Spiral structures of meiotic chromosomes in plants

Abstract

The structure of meiotic chromosomes in Lilium longiflorum was studied using aceto-carmine squash preparations and ammonium vapour technicue for artificial uncoiling of chromosomes. Ammonium vapour treatment uncovers minor spiral within major spiral in chromosomes at the first meiotic anaphase, i. e. chromosomes at the first meiotic division show spiral-within-spiral structure. At the second division, ammonium vapour treatment reveals apparently single fine gyred spiral, chromosomes showing four times more gyres than chromosomes at the first meiotic division. Calculated chromonema lengths were estimated to be about 1:7–10 in chromosomes at the first meiotic division and 1:2–3 in chromosomes at the second meiotic division as compared to respective pachytene chromosome length. On the basis of the dimensions of chromonemata it was concluded that the 1:7–10-chromonema of the first meiotic division is the secondary spiralizing product of the 1:2-chromonema, which shows 1:3.6 package within 1:7–10-chromonema. Comparison with data available of chromonema lengths at both meiotic and mitotic divisions revealed lengths of basic or primary chromonemata at meiotic divisions to be 1:2 and at mitotic divisions 1:2 or 1:1 of respective pachytene chromosome length. Thus the secondary chromonema (1:7–10) and its spiralization product major spiral represent higher order structures, the existence of which is confined to the first meiotic division in a limited group of plants. The data available of chromonema package within chromosomes suggest 1:3 and 1:6 to be basic packing ratios of chromonemal spirals within chromosomes.