Breeding

9. Keep in mind that, since the FPsc variety has been extensively inbred, seeds produced by true-breeding lines (whether wild type FPsc or homozygous mutants) will breed true via self-pollination. The genetic uniformity of FPsc plants facilitates genetic analysis of mutant alleles since, in segregating seed families, we intend that the only difference is the presence or absence of particular allelic variants, whose effect on plant phenotype can be assessed in an otherwise uniform genetic background. A practical benefit also ensues: provided that you and your students take care to keep plants in optimal growing conditions through the life cycle, seeds harvested in excess of those needed for immediate experiments can be used as starting materials for future implementation of classroom exercises.

An important exception to this general rule applies to seed stocks distributed as FPsc/R500 hybrids. R500 is a naturally selfing, thoroughly inbred line of B. rapa that is grown in areas of India and Nepal and whose seeds are used as a source of cooking oil or of condiments and spices. R500 was selected as the polymorphic partner to FPsc in our resources. The polymorphic phenotypic attributes range from the macroscopic (plant size, growth habit to molecular levels (R500 is the source of PCR fragment length polymorphisms that we use in genetic mapping experiments. Most of the seed stocks we distribute are in the form of F2 families derived from an initial FPsc mutant x R500 cross in which mutant alleles from the FPsc parent will be segregating. However, it is important to keep in mind that many (likely thousands) of allelic variants at other loci will also be segregating in such families and so each F2 plant will have a unique genetic constitution. This genetic variation is essential in order to extend activities and exercises from classical (Mendelian) to molecular genetic analysis but may complicate or prevent propagation of seed lineages over time, since the derivative lines will not be true-breeding.