Molecular basis of Mendel's wrinkled seed character

In the year 1990, a group of scientists, working at John Innes Institute at Norwich (U.K.), cloned the pea gene r (rugosus), which determines whether the seed is round or wrinkled. It was shown that an isoform of 'starch branching enzyme' (SBE I)is present in round (RR or Rr) seeds, but absent in wrinkled (rr) seeds. By carefully planned experiments it was shown that there was 100% co-segregation of the absence of SBE I with r locus, suggesting that the gene for SBE I is located at r locus. It was also shown that in rr lines, SBE I gene is interrupted by a small DNA sequence (0.8 kilobases) called 'transposon-like insertion', similar to Ac/Ds system in maize. Interruption of SBE I gene by insertion of a foreign sequence, gives an aberrant SBE I enzyme leading to metabolic disturbances in the biosynthesis of starch, lipid and protein. This results in an increase of free sugars due to failure of starch formation. This probably leads to higher osmotic pressure and hence higher water content and larger cell volume earlier observed in rr seeds. The seeds lose large proportion of their volume on maturation leading to shrinkage in volume. Since testa does not shrink with the cotyledons, seeds become wrinkled.

In most cases of the study of Mendelian traits, we seldom know the molecular basis, so that the students do not ask the question : how does a gene control the phenotype? With the availability of recent techniques of molecular biology and recombinant DNA, many traits are now being examined for the molecular basis of their development. This approach is receiving support from the recent emergence of 'reverse genetics' (from DNA to phenotype) as opposed, to forward or classical genetics (from phenotype to DNA).