A quantitative trait locus (QTL) controlling shoot differentiation from immature embryo callus was identified by linkage analysis with morphological and isozyme markers in barley, Hordeum vulgare L. Immature embryos were isolated from cvs 'Azumamugi' (difficult to differentiate), 'Kanto Nakate Gold' (easy to differentiate), their hybrids (F1) and a backcross population derived from a cross 'Azumamugi' x F1. The embryos were cultured in vitro for callus initiation and subsequent shoot differentiation. The shoot differentiation rate was closely associated with ear type (v locus), isocitrate dehydrogenase isozyme (Idh-2), and esterase isozyme (Est-11). These markers were found to reside in a chromosome segment of approximately 30cM on chromosome 2. Recombination frequency was 9.9% between v and a proposed QTL named "Shd1" (shoot differentiation), 11.5% between Idh-2 and Shd1, and 21.3% between Est-11 and Shd1. All data showed the Idh-2, v, Shdl and Est-11 loci to be arranged in this order from proximal to distal on the long arm of chromosome 2.

The locus Shd1, which we previously mapped to the long arm of chromosome 2 of Hordeum vulgare L., controls the differentiation of shoots from immature barley embryo callus. The locus has major effects and its action explains more than 65% of the total genetic variance in the shoot-differentiation rate. The allele of cultivar Kanto Nakate Gold designated Shd1K has a significant positive effect on the shoot-differentiation rate, whereas Shd1A of cultivar Azumamugi does not promote shoot differentiation. To identify gene products and characterize the function of Shd1, a set of near-isogenic lines is essential. In this study we produced BC5F1 plants by repeated backcrossing of 'Azumamugi' to F1 plants ('Azumamugi' x 'Kanto Nakate Gold'). The BC5F1 plants were examined for their RFLP genotype and for the shoot-differentiation ability of immature embryo-derived callus. The results indicated that the Shd1 locus was located in a chromosomal region between MWG2081 and MWG503 that flanks the MWG801, cMWG699, v (ear type), and MWG865 loci. Shd1K from 'Kanto Nakate Gold' functions effectively in the genetic background of 'Azumamugi', an indication that backcross breeding is possible for production of near-isogenic lines that would be very suitable for tissue culture.

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