Genetic Systems I | 167 



mechanism of protoorganisms, each "individual" would presumably 

 have been difiFerent from all others. Fusion would occasionally com- 

 bine complementary "genotypes," and this may have been the foun- 

 dation of a selective advantage of fusion. On the basis of the 

 speculations presented in Chap. 1, it may be suggested that the 

 stabilization of the genetic mechanism of early protoorganisms in- 

 volved loss of superfluous genetic material or its assumption of new 

 functions; association of the DNA nucleotides with protein mole- 

 cules to form chromosomes; and the restriction of gene function so 

 that expressivity became less variable and control more precise. The 

 variety of genetic systems in plants and animals includes manv bi- 

 zarre and unusual phenomena. Their common features suggest that, 

 in nearly all instances, the advantages of diploidy and of recombina- 

 tion have been combined. Meiosis of the nuclear genetic material 

 and fusion of cells are combined in a life cycle of varying degrees 

 of complexity. This combination appears to have arisen inde- 

 pendently in a number of diff^erent ways in plants and animals. 



Stebbins has concluded that the wide occurrence of haploidv in 

 the flagellates and filamentous algae is a result of their short and 

 simple development and their rapid rate of reproduction, which 

 makes the establishment of complex gene-developmental systems 

 less important. The selective value of buffering and long-term storage 

 of recessives would also be less important. With increasing complex- 

 ity have come increased length of the developmental period and 

 concomitant lengthening of the life cycle. The build-up of inte- 

 grated gene complexes with ontogenetic buffering and genetic home- 

 ostasis is thus favored, and the diploid state has high selective value. 



Diploid Life Cycles and Alternation of Generations 



Some groups of Protozoa are predominantly diploid, with complex 

 mating behavior effecting recombination. Certain groups of algae, 

 notably some of the brown algae (Phaeophycophyta), diatoms 

 (Bacillariophyceae), and some green algae (Siphonales), also are 

 diploid during most of their life cycle. Meiosis results in the produc- 

 tion of haploid gametes. The Metazoa, of course, ha\c tlic same sort 

 of genetic system. 



Presumably this diploid life cycle arose independently several 

 times from organisms with a predominantly haploid cycle. In addi- 

 tion, there also arose in plants and in fungi life cycles that involve 

 alternation of generations. In these forms there is a regular cycle of 

 haploid individuals that produce gametes and diploid individuals 

 that produce asexual spores. Gametogenesis takes place by mitosis. 



