Endosperm 443 



the angiosperm seed and is interesting genetically from several 

 angles. 



In maize, several genes are present which affect the endo- 

 sperm. One gene, Y, produces a yellow color in the endosperm 

 whereas its allele, y, produces no color. If a yy female is crossed 

 with a FF male, the embryo in the seed is genotypically Yy. 

 The endosperm is Yyy having received a F gene in a sperm nu- 

 cleus and two y genes in the two polar nuclei. Since the F gene 

 affects the endosperm, the color of the endosperm will be yellow. 

 This is actually only an example of a gene that exerts its influ- 

 ence on the endosperm instead of on part of the mature plant. 

 It happens that the F gene is dominant over two y genes, but 

 this is not true of all genes affecting the endosperm. This situ- 

 ation is interesting historically because it was formerly consid- 

 ered to be an example of the direct influence of the male. As 

 such, this phenomenon was termed xenia. 



The same result is found for the sugary gene (su) in maize and 

 its allele (Su) for starchy. If a row of sweet corn (susu) is 

 planted next to one of field corn (SuSu), pollen from the field 

 corn may blow on to the silks of the sweet corn. Because Su is 

 dominant over two doses of su, and because these genes affect 

 the endosperm, seeds which are fertilized by this pollen will 

 have starchy rather than sugary endosperm. In this example, 

 xenia is again observed because of the dominance relations, for 

 if one dominant gene were dominant over one recessive and not 

 two, the endosperm would show the character of the female 

 parent instead of the male. 



The importance of the endosperm has also been shown in 

 certain hybrids where the abnormal development of the endo- 

 sperm will cause the hybrid seed to fail to develop. Brink and 

 Cooper showed that seeds of the hybrid Nicotiana rustica X ■^• 

 glutinosa abort early in development whereas those of N. rustica 

 X A^. tahacum usually abort at a later stage of development, 

 although a few develop to the stage where they can germinate. 

 A comparison of these two hybrids with seeds of N. rustica 

 shows that the embryo is probably viable in all three but com- 

 plete or partial failure of the endosperm in the hybrids usually 

 interferes with the nourishment of the developing embryo. After 

 double fertilization the endosperm normally begins to develop 

 and apparently also secretes some growth-promoting substances 



