On Recombination Among In ( 2 L ) t , a - Gpdh and Adh in Drosophila melanogaster

The occurrence and patterns of linkage disequilibrium between an inversion and allozymic loci within it or nearby have been used in attempts to discriminate among different hypotheses for the maintenance of variability. The interpretation of the data on the best-documented case, that of the nonrandom association between In(2L)t and a-Gpdh or Adh in the second chromosome of Drosophila melanogaster, has been done on the basis that recombination between a-Gpdh and Adh is almost entirely due to the recombination between Zn(2L)t and the locus within it (a-Gpdh), the recombination between the inversion and the nearby locus (Adh) being negligible. In this paper, we show that the pattern of recombination is just the opposite. BSERVATIONS of linkage disequilibrium be0 tween an inversion and allozymic loci within it or nearby has variously been proposed to be proof of selection working upon the system (PRAKASH and LEWONTIN 1968), as a relic of an unique event (NEI 1975), and as the consequence of random drift with or without selection acting upon the inversion (NEI and LI 1980). These alternative explanations can be distinguished by collating the observed patterns of those associations with those expected under the different hypotheses. These expectations rely heavily on the recombination fraction (rf) between the markers that are considered. The best-documented case is that of the nonrandom association between Zn(2L)t and a-Gpdh (ZZ, 17.8) or Adh (ZZ, 50.1) in the second chromosome of Drosophila melanogaster. In this paper, we report our results on the recombination among these markers. MATERIALS AND METHODS Experimental stocks: A near isogenic line for the second chromosome with In(2L)t, a-GpdhF AdhS phenotype extracted from the population Aspe (MALPICA and VASSALLO 1980) using Cy0 (LINDSLEY and GRELL 1968) as balancer was used throughout this study. No instability was observed in the progeny of the crosses. Flies were reared at 25" on cornmeal-molasses medium. Enzyme assays: Horizontal starch gel electrophoresis with a buffer system Tris-versene-borate (0.05 M, pH = 8, for the gel, 0.5 M in the tanks) was used. Staining recipes were those of SHAW and PRASAD (1970). Cytological analysis: Recombinant chromosomes were extracted as above. These near-isogenic stocks were crossed with al 6 pr cn stock carrying the standard arrangement. From these crosses, third-instar larvae were scored for inversions by temporary propionic-orceine-carmine squash preparations of salivary gland chromosomes (BECKER 1962). In all cases the observed inversion had the same breaking Genetics 115: 141-142 (January, 1987). ae dp Ce uGpdh b Adh pt cn