Genetics of an L2 Venation Mutant in Drosophila Melanogaster I. Mode Op Inheritance and Expression-

The results indicate the mutant phenotype is recessive; high penetrance being due to the homozygosity of factors found on both the second and third chromosomes. Selection for high and low lines of expression was initiated and after nine generations of selection for a high line and six generations for a low line, a significant difference between the two was obtained. Analysis of crosses between high and low lines, Fi crosses, and backcrosses to both parental lines gives evidence for additive action of the polygenes controlling expression of the second longitudinal vein (L2). Some thirty mutant loci are known in Drosophila melanogaster (Braver, 1956) which produce an interruption of the longitudinal veins of the wing (Bridges and Brehme, 1944). Twelve of these have an effect on the second longitudinal vein (L2). Five of these twelve confine their effect solely to L2. It seemed of some interest therefore, to investigate the mode of inheritance and expression of an L2 mutant phenotype arising in a wild-type stock maintained in mass culture at the Virginia Polytechnic Institute. The interest in a study of the genetic and environmental variables involved in the realization of L2 interruption is further heightened by the apparent differences in the developmental events involved in the establishment of the second vein from those associated with the other longitudinal veins as revealed by the descriptive (Waddington, 1939) and experimental (Lees, 1941) studies of venation development in Drosophila melanogaster. MATERIALS AND METHODS The standard culturing medium of corn meal, agar, dextrose, and yeast, including the addition of the mold preventitive, tegosept M, was used. All crosses, except when otherwise noted, involve single pair replications, the parents of a given mating being placed initially in shell vials and transferred one day later to half pint bottles. All cultures were incubated at 26°C±1°C. The following list comprises the stocks used in the investigation. 1. Oregon-R, a highly inbred wild type stock maintained in mass culture at The Ohio State University. 2. Cy/Pm; H/Sb, a balanced lethal stock with dominant markers for the second and third chromosomes. 3. The mutant stock being studied. The Oregon-R stock was used in the determination of the mode of inheritance of the mutant. The Cy/Pm; H/Sb stock was used in the chromosome localization tests. The marker genes in the latter stock show the following characteristics: Curly (Cy)—a mutant associated with a second chromosome inversion, lethal as a homozygote and producing a strong upward curling of the wings in the heterozygous state. Plum (Pm)—a mutant associated with a second chromosome inversion, lethal as a homozygote and producing a reddish brown eye flecked with darker spots as a heterozygote. Hairless (H)—a homozygous lethal mutant associated Based on a thesis submitted to the Graduate School of The Ohio State University in partial fulfillment of the requirements for the M.Sc. degree. Manuscript received August 24, 1963. Present address: Department of Biology, Fairleigh Dickinson University; Madison, New Jersey. THE OHIO JOURNAL OF SCIENCE 66(3): 340, May, 1966. No. 3 L2 VENATION MUTANT IN DROSOPHILA 341 with a third chromosome inversion, causing an absence of certain bristles and often producing interruption of the fourth and fifth (L4, L5) longitudinal veins and occasionally causing a break in L2. Stubble (Sb)—a third chromosome mutant causing a shortening and thickening of the bristles of the fly as a heterozygote and lethality as a homozygote. The degree of interruption of the L2 vein was determined for the various progeny according to the following scheme, the score for a given individual being determined as the mean of the interruption of the left and right wings. The flies were classified under a binocular microscope (27 X) into one of six classes (fig. 1) based on the percentage of L2 missing, relative to the length of the vein extending from the junction of the second and third longitudinal veins to the junction of the CLASS 3 CLASS 4 CLASS 5 FIGURE 1. Classification of the degree of interruption of the second longitudinal vein. second vein with the wing margin (distance "a" in fig. 1). the six classes are as follows: Class 0 Normal, wild type L2 vein. Class 1 0 to 20% of L2 absent. Class 2 20 to 40% of L2 absent. Class 3 40 to 60% of L2 absent. Class 4 60 to 80% of L2 absent. Class 5 80 to 100% of L2 absent. The specifications of 342 JAMES H. CARLSON Vol. 66 RESULTS AND DISCUSSION Penetrance and Expression of the Mutant Stock The percentage of individuals of the mutant stock having interrupted veins (the penetrance) at 26°C was very high, being 99.86 per cent. The distribution by sex into the various classes of phenotypic expression described under the section on methods (fig. 1) is summarized in table 1. Also included in this table is the percentage distribution relative to the various classes for the combined total of males and females. A very low percentage of the flies at this temperature (26° C) fell into the extreme classes (0 and 5). The majority of the flies fell in class 2. The mean L2 interruption for combined males and females was 33.24 per cent, calculated by the following method. Each class was assigned its midpoint value on the interruption scale and this value was then multiplied by the total number of flies in that class. Summation of these numbers for all classes, divided by the grand total of flies, yielded the average amount of interruption. A chi-square test of the significance of the differences between the male and female distributions of table 1 yielded a p value less than 0.01 that this difference could be due to chance alone. Classes 0 and 1 were combined as were classes 3, 4, and 5 due to the small numbers. TABLE 1 Distribution of Li interruption at 26° C in an unselected mutant stock