PHYLCX^ENY 



under such circumstances is not clear. A "hunger theory of sex" has been 

 proposed, according to which the gametes are simply undersized spores, 

 individually lacking the food and energy necessary to complete develop- 

 ment. They therefore pool their resources by means of two-by-two fusions, 

 and thus each zygote obtains a sufficient supply of the materials necessary 

 for development. This theory would be more satisfying were it not for the 

 fact that the four zoospores produced by the zygote are also very small 

 cells, yet they complete their development satisfactorily without further 

 fusions. Whatever the original stimulus to sexual reproduction may have 

 been, the great selective advantages which have made it so nearly a uni- 

 versal property of all higher plants and animals are clear. Sexual repro- 

 duction causes a relatively rapid reshuffling of the various possible re- 

 combinations of characters within a species, with the possibility that the 

 most favorable combinations may be formed and tested by natural selec- 

 tion. Further, diploidy, which results from sexual reproduction, makes it 

 possible to accumulate a store of genetic variability in the heterozygous 

 state. Diploidy may also be physiologically advantageous because of the 

 increased production of nuclear enzymes, or in other less obvious ways. 



The degree of sexual difterentiation varies greatly in different species 

 of Chlamijdomonas. The adults are morphologically identical, and cannot 

 be sexed by inspection. In some isogamous species, the gametes are also 

 morphologically identical. Yet they are physiologically differentiated, for 

 fertilization is possible between some pairs of clones ( asexually produced 

 descendants of a single cell) but not between others. The male-female 

 alternative of higher plants and animals is not present here, for as many 

 as eight mating types (sexes?) have been identified in some species of 

 Chlaniifdonionas. Moevus believes that all of these are based upon vary- 

 ing concentrations of the isomeres of a single substance, namely the cis- 

 and trans- forms of dimethyl crocetin ( a fairly simple hvdrocarb(Mi, which 

 is, curiously, based upon isoprene, the unit of structure of svnthetic rub- 

 ber). In other species, both macrogametes (egg-like) and microgametes 

 (sperm-like) are produced, both of these being flagellate and actively 

 motile. In some of these species, all possible types of fertilization ( large- 

 large, large— small, and small— small) occur, while in others, one macro- 

 gamete and one microgamete always form a pair. In such cases, one 

 speaks of heterogamy. Finally, there are species in which the macrogam- 

 ete is nonflagellate and must be sought by the flagellate microgametes, 

 thus approximating the condition of egg and sperm in the higher animals, 

 or ovule and pollen in the higher plants (oogamy). 



Closely related to CJihinitjdomonas is the familv Volvocidae, known to 

 all students of elementary biology as the standard example of an evolu- 

 tionary trt^nd toward increasing complexity of colonies. The simplest 

 members of this family (Goiiiu)n) consist of four to sixteen or thirty-two 

 identical cells, any of which can form gametes. Hut these are advanced 

 beyond Chlamijdomonas in that the gametes are always niorphologicallv 

 difiFerent from the vegetative cells. The most specializ(xl member of the 

 family, Volvox, comprises as manv as 40,000 cells arranged as a hollow 

 sphere (Figure 34). Most of these cells are purely vegetative and sub- 



106 



