Genetics of starch granule size distribution in tetraploid and hexaploid wheats

Abstract

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.