126 SEX IN MICROORGANISMS 



tions of its formation. In C. eugametos, such zoospores are said to be 

 immediately capable of acting as gametes (Moewus, 1933). 



A few attempts have been made to follow meiotic segregation 

 among the products of zygote germination. Pascher (1916) succeeded 

 in crossing two 1 0-chromosome species of Chlamy domonas and in 

 effecting some analysis of genetic characters among the progeny: it 

 is claimed (Pascher, 1918b) that these early experiments were the 

 first to establish directly the fact of Mendelian segregation in any 

 plant. 



Clones may be grown from zoospores produced after zygotic 

 meiosis, and in such cultures sex (or mating type) has been shown to 

 segregate equally in heterothallic species of Gonium and Eudorina 

 (Schreiber, 1925), Chlorogonium (Schulze, 1927), Dwialiella 

 (Lerche, 1937), Chlamy domonas eugametos (Moewus, 1933), C. vari- 

 abilis (Behlau, 1939), C. reijihardi (Smith and Regnery 1950), and 

 C. moeivusii (Lewin, 1950b). However, the occasional formation 

 of homothallic and neuter strains, variously attributed to cytologically 

 demonstrable non-disjunction of sex chromosomes (Hartmann, 1934), 

 to crossing-over between sex loci, and to mutations affecting sex 

 determination and behavior, have been reported in a number of algae 

 investigated by Moewus (see reviews by Jennings, 1941; Beadle, 

 1945; and Sonneborn, 1951; also Moewus, 1944). Philip and Haldane 

 (1939) have criticized some of these results: Moewus' replies to 

 these criticisms (1941, 1943) are far from satisfactory. 



In Dimaliella, Lerche (1937) observed that zygotes would oc- 

 casionally give rise on germination to only two zoospores, cytokinesis 

 apparently coinciding with the first meiotic division. By taking ad- 

 vantage of such exceptional cases, she was able to distinguish first 

 from second division segregations and thus to estimate the map dis- 

 tances of two unlinked loci, for mating type and for a morphological 

 marker from for/na ^'oblonga,^^ from their centromeres. Moewus 

 (1940b) claimed to have mapped forty-two characters on the ten 

 chromosomes which he observed in Chlamy domojias eugametos and 

 its allies, in which crossing-over invariably took place between whole 

 chromosomes, as he had found also to be the case in Protosiphon, 

 Tolytoma, and Brachiomonas. However, he illustrated (1936) chias- 

 mata which appear similar to those of other organisms and stated 

 (1944) that, in nature, crossing-over occurs in the four-strand stage. 

 Several serious criticisms of these reports can be put forward, as has 



