Evidence for functional differentiation among Drosophila septins in cytokinesis and cellularization

The septins are a conserved family of proteins that are involved in cytokin esis and other aspects of cell-surface organization. In Drosophila melanoga ster, null mutations in the pnut septin gene are recessive lethal, but homo zygous pnut mutants complete embryogenesis and survive until the pupal stag e. Because the completion of cellularization and other aspects of early dev elopment seemed likely to be due to maternally contributed Pnut product, we attempted to generate embryos lacking the maternal contribution in order t o explore the roles of Pnut in these processes. We used two methods, the pr oduction of germline clones homozygous for a pnut mutation and the rescue o f pnut homozygous mutant flies by a pnut(+) transgene under control of the hsp70 promoter. Remarkably, the pnut germline-clone females produced eggs, indicating that stem-cell and cystoblast divisions in the female germline d o not require Pnut. Moreover, the Pnut-deficient embryos obtained by either method completed early syncytial development and began cellularization of the embryo normally. However, during the later stages of cellularization, t he organization of the actin cytoskeleton at the leading edge of the invagi nating furrows became progressively more abnormal, and the embryos displaye d widespread defects in cell and embryo morphology beginning at gastrulatio n. Examination of two other septins showed that Sep1 was not detectable at the cellularization front in the Pnut-deficient embryos, whereas Sep2 was s till present in normal levels. Thus, it is possible that Sep2 (perhaps in c onjunction with other septins such as Sep4 and Sep5) fulfills an essential septin role during the organization and initial ingression of the cellulari zation furrow even in the absence of Pnut and Sep1. Together, the results s uggest that some cell-division events in Drosophila do not require septin f unction, that there is functional differentiation among the Drosophila sept ins, or both.