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Inversion polymorphism in Drosophila subobscura from two different habitats from the mountain of Goc

2003; BioMed Central; Volume: 138; Issue: 3 Linguagem: Inglês

10.1034/j.1601-5223.2003.01747.x

1601-5223

Marko Andjelković, Vuk Savković, Predrag Kalajdzic,

Insect-Plant Interactions and Control

Drosophila subobscura possesses rich inversion polymorphism on all of its five long chromosomes. This polymorphism shows clear-cut geographical and less clear annual variation, while there is little evidence for altitudinal, seasonal and daily variation (summarized by Krimbas 1992, 1993). Data on microgeographic and habitat variability are also scarce. All of these aspects of potential variability in D. subobscura inversion polymorphism are to a certain degree associated with the variety and dynamics of ecological factors. Since the Balkan Peninsula is characterized by a long-standing and extensive ecological diversity, it provides opportunities for detailed research of very different and dynamic habitats. Drosophila subobscura flies for the present study were collected on mountain Goč, situated between 43°33′–43°35′ N and 18°15′–18°40′ E in central Serbia. Two local populations were sampled simultaneously, at the end of June, 1998, and one local population in June, 2001, using fermented fruit baits. The local populations were from two forest communities (topographically about 4 km apart), at about 700 m above sea level: Abieto-fagetum and Fraxineto-quercetum, hereafter referred to as “beech” and “oak” woods. Local differences in topography and soil composition, as well as distribution of dominant trees within “beech” and “oak” woods modify micro-climates considerably (Gajić 1984). In other words, these forest communities represent two ecologically different habitats. Analysis of inversion polymorphism was carried out with wild captured males. The males were individually crossed with virgin females from the Küsnacht laboratory stock, which is homozygous for standard gene arrangement at all five large chromosomes. Salivary glands from third-instar larvae were squashed and chromosomes stained with aceto-orcein solution. Eight larvae were analyzed from the progeny of each of the crosses performed. The frequencies (in percentages) of the gene arrangements in D. subobscura samples from two natural habitats, “beech” and “oak” woods on mountain Goč, are given in Table 1. Data from year 1998 are compared between habitats. The homogeneity test indicates no statistically significant differences in gene arrangement frequencies, at all five chromosomes of D. subobscura between two habitats (χ2=18.309; df=15; p>0.05). An analysis of differences in frequencies of individual chromosome arrangements yields statistical significance for the J chromosome (χ2=5.988; df=1; p<0.01). The difference is due to a significantly higher frequency of the JSt arrangement (Z=3.52; α=0.01) and significantly lower frequency of the J1 arrangement (Z=3.54; α=0.001) in the “beech” than in the “oak” habitat, respectiverly. The frequencies of particular gene arrangements on chromosomes A, U and E differ significantly between the two habitats. The gene arrangements A2 (Z=2.62; α=0.01) and USt (Z=2.17; α=0.05) were more frequent in “beech” woods, while gene arrangements A1 (Z=2.38; α=0.05) and E8 (Z=2.70; α=0.01) were more frequent in flies caught in “oak” woods. The gene arrangement U1 was found only in “oak”, while the inversion complex E1+2 only in “beech” woods. The general range of inversion polymorphism in D. subobscura populations from those two habitats is rather similar, which is reflected through nearly identical values of both the degree of inversion heterozygosity and the IFR. An analysis of inversion polymorphism of D. subobscura in “beech” and “oak” habitats on mountain Jastrebac (Serbia), (Živanović et al. 1995), also showed clear differences in the frequencies of certain gene arrangements between habitats. Gene arrangements A1 and USt had identical frequency patterns as presented here, while other karyotypic differences between “beech” and “oak” woods were attributable to other gene arrangements. Those Results reflect geographical interpopulation differences in D. subobscura. The observed karyotypic variability could, in part, be viewed in relation to microclimate differences between two habitats. The average daily light intensity in “beech” wood was lower than in “oak” woods, the average temperature was lower, and relative average humidity was higher (Gajić 1984, and pers. obs.). All those ecological abiotic factors vary during the day, but the variations were of the smaller range in “beech” woods. Menozzi and Krimbas (1991) said that most of the gene arrangements that contribute to north-south cline in Europe show highly significant correlation with temperature, but not with humidity. Therefore, most studies of the effect of environment on the frequencies of gene arrangements in D. subobscura have focused on temperature. These studies generally suggest that temperature is probably not a relevant selective factor by itself, but some selective agent must be related to it (Krimbas 1967; Zapata et al. 1986; Rodriguez-Trelles et al. 1996; Dorcas-J and Prevosti 1996). The significantly higher frequency of USt gene arrangement in “beech” wood agrees with the Results which show that this inversion type is more frequent under lower temperature conditions (De Frutos and Prevosti 1984; Gosteli 1990). The significantly longer wings of D. subobscura from “beech” if compared to the flies from the “oak” habitat (Stamenković-Radak, unpubl. data) indirectly suggest that “beech” wood is a relatively colder habitat. Wing length, a strongly correlated character with thorax length (Krimbas and Loukas 1980), displays a north-south cline in some parts of Europe (colder climates vs larger wings) (Prevosti 1955; Misra 1966). Also, the St gene arrangement was much more frequent in lines selected for an increased wing length (Prevosti 1967). Comparison of gene arrangement frequencies on all five chromosomes from the “beech” woods between 1998 and 2001 samples (Table 1) revealed significant differences (χ2=68.068; df=14; p<0.001). There are statistically significant differences of gene arrangements between two years (chromosome A: χ2=23.715; df=2; p<0.001; chromosome J: χ2=7.153; df=1; p<0.001; chromosome U: χ2=16.469; df=2; p<0.001; chromosome E: χ2=15.971; df=3; p<0.001) within each chromosome, except the O-chromosome. Temporal changes in chromosomal polymorphism of D. subobscura are reported for other populations studied over several years, or a much longer period (De Frutos and Prevosti 1984; Krimbas and Alevizos 1973; Gosteli 1990; Živanović et al. 1995; Dorcas-J and Prevosti 1996; Rodriguez-Trelles et al. 1996). Although inversion polymorphism of D. subobscura shows, to some extent, erratic temporal changes, in all populations studied, a proportion of standard gene arrangements showed a decreasing trend on most chromosomes, while other, primarily more complex gene arrangements, were more frequent. Such a temporal pattern is partly seen in the “beech” population from Mt. Goč. Gene arrangements JSt (Z=3.74; α=0.01) and ESt (Z=3.46; α=0.01) have significantly lower frequencies in the year 2001 than in 1998, while the decrease in frequencies of USt and OSt is not significant. Gene arrangement ASt is significantly more frequent in the year 2001 than in 1998. (Z=6.50; α=0.01). In most cases, the complex gene arrangements did not have a uniform increasing trend. While the frequencies of inversion complexes U1+2 (Z=5.84; α=0.01) and O3+4 (Z=3.20; α=0.01), and simple gene arrangements J1 (Z=3.80; α=0.01) and E8 (Z=5.22; α=0.01), significantly increase, the frequencies of A1 (Z=5.82; α=0.01), U1+2+6 (Z=5.10; α=0.01) and O3+4+1 (Z=2.06; α=0.05) are significantly lower in 2001. Those relative discrepancies from the general pattern in European D. subobscura populations are probably caused by a specific complex of ecological factors and genetic structure of local populations. The Results presented here, related to spatial changes, i.e. inter-habitat differences, and temporal changes, i.e. inter-annual differences in the inversion polymorphism of D. subobscura, indicate the relationship between gene arrangement frequencies and some environmental factors. A certain degree of association of this kind of polymorphism with environmental variability corresponds to flexible, as contrasted to rigid (Dobzhansky 1962), type of polymorphism. The inversion polymorphism of Drosophila subobscura is not only the most extensive in any Drosophila species; in addition we have good records going back to the 1950s from many European countries. We have here shown that this polymoprphism can be used in microevolutionary studies. It may be a very useful tool in monitoring climate changes (Rodriguez-Trelles et al. 1998). This study was supported by the Ministry of Science, Technologies and Development of Serbia (Contract No. 1527).