Chapter 15, Problem 18P

Plant breeders have long appreciated the phenomenon called hybrid vigor or heterosis, in which hybrids formed between two inbred strains have increased vigor and crop yield relative to the two parental strains. Starting in the 1930s, seed companies exploited the cytoplasmic male sterility (CMS) phenomenon in corn that was described in Problem 17 so that they could cheaply produce hybrid corn seed to sell to farmers. This type of CMS

is caused by mutant mitochondrial genomes that prevent pollen formation.

a.	How would CMS aid seed companies in producing hybrid corn seed?
	Dominant Rf alleles of a nuclear gene called Restorer suppress the CMS phenotype, so that Rf-containing plants with mutant mitochondrial genomes are male fertile.
a.	Describe a cross generating hybrid corn seed that would grow into fertile (self-fertilizing) plants. (Farmers planting hybrid seed want fertile plants because corn kernels result from fertilized ovules.)
b	One of the historical challenges in the commercialization of hybrid corn produced through CMS was the maintenance of strains with CMS mitochondria: How could the seed companies keep producing male sterile corn plants if these plants never themselves produced pollen? Suggest a strategy by which the seed companies could continue to obtain male sterile plants every breeding season.
c.	Are there any potential disadvantages to the use of hybrid corn? If so, what issues might arise?