732 INHERITANCE 



Inheritance in Biparental Reproduction 



Biparental or sexual reproduction includes the processes that lead to 

 the formation of new individuals from the united parts of two earlier 

 individuals. Biparental reproduction in the Protozoa occurs in two main 

 types, known respectively as copulation and conjugation. Copulation is 

 characteristic of haploid species; in it two haploid individuals unite com- 

 pletely to form a diploid zygote, which later divides with reduction, to 

 form haploid individuals again. This method occurs in the Flagellata. 

 In conjugation two diploid individuals exchange halves of their nuclei, 

 including a haploid set of chromosomes; then the two separate and each 

 continues thereafter to multiply by fission. This is the characteristic 

 method in the Ciliata. 



BIPARENTAL INHERITANCE IN HAPLOIDS: FLAGELLATA 



Knowledge of the genetics of flagellates is largely due to the recent 

 work of Moewus (1932-38). 



In flagellates the single motile individual is haploid. In copulation two 

 such haploid cells unite completely, to form a diploid cell. The two 

 hapoid cells thus correspond functionally to two gametes, while the dip- 

 loid cell is the zygote. The zygote is inactive; it secretes a wall about 

 itself and becomes a cyst. Later, under favorable conditions, the diploid 

 cyst divides twice by the two "maturation divisions." At one of these 

 divisions chromosome reduction occurs, so that the four cells formed 

 are haploid. At times additional cell divisions occur before the cells 

 emerge from the cyst. The cyst wall dissolves and the haploid cells are 

 freed. Each develops flagella and swims about as a free individual. 

 On emerging from the cyst, the individuals are often called swarmers 

 or swarm cells; each is potentially a gamete. These free cells commonly 

 multiply vegetatively for many generations, the descendants of each 

 original swarm cell forming a clone. 



In any species or variety the haploid individuals or gametes are dif- 

 ferentiated into two sexes; details as to this will be found in the chapter 

 on "Sexuality in Unicellular Organisms" (Chapter XIV). In some spe- 

 cies or varieties all members of the same clone are of the same sex, the 

 different sexes being in difl^erent clones (dioecious species or races). In 

 others both sexes are found among the individuals of a single clone 



