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Materials and methods

Eight common wheat cultivars, six intervarietal chromosome substitution lines of cultivar Chinese Spring (CS) in which homoeologous group 5 chromosomes were substituted with those of Hope and Cheyenne, and their donor cultivars, as shown in Table1 were crossed with Aegilops squarrosa var. strangulata (strain No. KU 20-9) and rye, Secale cereale cv. Petkus under greenhouse and field conditions. Wheat cultivars were chosen from those shown to have high and low crossability with rye in our previous study (Koba and Shimada 1992). Normal line of Ae. squarrosa was used in the greenhouse, while in the field an early mutant genotype of Ae. squarrosa which was isolated by Dr. T. Makino, National Agricultural Research Center, Tsukuba, Japan, was used for the crosses, except for the cross with Hope, because of its coincidence of the flowering period with those of the wheat lines.

CS/Hope and CS/Cheyenne chromosome substitution lines, rye cv. Petkus, and normal and mutant lines of Ae. squarrosa were kindly provided by late Dr. E. R. Sears, U.S.A., Kihara Institute for Biological Research, Laboratory of Plant Breeding, Tottori University and Dr. T. Makino, respectively.

In order to adjust the flowering period for the crosses in the greenhouse, seedlings of wheat, rye and Ae. squarrosa were kept in cold chamber at 2oC for at least one month for vernalization, then transferred periodically into the greenhouse and grown under long day photoperiod (15h) condition at 15oC and 25oC at night and day, respectively. Twenty to twenty-four florets of the first and second florets in a wheat spike were emasuculated two or three days prior to anthesis. Simultaneously, only for the crosses with Ae. squarrosa, a 100 mg/l 2,4-D was injected into upper one or two internodes of the wheat culms in order to accelerate the development of the hybrid embryos. The efficiency of treatment of 2,4-D for wide hybridization in common wheat was shown by Inagaki (1986) and Koba et al (1991). In the case of crosses with rye, fresh pollen grains were collected in a petri dish and a small brush was used for pollination. In the field, the same procedures were used for crossing.

At 15 to 20 days after pollination, all of the developed ovules were dissected and presence or absence of the hybrid embyros were examined in the case of common wheat x Ae. squarrosa, since the endosperm was not formed or incompletely formed and ovules were filled with liquid. Thus the crossability of this cross was estimated as the ratio of number of embryos formed to number of florets pollinated. In the case of common wheat x rye, number of seeds obtained in each cross was counted four weeks after the crosses and the ratios to number of florets pollinated were calculated. Statistical analyses were carried out by using transformed values to angles of the percentages in individual spikes.


Results

Crossabilities of eight wheat cultivars with Ae. squarrosa and rye under greenhouse and field conditions are shown in Table 1. Significant differences between wheat cultivars in crossabilities with both Ae. squarrosa and rye were observed after analysis of variance in which variation between spikes within cultivars was regarded as error (Table 2). Although a difference between the crossabilities in the greenhouse and in the field was observed in each wheat cultivar, similar clear differences were observed in both conditions in both crosses with Ae. squarrosa and rye. Six wheat cultivars except Aobakomugi and Nanbukomugi showed relatively higher crossabilities with both Ae. squarrosa and rye, while Aobakomugi and Nanbukornugi showed lower crossabilities with both Ae. squarrosa and rye, although Aobakomugi showed rather higher crossability with Ae. squarrosa in the greenhouse than that in the field. These data indicate that crossabilities of common wheat cultivars with Ae. squarrosa and rye are controlled by the same genetic factor(s).

Six chromosome substitution lines of Chinese Spring in which a pair of each homoeologous group 5 chromosomes are substituted with their respective homologous chromosomes of cultivars Hope and Cheyenne, and their two donor cultivars were crossed with Ae. squarrosa and rye in the field (lower part in Table 1). In both crosses, significant differences were observed between the lines including Chinese Spring (lower part in Table 2). Donor cultivars of the substitution lines of homoeologous group 5 chromosomes, Hope and Cheyenne, showed significantly lower crossabilities than that of Chinese Spring with both Ae. squarrosa and rye, Hope being lower than Cheyenne, showing that Hope and Cheyenne possess genetic factors suppressing crossability not only with rye but also with Ae. squarrosa.

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