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Results and discussion

The intergeneric F1 hybrids of P. huashanica with T. aestivum cv. J-11 are theoretically expected to have 28 chromosomes. The majority of the PMCs observed met the expectation (Table1, Fig.1 (1)). The meiotic data of the normal F1 hybrids and their parents indicate that P. huashanica shares no common genome with T. aestivum (Sun et al. 1992a). However, as shown in Table1, unusual chromosome numbers were observed. The chromosome number was fewer than 28 in 18.8% of the PMCs observed; and it was more than 28 in 7.2% of the PMCs. The highest chromosome number was more than one hundred (Fig.1 (8)).

Microspore formation was observed in intergeneric hybrids of T. aestivum with P. huashanica. Under certain circumstances, chromonemata were migrating from one PMC into immediate neighboring PMCs through small openings, which might be just common plasmodesmata reported in many literatures., In some cases, chromonemata were seen passing from one PMC to another through a big opening (Fig.1 (3)). If microsporocytes were arranged closely together, conjugation openings were formed at these conjoined points. The nucleus, chromatin mass or chromonemata (Fig.1 (3, 5, 6)) could migrate through the opening into immediate neighbouring microsporocytes. The conjugation opening observed in this hybrid was similar to that in Roegneria ciliaris x P. huashanica hybrid (Yen et al. 1993). In that F1 hybrid of R. ciliaris x P. huashanica, meiocytes were arranged very loosely. Outgrowth of bud-like structure came into contact with other pollen mother cells and became fused together, forming conjugation tube between cells (Yen et al. 1993). In this hybrid, meiocytes were arranged very closely. Thus, the cells came into contact with other so closely that there was no space for conjugation tube formation. A conjugation opening had the same function as a conjugation tube, but they differed in morphology (Yen et al. 1993). If the chromatin mass migrates through an opening as small as plasmodesma, this is cytomixis, which has been reported by many authors. In this hybrid, we observed that the chromatin material of premeiotic PMCs could migrate through the conjugation opening (Fig.1 (2, 3)) and from coenocytes (Fig.1 (7)). It has been observed that the chromatin of the pollen grains migrates through the conjugation opening after meiosis (Fig.1 (4, 5, 6)). Fig.1 (4) shows that the walls of young pollen grains (a) and (b) are dissolving, the nucleus of pollen grain (a) is elongating toward pollen grain (b), the young pollen grain walls of (c) and (d) have dissolved and formed a giant pollen grain. The nucleus migration is taking place between pollen grains (c) and (d), and the two nuclei are fused together.

Coenocytism in microsporogenesis is not rare in intergeneric hybrids in Triticeae (Kagawa 1929; Kihara and Lilienfeld 1934; Villax and Mota 1953; Nakajima 1954a, b). According to Price (1956), coenocytes were formed: i) by the failure of cytokinesis in the mitotic division preceding to meiosis, ii) through the passage of a nucleus from one PMC into another, and iii) through the fusion of PMCs as reported also by Mehra and Kalia (1973). Our observation showed that the coenocytes were formed through the nucleate materials transferring of PMCs at different stages of meiosis. Wang (1988) reported coenocytism in the P. huashanica x Secale montanum hybrids and observed an average of 4.44 nuclei, ranging from 2 to 25 per PMC. Yen et al. (1993) reported that 19.92% of the PMCs observed in the R. ciliaris x P. huashanica hybrids were found to be coenocytes containing 2 to 11 nuclei. In the present study, 52.0% of the PMCs observed were coenocytes containing 2 to 4 nuclei (Table 2).

If a coenocytes has synchronized nuclei (Fig.1 (7)), the chromosome number could be doubled or redoubled by nuclei fusion and a unified high level polyploid nucleus could be formed, although in some cases nuclei may not remain fused in coenocytes (Fig.1 (7)). PMCs that contained probably more than one hundred chromosomes (the high density of chromosomes made an accurate count impossible) were observed (Fig.1 (8)). The decaploid PMC (Table1) could form a pentaploid tetrad after meiosis. This might be a way in which spontaneous chromosome number doubling occurs. Thus, high level autoallopolyploid Leymus and Elytrigia species might have evolved by this mechanism.

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