120 SEX IN MICROORGANISAIS 



Planozygotes 



Once pairing has been initiated, the course of events leading to 

 zygote formation differs according to species. In some the cells come 

 to rest almost at once, whereas in others there is a further period of 

 motility, possibly with the biological advantage of locating the zygo- 

 spore in a suitable resting place. In most cases this motile stage occurs 

 after cytogamy and perhaps also caryogamy, so that we are justified 

 in referring to it as a planozygote. The axes of the cells usually swing 

 round until the gametes are laterally appressed, and then not only do 

 the cells fuse, but also the two pairs of flagella become coordinated 

 into a single integrated motor apparatus, so that a quadriflagcllatc 

 planozygote is formed, resembling a cell of Carteria. In fact, Behlau 

 (1939) has shown that planozygotes of Chlamy domonas variabilis 

 were apparently described by Jacobsen as ''''Carteria ovata^'' and, fol- 

 lowing Korschikoff (1926), Behlau considers that ''''Chlorobotrys 

 gracilliina^' and ''''Tetradonta variabilis^'' are likewise diplophase cells 

 of Chla77iydobotrys gracilis and Chloromonas paradoxa respectively. 



According to Teodoresco (1905) the planozygotes of Dwmliella 

 are biflagellate, one pair of flagella being shed after copulation. In 

 Carteria iyengarii, the planozygote, formed by the fusion of a naked, 

 non-motile egg cell with a quadriflagellate microgamete, is propelled 

 from some time by the flagella of the male cell (Ramanathan, 1942) 

 like that recorded in the phycomycete Monoblepbarella (Sparrow, 

 1939). Such planozygotes may remain actively swimming for some 

 hours in Protosiphon (Klebs, 1896), Fhyllocardiiim (Korschikoff, 

 1927), and Tetraspora (Klyver, 1929), or, in dim light, for as long as 

 1 to 3 weeks in Chlamy domonas paradoxa and Chlamy dobotrys stellata 

 (Gerloff, 1940; Strehlow, 1928-29) and Phylloinonas striata (Kor- 

 schikoff, 1926). In Raciborskiella, according to Wislouch (1924-25), 

 the planozygotes are even capable of diploid mitosis! 



Vis-A-vis Pairs 



An unusual exception to the general pattern of planozygote 

 formation is furnished by Chlamy domonas eiigajnetos (Moewus, 

 1933; Mitra, 1951) and C. moewnsii (Gerloff, 1940). In these forms, 

 after gamete pairing has been initiated, cytogamy is arrested for a 

 period, the cell pairs continuing to swim actively for some hours 



