Genetics of starch granule size distribution in tetraploid and hexaploid wheats

Wheat endosperm starch is deposited in large, A-type granules and smaller B-type and C-type granules. The quantitative genetics of starch granule size distribution was investigated in 2 ways. Complete diallel crosses, with 5 parents each, were prepared in tetraploid wheat, Triticum turgidum, and hexaploid wheat, Triticum aestivum. Parent and F1 plants were grown in controlled environment growth chambers with 18�C days and 13�C nights to provide parent and F2 grains from uniform conditions. In the same conditions, the basic generations of parent, F1, F2, and backcross of 6 other individual crosses were grown and F1 and backcross grains were freshly generated on these plants. Starch granule size distribution was determined in parent and F2 grains in the diallels and all possible generations in the other crosses. Granules of <10 μm diameter were considered 'B granules' (thus including C granules), and B-granule content was expressed as a percentage of total starch volume. The modal A-granule diameter was also determined.B-granule content varied widely in both species, whereas modal A-granule diameter was much more variable in tetraploids than in hexaploids. Additive gene action was more important than dominance in determining B-granule content in both species and A-granule diameter in tetraploids, whereas dominance was more important for A-granule diameter in hexaploids. Dominance acted to increase or to decrease B-granule content, depending on the cross. According to variance–covariance analysis, the line with the most dominant alleles in the hexaploids was the one lowest in B granules, but in the tetraploids it was the one highest in B granules. Digenic interactions affected B-granule content and A-granule diameter in all of the analyses of the basic generations, and nucleocytoplasmic interactions affected these traits in most of the crosses. Diallel analyses of the F2 generations, in contrast, showed a limited importance of epistasis. Cytoplasmic effects made small but significant contributions to the variation in B-granule content in some of the crosses. Variation in B-granule content and A-granule diameter appeared to be affected by different gene actions and were therefore likely to be susceptible to independent manipulations in a breeding program.