Chapter 3, Problem 16P

In certain plant species such as tomatoes and petunias, a highly polymorphic incompatibility gene S with more than 100 known alleles prevents self-fertilization and promotes outbreeding. In this form of incompatibility, a plant cannot accept sperm carrying an allele identical to either of its own incompatibility alleles. If, for example, pollen carrying sperm with allele S1 of the incompatibility gene lands onto the stigma (a female organ) of a plant that also carries the S1 allele, the sperm cannot fertilize any eggs in that plant. (This phenomenon occurs because the pollen grain on the stigma cannot grow a pollen tube to allow the sperm to unite with the egg.)

For the following crosses, indicate whether any progeny would be produced, and if so, list all possible genotypes of these progeny.

a.	♂ S1 S2 × ♀ S1 S2
b.	♂ S1 S2 × ; ♀ S2 S3
c.	♂ S1 S2 × ♀ S3 S4
d.	Explain how this mechanism of incompatibility would prevent plant self-fertilization.
e.	How does this incompatibility system ensure that all plants will be heterozygotes for different alleles of the S gene?
f.	How do you know that peas are not governed by this incompatibility mechanism?
g.	Explain why evolution would favor the emergence of new incompatibility alleles, making the gene increasingly polymorphic in populations of tomatoes or petunias.