D.W. Mornhinweg, D.R. Porter and J.A. Webster
USDA-ARS, Stillwater, OK
The entire USDA-ARS Small Grains Germplasm Collection of Hordeum vulgare has been screened for Russian wheat aphid (RWA) resistance. One hundred nine accessions from over 20 collection sites have been identified with some level of RWA resistance. Resistance has been found in both 2-row and 6-row types and both spring and winter types. Resistant lines have been developed from all 109 accessions with resistance varying from resistant (1-3 on Webster's scale of 1-9) to moderately resistant or moderately susceptible (4-6 on Webster's scale). Genetic studies to determine the inheritance of RWA resistance in each of the 109 lines is ongoing both to aid breeders in their use of released germplasm and to determine genetic diversity towards future release of barley germplasm lines with different genes for RWA resistance.
A complete inheritance study includes parents, F1, RF1, 150 BC to each parent, 200, F2 and 300 F2-derived F3 families. To date, nine of these studies have been completed and two interpreted resulting in the release of two germplasm lines, STARS-9301B and STARS-9577B, differing in inheritance for RWA resistance. Inheritance of RWA resistance in STARS-9301B was found to be controlled by recessive epistasis of a dominant gene on an incompletely dominant gene. Genetic control of RWA resistance in STARS-9577B was found to be controlled by two dominant genes with recessive epistasis. Genetic diversity studies to determine if these lines share a gene are ongoing.
Interpretation of genetic analysis on the other seven lines has not been completed, yet the number of homozygous resistant, homozygous intermediate and homozygous susceptible F2-derived F3 families for these lines indicate genetic differences. Studies to determine the mechanisms of resistance in each of these nine lines also indicate genetic differences in the lines. For some lines the major mechanism of resistance is tolerance, while for others it is antixenosis or antibiosis or combinations of the three.
Early indications show multiple gene control in all lines studied to date. The search for a germplasm line with simple inheritance is ongoing. Inheritance and genetic diversity studies will continue for the 109 RWA-resistant lines. Release of RWA-resistant barley germplasm lines differing in their source of resistance, mechanism of resistance, agronomic adaptation, and endproduct use will also continue.