Knob DNA in maize: its relationship with combining ability for yield and maturity in different environments
--Sajjad R. Chughtai, J. Crossa*, H.I. Javed, M. Aslam and M. Hussain

Heterochromatic DNA in maize and its close relatives in the Maydeae is present in the form of highly condensed knobs at fixed locations on the chromosomes. The racial composition and geographic distribution of knobs is not random. Different sets of knobs are present in different races of maize. The high knob races are restricted to the lowlands while low knob races are restricted to the highland areas. Also, the knob frequency decreases with increasing latitude. Apparently, the amount of knob heterochromatin is related to the growing season and thus to the adaptation of maize to its environment.

Crossa et al. (Crop. Sci. 30:1182-1190, 1990) published extensive data on combining ability among 25 Mexican races of maize. The hybrids among these races were developed and evaluated at three different altitudes (1300, 1800 and 2249 meters) in Mexico during the early sixties. In collaboration with Dr. J. Crossa from CIMMYT, Mexico, we have been analyzing these data with respect to the knob composition of races and their hybrids based on knob data published by McClintock et al. (Chromosome constitution of races of maize: Collegio de Postgraduados, Chapingo, Mexico, 1981). Earlier (MNL 67:49-50, 1993), we concluded that depending on the environment, the number of homozygous or heterozygous knobs determined the hybrid performance. Now some more data along with days to pollen shedding are presented to corroborate our earlier conclusion regarding a relationship between knob DNA and combining ability for yield and maturity. It is very important to note that the relationship between knob DNA and combining ability for yield and maturity largely depends on the environment. At low altitude, both the top 50 and bottom 50 hybrids took about 78 days to mid-flowering. The main difference was in the number of knobs. The top 50 and bottom 50 hybrids on the average had 7.8 and 2.44 homozygous, and 3.46 and 5.56 heterozygous knobs, respectively. At intermediate elevation, the top 50 hybrids, on the average, were 5 days later in pollen shedding than the bottom 50. Regarding their knob composition, the top 50 had 6.84 homozygous and 3.66 heterozygous knobs compared with 3.86 homozygous and 5.04 heterozygous knobs in the bottom 50 hybrids. At high altitude, the top 50 and bottom 50 hybrids took 91 and 105 days to pollen shedding, respectively, and had 4.06 and 8.26 homozygous, and 3.64 and 5.58 heterozygous knobs, respectively. At high altitudes, the yield increased with increase in the number of heterozygous knobs though the maturity was reduced. The top 50 hybrids have a higher number of ears per plant than the respective bottom 50 hybrids at all elevations (Table 1).

Table 1. Number of knobs, days to flower, ears per plant and grain yield in the top and bottom 50 hybrids among Mexican races grown at low (1300 m), medium (1800 m) and high (2249 m) altitudes.
 

		Number of Knobs
Altitude	Rank	Homozygous	Heterozygous	Days to flower	Ears/plant	Yield (T/ha)
Low	Top 50	7.82	3.46	78	1.30	5.86
	Bottom	2.44	5.56	77	1.08	2.65
Medium	Top 50	6.84	3.66	88	1.62	7.63
	Bottom	3.86	5.04	83	1.51	3.65
High	Top 50	4.06	5.58	91	1.66	5.84
	Bottom	8.26	3.64	106	1.12	1.65

Table 2. Correlation between days to flower (DF), ears per plant (EP) and grain yield (GY) in hybrids grown at low (1300 m), medium (1800 m) and high (2249 m ) altitudes.
 

	Altitude	DF	EP	GY
DF	Low	1.00
	Medium	1.00
	High	1.00
EP	Low	-0.495**	1.00
	Medium	-0.215**	1.00
	High	-0.510**	1.00
GY	Low	-0.067 NS	0.270**	1.00
	Medium	-0.138*	0.194**	1.00
	High	-0.363**	0.561**	1.00

NS = nonsignificant; * = significant; ** = highly significant

Correlations between yield, maturity and prolificacy on 300 hybrids and 25 parental races grown at three different altitudes (Table 2) elucidate some interesting relationships. Prolificacy is negatively but highly significantly correlated with maturity at all sites. In other words, earlier maturing hybrids have a highly significant probability of producing a second ear. Maturity and yield show different relationships at different altitudes (environments). They are positively but non-significantly correlated (r=0.067) at low, positively and significantly correlated (r=0.138) at intermediate, but negatively and highly significantly correlated (r=-0.363) at high altitude.

Our earlier explanation (MNL 67:49-50, 1993) of the relationship between knob composition and hybrid performance was that in the temperate environments, knob heterozygotes (like the knobless genotypes) are early in maturity and thus well adapted to the local agroclimatic conditions. We thought that knob homozygosity delayed plant development in cooler climates probably through the cis-acting position effect. The data presented in this communication strongly support this contention. We (Maydica 32:171-187, 1987; S.R. Chughtai, Ph.D. Thesis, University of Illinois, 1988; SABRAO J. 21:21-26, 1989) proposed the existence of a DNA binding protein which specifically binds to the knob DNA and thus changes the expression of the bracketing genes by a spreading or position effect. Our observation that treatment of root tips with Quinacrine (a DNA binding agent) inhibits knob condensation even at low temperatures indicates the presence of the putative protein. Quinacrine probably prevents the binding of a knob specific protein which recognized the A-T rich stretches (GAAT or GAAAAT) in knob DNA. The effect of this putative DNA binding protein is temperature dependent. Since knob DNA is highly condensed at low temperatures, the spreading or position effect is pronounced at high altitude (temperate environments) but not at the low altitude (warmer climates). In such climates, knob homozygotes are better adapted than the knob heterozygotes though they have similar maturities. Whatever the explanation may be, it is important that knob composition of hybrids (as well as landraces and varieties) of maize apparently plays an important role in their adaptation to the environmental conditions. Consideration of this factor in breeding of maize for various target environments and purposes will ensure success more than random crossing efforts and selection. For example, breeding for cold tolerance must ensure knob heterozygosity and early maturity by choosing one of the parents with low knob number and early maturity. In other words, the early maturity of the knobless or low-knob genotypes can be coupled with the high yield of the late-maturity high-knob genotypes thus breaking the linkage between yield and maturity. 

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