No abstract

The existence of a sorghum (Sorghum bicolor Moench) thermo-sensitivity gene has been proposed to account for the acceleration of flower initiation when plants are exposed to a temperature lower than 20°C. Four varieties and their F 1 , F 2 , F 3 and BC 1 F 1 populations were evaluated for days to emergence of flag leaf (DEFL) under the field condition of a long daylength (> 13 h) and temperatures over 20°C for at least 40 days after sowing. The F 1 hybrids, Hiromidori (Norin Ko-2) : 390 (early) × Regs.Hegari (late) and Natsuibuki (Norin Ko-9) : MS175 (late) × Daikoukaku (early), showed the same flowering response as Regs.Hegari and MS175 which show a delay in DEFL under temperatures over 20°C, suggesting the presence of a dominant thermo-sensitivity gene. In two different F 2 populations, the segregation of early and late plants for DEFL fitted in with the expected ratio of 1 : 3, indicating that a single dominant gene was controlling the phenotype. The inheritance mode was confirmed in two different BC 1 F 1 populations, where the expected ratio of 1 : 1 (early : late) was observed in the segregation of BC 1 F 1 of 390 × F 1 and F 1 × Daikoukaku. As expected, no segregation was observed in BC 1 F 1 populations of F 1 × Regs.Hegari and MS175 × F 1 . These results suggest that the sorghum thermo-sensitivity for flower initiation is controlled by a monogenic dominant gene of late over early, and the symbol TT is assigned to the genotype in which flower initiation is accelerated by the exposure to a temperature lower than 20°C.

The existence of a sorghum ( Sorghum bicolor Moench) photoperiod sensitivity gene has been proposed to account for postponing flower initiation until Tentaka and Kazetachi meet with short daylengths below 12.5 h. To investigate the inheritance of this gene, three parental varieties, populations of Tentaka and Kazetachi were evaluated for days-to-emergence of flag leaf (DEFL) under the field condition of long daylength (>13 h) and minimum temperatures over 20 ° C for at least 51 days after sowing in 1999, 2000 and 2001. The F 1 hybrids, Tentaka (MS79 × Chohin232) and Kazetachi (MS138 × Chohin232), showed strong photoperiod sensitivity and very late DEFL, in spite of the photoperiod insensitivity of their parental varieties. In two different F 2 populations, the segregation of early, middle and late plants for DEFL fitted in with the expected ratio of 7 : 30 : 27 under the three genes hypothesis, TtD 1 d 1 D 2 d 2 for Tentaka and Kazetachi. Additionally, the expected ratio of early : middle : late (1 : 2 : 1) was observed in the segregation of BC 1 F 1 of Tentaka × Chohin232 and Kazetachi × Chohin232 in 2000. These results suggest that two photoperiod sensitivity genes, D 1 and D 2 which show complementary dominant effects, are existent and the flowering phenotype of Tentaka and Kazetachi are expressed by the interactive operation both of a dominant thermo-sensitivity gene ( T ) and complementary dominant photoperiod sensitivity genes ( D 1 , D 2 ), that is, the genotype TtD 1 d 1 D 2 d 2 .

Hybrid lethality, a postzygotic mechanism of reproductive isolation, is a phenomenon that causes the death of F1 hybrid seedlings. Hybrid lethality is generally caused by the epistatic interaction of two or more loci. In the genus Nicotiana, N. debneyi has the dominant allele Hla1-1 at the HLA1 locus that causes hybrid lethality in F1 hybrid seedlings by interaction with N. tabacum allele(s). Here, we mapped the HLA1 locus using the F2 population segregating for the Hla1-1 allele derived from the interspecific cross between N. debneyi and N. fragrans. To map HLA1, several DNA markers including random amplified polymorphic DNA, amplified fragment length polymorphism, and simple sequence repeat markers, were used. Additionally, DNA markers were developed based on disease resistance gene homologs identified from the genome sequence of N. benthamiana. Linkage analysis revealed that HLA1 was located between two cleaved amplified polymorphic sequence markers Nb14-CAPS and NbRGH1-CAPS at a distance of 10.8 and 10.9 cM, respectively. The distance between these markers was equivalent to a 682 kb interval in the genome sequence of N. benthamiana.

This study aimed to clarify the flowering habit in sorghum varieties with a dominant thermosensitivity gene. The number of days from sowing to floral initiation in four varieties with a dominant thermosensitivity gene (TT and Tt) and two varieties of which flower initiation does not depend strongly on day‐length and night temperatures were evaluated under different day‐length and temperature conditions. The day‐length treatment at 25°C indicated that anything over a 12.5‐h day‐length has a long day effect on the flowering of the sorghum varieties examined. Under a long day condition of a 12.5‐h day‐length, floral initiation was accelerated under 30°C/20°C (day/night) conditions, compared to 25°C (constant) conditions in the varieties with a dominant thermosensitivity gene. Under a 15‐h day‐length, floral initiation was accelerated at night temperatures of 20°C and 15°C instead of at the constant 25°C. These results suggest that floral initiation was accelerated by a night temperature of lower than 20°C, even under long day‐length conditions in sorghum varieties with a dominant thermosensitivity gene.