Results and Discussion

Fertility and seed shriveling of alloplasmic aneuploids carrying Ae. uniaristata cytoplasm

By crosses between (uniaristata)-CS as female and nulli-tetrasomic 1B-1D, 1D-1A and 1D-1B, alloplasmic mono-trisomic 1B-1D, 1D-1A and 1D-1B were produced. These monotrisomics were self-pollinated or crossed as female to normal CS and ditelocentric 1BL, 1BS, 1DL and 1DS, and the various alloplasmic aneuploid lines listed in Table 1 were obtained. Chromosome constitution, meiotic chromosome configuration, and pollen and selfed seed fertility of the alloplasmic aneuploid lines are shown in Table 1.

The alloplasmic aneuploid lines having the short arm of chromosome 1B in the hemizygous state, i.e., mono-trisomic 1B-1D and monotelodisomic 1BL, show low pollen and selfed seed fertility (Table 1). This finding supports a previous report (MUKAI 1983) that a major fertility-restoring gene, Rfun1, is located on the short arm of chromosome 1B. The lines having the long arm of chromosome 1D in the hemizygous state also show low pollen and seed fertility except selfed seed fertility of mono-trisomic 1D-1B and monotelodisomic 1DS. The remarkable point for these lines, is the production of many shriveled seeds by selfing (Table 1). On the contrary, no shriveled seed was produced by the hemizygotes of 1BS aas well as 1BL or 1DS. These findings indicate that a gene(s) controlling seed shriveling is located on the long arm of chromosome 1D.

Chromosome constitution of the plants derived from the shriveled seeds

Table 2 shows the chromosome constitution of plants derived from the shriveled and plump seeds obtained by the self-pollination of alloplasmic monosomic 1D and monotelodisomic 1DS. The chromosome constitution is closely related to the seed morphology : All the plants derived from the shriveled seeds except one lacked a long arm of chromosome 1D, i.e., they were monosomic 1D or monotelodisomic 1DS, whereas all plants from plump seeds except one had normal 42 chromosomes. This indicates that female gametes lacking chromosome arm 1DL produce shriveled seeds after fertilization. In other words, the long arm of chromosome 1D has a gene(s) that suppresses gametophytically seed shriveling caused by the Ae. uniaristata cytoplasm. Shriveling of the seed from which the former exceptional plant having 42 chromosomes was obtained should have been due to some other cause than the deletion of 1DL arm. The latter exceptional plant having 41 chromosome which was produced from a plump seed seems to have a shifted monosome.

No plants lacking a pair of chromosome arm 1DL, i.e., nullisomic 1Dor ditelocentric 1DS were obtained. This suggests that pollen grains lacking 1DL become sterile. Low pollen fartility of the hemizygotes for chromosome arm 1DL supports it. Occurrence of monosomics in the progeny of the monotelodisomic 1DS must be attributed to univalent elimination following the partial asynapsis between chromosome 1D and telosome 1DS.

The present seed shriveling manifested by the combination of Ae. uniaristata cytoplasm and absence of chromosome 1D can be used for screening monosomic 1D without cytological observation.

Relationship between Rfunl for fertility restoration on chromosome 1B and suppressor for seed shriveling on chromosome 1D

MAAN (1978) reported that an Ae. uniaristata chromosome was transmitted preferentially through recurrent backcrosses which were made in order to produce alloplasmic durum wheat having Ae. uniaristata cytoplasm, because the seeds not carrying this chromosome (2n=28) became shriveled and inviable. The shriveled seeds in the present study had viability in contrast to MAAN's results because they have a long arm of chromosome arm 1D transmitted from the pollen.

The short arm of chromosome 1B where Rfun1 is located has fertility-restoring genes for many other cytoplasmr : Rf3, Rfu1, Rfv1, Rfo2 and Rfm1 for G, C^u, S^v, M^o and Mt type cytoplasms, respectively (TAHIR & TSUNEWAKI 1969 ; TSUNEWAKI 1974 ; MUKAI & TSUNEWAKI 1979 ; TSUNEWAKI 1982 ; TSUJIMOTO & TSUNEWAKI 1984). The long arm of crromosome 1D has a gene for microgametophytic viability against the Ae. squarrosa cytoplasm, which was designated Mgv by TSUJI & KOBA (1983). Combining this and the above-mentioned MAAN's (1978) results, seed shriveling seems to be caused by the absence of Mgv in the egg cell having the Ae. uniaristata cytoplasm.

The results of isozyme and seed protein studies revealed that the short arm of chromosome 1B is homoeologous to the short arm of chromosome 1D (PAYNE et al. 1982 ; CHOJECKI & GALE 1982). Consequently, Rf genes on chromosome arm 1BS and Mgv on 1DL can not be homoeologous with each other although both suppress the inviablity caused by some Aegilops cytoplasms. Hence, it is reasonable to distinguish the Mgv gene from Rf genes.