Clustering among loci underlying soybean resistance to Fusarium solani, SDS and SCN in near-isogenic lines

In the soybean [Glycine max (L.) Merr.] cultivar 'Forrest' a single chromos omal region underlies coinheritance of field resistance of the sudden-death syn drome (SDS), caused by the fungus Fusarium solani (Mart.) Sacc. f. sp. glycines (Burk.) Snyd, & Hans. and soybean cyst nematode (SCN) race 3 (cau sed by Heterodera glycines Ichinohe). Our objectives were to verify that co -inheritance was derived from a single chromosomal region in near-isogenic lines and to separate component gene clusters. DNA markers were compared wi th a SDS leaf-scorch index (DX), F. solani root-infection severity (IS) and a SCN index of parasitism (IP) among 80 near-isogenic lines (NILs). The ge nomic region identified by the RFLP marker Bng122D was strongly associated (0.0004 less than or equal to P less than or equal to 0.006) with mean SDS DX (R-2 > 16-38%) and IS (R-2 > 38-73%), but only marginally associated wit h resistance to SCN, However, the linked (4.3-7.4 cM) microsatellite marker SATT309 was strongly associated with both resistance to SCN (0.0001 less t han or equal to P less than or equal to 0.0003; R-2 > 24-97%) and mean leaf DX (0.0001 less than or equal to P less than or equal to 0.0003; R-2 > 25- 63%), but not root IS. Recombination events among markers and traits enable d separation of the qualitative loci underlying resistance to SDS and SCN. Our data showed that resistance to SDS DX, SDS IS and SCN IP in Forrest may be caused by four genes in a cluster with two pairs in close linkage or by a two-gene cluster with each gene displaying pleiotropy, one conditioning SDS IS and DX and the other SCN LP and SDS DX.