542 EVOLUTION OF ANIMALS Part V 



SUMMER 



Much food. Great population 

 Every unit a female A 

 reproducing asexually f 



LATE SUMMER 



Asexual reproduction 

 decreases 



0-> *=VA_^ ^ 



^ ^^^ ^ Sexual reproduction 



^ / (J) begins 



SPRING \U A FALL 



More food f \S\ S produces 



More rotifers rx V/ some eggs (m) 



Resting eggs W\ $ motes with M / I 'hat unfertilized 



develop into \) and produces / i develop into 



stem mothers \ fertilized / moles. |||V1 



winter eggs 



Al \ 



© 



<^ 



sperm 



WINTER 



Food sparse. Population reduced 

 to fertilized resting eggs, ^y^ 



Fig. 27.9. Annual succession of generations typical of summer rotifers. Begin- 

 ning with the stem mothers {A I) of spring there are successive generations that 

 consist only of parthenogenetic females (A), all producing female young (eggs, 

 /) an economical arrangement for great multiplication. Under changed con- 

 ditions that occur in the fall a generation of sexual females (S) arises whose 

 unfertilized eggs (m) develop into males (M). These males mate with the sexual 

 females (5) of their mothers' generation and produce the fertilized resting or 

 "winter" eggs {W) from which stem mothers (Al) develop. In the spring the stem 

 mothers produce parthenogenetic females and the cycle begins again. (Based on 

 data for Lecane inermis by H. R. Miller, Biol. Bull, 60:345-380, 1931. 



produced from unfertilized eggs containing the diploid number of chromo- 

 somes. Then, with a change in the environment, such as temperature, or food, 

 or others not fully understood, generations of sexual females appear that bear 

 especially small eggs. They contain the haploid number of chromosomes and 

 develop parthenogenetically into males. These males mate with the sexual 

 females, actually the generation of their mothers. The fertilized eggs that result 

 become the thick-shelled resting or "winter" eggs. They contain the haploid 

 number of chromosomes from the male plus the haploid number from the 

 sexual female, and thus carry a biparental inheritance. After a resting period 

 they develop into the stem mothers. 



Seasonal Differences in Reproductive Cycles. There are striking sea- 



