Heteromorphic Types - Intra specific Incompatibility
Plants of the same species produce flowers that differ in morphology. This involves two (distyly) or three (tristyly) morphologically distinct types of flowers showing similar breeding behavior (mating type) within a species. The difference in the mating types lies in the position of stigma and anthers (heterostyly). Distyly is under the control of a single gene complex with two alleles, S (for short style-dominant) and s (for long style-recessive). Long style individuals are, therefore, homozygous recessive (ss) and short style are heterozygous (Ss). This ensures approximately equal ratio of short and long-styled individuals from compatible crosses.
Tristyly refers to three floral morphs with individuals having either long, mid- or short- styled flowers. Each type has stamens of two heights corresponding to the height of stigma in the other two forms. Successful pollination results only between forms having stigma and stamens of the same height. Two genes, M and S with two alleles each control tristyly. Gene S is epistatic to M. Homozygous recessive for both the genes (ssmm) determine long style, mid-style have ssMM or ssMm and short-styled flowers carry the genes SsMm or Ssmm or SsMM. In both di- and trimorphic forms sporophytic incompatibility operates.