Genes Genet. Syst. 82: 241-248 (2007)
Cytological evidence for chromosome elimination in wheat x Imperata cylindrica hybrids
Norio Komeda, Harinder K. Chaudhary, Go Suzuki and Yasuhiko Mukai

Haploid induction of wheat by crossing with Imperata cylindrica pollen is an efficient method for doubled haploid breeding. We investigated the process of wheat haploid formation after crossing with I. cylindrica. Our cytological observations of zygotes showed the successful fertilization of parental gametes. Wheat haploids were formed by complete elimination of I. cylindrica chromosomes. Missegregation of I. cylindrica chromosomes was observed in the first cell division of zygote. At metaphase I. cylindrica chromosomes did not congress onto the equatorial plate. The sister chromosomes did not move toward the poles during anaphase, though their cohesion was released normally. I. cylindrica chromosomes were still in the cytoplasm at telophase and eliminated from daughter nuclei. After two-celled stage, we could find no I. cylindrica chromosome in the nuclei but micronuclei containing I. cylindrica chromatin in the cytoplasm. These observations indicate that I. cylindrica chromosomes are completely eliminated from nuclei in the first cell division probably due to lack of functional kinetochores.

INTRODUCTION

Interspecific hybrids often show various abnormalities in their development due to the incompatibilities between their parental genomes (Jones and Pasakinskiene, 2005). Several hybrids are karyotypically unstable. Uniparental chromosomes, paternal in almost all cases examined, are eliminated partially or completely from the hybrid nuclei. This phenomenon is called as "chromosome elimination" and has been observed in several interspecific hybrids (Kasha and Kao, 1970; Barclay, 1975; Laurie and Bennett, 1988, 1989; Laurie, 1989a; Riera-Lizarazu et al., 1996; Fujiwara et al., 1997).

The combination of parental species mainly determines the degree of chromosome elimination. For example, in oat x maize, and wheat x barley (Thomas and Pickering, 1983; Riera-Lizarazu et al., 1996), paternal chromosomes are eliminated incompletely and several chromosomes can be found in mature plants. Complete elimination of paternal chromosomes occurs in wheat x Hordeum bulbosum, maize, pearl millet, or sorghum, and barley x H. bulbosum, and so on (Kasha and Kao, 1970; Barclay, 1975; Laurie and Bennett, 1988, 1989; Laurie, 1989a). In these crosses, complete loss of paternal chromosomes results in the production of maternal haploids.

Cytological analyses performed to reveal the mechanism of chromosome elimination enunciated similar mitotic abnormalities in many hybrids (Laurie and Bennett, 1988, 1989; Laurie, 1989a; Mochida et al., 2004). Chromosomes being eliminated tend to be apart from the equatorial plate at metaphase. Sister chromosomes fail to move toward the poles at anaphase and are retained in the cytoplasm to be eliminated from the nuclei. These chromosomes form extra-micronuclei and are finally degraded. In addition to mitosis dependent elimination, another pathway of chromosome elimination was proposed in wheat x pearl millet crosses (Gernand et al. 2005). Micronuclei containing pearl millet chromatin were directly budded and eliminated from interphase nuclei. There might be an active mechanism to distinguish and to eliminate alien chromatin from nuclei. It remains to be seen whether such elimination pathway is common in other hybrids.

Partial Hybrids