Unterschiedliche Teilungsverläufe bei den Phytomonaden (Chlorophyta)

Abstract

Summary The Phytomonads have like other flagellates heteropolar cells. The extremity commonly bearing the flagella is marked off as the anterior (apical), whilst the opposite one is the posterior (antapical) pole. The typical division of the flagellates is longitudinal, it takes place along the longitudinal plane. In walled flagellates (Chlamydomonadales, Volvocales) the division takes place inside of the cell wall (sporangium). The plane of division is here mostly longitudinal, but in many cases before or during the division the protoplast rotates through a right angle. That's why the plane of division comes to be transverse to the old cell, but the protoplast itself divides always longitudinally (seemingly transverse division). In few cases the walled flagellates divide transversely (true transverse division). Chlorogonium is one of the best known genera in this way. The division takes place along the transverse plane of the cell and protoplast — vertically to the longitudinal axis. In spite of the inner change of cell polarity, given by the new basal bodies during the division, the old flagella together with the stigma persist in the original position. They keep their function during the whole process of division (schizogonie) and also during the differentiation of the zoospores. The sporangium is motile and phototactic even all zoospores are formed and possess their own flagella. The anterior part of the old protoplast and later the anterior zoospore keeps the old flagella in its anterior pole while the new ones appear in the opposite pole. At last the old flagella are thrown off and the new ones take over the function of motility. The extremities have changed their position and function, they have turned round. A similar transverse division occurs as well in the genus Haematococcus as in Volvox rousseletii . In the last species the single zoospore escapes during the germination of the zygote. A true transverse division in the genus Chlamydomonas was not known up to the present time and there were doubts about it. Now as well a semitransverse as a true transverse division were found. Comparing the progress of the division in the Phytomonads we are able to distinguish 4 types: I. The schizotomous longitudinal division of naked Phytomonads into two morphologically equal sister cells. The flagella are distributed among the sister cells. II. The schizogonous (multiple) longitudinal division of the walled Chlamydomonadales and Volvocales into morphologically equal sister cells which number is a power of two. The flagella are drawn in or thrown off. III. The semitransverse division of walled Chlamydomonadales into morphologically unequal parts of the protoplast which number is a power of two. From the number of the daughter protoplasts one is the rest of the mother cell. The protoplast rotates after the first division, but the old flagella are connected with the rest of the mother cell. IV. The true transverse division without rotation of the protoplast into morphologically unequal parts of the protoplast. The number of the daughter protoplasts is a power of two, but one of them is the rest of the mother cell. The old flagella are connected with the rest of the mother cell. This type of division is up today known only in few genera. Both the semitransverse and transverse division look like a variant of unequal cell division. The rest of the mother protoplast remains as long as all zoospores are differentiated and able to exist independently. At the end of the process the rest of the mother protoplast gets a new wall and turns round to a zoospore. It is possible that the transverse division shows the way of coenobia formation in the Volvocales and the differentiation of the cells into coenobia. In any way the modus of division should be an important character in the classification of the Chlamydomona-dales.