Lineages of 'Sikkim Primitives' (SP)

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

Cytological characterisation of eight SP maize strains collected from different parts of the NEH region was done. These included three strains from Meghalaya (M-1, M-15, M-25), two from Tripura (T-2, T-26), two from Sikkim (S-27, S-44) and one from eastern Nepal (SP Nepal).

Pachytene analysis of the PMCs of these SP strains fixed in aceto-alcohol (3:1) was done to study the knob composition of these strains, and later on they were compared with the standard knob composition of American primitive races given by Kato (1965).

Altogether, 16 knob forming positions were identified on different chromosomes (Table 1). The minimum knob number was in the M-25 and T-26 collections, whereas T-2 from Tripura had the highest knob number (11). The most frequent knob forming positions in SP strains were 2L, 4L, 6S, 8La and 9ST. Less frequent knob forming positions were 1ST, 3ST, 6Lb and 9L. Knobs were rarely present at 2S, 3L, 5L, 6LC, 7ST, 7L and 8Lb positions. Chromosome 10 was completely knobless in all SP strains. Generally speaking, SP strains from Nepal and Sikkim possessed mostly large knobs in comparison to those of Meghalaya and Tripura.

A terminal knob on the short arm (1ST) of chromosome 1 was present in two Meghalaya collections, out of which M-1 possessed a large knob whereas M-15 had a medium sized knob at this position. An interstitial knob on the short arm of chromosome 2 at the 2S position was missing in all of the SP collections, except one large knob at this position in S-27. An interstitial knob on the long arm of chromosome 2 at the 2L position was very frequent in occurrence, being present in about 75% of the SP strains. The SP collection from Nepal possessed a very large 2La knob, whereas those of the Meghalaya and Tripura collections possessed a large to medium sized knob. A knob on the 3ST position was present in only 2 out of eight collections, namely S-44 and SP (Nepal), whereas a knob on the long arm was present in the SP (Nepal) collection only. The Nepal collection possessed a large sized knob on both arms of chromosome 3, whereas S-44, from Sikkim, possessed a medium sized knob on the short arm only.

A knob at the 4L position was present in all but M-25 SP strains. A large cylindrical knob was present in the Nepal collection, whereas a medium sized round knob was present in S-27 and S-44 from Sikkim, and T-2 and T-26 from Tripura. The Meghalaya collections M-1 and M-15 possessed a large sized 4L knob.

A medium sized 5L knob was present in T-2 only. A large or medium sized knob at the 6S position near NOR was invariably present in all the collections studied.

An interstitial knob on the long arm of chromosome 6 at the 6Lb position was present in only three collections, namely SP (Nepal), T-2 and T-26. T-2 possessed one more small knob on the 6La position also. This 6La knob was absent in all other chromosomes.

Table 1. Knob composition in 'Sikkim Primitives'.
 
   
Knob positions
Collections Total knob no. 1ST 2S 2La 3ST 3L 4L 5L 6S 6Lb 6Lc 7ST 7L 8La 8Lb 9ST 9La
M-1 6 + - + - - + - + - - - + - - + -
M-15 6 + - + - - + - + - - - - + - + -
M-25 4 - - - - - - - + - - - + + - + -
S-27 8 - + + - - + - + - - - + + - + +
S-44 6 - - + + - + - + - - - - + - + -
SP (Nepal) 8 - - + + + + - + + - + - - - + -
T-2 11 - - + - - + + + + + - + + + + +
T-26 4 - - - - - + - + + - - - + - - -
Frequency out of 8 collections   2 1 6 2 1 7 1 8 3 1 1 4 6 1 7 2
% frequency of knobs   25.0 12.5 75.0 25.0 12.5 87.5 100.0 37.5 12.5 12.5 12.5 50.0 75.0 12.5 87.5 25.0
1, 2 . . . 9 = chromosomes; S, L = short and long arm of the chromosomes; T = terminal knob positions; b, c = knob positions on the same chromosomes; + = presence of knob; - = absence of knob
 

A terminal knob on the short arm of chromosome 7 at the 7ST position was absent in all collections except that of a small knob in SP (Nepal). However, 50% of the collections studied possessed a knob at the 7L position. The knob at this position was mostly large, cylindrical or round.

A knob at the 8La position was present in 6 out of 8 collections studied, being absent in M-1 and SP (Nepal) collections. The Meghalaya collections had a medium sized knob at this position, whereas the Sikkim collections, as well as T-2 from Tripura, had a large knob, but T-26 possessed a small knob.

A terminal knob at the 9ST position on the short arm of chromosome 9 was present in all except T-26. This 9ST knob was mostly large cylindrical/conical, but was of medium size in S-44. The interstitial knob at the 9La position was present in two collections only, T-2 having a large sized knob whereas a medium sized knob was present in T-2.

On the basis of knob number, these SP strains can be grouped into two categories: a) low knob number group (4 knobs) comprised of M-25 and T-26, possessing common knobs at 6S and 8La, in addition to some other knobs at the 4L, 7L and 9ST positions; and b) high knob number group (6-11) comprised of M-1, M-15, S-27, S-44, SP (Nepal) and T-2. Frequent knob positions in the second group are 2L, 4L, 6S, 6Lb, ,8La, 9ST, whereas knobs are present with lower frequency on 1ST, 2S, 3ST, 3L, 5L, 6Lc, 7ST, 7L, 8Lb and 9L.

When these two SP groups are compared with those of American primitive races, the low knob number SP group appears to have its linkage from Confite Morocho of Peru and/or Palomero Toluqueno of Mexico. On the other hand, the high knob number group of SP strains possesses close similarity to the Nal-Tel-Chapalote complex.

It appears that SP maize still harbours some genes of teosinte, which were introgressed into it much before its prehistoric introduction into the NEH region. These teosinte genes appear to be responsible for much delayed flowering, prolificacy, sturdier and longer stem, and inclusion of a male spike at the tip of the female inflorescence. Such a contention gets support from the maize X teosinte crosses currently being carried out in our laboratory (Ramesha, 1991, personal communication), in which F1 hybrids show most of the characters of SP strains. Hence Beadle's assertion (e.f. Sachan and Sarkar, 1986) that SP maize can be resynthesised appears to hold true.


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