Backcrossing of nematode-resistant sugar beet: a second nematode resistance gene at the locus containing Hs1pro-1?

Abstract

Beet cyst nematode-resistant sugar beet plants, containing the Hs1pro-1 locus from Beta procumbens, show a female transmission frequency of the resistance of ca. 90%. Such plants often suffer from tumour formation on leaves and root systems, and from the occurrence of a so-called ‘multi-top’ phenotype. With the aim of obtaining resistant sugar beet material lacking these negative traits, nematode-resistant plants with a reduced size of the chromosome segment of the wild beet that carries the Hs1pro-1 gene were selected from backcrosses between the resistant stocks B883 or AN1-65-2 and susceptible sugar beet (Beta vulgaris). Analysis of such plants, referred to as Sat-minus plants, showed that the transmission frequency of the resistance to subsequent generations had dropped dramatically to ca. 0.5%. The multi-top phenotype was still present in the newly selected material, indicating that improvement of the resistant sugar beet material by further backcrossing will be hard to achieve. Two of the selected resistant offspring plants were analysed at the molecular level. With the aid of AFLP markers it was found that the size of the alien chromosome segment had decreased to 35% and 17% of the original size, respectively. Surprisingly, both plants had lost the Hs1pro-1 nematode resistance gene that recently was isolated from the original introgression material. This shows that more than one gene conferring resistance must be present in the locus in B883 and AN1-65-2 carrying the resistance gene Hs1pro-1.