Mapping of a major locus involved in shoot growth habit in hexaploid sweetpotato using bulked-segregant analysis

The traits of shoot growth habit differ between sweetpotato (Ipomoea batatas) and its wild ancestor (Ipomoea trifida). In general, sweetpotatoes have thick stems without twining, while I. trifida have slender twining stems. Anatomical observation in this study showed that this difference is caused by the difference in the size and number of cells between the stems of sweetpotato and those of I. trifida. To reveal the genetic basis of the difference in shoot phenotype, F₁ progeny were produced by crossing sweetpotato (Konaishin) and I. trifida (K123-11), and the G-statistic method of bulked-segregant analysis was used to investigate stem-twining ability as a representative trait of shoot growth habit. As a result, a major quantitative trait locus (qSgh) related to shoot growth was successfully detected at 12.37–14.12 Mb in Chr13 of the reference genome. Genotyping F₁ individuals using a PCR-based SNP marker designed for qSgh supported the results of bulked-segregant analysis and further suggested that qSgh had a dosage effect on stem diameter. Based on these results, we propose that the G-statistic method is an effective approach for bulked-segregant analysis in polyploid species, including sweetpotato. Additionally, some candidate genes in qSgh were found by comparative analysis of the genome and transcriptome between sweetpotato and I. trifida. At least two of these, Iba_chr13aCG7290 and Iba_chr13cCG9960, are likely involved in radial growth of the stem in sweetpotato. The results of this study provide new insight into the transition of shoot phenotype from I. trifida to sweetpotato.