Induced mutagenesis is one of the promising techniques in maize breeding. Mutations can result from the pollination of plants with high dose gamma-irradiated pollen, the latter being a mutagen which provides a wide range of variegated mutations.
Cytological studies constitute an important component part in genetic breeding programs involving the development of new original forms with the aid of the methods of experimental mutagenesis. A normal course of meiosis is one of the factors which determines a stable status of the forms. Abnormal reductional divisions can lead to deviations in offspring contributing to the variability of traits in higher plants.
No studies on the indirect action of high gamma-irradiation doses via irradiated pollen have been conducted and the consequences of this treatment have not been studied in subsequent generations. Given the destabilization of meiosis in experimental plants,the effect of the irradiated pollen on morphologic process might be expected in maize.
The recipient stock P-346, and pollen of the donor marker stocks K167 bm2, su1 s4 bm2 g1, Doina, and Doina br2 were taken as objects of the cytologic studies.
The examination of meiosis in maize microsporogenesis showed that the reductional division was occurring in it, mainly, in a proper way. All stages of early (leptotene, zygotene, pachytene) and later prophases are present. The normal parity of chromosomes ensures meiotic regularity. Ten bivalents, including seven of the closed type and three open, are observed, which then line up along the equatorial plate in metaphase 1 and separate normally (10 + 10 ) to both poles in anaphase 1.
Cytological examination of the first generation samples of stock P-346 treated with the irradiated pollen of the donor stocks K167 bm2, Doina, Doina br2, su1 s4 bm2 g1 discovered no structural mutations. However, chlorophyll mutations from pollination of the recipient with the K167 bm2 pollen irradiated with doses of 500, 800, and 1000 Gr, and a mutation of the barren stalk type, as well as of the golden type when the pollen was irradiated with 800 Gr suggests the existence of point mutations in the samples under study. This is proved indirectly by the absence of sterile pollen in these mutants.
A small number of second generation plants exhibited deviations from the norm in reduction division following pollination with high dose irradiated pollen K167 bm2 in the dose range between 500 and 1000 Gr.
A relative synchrony is normal for the first meiosis in the maize anther. Some asynchrony is observed only during the second division. Treatment with irradiated pollen causes the initiation of meiotic asynchrony as soon as prophase 1. At the stage of metaphase 1 the origination of univalents, chromosome acceleration, and early chromosome divergence are observed in some cells of the form studied. Examination of meiotic anaphase 1 revealed lagging of bivalents.
The pattern of meiosis during microsporogenesis upon this treatment is as follows: in many cells, univalents were formed at the stages of metaphase, anaphase and telophase of both divisions. In most cells, they are well-discernible in diakinesis at this stage, and from one to four univalents can be scored in different meiocytes. The number of anomalous tetrads increased to 24.31±0.86% against 3.41±0.17% in the control. Due to this, the sizes and shape of the pollen grains varied strongly, the pollen appeared to be viable only in half--41.54±0.20% in comparision with 89.72±0.38 in the control.
In the control plants, meiosis mainly had a normal course, with insignificant deviations having no effect on the pollen quality. They are characterized by the regularity of tetrads, evenness of pollen grain size and high fertility.
Cytological examination of the second generation samples produced through the pollination of the stocks Doina and Doina br2 with the high dose irradiated pollen revealed no structural changes in chromosomes except the sample produced through pollination with the Doina br2 pollen irradiated at a dose of 300 Gr. This sample was characterized by structural changes at all stages.
At the stage of metaphase, from one to three univalents could be counted in different meiocytes. In some cases, a delay of bivalent divergence occurred, and lagging bivalents were observed in the first and second anaphases. By the end of the meiotic division, two-nucleolus pollen was observed with a frequency of 5.01±0.41%, not seen in the control.
Unforeseen disorders were found in the second generation of P-346 pollinated with highly irradiated su1 s4 bm2 g1. The disorders started from prophase 1. Unexpectedly frequent, in comparison with the control, chromosome breaks occurred at the doses of 300, 800, 1000 Gr with a frequency of 15.8±1.52%, 20.34±1.81%, and 36.51±2.41%,respectively. Terminal deletions, chromosome nondisjunction, and frequent chromosome breaks at various irradiation doses were marked during diakinesis. Chromosome lagging and individual fragments were observed in metaphase 1 and anaphase 1, respectively. In the samples at the dose of 1000 Gr, along with frequent chromosome breaks, upset of nucleolus formation with a frequency of 1.83% and heteromorphic bivalents with a frequency of 0.96% were observed during diakinesis due to the different lengths of the conjugating chromosomes. These types of upsets are characteristic of meiotic mutants with disorders in spiralization and partial asynapsis.
Further development of microspores varied owing to anomalies in meiosis.
Different stages of degeneration were marked in a part of microspores,
the pollen fertility was quite low in the experimental plants and oscillated
from 46.82% to 64.73%. In the control plants, the pollen remained highly
fertile, 90.12-94.54%. The abnormal meiosis in maize suggests that pollination
with the high dose irradiated pollen upsets the coordinated program of
the occurrence of meiotic processes, which leads to the production of anomalous
kernels or a specific level of sterility of maize plants, both male and
female ones. The indirect action of the ionizing irradiation via the high
dose irradiated pollen results eventually in the fixation of a considerable
number of mutational changes in the genotype with aid of both point and
to the MNL 72 On-Line Index
Return to the Maize Newsletter Index
Return to the Maize Genome Database Page