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Materials and methods

Triple Dirk (B), Pugsley's near-isogenic line carrying Vrn2, was crossed with a set of monosomic lines of a winter type wheat cultivar Dwarf A which was kindly supplied by Dr. A. J. Worland, John Innes Centre, UK. Monosomic F₁ plants were selfed for each cross combination. F₂populations were sown on 18th September and grown in a glasshouse of Kochi University, Japan, under a regime of 24 hour day length without heating. They were sown in soil-filled containers at a spacing of 2 cm (between hills) x 3.5 cm (between rows). The number of days from sowing to flag leaf unfolding or the number of vegetative rosette segregants was determined. Plants successfully unfolded flag leaf were regarded as spring type, and those remained vegetative rosette as winter type.

Results and discussion

Frequency distribution of the number of days from sowing to flag leaf unfolding was shown in Fig.1, for F₂populations of two cross combinations between Triple Dirk (B) and monosomics 5B^L-7B^L (Fig. l a) and 5B^s-7B^s (Fig. 1b). Days from sowing to flag leaf unfolding ranged from 45 to 136 in both populations, and these segregants were regarded as spring type which carried Vrn2 in homozygous, heterozygous or hemizygous condition. On the contrary, eight and thirty-four plants did not unfold flag leaf within 150 days after sowing, and they were regarded as winter type.

Segregation ratio of spring and winter type plants fitted well to an expected ratio of 3:1 in most cross combinations, though monosomics for 2A and 3A were not tested (Table 1). Significantly deviated segregation ratio was observed in the cross with monosomics 7A and 5B^L-7B^L P<0.01). However, the deviation was caused by over segregation of winter type in the former. Segregation of winter type was significantly less than expected, 8/107, and thus was concluded that Vrn2 should be located on 5B^L-7B^L chromosome.

Reciprocal translocation between 5B and 7B chromosome is known in European wheat cultivars including Dwarf A (Lange et al. 1987), and thus monosomic lines of Chinese Spring should be preferable for the analysis of genes on groups 5 and 7 chromosomes. However, in F₂ population of the cross combinations between CS monosomics and Vrn2 or Vrn4 carrier, frequency of winter type segregants will be 1/16 in non- critical cross and a few percent in critical cross, respectively. Accordingly it is difficult to distinguish the two types of segregation ratio. From these reasons, monosomic lines of Dwarf A were used in the present study, and Vrn2 was successfully identified on either 5B^Lor 7B^L .

For vernalization response, five genes are known in common wheat, three genes in barley, and one gene in rye, respectively. Synteny of group 5 chromosomes is well known among these crop species, and Vrn genes on 5A, 5D (wheat), 5H (barley), and 5R (rye) are regarded as orthologous gene of Vrn-1 (Plaschke et al. 1993; Nelson et al. 1995; Laurie et al. 1995). Additional recessive gene for spring growth habit has been located on 5A chromosome of T. monococcum, and proved to be on translocated segment from 4AL (Dubcovsky et al. 1998). Its synteny with barley sh gene on 4H chromosome was confirmed, and these two genes are regarded as orthologous gene of Vrn-2. Besides these two orthologous genes, Sh3 on 1H (Takahashi and Yasuda 1971) and Vrn5 on 7B^s (Law 1966) have been reported. However, no vernalization response gene have been located on 7B^L . It was therefore suggested that Vrn2 should be equivalent to Vrn-B1 located on 5B^L. Further analysis is now going on to locate Vrn2 on RFLP map of 5B^L, by converting RFLP markers to PCR markers.

Most of attentions have been so far paid to orthologous genes of Vrn-1 on group 5 chromosomes, since orthologous genes corresponding to sh (4H) and Sh3 (1H) are not known in common wheat. However, as mentioned above, the second gene corresponding to sh has been identified in T. monococcum (Dubcovsky et al. 1998), and digenic segregation of growth habit has been reported in Aegilops tauschii (Goncharov and Chikida 1995). Even in common wheat, local landraces carrying Vrn gene(s), other than Vrn-A1, Vrn-B1, Vrn-D1 and Vrn4, have been identified in various areas (Iwaki and Kato 1998). Detailed analysis of these landraces might lead to the identification of orthologous genes, Vrn-3 and Vrn-2, on groups 1 and 4 chromosome.

Acknowledgments

We would like to thank Dr. A. J. Worland , John Innes Centre, UK, for kindly supplying the seeds of monosomic lines of Dwarf A.

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