental conditions; neither of the two loci showed heterozygote superiority in viability or productivity, while the inversion heterozygotes showed a superior productivity compared to the corresponding homozygotes in the fluctuating environment. These findings are compatible with the hypothesis that polymorphic isozyme genes are maintained by random drift of neutral genes in natural populations, and that association with linked inversions is a historical accident.     

Inversion polymorphism in malarial mosquito Anopheles messeae natural populations from the north-eastern region of the range

A cytogenetic study of eight natural populations of Anopheles messeae from the north-eastern part of the species areal was conducted. Complete predominance of homozygotes XL11 and 3R11 in the northern populations was observed. Change in the chromosome 2 inversion frequencies from south northwards was shown. The 2R11 variant, which was not observed in the southern region, was found in the northern populations. These results indicate the maintenance of chromosome frequency of the distribution of inversions XL1 and 3R (in longitude) and 2R1 (in latitude). The inversion frequency distribution in the examined part of the areal have been preserved for a long time.     

Homogeneity of common cosmopolitan inversion frequencies in Southeast AsianDrosophila melanogaster

East AsianDrosophila melanogaster are known for great variation in morphological and physiological characters among populations, variation that is believed to be maintained by genetic drift. To understand the genetic properties of AsianD. melanogaster populations, we initiated a population genetic study of chromosome inversion polymorphisms in hitherto unanalysed population samples from Southeast (SE) Asia. We generally found a high frequency of each of the four common cosmopolitan inversions in comparison to populations from Africa, Asia, and Australia. In contrast to the great phenotypic variation among Asian populations, however, we could not detect differences in inversion frequencies among populations. Furthermore, we observed neither correlations of inversion frequencies with population latitude and longitude, nor evidence for linkage disequilibrium between different inversion loci. We propose two explanations for the observed genetic homogeneity among these SE AsianD. melanogaster populations: (i) the observed pattern simply reflects the retention of ancestral polymorphisms originating from a panmictic population that was once present on a large single landmass (Sundaland), and/or is a consequence of high recent gene flow between populations; and (ii) it is caused by selective forces (e.g. balancing selection).     

Population genetics of Drosophila ananassae: genetic differentiation among Indian natural populations at the level of inversion polymorphism

The present study, which is one of the longest temporal (two decades) and largest spatial (different parts of India covered) investigations on inversion polymorphism in natural populations of D. ananassae, was undertaken to understand the dynamics of inversion polymorphism in a broad and comprehensive manner. Forty-five natural populations from different ecogeographic regions of the country (covering the regions from Kashmir to Kanniyakumari and Gujarat to Nagaland) were analysed for chromosomal inversions. All the populations show the presence of the three cosmopolitan inversions, frequencies of which vary among the populations analysed. Simple correlations between frequencies of different inversions and regression analysis of inversion frequencies with latitude, longitude and altitude were insignificant. This reinforces the concept of rigid polymorphism in D. ananassae. Genetic divergence (spatial and temporal) at the level of chromosomal polymorphism among natural populations was calculated. Results show spatial divergence but no temporal divergence. Rigid polymorphic systems of D. ananassae did not show long-term directional trends. On the basis of the present study, and after including comparisons with the studies conducted more than two decades ago, the most important conclusion to be drawn is that the three cosmopolitan inversions in D. ananassae segregate within populations at fairly similar frequencies, and the general geographic pattern has remained constant.     

Chromosome Inversion Polymorphisms in DROSOPHILA MELANOGASTER. I. Latitudinal Clines and Associations between Inversions in Australasian Populations

Nineteen Australasian populations of Drosophila melanogaster have been screened for chromosome inversion polymorphisms. All 15 of the inversion types found are paracentric and autosomal, but only four of these, one on each of the major autosome arms, are common and cosmopolitan. North-south clines occur, with the frequencies of all four of the common cosmopolitan inversions increasing toward the equator. These clines in the Southern Hemisphere mirror north-south clines in the Northern Hemisphere, where the frequencies of all four of the common cosmopolitan inversions again increase towards the equator.-While few of the Australasian populations show significant disequilibrium between linked common cosmopolitan inversions, those that do invariably have excesses of coupling gametes, which is consistent with other reports. We also find nonrandom associations between the two major autosomes, with the northern populations in Australasia (those with high inversion frequencies) tending to be deficient in gametes with common cosmopolitan inversions on both major autosomes, while the southern populations in Australasia (low inversion frequencies) tend to have an excess of this class of gametes.-The clines and the nonrandom associations between the two major autosomes are best interpreted in terms of selection operating to maintain the common cosmopolitan inversion polymorphisms in natural populations of D. melanogaster.     

Chromosome inversion polymorphisms in Drosophila melanogaster II. Geographic clines and climatic associations in Australasia,North America and Asia

The four paracentric autosomal chromosome inversions In(2L)t, In(2R)NS, In(3L)P and In(3R)R are commonly polymorphic in natural populations of D. melanogaster in Australasia, North America and Asia, with latitudinal clines in the frequencies of each inversion in each region. In each region inversion frequency decreases with increasing distance from the equator, although the precise relationship between frequency and latitude varies between inversions and, for In(2L)t and In(2R)NS, among regions. Each inversion also shows a longitudinal cline in at least one region but none show such a cline in all three. Although no inversion's frequency is associated with the same climatic variable in all three regions, inversion frequencies are generally positively related to annual maximum temperature and, more particularly, minimum temperature and minimum rainfall. The directions of the latitudinal clines and the climatic associations are consonant with evidence from D. melanogaster that inversion frequencies decline in winter. They are also consonant with evidence from some other Drosophila species that inversion heterozygosities are lower at the geographic margins than at the centre of the species' ranges.     

On the degree of genetic divergence in Drosophila ananassae populations transferred to laboratory conditions

Twelve natural populations of Drosophila ananassae were sampled and laboratory populations were derived. All the populations were maintained in food bottles in the laboratory for ten generations by transferring fifty flies (females and males in equal number) in each generation. After ten generations they were analysed chromosomally to determine the frequency of different chromosome arrangements. The results show that there is significant variation in the frequencies of chromosome arrangements and in the level of inversion heterozygosity. Although some of the populations became mo-nomorphic for certain inversions, in general all populations remained polymorphic even after ten generations. The degree of genetic differentiation in the populations after they were transferred to laboratory conditions has been estimated by calculating genetic identity and distance between the initial and final populations based on the differences in chromosome arrangement frequencies. The estimates of I and D suggest that there is considerable variation in the degree of genetic divergence in D. ananassae populations. Some populations have remained unchanged while others have diverged to a considerable extent.     

Inversion polymorphism in Drosophila obscura

The inversion polymorphism of Drosophila obscura Fallen, a European species of the obscura group of the subgenus Sophophora, is described. A total of 21 inversions have been recorded; they are located in the five large chromosomes of the species (a dot chromosome is also present) and form 25 gene arrangements present in the species' natural populations. Strains from five different countries were studied. Two of these inversions were found to be pericentric, and the remaining were paracentric. The presence of "hot" points (multibreaks) was noticed. The distribution of the relative lengths of inversions conformed to the Van Valen-Levins distribution, contrary to what happens in D. subobscura. Observations also showed that there is no crossover inhibition between nonoverlapping inversions. The phylogeny of chromosome C was reconstructed; the existence of several arrangements not found was postulated; and the primitive gene arrangement, linking D. obscura to its closely related species was identified, as well as the primitive gene arrangements of the other chromosomes. Photographic maps of the chromosomes of D. obscura are provided here.     

Cytogenetic features of the blood-sucking 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 disk sequence. 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).     

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 ISOFEMALE LINES AND CAGE POPULATIONS OF DROSOPHILA MELANOGASTER

Drosophila melanogaster populations in nature usually carry inversion polymorphisms. When they were transferred to and maintained in the laboratory as large cage populations, frequencies of polymorphic inversions were drastically decreased and finally eliminated. This “cage effect” was observed irrespective of the geographical origin of the population or the initial frequency of each inversion. The decrease and elimination of inversions in the cage was not overcome by changing conditions such as medium, temperature, or the number of isofemale lines (40-600) introduced. On the other hand, in the sets of isofemale lines derived from the same geographical origins as the cage populations, each of which was maintained as a small vial population, the inversion frequencies, though decreased from the initial frequencies, were kept at significantly high levels. The cage populations initiated with one or two isofemale lines also maintained the inversion polymorphisms that were as high as vial populations.     

Viabilities of originally natural O-chromosomal inversion homo- and heterokaryotypes in Drosophila subobscura

The influence of epistatic interactions of lethal and non-lethal genes upon viability of Drosophila inversion karyotypes is poorly known. In this paper we present comparative results for viabilities of 21 originally natural O-inversion homo- and 38 heterokaryotypes in a D. subobscura population. We observed strong heterotic effect in viability of O-lethal heterozygotes irrespective of different inversion backgrounds, which indicates a mechanism for protection of a great number of lethal genes. In O-non-lethal heterozygotes the heterotic effect in viability was absent irrespective of different inversion backgrounds. Our results showed a great extent of genetic load and high abundance of O-chromosomal arrangements in the population analyzed. It belongs to the set of central European populations. An epistasis of lethal genes present in O-inversion hetero- and not present in O-inversion homokaryotypes of moderate or low frequencies could be good example for co-adaptation of chromosomal inversions with regard to the genetic load. This represents a more efficient mechanism for limitation of genetic load than alternative mechanisms for protection of lethals. Except for lethal genes, possible epistatic interactions of mildly deleterious (subvital) genes, could also be responsible for limiting the extent of genetic load in natural D. subobscura populations. We demonstrated a non-random distribution of several combinations of viability classes among different O-inversion homo- and heterokaryotypes. As a consequence of that, the viabilities of the O-inversion homokaryotypes compared to heterokaryotypes were significantly higher at low frequencies than in moderate or high frequencies. This shows frequency-dependence as a mechanism of balancing selection for protection of O-chromosomal inversions in natural D. subobscura populations. In addition, the viabilities of the O-inversion homokaryotypes of lower frequency, compared to homokaryotypes of moderate or high frequency, were significantly higher. This again indicates the existence of supergene selection as another mechanism for protection of chromosomal inversions, as co-adapted complexes in natural D. subobscura populations.     

Inversion polymorphism in malarial mosquito <Emphasis Type="Italic">Anopheles messeae</Emphasis> natural populations from the north-eastern region of the range

A cytogenetic study of eight natural populations of Anopheles messeae from the north-eastern part of the species areal was conducted. Complete predominance of homozygotes XL₁₁ and 3R₁₁ in the northern populations was observed. Change in the chromosome 2 inversion frequencies from south northwards was shown. The 2R₁₁ variant, which was not observed in the southern region, was found in the northern populations. These results indicate the maintenance of chromosome frequency of the distribution of inversions XL₁ and 3R₁ (in longitude) and 2R₁ (in latitude). The inversion frequency distribution in the examined part of the areal have been preserved for a long time.     

Selection in heterogeneous environments maintains the gene arrangement polymorphism of Drosophila pseudoobscura

Chromosomal rearrangements may play an important role in how populations adapt to a local environment. The gene arrangement polymorphism on the third chromosome of Drosophila pseudoobscura is a model system to help determine the role that inversions play in the evolution of this species. The gene arrangements are the likely target of strong selection because they form classical clines across diverse geographic habitats, they cycle in frequency over seasons, and they form stable equilibria in population cages. A numerical approach was developed to estimate the fitness sets for 15 gene arrangement karyotypes in six niches based on a model of selection-migration balance. Gene arrangement frequencies in the six different niches were able to reach a stable meta-population equilibrium that matched the observed gene arrangement frequencies when recursions used the estimated fitnesses with a variety of initial inversion frequencies. These analyses show that a complex pattern of selection is operating in the six niches to maintain the D. pseudoobscura gene arrangement polymorphism. Models of local adaptation predict that the new inversion mutations were able to invade populations because they held combinations of two to 13 local adaptation loci together.     

Direct evidence for suppression of recombination within two pericentric inversions in humans: a new sperm-FISH technique.

Crossover within a pericentric inversion produces reciprocal recombinant chromosomes that are duplicated/deficient for all chromatin distal to the breakpoints. In view of this fact, a new technique is presented for estimating the frequency of recombination within pericentric inversions. YAC probes were selected from within the q- and p-arm flanking regions of two human inversions, and two-color FISH analysis was performed on sperm from heterozygous inversion carriers. A total of 6,006 sperm were analyzed for chromosome 1 inversion (p31q12), and 3,168 were analyzed for chromosome 8 inversion (p23q22). Both inversions displayed suppression of crossing-over, although the amount of suppression differed between the two inversions. The recombination frequency of 13.1% recorded for chromosome 8 inversion was similar to the frequency of 11.4% previously estimated by the human/hamster-fusion method. For chromosome 1 inversion, the recombination frequency of 0. 4% reported here was below the limits of detection of the fusion technique. The simplicity of the FISH technique and the ease of scoring facilitate analysis of a sample-population size much larger than previously had been possible.     

Increase in the degree of inversion polymorphism in Drosophila bipectinata populations transfereed to laboratory conditions

Thirteen laboratory populations (six mass cultures and seven isofemale lines) of Drosophila bipectinata were established from files collected from six different geographic localities (four in the north and two in the south) in India. These mass cultures and isofemale lines were maintained in food bottles in the laboratory for varying number of generations by transferring about 50 files (females and males in equal number) in each generation. After several generations, all the laboratory populations were analysed chromosomally to determine the frequency of different inversions. The results indicate that all the laboratory populations remained chromosomally polymorphic due to the persistence of inversions which were originally present ion the populations. The quantitative data on the frequency of inverted gene orders and the level of inversions heterozygosity in laboratory populations are compared with those in corresponding natural populations. This comparison clearly shows that there is a considerable increase in the frequency of inverted gene orders and the level of inversion heterozygosity in laboratory populations when compared with corresponding natural populations. This is most likely to be due to selective advantage of inversion heterozygotes under stringent competition in laboratory populations. Thus heterotic buffering is associated with chromosome inversion of Drosophila bipectinata.     

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).