Myosin as a constituent of the Drosophila egg cortex

Abstract

MYOSIN and actin have been found in many cell types, including the eggs of Amphibia¹, sea urchins², starfish³ and rats⁴. Their distribution correlates well with the distribution of microfilaments in structures such as the contractile ring, and these proteins are believed to have a major role in cell cleavage and plasmalemma elongation⁵. They have not previously been recorded in Drosophila eggs and embryos. The egg of Drosophila develops as a syncytium with about 6,000 nuclei present in a common cytoplasm⁶. Subsequently, nearly all the nuclei are simultaneously surrounded by plasmalemmas which grow down into the cortex of the egg. Because of this, and because of the existence of several mutants which specifically affect cellularisation^7,8, Drosophila eggs are a particularly appropriate system in which to study cell membrane formation. The simultaneous cellularisation of the whole of the egg cortex would seem to require the existence of large numbers of microfilaments but these have not yet been visualised in the electron microscope⁹. We report here the finding of a continuous band of myosin immediately below the egg plasmalemma which could well be associated with massed microfilament bundles. This is laid down by the time of fertilisation and remains continuous until the arrival of the nuclei in the cortex. The structure of the band and its relationship to early developmental events are described.