280 PROTOZOA 



That other varieties may exist with different breeding systems 

 is perhaps suggested by Kay's report of what might be autog- 

 amy. 



Obligatory Inbreeding and Asexual Protozoa 



The Transition from Sexual to Asexual Reproduction in the 

 Flagellates of Wood Roaches and Termites. The Ciliates illus- 

 trated gradations from extreme outbreeders to nearly the extreme 

 of inbreeding. The highly evolved and morphologically complex 

 Flagellates that live in the gut of wood roaches and termites 

 illustrate the transition from obligatory close inbreeding to 

 asexual reproduction. Cleveland (1934-54) has reported ex- 

 tensively on the Flagellates of wood roaches. They undergo 

 sexual reproduction at or near the time the host moults. Only at 

 this time do the roaches pass fecal pellets laden with the Flagel- 

 lates; and at the same time the eggs, which are laid earlier, 

 hatch. Sexual processes in the Flagellates are thus timed to 

 coincide with transmission to new individual hosts. 



That the two processes should be timed in this way is hardly 

 without meaning. Obviously this would make possible selection 

 of those genetic variations in the symbionts which happen to be 

 adapted to variations of the individual infected host. Yet, as I 

 shall at once show, there is little opportunity for either host or 

 symbiont variability. It would therefore seem more likely that 

 the present correlation in time of symbiont sexual processes and 

 transmission to new hosts is a relic of earlier conditions when 

 both symbiont and host were more variable than they are to- 

 day. 



The wood roach lives in small isolated colonies and migration 

 seems to be very limited. Under such conditions, the amount of 

 genetic variability expected among the individuals of a single 

 roach colony would be small. Correspondingly there would be 

 little need for genetic variability among the Flagellate symbionts 

 living in so constant a milieu. 



In agreement with this, Cleveland's accounts of the sexual 

 processes in the Flagellates indicate little opportunity for genetic 

 recombination. Although some of the Flagellates, such as Euco- 



