Two Independent Inversions in the X-chromosome of <i>Drosophila virilis</i> and Their Effects on Crossing Over and Disjunction

Abstract

1. A strain of D. virilis, having two independent inversions in the X-chromosome In (X) Spd, were obtained by X-ray treatment. The extents of the inversions are illustrated in Figs. 1, 2. From this original strain two secondary strains In (X) Sp and In (X) Sd, each with one of these inversions, have been derived through the crossing over in the intercalary section.2. All the In(X) Spd, In (X) Sd and In (X) Sp strains greatly ;suppress the crossing over in the regions included in the inversions. That of the intercalary section in In (X.) Spd is also suppressed to no smaller degree. The crossing over in the uninverted distal sections in all the three strains is suppressed very little, except possibly that in the directly adjacent regions, The crossing over in the section proximal to In (X) Sd is suppressed more markedly.3. The heterozygous In (X) Spd strain throws 13-14 times as many primary exceptions of either sex as the normal strain, the sex-ratio among the exceptions being 1 _??_:5.4 _??_ The In (X) Sd strain throws nearly half as many primary exceptions as does the In (X) Spd strain. The In (X) Sp strain does not produce any greater number of exceptions than the normal strain.4. The secondary non-disjunctions are 10 times as frequent in the heterozygous In (X) Spd strain as in the normal strain, the sexratio among the exceptions being 1 _??_:1.24 _??_ Some superfemales and some XO males were found among the progeny of the XXY females.5. The reduction of crossing over by the heterozygous inversion is apparently due to various causes. The fact that the breakage points of an inversion have a common property with the centromere, in that the pairing chromosomes remain in asynapsis in their proximities, is to be counted among them.6. The primary non-disjunction is correlated somehow with the suppression of crossing over especially in the middle region of the chromosome, where chiasmata are most concentrated.7. The suppression of chiasmata plays a more important role for the secondary non-disjunction than for the primary non-disjunction. The comparative scarcity of secondary exceptions and the marked disparity of sexes among them in D. virilis, seem to be correlated with the more intimate regional affinity of the X-chromosomes in this species than in D. melanogaster.