Structure, frequency and distribution of P elements in relation to P—M hybrid dysgenic male recombination inDrosophila melanogaster

The frequency and distribution of P elements were investigated in the third chromosomes of two wild-type strains ofDrosophila melanogasterusingin situhybridization of biotinylated probes to the polytene chromosomes. The relationship between these data and the extent of hybrid dysgenesis was determined through assays of egg production, egg hatchability (F2embryo lethality),snwdestabilization and male recombination along the third chromosome. The results suggest that P-element distribution, frequency and structure are all contributory factors in the regulation of hybrid dysgenesis. Texas 6 was shown consistently to be a stronger P strain than Texas 1, eliciting greater reductions in fertility, more extensivesnwdestabilization and higher frequencies of male recombination. Clustering of male recombination events, arising from pre-meiotic crossing over, was evident among the dysgenic progeny of each strain. Male recombination andsnwdestabilization were independently distributed among the dysgenic males studied, suggesting that these traits represent separate P-mediated functions. The third chromosome male recombination maps produced by the two strains differed significantly from each other and from the published female meiotic and polytene chromosome maps. Male recombination breakpoints were associated with the original distribution of P sequences in the two strains and the results suggest that this relationship may be closer for potentially complete P factors than for P sequences in general. An analysis of sub-lines derived from individual recombinant males revealed that chromosomal breakpoints could also be associated with novel insertions following P-element transposition.

Effect of extra Y chromosome on number and fluctuating asymmetry of sternopleural bristles in Drosophila melanogaster

The effect of the extra Y chromosome on the number and fluctuating asymmetry of sternopleural bristles in Drosophila melanogaster was studied at two temperatures, 18 degrees C and 25 degrees C, in the stock In(l)wm4/y+Y. The extra Y chromosome significantly increased the number of sternopleural bristles especially in the lateral areas of the sternopleura, which hampered the analysis of the effect of the extra Y chromosome on fluctuating asymmetry. The formula used was one which assumes that the fluctuating asymmetry is dependent on the total number of bristles. It was found that the extra Y chromosome increased fluctuating asymmetry at 18 degrees C but decreased it at 25 degrees C. There was less fluctuating asymmetry in females than in males. It appears that the extra Y chromosome has an effect on the temperature which is most favourable to the development of the sternopleural bristles.

Consequences of stabilising selection for polygenic variation

A number of earlier authors have investigated the consequences for fitness of gene differences affecting the expression of a primary character where stabilising selection is acting in favour of an optimal expression. Assuming that the mean expression must be at or close to the optimum, deviations from the mean have been taken as measures of the deviation from the optimum, and the conclusion reached that, random drift apart, stabilising selection must ultimately lead to fixation of the commoner allele. It is now shown that this approach is incorrect: in an illustrative example the mean cannot be taken as synonymous with the optimum except in the trivial case where they have precisely the same value. Where the mean departs from the optimum, even by sampling variation only, continuing selection for it is effectively self-propagating and directional away from the optimum. Deviation from the optimum itself must be used in investigating the consequences of stabilising selection. The model used here is based on the biometrical description of the effects of a gene difference on a quantitative character as used by Mather and Jinks (1982). This includes a parameter, h, which allows dominance of any magnitude in either direction to be taken into account, and is adapted for the present purpose by introducing an additional parameter, m, measuring the departure from the optimum of the mid-point, or mid-parent, which is used biometrically as the origin for measurement of the gene effects. Assuming random mating and the absence of epistasis in the effects on the primary character, it is shown that stabilising selection acting on a pair of alleles (A and a) can have any one of three possible outcomes depending on the relative values of the m and h which characterise the effects of the gene difference on the primary character, viz: (i) a stable equilibrium in the population where in respect of the primary character Aa is nearer to the optimum than both homozygotes; (ii) fixation of the fitter allele where Aa is intermediate between the homozygotes in its departure from the optimum; (iii) a theoretical unstable equilibrium leading to fixation of the commoner allele, where Aa departs further from the optimum than both homozygotes. Only outcome (i) can lead to the conservation of variation in the population. Preliminary consideration is also given to the interaction in effect on fitness of two gene pairs affecting the same character and segregating simultaneously in the population.(ABSTRACT TRUNCATED AT 400 WORDS)