Reproduction and Life-Cycles 81 



(zygotic meiosis), or in one of the pregamic divisions in conjugation 

 (conjugant meiosis). The type of meiosis varies in different Protozoa 

 (Table 2. 1). Available data indicate that the Heliozoida, Foraminiferida, 

 Cnidosporidia, and Ciliophora are diploid throughout most of the life- 

 cycle. Among the Mycetozoida, some of the Plasmodiophorina are said 

 to be predominantly haploid. Nuclear fusion, supposedly occurring at 

 the end of the vegetative phase, may be followed immediately by meiosis 

 (116, 236). In such cases, meiosis might be considered zygotic, although 

 the uninucleate haploid products promptly encyst, becoming "spores." 

 Some of the Eumycetozoina are believed to undergo syngamy just before 

 development of the plasmodium begins, and presumably are diploid 

 throughout the vegetative phase. In such cases, meiosis apparently pre- 

 cedes the formation of "spores," which give rise to the gametes after ex- 

 cystment. The Coccidia and a number of the Gregarinida are haploid 

 organisms, although a few of the gregarines seem to be diploid. Among the 

 flagellates, gametic meiosis has been reported in two species, and zygotic 

 meiosis in a number of others. 



Syngamy 



In addition to many established cases of syngamy (Table 2. 1) in 

 Protozoa, a nimiber of descriptions need confirmation. The lack of critical 

 evidence does not in itself justify dismissal of such reports. Syngamy in 

 Zoomastigophorea was described occasionally in the older literature but 

 most protozoologists remained unconvinced. The investigations of Cleve- 

 land (62, 64, 65, 65a, 65b) have supplied cytological evidence that was 

 previously lacking. Certain descriptions of syngamy in trypanosomes (86, 

 87) do not approach the cytological standards set by Cleveland. However, 

 the trypanosomes are not particularly favorable material for studying 

 chromosomal behavior and the accumulation of adequate evidence will 

 be correspondingly difficult. 



The status of sexual phenomena in Phytomastigophorea other than 

 the Phytomonadida remains uncertain. A fairly recent description (21) 

 of syngamy in Euglena has not been confirmed, and the often cited case 

 of "Copromonas subtilis" (75) is questionable. In "C. subtilis" the so- 

 called reduction-divisions involved the extrusion of small granules ("polar 

 bodies") from the nucleus, whereas meiosis, as demonstrated in many 

 Protozoa, is a genuine nuclear division. Other reports of syngamy in 

 Euglenida also offer inadequate evidence. Among the Dinoflagellida, 

 syngamy has been reported in Ceratium hirundinella (85), Coccodinium 

 mesnili (41) and Noctiluca milaris (104). Syngamy and formation of 

 zygotes have been described in Glenodinium lubiniensijorme , a hetero- 

 thallic species which apparently undergoes zygotic meiosis (74). These 

 accounts receive additional support from a brief account of meiosis in 

 Paradinium poucheti (40). 



