Self-Sterility 277 



in the cross s'^a / s^a X s'^a / s^A, but all will be aa if the cross 

 is s^a / s^a X s^A / s^a. However, the reciprocal of each of 

 these crosses will segregate into a ratio of lAci : laa. As at least 

 176 species of plants representing 55 families of both monocoty- 

 ledons and dicotyledons are known to be self-sterile, the geneticist 

 must be prepared to encounter disturbed ratios as the result of 

 linkage of morphological genes and self-sterility alleles, for un- 

 doubtedly many more such cases will be discovered than have 

 already been found. 



In addition to the self-sterility alleles, alleles at the same locus 

 but producing self-fertility have been found. In Nicotiana San- 

 derae and N. alata grandiflora, East and Yarnell discovered 

 sixteen alleles at the s locus. A number of these alleles are 

 characterized by different ratios of pollen tube growth after 

 selfing, but in only one was the rate of growth sufficiently great 

 to produce self-fertility. Fifteen of these alleles (s^-s^^) are al- 

 leles for self-sterility, but the sixteenth, s^, is a gene for self- 

 fertility. If an s^s^ plant is selfed, the s^ pollen tubes are not 

 accelerated; but the s^ pollen tubes grow rapidly enough to set 

 seed since the s^ gene in the style does not act as a barrier. Such 

 a self-pollination, therefore, would be fertile and the offspring 

 would be s^s^ and sV. Plants which have the s^ gene are, there- 

 fore, self-fertile. 



The large number of different alleles at this locus is interesting. 

 It is presumed that all arose as point mutations at various times. 

 Although 16 alleles were identified. East himself states that this 

 number does not indicate the actual number that might be found 

 from wild material. A study of the number of alleles in wild 

 populations of Oenothera organensis was made by Emerson. In 

 this species of evening primrose which inhabits the Organ Moun- 

 tains in New^ Mexico, 37 self-sterility alleles were discovered in 

 about 500 plants. All behaved as a series of multiple alleles 

 and all apparently represent a different gene mutation at the 

 same locus of the same chromosome. In red clover, Williams 

 found 37 alleles in a series of 40 plants and 41 s-alleles in another 

 series which consisted of 48 plants. In red clover, Atwood 

 crossed 49 plants from each of two populations with a s'^s'^ plant. 

 One plant was chosen from the offspring of each cross, and all 

 these plants were crossed together to test the number of dif- 



