Chromosome breakage in wheat plants doubly hemizygous for the gametocidal genes derived from Aegilops speltoides and Ae. sharonensis.

Shuhei Nasuda

Laboratory of Plant Genetics, Graduate School of Agriculture, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto 606-8502 JAPAN

Corresponding author: Shuhei Nasuda

    e-mail: nasushu@kais.kyoto-u.ac.jp

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Gametocidal (Gc) genes, introduced to common wheat from Aegilops species, causes chromosome breakage (for review, see Endo 1990). Cytological observations revealed that chromosome breakage is a post-meiotic event in plants hemizygous for a Gc gene (Finch et al. 1984; Nasuda et al. 1999).  Tsujimoto (1995) summarized available data and described interaction between different Gc genes.  Regarding to the interaction between Gc genes on homoeologous group-2 and -4 chromosomes, he concluded that Gc genes on group-4 chromosomes are epistatic to those on group-2 chromosomes.  Moreover, he pointed out that chromosome breakage was enhanced in plants doubly hemizygous for Gc genes on different homoeologous groups.  These conclusions were drawn from cytological observations of structurally aberrant chromosomes in the progeny.

Here, I describe chromosome breakage occurred in a doubly hemizygous wheat plants for Gc genes from chromosomes 2S of Ae. speltoides and 4S^sh of Ae. sharonensis.  These Gc genes were translocated to the tips of long arms of homoeologous wheat chromosomes, being described as T2B-2S and T4B-4S^sh by Nasuda et al. (1999).  Cytological observation of male gametogenesis of a double-monosomic substitution line (dMT2B-2S/4B-4S^sh) revealed that there are three kinds of pollen grain with different degrees of chromosome breakage; ‘normal (none)’, ‘severe’, and ‘moderate’ (Figure 1).  In normal pollen grains (Figure 1, top), no chromosome fragments and/or bridges were observed in anaphase of first postmeiotic mitosis. The pollen grains belonged to ‘severe’ chromosome breakage had many chromosome fragments between spindle poles (Figure 1, middle) and the pollen grains categorized to ‘moderate’ chromosome breakage had only two or three chromosome fragments (Figure 1, bottom).  In total, I observed 110 pollen grains. The numbers of pollen grains in each category were; 33 ‘normal’, 55 ‘moderate’, and 25 ‘severe’.  These numbers well fit to the segregation ration of 1:2:1 (χ²=1.491, df=2), indicating that ‘severe ‘chromosome breakage occurred in gametophytes lacking both Gc chromosomes.

References

Endo TR (1990) Gametocidal chromosomes and their induction of chromosome mutations in wheat. Jpn J Genet 60: 125-135.

Finch RA, Miller TE, Bennett MD (1984) “Cuckoo” Aegilops addition chromosome in wheat ensures its transmission by causing chromosome breaks in meiospores lacking it. Chromosoma 90: 84-88.

Nasuda S, Friebe B, Gill BS (1999) Gametocidal genes induces chromosome breakage in the interphase prior to the first mitotic cell division of the male gametophyte in wheat. Genetics 149: 1115-1124.

Tsujimoto H (1995) Gametocidal genes in wheat and its relatives. IV. Functional relationships between six gametocidal genes. Genome 38: 283-289.