Knobs in northeastern Himalayan maize

--M. Kumar and J. K. S. Sachan

Knob composition of 50 maize collections from different areas of the northeastern Himalayas (NEH) was studied through pachytene analysis of PMCs. Emerging tassels of these strains were fixed in glacial acetic acid-ethanol (1:3) during 1989 and 1990 at the Division of Genetics, IARI, New Delhi. These maize collections belonged to Assam (3), Arunanchal Pradesh (2), Meghalaya (15), Nagaland (12), Sikkim (18) and Tripura (10). Pachytene analysis of PMCs was done by making temporary aceto-carmine squash preparations. Camera lucida drawings of individual chromosomes were made and these were identified on the basis of length, arm ratio, position of knobs and prominent chromomeres and pattern of centromeric heterochromatin. Knob positions were identified on the basis of standard idiogram given by Longley and Kato (1965).

Twenty-six knob forming positions have been identified altogether in different collections of NEH maize. Mean knob number, range and total knob forming positions of these collections have been shown in Table 1. Frequency of occurrence of various knobs in different regions of NEH has been shown in Table 2.

Table 1. Mean knob number, range, total knobs and common knob forming positions in NEH maize.
 
Region No. of collections Mean knob no. Range Total knob positions Common knob forming position
Arunanchal Pradesh 2 8.5 7-10 11 3L,4L,6S,6Lb,8La,9ST
Assam 3 6.7 6-8 9 2La,4L,6S,6Lb,7L,8La,9ST
Meghalaya 15 5.7 4-8 16 2La,4L,6S,6Lb,7L,8La,9ST
Nagaland 10 7.1 5-9 18 2La,4L,6S,6Lb,8La,9ST
Sikkim 18 5.8 4-9 18 2La,4L,6S,8La,9ST
Tripura 10 7.3 4-11 20 2La,4L,6S,6Lb,8La,9ST

Table 2. Frequency in percent of knbos at each knob forming position in various regions of NEH.
 
Knob positions Assam Arunanchal Meghalaya Nagaland Sikkim Tripura Overall frequency
1Sa - 50.00 6.70 7.68 11.10 7.68 9.20
1ST - - 20.00 - - - 4.60
1La - 50.00 6.70 15.37 22.20 15.37 15.38
1Lb - - - - - 7.68 1.53
2S - - - 7.68 5.50 7.68 4.60
2La 100.00 50.00 60.00 84.60 55.60 76.92 69.23
2Lb - - - - - 7.68 1.53
2LT - - 6.70 - - - 1.53
3ST - - - - 22.22 15.38 10.76
3L - 100.00 20.00 23.07 38.90 30.77 32.07
4S - - - - 5.50 - 1.53
4L 66.66 100.00 60.00 69.23 66.70 84.61 72.30
5L 33.33 - 6.70 - 11.10 30.77 13.84
6S 100.00 100.00 100.00 100.00 100.00 100.00 100.00
6La - - 6.70 - - - 1.53
6Lb 66.60 100.00 46.70 76.92 22.22 92.30 58.46
6Lc - - 20.00 23.07 16.70 23.07 18.46
7ST 33.33 50.00 - 23.07 5.50 15.38 13.84
7L 100.00 50.00 46.70 61.53 33.33 46.15 47.69
8La 100.00 100.00 86.60 76.90 66.70 61.53 75.38
8Lb - - 6.70 37.69 5.50 7.68 12.30
9ST 66.66 100.00 80.00 84.60 77.80 92.30 84.61
9La - - - 15.37 16.60 7.68 9.02
9Lb - - - 7.68 - - 1.53
10La - - - 7.68 - - 1.53
K10 - - - 7.68 - 7.68 3.07
Total knob position 9 11 16 18 18 20 26
1, 2 . . . 10 = chromosomes; K10 = abnormal chromosome 10; S = short arm; L = long arm; T = terminal knob position; a, b, c = knob positions on the same chromosome; + = presence of knob; - = absence of knob.

The presence of some new and unusual knob positions in NEH maize, hitherto unknown in American maize races, have been identified in the present study. These positions are 1Lb, 2Lb, 2LT and 9Lb, all having small sized knobs except the medium sized knob at the 2LT position. Knobs at these positions have also been reported in Kashmir maize (Jotshi and Patel, 1983). All of these new knob positions have varied geographical distribution in NEH. 1Lb and 2Lb knobs, present distally on the long arms of chromosomes 1 and 2, are restricted to only two collections from Tripura, namely T-7 and T-17, respectively; whereas a 2LT knob, on the terminal position of the long arm of chromosome 2, is present in the M-240 collection from Meghalaya, and a 9Lb knob in N-52 from Nagaland. It is interesting to note that knobs at both the 2La as well as the 2LT positions on the long arm of chromosome 2 have been observed in different plants of the same strain. The presence of these new knob positions in different regions suggests their adaptive value and indicates the uniqueness of NEH maize.

Thus, the NEH knob complex may be comprised of frequent knob positions 2La, 4L, 6S, 6Lb, 7L, 8La and 9ST. Less frequent knobs at 1La, 3L, 5L, 6Lc, 7ST, 8L and 9L are also present in this region (Table 2). The presence of these most frequent knob positions in a majority of strains studied indicates that they were present in the original introduction into the NEH region. Fixation of these knobs with variable frequency, in a majority of the NEH strains, speaks of their adaptive value. The absence of some of these knobs in some strains can be explained on the basis of genetic drift.

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors

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