Construction of high-density genetic maps and detection of QTLs associated with Huanglongbing infection in citrus

Abstract

No true resistance to Huanglongbing (HLB), a citrus disease associated with infection of Candidatus Liberibacter asiaticus (CLas), is found within commercial citrus cultivars, though trifoliate orange ( Poncirus trifoliata ) has been described as resistant or tolerant. Through genotyping an intergeneric F ₁ population by Genotyping-by-Sequencing, high-density SNP-based genetic maps were constructed separately for trifoliate orange and sweet orange ( Citrus sinensis ). Both genetic maps exhibited high synteny and high coverage of citrus genome. After exposure to intense HLB pressure for two years, Ct value of qPCR for CLas detection in leaves throughout ten time points during the next three years was above 35 in trifoliate oranges, under 28 in sweet oranges, and ranged from 24 to 38 and exhibited obvious segregation among progenies. Phenotypic data of percentage of healthy trees showed high correlation with the Ct value. By mapping the two traits at all time points, a total of nine clusters of QTLs were detected, of which five, respectively located on LG-t7 and LG-t8 of trifoliate orange map and LG-s3, LG-s5 and LG-s9 of sweet orange map, collectively explained a major part of the phenotypic variation. This study provides a starting point for citrus breeding to support long-term control of this devastating disease.