An apomict is valuable plant breeding material, but obligate apornictic mutants have not yet been obtained in barley. Even if an apomict is induced, its genetic behavior cannot be distinguished from the self-propagation of a mutant with other genetic causes showing a similar behavior of inheritance. Thus, we propose an improved method to select for obligate apomictic mutants.

The male-sterile line produced by Falk and Kasha (1982) is useful for detecting apomixis. The male sterile gene (msg6) of this line is closely linked to the shrunken endosperm gene (sex1). If obligate apomixis is induced in male-sterile plants, seed fertility of the malesterile plants is extremely high and uniform shrunken seeds are produced on the male-sterile plants.

Air-dried shrunken seeds of the male sterile line were exposed to gamma rays (30kR) and an open-pollinated progeny test was carried out. Three plants suspected of reproducing by apomixis were obtained frorn 137 MI plants. Seed fertilities and shrunken seed frequencies were recorded by row, because chimeras are known to occur by row in plants treated with a mutagen (Makino et al., 1993). Some rows showed high seed fertility and set uniform shrunken seeds. One six-rowed MI plant showed high seed fertility (46.3%) and a high percentage (12.5%) of rows with uniform shrunken seeds. However, uniform shrunken seeds on MI rows do not always indicate apomixis. There is also a possibility that the uniform shrunken seeds result from a fertility-restoring mutation at the gene msg6.

Uniform progeny from a heterozygous plant is one of the best indicators of obligate apomixis (Hanna, 1991). Air-dried seeds of heterozygotes for sex1-msg6 were exposed to gamma rays (30kR) and a progeny test was carried out. The best seed-setting spike was picked from every plant and the shrunken-seed frequency was scored by row. Some rows (19.1%) were suspected of reproducing by apomixis based on absence of shrunken seeds. But the segregation rate of a single recessive character (25%) is too low for obligate apomixis to be definitively confirmed in a row composed of 10-15 spikelets.

An increased probability of detecting the homozygous genotype is needed to confirm the presence of obligate apomixis. For this purpose, we considered developing heterozygotes for three genes: the shrunken endosperm gene, the waxy gene (wx) and the blue aleurone gene (Bl). These three genes are located on different chromosomes, and all express xenia. We crossed the above male-sterile line with a naked barley cultivar having waxy and blue aleurone genes. The naked trait is more desirable than the hulled one because it is easier to score the xenia traits. Heterozygotes for three genes were found in some F2 plants. We expect to obtain male sterile plants with the homozygous condition for sex1-msg6, wx, and Bl in the next generation. Heterozygous plants (sex1-msg6/Sex1-Msg6, wx/WX, and Bl/bl) will be obtained by natural or artificial crossing between homozygotes of sexl-msg6, wx, and Bl and homozygotes of Sex1-Msg6, Wx, and bl. By using these heterozygotes, a detection and a confirmation of obligate apomictic mutants will be possible before sowing M2 seeds.

References:

Falk, D. E. and K. J. Kasha 1982. Registration of a shrunken endosperm, male-sterile germplasm to facilitate hybridization in barley. Crop Science 22:450.

Makino, T. M. Furusho, T. Fukuoka and 0. Yatou 1993. On the selection of induced mutants of distorted segregation using genetic male sterile line and multiple marker lines in barley. Japan J. Breed. 43 (Suppl. 1): 188.

Hanna, W. W. 1991. Apomixis in crop plants - cytogenetic basis and role in plant breeding. In: Chromosome Engineering in Plants: Genetics, Breeding, Evolution, Part A. (Gupta, P. K. and Tsuchiya, G. eds.)., Elsevier Science Publishers B.V., Amsterdam, pp. 229-242.