Frame-shift mutation of InCO might cause early flowering of Japanese morning glory and might have contributed to northward expansion

Japanese morning glory (Ipomoea nil), a short day plant, has been used for studying flowering times. Here, quantitative trait loci (QTL) analysis for days from sowing to flowering (DTF) of F2 between I. nil var. Tokyo Kokei Standard (TKS) and I. hederacea line var. Q65, an early flowering variety, revealed four QTLs: QTL Ipomoea Flowering 1-4 (qIF1-4). The position of qIF3, which had the most significant effect among the four QTLs, corresponds with that of I. nil (or I. hederacea) CONSTANS (InCO/IhCO) in the linkage map. There is a single-base In/Del in the coding sequence of InCO/IhCO. The single-base deletion (SBD) causes a frame-shift mutation and loss of function in TKS allele (inco-1). I. nil accessions bearing inco-1 tend to flower early, similarly to rice varieties bearing the loss of function allele of CO ortholog, hd1. Consequently, inco-1 was inferred to reduce DTF. This inferred effect of inco-1 corresponds with the effect of the TKS allele of qIF3. Because inco-1 is found exclusively in Asian accessions, the SBD in inco-1 might have played an important role in the expansion of Japanese morning glories, originally native to the tropical regions of the Americas, into temperate Asia.