FISH to meiotic pachytene chromosomes of tomato locates the root knot nematode resistance gene Mi-1 and the acid phosphatase gene Aps-1 near the junction of euchromatin and pericentromeric heterochromatin of chromosome arms 6S and 6L, respectively

The root-knot nematode resistance gene Mi-1 in tomato has long been thought to be located in the pericentromeric heterochromatin region of the long ar m of chromosome 6 because of its very tight genetic linkage (approx. 1 cM) to the markers Aps-1 (Acid phosphatase 1) and yv (yellow virescent). Using Mi-BAC clones and an Aps-1 YAC clone in fluorescence in situ hybridisation (FISH) to pachytene chromosomes we now provide direct physical evidence sho wing that Mi-1 is located at the border of the euchromatin and heterochroma tin regions in the short arm (6S) and Aps-1 in the pericentromeric heteroch romatin of the long arm (6L) close to the euchromatin. Taking into account both the estimated DNA content of hetero- and euchromatin regions and the c ompactness of the tomato chromosomes at pachytene (2 Mb/mu m), our data sug gest that Mi-1 and Aps-1 are at least 40 Mb apart, a base pair-to-centiMorg an relationship that is more than 50-fold higher than the average value of 750 kb/cM of the tomato genome. An integrated cytogenetic-molecular map of chromosome 6 is presented that provides a framework for physical mapping.