168 1 The Process of Evolution 



Sporogenesis involves meiosis and resultant recombination. In some 

 algae and fungi, the alternating generations are isomorphic (indis- 

 tinguishable morphologically ) . 



With increasing complexity, there seems to be a tendency for 

 reduction of the haploid gametophyte generation in proportion to 

 the diploid sporophyte. Thus in most ferns the gametophyte is a 

 small thallus, usually a centimeter or less in diameter, whereas the 

 sporophyte may be quite massive. In gymnosperms and flowering 

 plants the gametophytic generation is reduced to relatively few 

 cells (the pollen grain is a male gametophyte) and the sporophyte 

 is the conspicuous stage. In mosses, on the other hand, the gameto- 

 phyte is the conspicuous stage. It is perhaps better to regard the 

 bryophytes (mosses and liverworts) as a specialized offshoot of 

 ancient terrestrial plants and not on the main phyletic line of the 

 vascular plants. 



No process comparable to alternation of generations is known to 

 occur in animals, with the possible exception of some Sporozoa. In 

 other animals the haploid phase is represented by the gametes only. 

 The Coelenterata have the so-called alternating generations of 

 medusae and polyps, but these are morphologically, not cytologi- 

 cally, different. 



The fungi, as a group, have a number of distinctive genetic sys- 

 tems of interest here in so far as they shed light on the selective 

 forces involved in the evolution of genetic systems. These highly 

 specialized organisms are poorly understood cytogenetically. The oc- 

 currence of somatic crossing-over and systems of multiple-mating 

 types attests to the selective advantage of recombination. The water 

 molds, Phycomycetes, which are filamentous and without cross cell 

 walls, build up numerous haploid nuclei in the common cytoplasm 

 of the filaments or hyphae. Within the mycelium of these fungi 

 genetically different populations of nuclei may arise, producing the 

 so-called heterokaryotic state. 



Mushrooms and toadstools (Basidiomycetes), the most complex of 

 the fungi, and some of the sac fungi ( Ascomycetes ) show an inter- 

 esting parallel with the evolution of diploidy in plants and animals. 

 Haploid mycelia, often with specific mating types, develop in the 

 soil. The cellular hyphae of different mycelia, coming into contact, 

 may fuse. Eventually a mycelium results in which each cell has two 

 haploid nuclei— one from each haploid mycelium— which do not 

 fuse until reproductive structures are formed. This special sort of 

 diploidy is known as dikaryosis. Just as diploidy is associated with 

 developmental and structural complexity in plants and animals. 



