Genetica 75: 11-22, 1987 ~-.~ Dr W. Jnuk Publishers, l)ordrecht - Printed in the Netherlands 1 1 Patterns of puffing activity and chromosomal polymorphism subobscura IV. Position effect at the boundaries of the El2 inversion in Drosophila R. De Frutos, A. Latorre & L. Pascual Departamento de Gen~tica, Facultad de Ciencias Biol6gicas, Dr. Moliner, 50, 46100 Bwjasot, Spain Received 10.12.1985 Accepted in revised form 31.5.1987 Abstract The puffing patterns in polytene E chromosomes of Drosophilasubobscura were followed in third-instar larvae and throughout the prepupa period. Two gene arrangements, E~t and E 1 2+9-12 were studied. A majority of puffs exhibit a similar pattern, but the puffs 61AC and 67AB behave differently in the two chromosomal ar- rangements, both in homozygotes and in heterozygotes. These two puffs are located at the end of the E12 in- version. This position effect is an interesting phenomenon that probably is not due to a heterochromatinization effect. Introduction Sturtevant (1928) introduced the term "position ef- fect" to designate a phenomenon, in which two genes located on the same chromosome show a stronger effect on phenotype than when they are on different chromosomes. He was referring to the Bar locus of Drosophila melanogaster. Later the term was extended to cover to all cases in which an alterat- ed location of a gene on the chromosome gives rise to changes in its activity. In most cases, this phenomenon is associated with a process of heter- ochromatinization, i.e. the change in activity results in general from a gene losing its activity wholly or partially when located on or near to the centromere heterochromatin. The study of the position effect as a result of heterochromatinization has been and re- mains an interesting question which can be ap- proached equally by means of a strictly genetic methodology, or from a cytogenic or molecular standpoint. It has been treated in a number of reviews, such as those of Lewis (1950), Hannah (1951), Spofford (1976), etc. The data presented here refer to the change in ac- tivity of two puffs in the E chromosome of Drosophila subobscura, located within the inversion Ex2. This is a case of position effect which cannot be explained in terms of heterochromatinization. The changes in location of the two regions owing the inversion E12 take place within the euchromatic regions, or at least, at a distance from the centromer- ic heterochromatin. Nonetheless, the presence of in- tercalary heterochromatin, which in some way produces a similar effect to the centromere heter- ochromatin on the gene expression, could be postu- lated. Material and methods Strains The strains of Drosophila subobscura used were: H271, R225, Ralll, and Ra121. H271 and Ralll are homozygous for the standard gene arrangement in the E chromosome (E~t); R225 and Ral21 are homozygous for the complex gene arrangement E1_2+9+12. The four stocks were obtained from in-