Fine Mapping of a Soybean Iron Deficiency Chlorosis Tolerance QTL from Fiskeby III.

Soybean (Glycine max) is particularly susceptible to iron deficiency chlorosis (IDC) when grown in high alkaline soils.   IDC tolerance historically has been difficult to achieve in the narrow U.S. breeding base.  'Fiskeby III' (PI 438471), a 00 maturity group Swedish edamame variety released in 1949, shows exceptional tolerance to a wide range of abiotic stresses, including IDC.  To investigate the abiotic stress tolerance of 'Fiskeby III', the USDA-ARS developed a 'Fiskeby III' x 'Mandarin (Ottawa)' (PI 548379) bi-parental mapping population, which was previously used to map stress tolerance QTL.  Major IDC tolerance QTL were discovered on chromosomes 5 and 6 in this population.  The QTL on chromosome 5 explained 17% of the phenotypic variation for IDC tolerance in the population.  Near isogenic lines (NILs) were developed from 20 heterogenous inbred families (HIFs) from the 'Fiskeby III' x 'Mandarin (Ottawa)' bi-parental mapping population. F6 families that were heterozygous in the 1.5 mega-base chromosome 5 QTL were advanced.  F7:8 plants that were homozygous for the 'Fiskeby III' or 'Mandarin (Ottawa)' genotype in the region were planted in an IDC nursery in 2015 to validate the QTL.  Plants homozygous for the 'Fiskeby III' genotype were significantly more tolerant than plants homozygous for the ‘Mandarin (Ottawa)’ genotype (p < 0.001).  Future directions include the development of analogous NIL pairs for the chromosome 6 QTL, and the identification of new recombinants for higher resolution of the chromosome 5 QTL, and backcrossing these traits into elite germplasm.