Principles of Plant Breeding (2+1)

Sporophytic incompatibility

• This was first described by Hughes and Babcock (1950) in Crepisand by Gerstel (1950) in Parthenium. This system is also governed by multiple alleles S gene. In the gametophytic system genes are not independent and show dominance relationship. The tube reaction is governed by genotype where they are produced, for example, both type of pollen produced by S1S2 plant would behave similar.
• Inhibition occurs at the stigmatic surface and the pollen does not germinate on stigma. At the incompatibility locus, there are two closely linked with genes affecting style length.
• a) Distily In primula there are two types of flower, pin type with style Short filament, large stigmatic cells and small pollen and “Thurm type” with short style, long filament small stigmatic cells and large pollen. Both types of flowers are seen incompatible but cross compatible. The thrum is governed by genes ss and pin by SS. The pollen of thrum plant, in spite their segregation into S and s types behave same.
• b) Tristyly in Lythrums there are three types of flowers; genetic constitution of the trace forms is as follows.

  Long style : mm ss

  Mid style : MM ss or MMss

  Short style : MMss, mmss, mmSS, MMSS, MMSs, MsSs
• Two independent loci S and M govern the style length. Plants with S have short style irrespective of constitutions. Plants with have medium styles if they have long styles if they have M. All the three stylar types are incompatible but cross compatible.