Requirement of Drosophila l(2)gl function for survival of the germline cells and organization of the follicle cells in a columnar epithelium during oogenesis

The lethal(2)giant larvae gene, or l(2)gl, encodes a widely expressed cytos keletal protein which acts in numerous biological processes during embryoge nesis and oogenesis, including cell proliferation, and morphogenetic moveme nts. Having identified the nucleotide change occurring in the l(2)gl(ts3) s equence, we produced by site-directed mutagenesis the identical change lead ing to the substitution of a serine by a phenylalanine at position 311 of p 127(l(2)gl) and introduced the modified l(2)gl(F311) gene into l(2)gl(-) fl ies. The transgene can fully rescue the development of l(2)gl flies raised at 22 degrees C but causes drastic effects on their development at 29 degre es C confirming the temperature sensitivity of the phenylalanine substituti on at position 311. Fertility of females, albeit not of males, was strongly affected. Temperature-shift experiments and microscopic examination of ova ries showed that the mutation blocked egg chamber development at the onset of vitellogenesis (stages 8-9) with growth arrest of the oocyte, incomplete follicle cell migration over the oocyte associated with abnormal organizat ion of the follicular epithelium, and apoptosis of the germline cells, as m easured by TUNEL assays. By comparison to wildtype, we found that p127(F311 ) is already reduced in amount at 22 degrees C and delocalized from the cyt oskeletal matrix, albeit without affecting the apical localization of myosi n II, a major partner of p127. At 29 degrees C, the level of p127(F311) is even more reduced and the distribution of myosin-II becomes markedly altere d at the apices of the follicle cells. These data indicate that during ooge nesis p127 plays a critical function at the onset of vitellogenesis and reg ulates growth of the oocyte, follicle cell migration over the oocyte and th eir organization in a palisadic epithelium, as well as viability of the ger mline cells.