Linkage of an Alternaria disease resistance gene in mandarin hybrids with RAPD fragments

The inheritance of resistance to a host-specific isolate (Shinn) of Alternaria alternata (Fr.:Fr.) Keissl. from 'Minneola' tangelo (across between Citrus paradisi Macf. 'Duncan' and C. reticulata Blanco 'Dancy') was shown to be controlled by a single recessive allele, aaM1, within the citrus genome. A backcross between resistant 'Clementine' mandarin (C. reticulata) and susceptible LB#8-10 (a hybrid of 'Clementine' mandarin and 'Minneola' tangelo) resulted in 61 resistant (R) and 58 susceptible (S) plants (chi(2) = 0.0756, P >= 0.05), but the reciprocal cross deviated from the expected 1R: 1S ratio (87 R and 36 S plants (chi(2) = 21.1463, P >= 0.05). A dominant allele, AaM1, of this resistance gene was found in a loose coupling phase linkage with two RAPD markers, P12(850) (15.3 cM) and AL3(1250) (36.7 cM), after JOINMAP computer analysis.