The sdw gene (Rasmusson, 1988) which confers prostrate habit and short straw has been extensively used by spring barley breeders in Europe, and increasingly in North America, to improve lodging resistance. It is a simple recessive gene, but its chromosome location is not known. The stock designated as carrying sdw is the six-rowed line 'Jotun'. This was crossed with a European cultivar 'Blenheim' to check that the semidwarf character in the two is allelic. F₁ plants all had prostrate habit, confirming that European prostrate cultivars carry sdw.

Observations in breeding material suggested that it might be linked with the rachilla hair gene, Ss, on chromosome 7. Two multiple marker stocks for chromosome 7 were obtained from the Barley Genetic Stock Center at Fort Collins (BGN 19:80-81). These were said to contain the following genes on chromosome 7:

The first problem encountered was that both stocks in fact had long-haired rachillas. A search was therefore made for breeding lines having short-haired rachillas as well as the semidwarf gene. A suitable line was found, and crossed to both marker stocks. F₂ populations from both crosses were grown in the field in 1992.

The second problem was that the mt2 gene was expressed when the parent stock was grown in a warm glasshouse, but was not visible in field conditions. Since the sdw gene is expressed in the field but not in the glasshouse, it was not possible to score the F₂'s for mt2.

The third problem encountered was that the va gene reduced plant vigor. Some vava homozygotes failed to survive, those that did were slow growing and remained vegetative for longer than normal. Their phenotype thus resembled the prostrate character of sdw plants, and it was impossible to score vava plants for the sdw gene. The cross involving Marker Stock 1 therefore had to be abandoned. It may, however, be noted that the green plants in this cross, i.e., those not homozygous for va, segregated 124 normal:33 sdw, which is not significantly different from a 3:1 ratio.

The cross involving Marker Stock 2 was scored for the genes sdw, r, s, and lb. Data for r, s, and sdw are presented in Table 1. The data show a slight deficit of homozygous rr genotypes. Chi-squared tests gave significant deviations from expected for Rr segregation and independence of r and s.

The results clearly show that, contrary to expectation, sdw is not linked with r or s. The maximum likelihood estimate for the recombination frequency between r and s on chromosome 7 is 29.5 ± 5.7%.

Classification for Lb/lb was felt to be unreliable, as the basal rachis internode was sometimes of intermediate length, and often varied from tiller to tiller on the same plant. Data for the segregation of this gene are, therefore, not presented, although theyappeared to confirm its linkage with the smooth awn gene r and its independence from sdw. Of the variegated plants which survived, most were of smooth awn phenotype, confirming the close linkage of r with va as well as lb on chromosome 7.

In addition to the marker genes for chromosome 7, Marker Stock 2 also carried the w gene for six-rowed ears on chromosome 2, enabling classification of the F₂'s for Vv segregation, with heterozygotes being recognizable as a distinct class (Table 2). Marker Stock 2 was also heterozygous for the dominant gene for black lemmas on chromosome 5. Some of the F₁ plants were black, and F₂'s from these plants were classified for this gene, Bb, (Table 3).

The maximum likelihood estimate for the recombination frequency between b and sdwis 36.0% ± 6.9%. In view of the rather small number of individuals, this evidence of linkage placing sdwon chromosome 5 should be regarded with caution.

Reference:

Rasmusson, D. C. 1988. BGS 468: Semidwarf sdw. BGN 18:87.