Meiotic abnormalities in NEH maize
--M. Kumar and J. K. S. Sachan
A total of 40 landraces of the NEH region of India have been screened by pachytene analysis. Nagaland collections N-24, N-29, N-37 and N-44 exhibited various meiotic abnormalities. In some of these strains univalents as well as bivalents were present at metaphase I. Abnormal segregation was observed at anaphase I in these as well as a few other NEH strains. Besides these, laggards, precocious segregation, stickiness, micronuclei and pentads instead of normal tetrads were also observed.
As per Prakken's (1943) classification, the Nagaland collections showing desynapsis are medium strong mutants, showing a high number of univalents and a high frequency of segregation abnormalities. The desynapsis has been shown to be under genetic control (Koduru and Rao, 1981), as well as under the control of temperature and physiological factors or nutrient levels. In this context, it is worth mentioning that these NEH maize strains were grown in New Delhi, far away from their native habitat. Markedly different climate and very high temperature in the growing season could have induced these synaptic variants in these strains that are possibly carrying the genes for synaptic mutants. Such synaptic variants have been reported earlier in Sikkim Primitive strains N-3 and N-4, which are also from Nagaland (Dash et al., 1986), as well as some maize varieties being grown in Indian plains (Sarma, 1983).
Occasionally laggards were also observed during anaphase I. The micronuclei observed during late telophase I must be due to these laggards. Micronuclei leading to polyad formation have also been observed in triticale (Gobran, 1980).
The pollen grains carrying meiotic abnormalities appeared to be non-functional as was evident from the constant 2n=20 chromosome count in all these strains. Absence of seed sterility observed in these strains can be explained due to normal megasporogenesis and subsequent fertilization by normal pollen grains.
Stickiness of chromosomes at anaphase I has been consistently observed in the present study. Such adhesion between two or more chromosomes occurs due to a recessive monogenic mutation (Beadle, 1937) in maize.
Abnormal meiotic patterns, the presence of additional RNA and in some cases a second nucleolus were earlier reported in a Sikkim Primitive strain (Peeters, 1982) growing in a place away from its natural habitat.
A cross-shaped structure, formed by the fusion of centromeres at pachytene, was consistently observed in maize strains of Sikkim and Meghalaya. However, subsequent meiotic events at metaphase I were found to be normal, hence the possibility of translocation heterozygotes was ruled out.
Such centromeric fusion has been reported earlier in NEH maize. Randomness of this centromeric fusion was evident from the involvement of any of the bivalents in it. The association between centromeres of non-homologous chromosomes was found to be less frequent in the present study.
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