This overview will first consider the biochemistry and cell biology of karst and then discuss its effects on phenotype.
Epithelial cells constitute many tissues of the fly.
In this notation the 2's refer to the fact that each tetramer contains two alpha and two beta subunits. Although the phenotype of the most severe karst mutant eyes resembles that of the EGF receptor mutation DER Ellipse , no dominant interaction has been observed between these loci. The fact that R7 is the missing photoreceptor has been confirmed by examining serial sections through karst mutant eyes. Since R1-6 extend the entire depth of the retina, while R7 extends only through distal regions and R8 replaces R7 in more proximal regions, this result is consistent with the specific absence of R7. The morphology of such clusters suggests that they may have arisen through fusion of ommatidia to another cluster containing only a small number of photoreceptors.