SEXUAL REPRODUCTION IN DIAIOAIS 97 



/ntioii of a species. The fact that diatoms arc diploid rather than 

 haploid means that each individual may have genie flexibility for any 

 trivcn trait. This, coupled with asexual reproduction, no doubt is an 

 important factor in establishing tlie \\ide distribution patterns of the 

 species. Of course, many other factors contribute to producing the 

 broad distributional pattern of diatom species (Patrick, 1948). This 

 association of a wide distribution pattern of a species with asexual 

 reproduction is also found in other algae, protozoa, and fungi. 



The effect of this reproduction pattern on the evolution of dia- 

 toms is hard to evaluate, for it seems to produce effects some of w^iich 

 Mould favor evolution while others would slow it down. Asexual re- 

 production by its very nature greatly reduces chromosomal change, 

 eliminates the accumulation of mutations which have occurred in var- 

 ious genotypes, and the recombination of genotypes. It is interesting 

 to note that in the Centrales, in which reproduction by fusion has 

 only rarely been observed, there are a fairly large percentage of spe- 

 cies which have remained constant since Miocene times. 



In the Pennales, how'ever, the effect of asexual reproduction in 

 reducing the rate of change in the species may not be so great. In 

 this group "sexual reproduction" has been observed in a greater num- 

 ber of species. As Stebbins (1950) points out, in organisms with 

 short generations the number of genie recombinations per generation 

 can be reduced without affecting the flexibility in terms of the num- 

 ber of gene combinations available in a given unit of chronological 

 time. Thus, if "sexual reproduction" occurs often enough, the genie 

 flexibility \y'\\\ be preserved. The question then is how frequently does 

 "sexual reproduction" occur. 



The structure of the diatom species offers a favorable condition 

 on which natural selection can operate. A species is composed of seg- 

 regated population units in the form of clones between w^iich gene 

 exchange through sexual reproduction occasionally takes place. It 

 must be remembered, however, that these segregated population units 

 consist of a single genotype rather than several genotypes, as is usually 

 the case in segregated population units of obligate sexual species. 



The rate of evolution in diatoms undoubtedly is influenced by 

 these various factors. Because of the varying frequency of sexual re- 

 production in different species, these algae are an interesting group 

 for the study of some of these basic problems. 



Note: Since this paper w^as written Geitler (1953a) has shown 

 that Denticula tenuis Kiitz. may produce auxospores by Normal 



