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  • 1
    ISSN: 1573-6857
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract In experimental populations ofDrosophila melanogaster lethal chromosomes with dominant markers and inversions were introduced and the frequency changes of the markers studied during a period of several generations. The base populations of the various experiments differed from each other with respect to their degree of heterozygosity. Monochromosomal populations were isogenic for a quasinormal + chromosome, dichromosomal populations contained the genetic material of two different + chromosomes, trichromosomal of three, tetrachromosomal of four, hexachromosomal of six and polychromosomal populations of many normal chromosomes. Marker chromosomes with the dominant genesLCy, Cy, Pm orD respectively were added to the populations with an initial frequency of 16,6 per cent. The fate of the dominant markers was different in different populations. In some the marker chromosome reached equilibrium frequencies, in others they were eliminated with variable speed. As a rule the lethal marker chromosomes were accepted by monochromosomal populations; however, they were eliminated from populations with a higher degree of heterozygosity. Since in all populations one genotype, namely the homozygote for the marker chromosome, was lethal, the adaptive values “c” of the +/LCy, +/Cy, +/Pm or +/D heterozygotes could easily be calculated from the experimental data. This “c” value can be used as a measure for the combining ability of the marker chromosomes. It could be shown that “c” depends on the degree of heterozygosity of a population or in other words that the average degree of heterozygosity of the marker free individuals determines the selection processes. An equation can be arrived at which fits the experimental results very well if superiority of heterozygous +/+ individuals over +/+ homozygotes is assumed. From that it was concluded that heterosis is the determining variable in our experiments. An attempt was undertaken in order to decide if in our case the observed heterosis was due to dominance or to overdominance. It was postulated that in di-, tri-, tetra- or hexachromosomal populations the adaptive values of the marker free normals should progressively increase if recessive detrimental genes are the cause of heterosis but not if heterozygosity on many loci leads to overdominance. The “a” values of the +/+ individuals were ealeulated from the frequency changes of the marker chromosomes for each subsequent two-generation period. Unfortunately only two different dichromosomal populations were available. These showed increasing adaptive values for the normals. The tri-, tetra-, and hexachromosomal populations, however, gave different results, some with increasing, some with fluctuating adaptive values. From that it was concluded that heterosis can be due in one case to dominance and in the other to overdominance. In either case, the recessive genetic load may be rather important as a determinating factor in the dynamics of populations.
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  • 2
    ISSN: 1573-6857
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
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  • 3
    ISSN: 1573-6857
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
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  • 4
    ISSN: 1573-6857
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
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  • 5
    ISSN: 1573-6857
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Parts of 28S (nuclear) and 12S (mitochondrial) ribosomal RNA of Palearctic, Nearctic and African species of the obscura group have been sequenced by the direct method of sequencing. Rates of nucleotide substitutions in both molecules were compared. The nucleotide divergence is higher in the mitochondrial rRNA. Average distances of species taken in pairwise were compared to results obtained with the melanogaster subgroup: the divergence of nuclear rRNA appears lower, that of the mtDNA higher whereas genetic distances (allozymes) and sncDNA distances are similar. Noticeable variability of evolutionary rates can be observed even in low taxonomical levels. Phylogenetic trees for the obscura group are in general agreement with those obtained with other characters.
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  • 6
    ISSN: 1573-6857
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Strains of Drosophila melanogaster were made isogenic for their second chromosomes by means of the marker strain LCy/Pm. One of these strains was used as a founder for a homozygous experimental population (W). All other strains were mixed and established a heterozygous population (LKW). Both populations were free of lethals in the beginning with respect to their second chromosomes. After they had been exposed to an X-ray irradiation of 7000 r they contained about 26 per cent newly induced lethal chromosomes. Whereas in the heterozygous population the lethal frequency decreased rather fast to 10 per cent, that of the homozygous population remained rather constant at 25 per cent during a period of 135 days. After a year of continuation, however, both populations reached the same lethal frequency of about 10 per cent. Allelism tests carried out after 10 generations revealed that there was a highly heterotic lethal factor in the homozygous population. After excluding this heterotic lethal from the calculations, the lethal frequencies of the two populations remained significantly different. It was assumed that the relative mean fitness of lethal heterozygotes was generally higher in the homo-than in the heterozygous populations. The results indicate that homozygous populations are much more capable of incorporation new mutations than heterozygous.
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  • 7
    ISSN: 1573-6857
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The chromosome O of Drosophila subobscura was studied with respect to genetic variability at three enzyme loci (Odh, Me, and Lap-4) and with respect to inversion polymorphism. Population samples were taken from seven localities along a north-south gradient from Sweden and Scotland to Tunisia. The chromosomal analysis revealed clinal frequency changes for gene arrangements from north to south. With the enzyme loci Odh and Me allele frequencies are similar throughout the distribution range. Both loci are located outside the common inversion complex O3+4. On the other hand, frequency changes parallel to those of the gene arrangements were observed for the alleles of the Lap-locus. Nonrandom associations between Lap-alleles and the superimposed gene arrangements OST, O3+4, O3+4+8, and O3+4+23 were found. These gene arrangements differ from each other with respect to allele frequencies at the Lap-locus but for a given gene arrangement the relative frequencies of Lap-alleles remain relatively constant along the north-south gradient. Thus allele frequencies at the Lap-locus can be predicted from inversion frequencies. These observations can be interpreted in such a way that the pattern of allozyme variation within gene arrangements is due to founder effects caused by the unique origin of inversions. The gene blocks in the different inversions seem to represent more or less separated gene pools. In polymorphic populations the coexistence of genetically differentiated inversions presumably gives rise to heterotic interaction.
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  • 8
    ISSN: 1573-6857
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract By means of the marker strain Va/Ba wild chromosomes O of Drosophila subobscura were extracted from eight natural populations situated on a north-south gradient from Sweden or Scotland to Tunesia. Lethal frequencies and viability effects of the wild chromosomes O were studied in homozygous and random heterozygous combinations. In accordance with results from other Drosophila species random heterozygotes were always more viable than homozygotes. The viability-determining polygene system proved, however, dominant to some degree. Geographic differences became apparent especially with respect to three different characteristics: (1) The lethal frequencies for chromosomes O from central populations are higher than for those from northern and southern marginal populations; (2) Mean viabilities of non-lethal homozygotes and random heterozygotes are lower for central than for marginal populations; (3) The increase of viability through heterozygosity is more pronounced in the northern populations than in the others. The differences are thought to be mainly due to differences in the adaptation strategy of marginal and central populations. The viability fitness components seem of more importance for the marginal populations while fertility components may be of greater weight under central conditions. The geographic variability of the viability polygene system is finally compared with that of other genetic traits in D. subobscura.
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  • 9
    ISSN: 1573-6857
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Enzyme loci, visible marker genes and λ-cloned DNA-sequences from a D. miranda library were mapped cytologically on the chromosome elements C and E of D. pseudoobscura and D. subobscura. New data are incorporated into the linkage maps of the two species. Homologous segments can now be localized in the polytene chromosomes with these markers. A comparison of the chromosome elements E of D. melanogaster and D. subobscura shows 12 conserved subsections which have been rearranged by paracentric inversions in the evolution of the two lineages.
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  • 10
    ISSN: 1573-6857
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract A highly repetitive satellite DNA sequence from the genome ofDrosophila tristis with a length of 181 bp has been cloned in the pUC plasmid. The sequence hybridizes to the telomeres of all chromosomes but the Y ofD. tristis and produces a ladderlike hybridization pattern with filterbound genomic DNA ofD. tristis digested with Eco RI or Pst I with the hybridization bands at fragment lengths in multiples of 181 bp. A similar pattern is found when the genomic DNA comes fromD. ambigua or, though less clear, fromD. microlabis. Additional bands appear in the zones of high fragment lengths, too. InD. obscura andD. kitumensis, however, the 181 bp sequence is found in fragments with a length of a few kb only. The 181 bp sequence is tandemly arranged in the genome ofD. tristis and has a copy number of about 82,000 per haploid genome (i.e. 10 per cent of the total DNA). A sequence comparison among four independently cloned copies of the family fromD. tristis and another homologous sequence fromD. obscura, found by chance, shows a one to six per cent variation in basepair composition. However, low divergence (only one per cent) between two copies ofD. tristis and between the one ofD. obscura and one ofD. tristis was observed, and high divergence (six per cent) between these two pairs. This is discussed and explained as the evolutionary consequence of an existing homogenization process by unequal crossing over.
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