Cytological expression of ig mutation in megagametophyte
--N. Enaleeva, O. Otkalo and V. Tyrnov

The cytological effect of the ig gene in a homozygous condition (in an embryo marker line background) was investigated. The technique of isolation of total megagametophytes (MGs) at early coenocytic and mature developmental stages was used. Different abnormalities of MG structure (55-98% of plants) were observed. The range of abnormality types was found to be wider than in the original ig W23 line, described by Lin (1978, 1981). The comparison of abnormality patterns, manifested at different stages of MG development, resulted in the conclusion that the ig gene expression occurs at the beginning of the postmeiotic MG developmental program. The major effect of the ig mutation seems to consist in disturbance of some cytoskeleton functions, relating to bipolar nuclei distribution during the coenocytic phase. It is very likely that in MG-genesis, the formation of a large central vacuole after the first mitotic division is suppressed or reduced. As a result, the nuclei are not arranged at the opposite ends of the coenocyte, but are situated at random. Because of the abnormal nuclei position, disturbance of the subsequent chain of events happens. In particular, the number of mitotic cycles increases, the nuclei are scattered or are in contact with each other, and the cytokinesis and following cell differentiation is irregular. The cell types depend on nuclear position at the moment of differentiation. In some cases the MG developmental program is blocked at different stages and MG nuclei start to degenerate. It may be assumed, therefore, that the Ig gene apparently controls only one key event of MG-genesis, and its mutation into ig gives rise to a disbalance of this developmental program, resulting in plural cytological patterns.
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