Identification and mapping of markers linked to the Mi gene for root-knot nematode resistance in peach.

An F2 population from a single F1 plant from the cross of peach [Prunus persica (L.) Batsch] rootstock cultivars Harrow Blood ('HB') x Okinawa ('Oki') was used to locate the Mi locus, which conditions resistance to Meloidogyne incognita (race 1) (Kofoid and White) Chitwood. These data and comparison of common markers among published genetic linkage maps placed the Mi locus on Prunus linkage group 2. Two restriction fragment length polymorphisms ('RFLPs') [linked at 4.8 and 6.8 centimorgan ('cM'), repulsion phase] and one random amplified polymorphic DNA ('RAPD') marker (linked at 9.5 cM, coupling phase) were linked to Mi. The RAPD marker was cloned, sequenced and converted to a polymerase chain reaction ('PCR')-based cleaved amplified polymorphic sequence ('CAPs') marker. Clones of resistance gene analogs ('RGA') developed from Oki were highly polymorphic when used as RFLP probes. The RGA's mapped to four linkage groups but clustered on two of the four linkage groups, providing limited coverage of the genome. Even so, they may be useful as markers for disease resistance genes that occur in other populations. The linkage maps of the HB x Oki F2 population and a peach x almond (Prunus amygdalus Batsch) F2 population were colinear in certain regions, however a significant number of markers mapped to different linkage groups among the two populations. The locus for the blood-flesh trait (red-violet mesocarp) mapped to the top of linkage group 4.