88 



THE ORGANISM: 



chromosomes (2n), one set coming from each parent. 

 The union of parental gametes is caUed fertilization. 



The origin of gametes with one set of chromosomes, 

 directly or indirectly, from a parent with two sets of 

 chromosomes necessitates a type of cell division called 

 meiosis. Meiosis is a process in which a cell with two 

 sets of chromosomes produces daughter cells with one 

 set of chromosomes. The significance of meiosis in 

 our present discussion is that it compensates for the 

 doubling efiTect of fertilization. 



Meiosis can occur in various stages of a life cycle. 

 For example, in man meiosis produces sex cells, male 

 sperms and female eggs. However, in plants having 

 sporophyte and gametophyte generations, meiosis 

 produces spores. Then these spores grow into 

 gametophytes and sex cells are formed by mitosis. 

 Therefore, meiosis may be somewhat remote from 

 sexual reproduction. 



Under the heading of sexual reproduction, as- 

 sociated phenomena may be distinguished (Figure 

 6.3). 



Isogamy and Heferogamy. Isogamy is the union of 

 two gametes of like size and structure, isogametes. 

 Neither gamete is distinguishable as egg or sperm but 



MEIOSIS, sex cell formation 



ISOGAMY 



HETEROGAMY 

 sperm 



isogametes zygote 



Figure 6.3 Types of sexuoi reproduction. 



functionally there are two parental gametes. The 

 process occurs in organisms of simpler form, such as 

 the algae. 



Most sexually reproducing plants and animals dis- 

 play heterogamy, a condition wherein egg and sperm, 

 or heterogameles, can be distinguished. The egg is 

 generally larger, spherical, and nonmotile; the sperm 

 is usually smaller, elongate, and highly mobile. 



Sexuality. In sexual reproduction with heterogamy, 

 a species may be biparental with separate male and 

 female sexes, or be hermaphroditic with individuals 

 containing both male and female sex organs. Her- 

 maphrodites are sometimes capable of fertilizing 

 their own eggs (e.g., flukes), but most require cross- 

 fertilization between two individuals. Some hermaph- 

 rodites do not display both sexes at the same time. 

 An oyster, for example, may start life as a male and 

 later become a female. All of these processes are 

 sexual. 



Allied to sexual reproduction because it probably 

 represents a breakdown of the process is partheno- 

 genesis. Commonly called "virgin birth," this is the 

 development of an egg without prior fertilization. 

 Because there is no fusion of unlike gametes, it is not 

 sexual reproduction. It is commonest in protistans 

 and algae but is found in many higher plants and in 

 animals. It may be the only reproductive process 

 known in a particular organism, or it may alternate 

 with sexual processes in the same organism. In 

 bees fertilization produces females and partheno- 

 genesis males, but in many aphids unfertilized eggs 

 develop and form either females, or males and fe- 

 males, depending upon the season of the year. At 

 certain times some of these aphids reproduce sexu- 

 ally, but other species of aphids are strictly 

 parthenogenetic. 



Alternation of Generations. This is a regular cycle 

 of interchange of structurally distinct adults, one 

 reproducing sexually and the other asexually. The 

 asexual generation is derived from and gives rise to 

 the sexual generation, hence the alternation. There- 

 fore, only every other generation reproduces by the 

 same means and has the same physical appearance. 

 In many plants this is what involves a sporophyte 

 and a gametophyte, two kinds of adults, in a single 

 species. In animals, alternation of generations is 

 typical of the coelenterates, but the asexual repro- 

 duction is by the budding of organisms rather than 

 true buds. 



Fertilization Site. The actual place of fertilization 



