Adult cap 



Resting cyst 



Fig. 10. Life cycle of Ace- 

 fabularia med'iferranea 

 (after Brachet, Biochemical 

 Cytology, p. 303). In the 

 laboratory, unlike in the 

 natural habitat, develop- 

 ment of the adult structure 

 from the zygote is direct, 

 without regression, and 

 takes only one year. 



Germinating cyst 



'^0 Nucleus 



Zygote 

 2 days 



to yield haploid daughter nuclei. Subsequent mitotic divisions produce a 

 great number of tiny haploid nuclei that stream away from the rhizoid 

 into the cap where they are gathered into large bodies with thick walls 

 called cysts. The cysts are eventually shed from the cap and lie dormant. 

 By this time, each nucleus within the cyst is surrounded by cytoplasm and 

 a cell membrane. They look like flagellate protozoa, being pear-shaped 

 with two large flagella protruding from the front end. Eventually the lid 

 of the cyst flips off, freeing the flagellate cells which swim about for a 

 time and then conjugate in pairs. The two cells become one, the two 

 haploid nuclei fuse to yield a diploid, and the zygote can then give rise 

 after 3 years to an adult alga with cap, stalk, and rhizoid to complete the 

 cycle. 



In nature, Acetabidaria can regenerate lost parts. For example, the 

 seasonal disappearance of the stalk is followed by the synthesis of a new 

 stalk, and the loss of the adult cap by mechanical abrasion is followed by 

 the formation of a new cap. The regenerative powers of Acetabularia have 

 been employed to study its morphogenesis and in particular the role 

 played by the nucleus in morphogenetic processes. One can ask whether 

 or not the nucleus has to be present at all for regeneration to occur. It can 

 in fact be shown that pieces of stalk lacking a nucleus (anucleate) can 

 produce new cell membranes, chloroplasts, a perfectly normal new cap, 

 and, in a few cases, even a new rhizoid— in short, everything but another 

 nucleus. After a few months all these parts die. In contrast, pieces con- 

 taining a nucleus can regenerate lost parts over and over again. These 

 activities are accompanied by extensive synthesis of proteins and nucleic 

 acids. Thus, we can conclude that an anucleate segment of Acetabularia 

 has stored reserves of material and energy sufficient to construct a com- 



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