272 



Multiple Alleles 



Self-sterile species have been known for a long time although 

 the genetic basis for self-sterility in most plants has been known 

 only since 1925. The botanist, Koelreuter, in 1764 probably 

 published the first discussion of self-sterility in plants, and 

 Castle in 1896 reported the first known case in animals. Darwin 

 considered the problem at length in 1876 but failed to realize 



the cross-sterility relationship among 

 plants from other than the same clone. 

 Darwin's philosophical view of the 

 problem considered that self-sterility 

 was a personal reaction of the individ- 

 ual plant brought about by a decrease 

 in the differentiation of the sexual ele- 

 ments of the same plant. With this 

 decrease in differentiation these sexual 

 elements tend to become sufficiently 

 alike so that they no longer will ferti- 

 lize one another. 



Several different hypotheses have 

 been advanced to explain the mechan- 

 ism which controls the inheritance of 

 self-sterility in various organisms. Al- 

 though it is possible that several differ- 

 ent mechanisms are instrumental in 

 different plants, one is undoubtedly 

 operative in almost all plants known 

 to be self-sterile. This mechanism, first 

 described independently by Prell and 

 by East and Mangelsdorf and during the following year by 

 Lehmann and Filzer, is often referred to as the "oppositional 

 factor hypothesis." 



In Chapter 4, the method of reproduction in angiosperms was 

 described in detail. If a pollen grain adheres to the stigma, a 

 pollen tube emerges from it and grows down into the ovary. It 

 grows toward an ovule, enters through the micropyle, and dis- 

 charges its contents, including the male gametes or sperm nu- 

 clei, into the embryo sac. East and Park have shown that self- 

 or cross-sterility or self- or cross-fertility are determined by 

 the rate of growth of these pollen tubes. In a fertile combina- 



FiG. 79. Sterile and fer- 

 tile combinations in the 

 self-sterile (self-incompat- 

 ible) Capsella grandiflora. 

 Left, a sterile combination 

 in which none of the ova- 

 ries has developed. Right, 

 a fertile cross in which all 

 the ovaries of the flowers 

 that were cross-polHnated 

 have developed into ma- 

 ture capsules and contain a 

 full complement of seeds. 

 (From Riley in Genetics.) 



