Evolution and improvement of cultivated amaranths. IV. Variation in pollen mitosis in the F1 amaranthus spinosus x A. Dubius

Abstract

While the two parents of the hybrid have 17 and 32 chromosomes respectively, the hybrid commensurate with its meiosis, has a variety of chromosome numbers in pollen grains. On the basis of their size, the grains may be grouped in two categories: macropollen (21-26μ; 77%) and micropollen (8-14μ; 23%). These contain 19-27 and 1-4 chromosomes respectively. Among the latter, 44% contain just one, 39% 2, 11% 4 and 6% 3 chromosomes. The micropollen grains are the result of laggards at anaphase I and II in the hybrid. The exine development in micro- and macropollen appears to be normal. None of the grains is effective in fertilization. However, a distinct difference in the rate of development in the two types of pollen has been seen from metaphase onwards, because in the absence of a directive influence of spindle, micropollen does not proceed beyond this stage. The strong developmental difference vanishes almost altogether when due to cold shocks, walls either fail to develop or are feebly developed among the constituents of a tetrad ensuing from a single pollen mother cell. The intertetrad synchrony between micro- and macropollen in such cases, is understandable because the loss in one component compensates the gain in the other and thus deficient nuclei can develop at the same rate as others because of the exchange of metabolites. The intratetrad synchrony or developmental coordination within an anther sac may be the result of feeble wall formation and/or cytomictic channels. The present case with n=1 is deficient to the extent of 94%. Evidently a single chromosome survives independently for some time, internally completes DNA synthesis and finally proceeds to metaphase. Associated with these changes there is external development of apparently normal exine.