A hybridization barrier leads to the inability of seed formation after intergeneric crossings between Brassica rapa and Raphanus sativus. Most B. rapa lines cannot set intergeneric hybrid seeds because of embryo breakdown, but a B. rapa line obtained from turnip cultivar 'Shogoin-kabu' is able to produce a large number of hybrid seeds as a maternal parent by crossings with R. sativus. In 'Shogoin-kabu' crossed with R. sativus, developments of embryos and endosperms were slower than those in intraspecific crossings, but some of them grew to mature seeds without embryo breakdown. Intergeneric hybrid seeds were obtained in a 'Shogoin-kabu' line at a rate of 0. 13 per pollinated flower, while no hybrid seeds were obtained in a line developed from Chinese cabbage cultivar 'Chiifu'. F1 hybrid plants between the lines of 'Shogoin-kabu' and 'Chiifu' set a larger number of hybrid seeds per flower, 0. 68, than both the parental lines. Quantitative trait loci (QTLs) for hybrid seed formation were analyzed after intergeneric crossings using two different F2 populations derived from the F1 hybrids, and three QTLs with significant logarithm of odds scores were detected. Among them, two QTLs, i. e., one in linkage group A10 and the other in linkage group A01, were detected in both the F2 populations. These two QTLs had contrary effects on the number of hybrid seeds. Epistatic interaction between these two QTLs was revealed. Possible candidate genes controlling hybrid seed formation ability in QTL regions were inferred using the published B. rapa genome sequences.
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