THE SHAKING B GENE IN DROSOPHILA REGULATES THE NUMBER OF GAP-JUNCTIONS BETWEEN PHOTORECEPTOR TERMINALS IN THE LAMINA

The molecular structure of insect gap junctions differs from that in v ertebrates, and in Drosophila is possibly encoded by the shaking B (= Passover) locus, shaking B-2 is a null allele that acts in the nervous system. In the shakB(2) mutant, one site of action are gap junctions between photoreceptor terminals in the cartridges of the lamina, benea th the compound eye, which we assayed from the number of close-apposit ion profiles in thin-section EM. The number of profiles in the Canton- S (C-S) wild type is about 0.5 per cartridge per section in distal and mid-lamina depths, and significantly less, about one quarter this val ue, closer to the brain, in the proximal lamina. In shakB(2), there ar e fewer profiles, approximately one quarter the number of appositions in distal and mid-lamina depths as in C-S, and their number does not d iffer significantly from those at the proximal depth in either the mut ant or wild type. Thus mutant action is associated with a reduced numb er of appositions at distal and mid-lamina depths. We propose that R1- R6 gap junctions are partitioned into at least two strata, proximal an d distal, and that two populations of gap junctions exist, one extendi ng throughout the lamina that does not require shakB, and a second at distal and mid-depth levels, which does. The number of gap junctions i s reduced in mutant shakB(2), and surviving appositions at distal and middle lamina depths possibly have wider clefts than in C-S. Gap junct ions are reduced equally between all R1-R6 terminals, so the two diffe rent types of junction proposed, shakB(2)- and non-shakB(2)-dependent, can apparently express in a single receptor terminal. (C) 1998 John W iley & Sons, Inc.