A combined RFLP and AFLP linkage map of upland rice (Oryza sativa L.) usedto identify QTLs for root-penetration ability

A combined RFLP and AFLP linkage map of an F-6 recombinant inbred populatio n, which was derived from a previously mapped F-2 of a cross between the tw o drought resistant upland rice varieties Bala and Azucena, is presented. T he map contains 101 RFLP and 34 AFLP markers on 17 linkage groups covering 1680 cM. Also presented is the approximate mapping position of a further fo ur RFLP and 75 AFLP markers, which either could not be given a unique place on the map or for which the available data is not sufficient to allow conf ident positioning, and the result of quantitative trait locus (QTL) mapping of traits related to root-penetration ability. Root penetration was assess ed by counting the number of root axes that penetrated a 3 mm-thick layer c onsisting of 80% wax and 20% white soft paraffin. Good root penetration wou ld be expected to increase drought resistance where soil strength is high. Single-marker analysis revealed seven QTLs for the number of roots which pe netrate the wax layer. In identical locations were seven QTLs for the ratio of penetrated to the total number of roots. Transgressive inheritance of p ositive alleles from Bala explained four of these QTLs. Comparison of the Q TLs identified here with previous reports of QTLs for root morphology sugge st that alleles which improve root penetration ability may also either make the roots longer or thicker.