THE CYTOSKELETON OF THE NAKED GREEN FLAGELLATE SPERMATOZOPSIS-SIMILIS(CHLOROPHYTA) - FLAGELLAR AND BASAL BODY DEVELOPMENTAL CYCLE

Flagellar and basal body development during cell division was studied in the biflagellate green alga Spermatozopsis similis Preisig et Melko nian by light microscopy of immobilized living cells, statistical anal ysis of flagellar lengths during the cell cycle, and electron microsco py of cells and isolated cytoskeletons. Interphase cells display two f lagella of unequal/subequal length. An eyespot located in an anterior lobe of the chloroplast is connected to the basal body bearing the sho rter flagellum by means of a five-stranded microtubular root. Until ce ll division, the two parental flagella attain the same length. During cell division, each cell forms two new flagella that grow to a length of 1.5 mu m before they are distributed in a semiconservative fashion together with the parental flagella to the two progeny cells at cytoki nesis. During the following interphase, the flagella newly formed duri ng the preceding cell division grow to attain the same length as the p arental flagella until the subsequent cell division. The shorter of th e two flagella of a cell thus represents the developmentally younger f lagellum, which transforms to the mature state during two consecutive cell cycles. Interphase cells display only two flagella-bearing basal bodies; two nascent basal bodies are formed during cell division and a re connected to the microtubular d-roots of respective parental basal bodies with which the newly formed basal bodies are later distributed to the progeny cells. During segregation, basal body pairs shift into the 11/5 o'clock direction, thus conserving the 1/7 o'clock configurat ion of basal body pairs of interphase cells. Prior to chloroplast and cell division, an eyespot is newly formed near the cell posterior an c lose association with a 1s microtubular root, while the parental eyesp ot is retained. During basal body segregation, eyespot-root connection s for both the old and newly formed eyespots are presumably lost, and new associations of the eyespots with the 2s roots of the newly formed basal bodies are established during cytokinesis. The significance of this ''eyespot-flagellar root developmental cycle'' for the absolute o rientation of the progeny cells is discussed.