Isolation of the male germ unit: organization and function in tobacco (Nicotiana tabacum L.)

Sperm cells are released from pollen tubes of tobacco as linked cells, associated with the vegetative nucleus in an assemblage known as the male germ unit (MGU). Using light microscopy, the MGU assemblage appears to be ensheathed by cytoplasmic material of the pollen tube, which may stabilize their association. Following their release, the shape of the sperm cells and vegetative nucleus changes from an ellipsoidal to a more spheroidal morphology. When most of the cytoplasmic material is dispersed, a boundary remains around the two sperm cells. Using scanning electron microscopy, the cytoplasmic material surrounding the MGU appears filamentous, sometimes twisted and rope-like. Based on these observations, the function of the MGU of tobacco is discussed.