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Evolution of hybrid dysgenesis potential following P element contamination in Drosophila melanogaster
M G Kidwell1, K Kimura, D M Black
1Department of Ecology and Evolutionary Biology, University of Arizona, Tucson 85721.
Abstract:
P elements were introduced into M strain genomes by chromosomal contamination (transposition) from P strain chromosomes under conditions of P-M hybrid dysgenesis. A number of independently maintained contaminated lines were subsequently monitored for their ability to induce gonadal (GD) sterility in the progeny of reference crosses, over a period of 60 generations, in two experiments. The efficiency of chromosomal contamination was high; all tested lines acquired P elements following the association of M and P chromosomes in the same genome for a single generation. All the contaminated lines also sustained an initial unstable phase, marked by high frequencies of transposition and sterility within lines, in the absence of P element regulation. Subsequently, each of the lines rapidly evolved to one of three relatively stable strain types whose phenotypic and molecular properties correspond rather closely to those of the P, Q and M' strains that have previously been characterized. The numbers and structures of P elements and the presence or absence of P element regulation during the early generations appeared to be critical factors determining the subsequent course of evolution. On the basis of GD sterility frequencies, both the mean level of P activity, and the average capacity for P element regulation, were reduced in lines raised at 25 degrees, relative to those raised at 20 degrees, during the early generations. This latter result is consistent with the expectation that natural selection will tend to modify the manifestation of dysgenic traits, such as high temperature sterility, which cause a reduction of fitness. However, overall, stochastic factors appeared to predominate in determining the course of evolution of individual lines.
Insights
P elements can contaminate M strain genomes, causing gonadal sterility. Over 60 generations, these lines evolved into stable P, Q, or M
Area of Science:
- Genetics
- Molecular Biology
- Evolutionary Biology
Background:
- P elements are transposable elements in Drosophila melanogaster.
- P-M hybrid dysgenesis is a phenomenon that occurs when P elements are mobilized in the germline of hybrids between P-strain and M-strain flies.
- This mobilization can lead to gonadal sterility and increased mutation rates.
Purpose of the Study:
- To investigate the long-term evolution of M strain genomes following P element contamination.
- To characterize the stability and properties of newly established P element-containing lines.
- To determine the factors influencing the evolutionary trajectory of these lines.
Main Methods:
- Introduction of P elements into M strain genomes via chromosomal contamination under P-M hybrid dysgenesis conditions.
- Monitoring of independently maintained contaminated lines for gonadal sterility induction over 60 generations.
- Analysis of P element numbers, structures, and regulatory capacity in evolved lines.
Main Results:
- High efficiency of P element contamination, with all tested lines acquiring P elements.
- Initial unstable phase characterized by high transposition and sterility frequencies.
- Rapid evolution into three stable strain types resembling previously characterized P, Q, and M' strains.
- P element characteristics and regulation in early generations critically influenced subsequent evolution.
- Reduced P activity and regulation at 25°C compared to 20°C, suggesting natural selection against sterility.
- Stochastic factors predominantly determined the evolutionary course of individual lines.
Conclusions:
- M strain genomes contaminated with P elements rapidly evolve towards stable states.
- The evolution is influenced by P element properties, regulatory mechanisms, temperature, and stochastic events.
- Natural selection appears to act against traits reducing fitness, such as high-temperature sterility.