512 The Determination of Sex 



somatic tissue of the plant. Since, however, male gametophytes 

 and gametes arise only in anthers, this abnormality would elimi- 

 nate the male reproductive phase of that monoecious plant and 

 would convert it into a female. In fact, by combining such genes 

 in a certain way, Jones has established a dioecious strain of 

 maize. 



There is a recessive gene on the second chromosome in maize 

 known as silkless, sk, which, when homozygous, causes an abor- 

 tion of the ovaries. The lateral inflorescences are otherwise 

 normal, but the spike is entirely barren. The elimination of 

 functioning female flowers makes sksk plants effectively male. 

 Furthermore, there are several recessive genes which affect the 

 tassel in such a way that in homozygotes the staminate flowers 

 are replaced with pistillate ones. They are the tassel seed, ts, 

 genes, and plants homozygous for any of them are effectively 

 female plants. It so happens that tassel seed-2, one of these 

 genes, is epistatic to silkless. Plants which possess both Sk and 

 Ts2 are normal; those that are sksk TS2TS2 or sksk Ts2ts2 are 

 male plants, since they are silkless; and plants that have one 

 or two Sk genes and are also ts2ts2 are female, since they have 

 the stamens absent from the tassel. However, sksk ts2ts2 plants 

 are not completely sterile but are functional females because the 

 homozygous silkless condition is not expressed when the plant is 

 homozygous for ts2' 



If an Sksk Ts2ts2 plant was selfed, the offspring would segre- 

 gate into a ratio of 9 normal monoecious {Sk TS2) : 3 silkless 

 male {sk TS2) : 4 tassel seed female {3 Sk ts2 + 1 s/c ts2). This 

 epistatic relationship makes it possible to maintain a dioecious 

 strain in which half the plants are female and half are male, pro- 

 vided the original female is sksk ts2ts2 and the male is sksk Ts2ts2. 

 If no outcrosses are made, this strain can be maintained per- 

 petually in the dioecious condition, and it has been kept so for a 

 number of generations by Jones, who created it. Since all plants 

 in the strain are sksk, the sex is "determined" by Ts2 or ts2. The 

 ts2 locus is the differential that throws the balance towards the 

 male or the female side. In a sense, then, the chromosome that 

 carries this gene has become a sex chromosome, and the male 

 is the heterogametic sex. By a proper selection of other genes 

 that suppress or stimulate the male or female structures, other 

 dioecious strains could be developed. 



