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Anne M. Gillen and Fred A. Bliss

An F2 population from a single F1 plant from the cross of peach [Prunus persica (L.) Batsch] rootstock cultivars Harrow Blood (HB) × 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 L. 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 × Oki F2 population and a peach × 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.

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Anne M. Gillen and Fredrick A. Bliss

Peach rootstock breeding may be accelerated by utilization of molecular markers linked to the root-knot nematode resistance locus (Mi) to screen segregating populations. A genetic linkage map was constructed using RFLP markers in an F2 population (PMP2) that is segregating for this locus. PMP2 is derived from a controlled cross of the relatively diverse peach rootstocks Harrow Blood (susceptible) and Okinawa (homozygous resistant). Bulked Segregant Analysis was applied using RAPD markers. A single small (227 base pairs) RAPD marker was found to be linked to the dominant resistant allele of Mi at a distance of 10 cM. This new marker joined the Mi locus to the RFLP linkage map and showed that two dominant RFLP markers are located between the RAPD marker and Mi. RFLPS are expensive, time-consuming and RAPD markers are unreliable, and therefore both are unsuitable for screening breeding populations. We attempted to convert the RAPD marker to a more breeder-friendly CAPS marker. The converted CAP marker was dominant. Attempts to convert the CAP marker to a co-dominant marker were not successful. The utility of the CAP marker was tested in an open pollinated F2 population derived from the F1 parent of PMP2 and in several rootstocks. The genetic linkage map was compared to other Prunus maps. The PMP2 linkage group containing the Mi locus can be related to the peach × almond linkage group which contains the phosphoglucomutase Pgm-1 locus.