Genetic Analysis of Heading Time in a Cross between Two Indica Varieties with Inhibitor Genes for Photoperiod Sensitivity.

Abstract

The previous studies showed that an early Indica variety Culture 340 from India carries a gene for photoperiod sensitivity with large effects at the Se-I locus and a recessive inhibitor gene (i-Se-1) for sensitivity as well, causing early heading even under long day conditions (IMUTA et al. 1991). On the other hand, an Indica variety Nanjing 11 from China was also presumed to carry a recessive inhibitor gene for Se-1 gene (ARAKI 1988). The present study was undertaken to determine if these two Indica varieties have the same inhibitor gene for Se-1, by investigating the inheritance of heading time in the F1 and F2 populations of a cross between Culture 340 and Nanjing 11 at Tsukuba in 1990. The heading time of the F1 was later than that of midparent and showed partial dominance for lateness (Fig.1). In the F2 population, the variation of heading time showed a clear-cut bimodal distribution with two peaks corresponding to the heading time of parents and no transgressive segregation for late heading (Fig.1). These results indicated that Culture 340 and Nanjing 11 have the same inhibitor gene (i-Se-1). Assuming a class boundary at 96 days from sowing in the F2, the observed ratio showed a good fit to a 3 late: 1 early ratio, suggesting one dominant gene for lateness. The F3 lines selected from both the early and late F2 plants were grown under the 9-and 14-hour photoperiod treatments, to investigate their photoperiod sensitivities (Fig.2). The F3 lines selected from the late F2 plants showed a photoperiod sensitivity similar to that of Nanjing 11. The result showed that Nanjing 11 has a pair of photoperiod sensitive gene. The genetic analysis was performed to determine if this gene is at the Se-I locus, by using segregation of genotypes for phosphoglucose isomerase isozyme locus Pgi-2, which is closely linked with the Se-1 locus. Since the Pgi-2 alleles were not associated with the two classes of heading time (Fig.2), it was concluded that this gene is independent of Se-1, and the gene isn't suppressed by the inhibitor gene. This new photoperiod-sensitive gene was designated Se-4 (t).