90 



THE ORGANISM: 



ment from zygote to diploid adult sporophyte. Note 

 also that each adult may be capable of reproducing 

 its own kind by asexual reproduction. 



HAPLOBIONTIC, ADULT DIPLOID, CYCLE 



In most animals, the life cycle does not include a 

 haploid adult and for that reason no asexual repro- 

 duction by such a form. Therefore, because any adult 

 is generally diploid, the life cycle is said to be diplonlic 

 or haplobiontic, adult diploid (Figure 6.5). How- 

 ever, this does not prevent two or even more struc- 

 turally diflTerent adult stages from existing. Repeated 

 asexual reproduction can give rise to many such hap- 

 lobiontic adults. For this reason, "haplobiontic" 

 refers to a single chromosomal complement in adults, 

 either one set or two, and not necessarily to a single 

 adult stage. When adults are haplobiontic the asex- 

 ual reproduction phase may or may not be present. 

 However, when the cycle is present in diploid animals 

 such as coelenterates, the different appearing adults 

 already mentioned can occur. However, these adults 

 differ from those of the generalized cycle. For ex- 

 ample, the zygote of coelenterates generally forms 

 a diploid adult which asexually reproduces by bud- 

 ding a somewhat plantlike colony of diploid adults. 

 This colony includes specialized individuals that bud 

 diploid jellyfishlike adults. The jellyfishes continue 

 the life cycle by entering the sexual phase at the stage 

 of meiosis, males producing sperms and females eggs, 

 and finally through fertilization of eggs zygotes are 

 formed. Therefore, adequate portrayal of certain 

 coelenterate modified haplobiontic, adult diploid, 

 cycles would show an asexual colony (adults) being 

 derived from the zygote, and the colony giving rise to 



HAPLOBIONTIC, ADULT DIPLOID, LIFE CYCLE 



.;.;.DiplophaSe:.;.;.;.;.;.;.:.X. 





Haplophase . 



::::(n) :::::: 



ve^. reprod.::::adult::::: .'. 



•:•:•:•:■:■:■:•:•:•:•:•:•:■:•:■:• zygote- gametes ; 



iiiiiiiii ^m. 



o:%-:-:-:-:-:v:v:v:v^^^ 



jellyfish adults. Also, all adult coelenterates would 

 be diploid. 



HAPLOBIONTIC, ADULT HAPLOID, CYCLE 



In plants, especially, the diplophase of the general- 

 ized life cycle might be suppressed (Figure 6.6). This 

 means that the cycle is modified into a haplobiontic 

 type, but the adult or adults are haploid. The other 

 term applied to this cycle in reference to the haploid 

 adult is haplonlic. 



HAPLOBIONTIC, ADULT HAPLOID, LIFE CYCLE 



.....Diplophase ......■.......•.•.■ 



E^ri.-ivX-ivivi-H^^^ meiosis* • : 



Haplophase . 



:(n): 



adult • • veg. reprod 

 gametes 



m 



vtxiv:-::::::::::::::::::::: zygote 



Figure 6.5 The haplobiontic, odult diploid, life cycle. 



Figure 6.6 The haplobiontic, adult haploid, life cycle. 



ASEXUAL CYCLE 



The last major kind of life cycle, the asexual cycle, 

 is most difficult to relate to the generalized, or diplo- 

 biontic type. However, the asexual cycle can be inter- 

 preted as an extreme contraction of the generalized 

 cycle. In the asexual cycle, all stages except the 

 adults and asexual reproduction of like adults are 

 absent. As a consequence, there is an asexual cycle, 

 adult diploid representing the generalized diplo- 

 phase and an asexual cycle, adult haploid repre- 

 senting the generalized haplophase (Figure 6.7). 



The various kinds of life cycles are found within 

 many phyla in both the plant and animal kingdoms. 

 However, different groups of organisms reveal definite 

 tendencies toward one or more of the major types. 

 Asexual cycles of both kinds are most typical of 

 bacteria and blue-green algae. Haplobiontic, adult 

 haploid, cycles are most often encountered in proto- 

 zoans and certain fungi. The haplobiontic, adult 

 diploid kind is the "typical" cycle in animals, but it 

 also has spotty occurrence in the protozoans, fungi, 

 and plants. Finally, the diplobiontic cycle is widely 



