RESISTANCE TO THE CEREAL CYST-NEMATODE (HETERODERA-AVENAE WOLL) TRANSFERRED FROM THE WILD GRASS AEGILOPS-VENTRICOSA TO HEXAPLOID WHEAT BY ASTEPPING-STONE PROCEDURE

Transfer of resistance to Heterodera avenae, the cereal cyst nematode (CCN), by a ''stepping-stone'' procedure from the wild grass Aegilops ventricosa to hexaploid wheat has been demonstrated. The number of nem atodes per plant was lower, and reached a plateau much earlier, in the resistant introgression line H93-8 (1-2 nematodes per plant) than in the recipient H10-15 wheat (14 16 nematodes per plant). Necrosis (hype rsensitive reaction) near the nematode, little cell fusion, and few, o ften degraded syncytia were observed in infested H93-8 roots, while ab undant, well-formed syncytia were present in the susceptible H10-15 wh eat. Line H93-8 was highly resistant to the two Spanish populations te sted, as well as the four French races (Fr1-Fr4), and the British path otype Ha11, but was susceptible to the Swedish pathotypes HgI and HgII I. Resistance was inherited as though determined by a single quasi-dom inant factor in the F2 generations resulting from crosses of H93-8 wit h H10-15 and with Loros, a resistant wheat carrying the gene Cre1 (syn . Ccn1). The resistance gene in H93-8 (Cre2 or Ccn2) is not allelic wi th respect to that in Loros. RFLPs and other markers, together with th e cytogenetical evidence, indicate that the Cre2 gene has been integra ted into a wheat chromosome without affecting its meiotic pairing abil ity. Introduction of Cre2 by backcrossing into a commercial wheat back groud increases grain yield when under challenge by the nematode and i s not detrimental in the absence of infestation.