Knob DNA in relation to combining ability
--Sajjad R. Chughtai, Habib I. Javed, Haq Nawaz Malik, M. Aslam and Dale M. Steffensen
Knobs are large blocks of highly repeated (heterochromatic) DNA found at fixed chromosomal locations of maize and its close relatives in the Maydeae. Knob DNA is composed of a tandemly arranged 180-base pair repeat. Knob frequency is negatively correlated with latitude and altitude. Different races (and even inbreds) possess different knob compositions.
Knobs have extensively been utilized for characterizing the races of maize. They have been shown to be associated with a large number of agronomic features of maize, including maturity and yield. Several studies indicate that knob frequency is closely associated with combining ability of maize genotypes. Generally, the best hybrids result from crossing high (or medium)-knob genotypes with knobless or low-knob genotypes. Interestingly, Reid's Yellow Dent and Lancaster Sure Crop, two germplasms with maximum contribution to hybrid corn in the U.S.A., both originated from crosses between high-knob southern dents and low-knob northern flints.
Recently, Crossa, Taba and Wellhausen (Crop Sci. 30:1182-1190, 1990) published extensive data on hybrids among 25 Mexican races of maize. Over 300 hybrids were evaluated at 3 different altitudes in Mexico during 1963 and 1964. Similarly, extensive data on the knob composition of maize from the Americas have been published by McClintock, B et al. (Chromosome Constitution of Races of Maize, Colegio de Postgraduados, Chapingo, Mexico, 1981). We have combined these yield and knob data to see if knob constitution is related to hybrid performance. A summary of these analyses is presented here.
At high altitude the top 40 hybrids have, on the average, more heterozygous (5.93) than homozygous (3.66) knobs, while the reverse is true for the bottom 40 hybrids (8.33 homozygous and 3.53 heterozygous knobs) (Table 1). At low and medium elevations, the top 40 hybrids have a higher number of homozygous (7.85 and 6.75) than heterozygous (3.55 and 3.63) knobs, while the reverse is true for the bottom 40 hybrids (5.4 and 4.90 heterozygous, and 2.25 and 3.60 homozygous knobs, respectively). In other words, knob heterozygosity is directly related to hybrid vigor. Thus, it is the knob condition (homozygous or heterozygous) which is related to combining ability rather than the frequency of knobs.
Among the top 40 hybrids, those between lowland and midland (LxM) races at low (48%), MxM at medium (35%), and MxH at high altitude (67%) were the most frequent (Table 2). Among the bottom 40, MxH (55%), HxH (32%) and LxL (43%) at these altitudes, respectively, were the most frequent. Thus, the racial composition of the top 40 hybrids is different from those of the bottom 40 at each altitude.
Table 1. Mean yield and number of homozygous and heterozygous knobs
in hybrids among Mexican races of maize.
|Number of knobsA|
|Altitude (m) of test site||Hybrid ranking||Homozygous||Heterozygous||YieldB (mg/ha)|
|High (2249)||Top 40||3.66||5.00||6.02|
|Medium (1800)||Top 40||6.75||3.63||7.77|
|Low (1300)||Top 40||7.85||3.55||5.92|
Table 2. Racial percent composition of hybrids among Mexican races
|Racial composition (%) of hybridsA|
|Altitude (m) of test site||Hybrid ranking||LxL||LxM||LxH||MxM||MxH||HxH|
|Low (1300)||Top 40||20||48||10||15||7||0|
|Medium (1800)||Top 40||3||25||12||35||25||0|
|High (2249)||Top 40||0||3||20||5||67||5|
These data also indicate that highland races combined well only at the high altitudes (in 92% of the top 40 hybrids) but not at low and medium altitudes (in 98% and 72% of the bottom 40 hybrids, respectively). The lowland races combined well at the low altitude (in 78% of the top 40 hybrids) but not at high altitude (in 88% of the bottom 40 hybrids). The midland races combined well at all locations, being involved in 70%, 85% and 75% of the top 40 hybrids at low, medium and high altitudes, respectively. That midland races combined well at high altitude suggests the potential utilization of the subtropical maize material in temperate regions. This may be of great interest to maize breeders in the U.S.A. and other temperate regions of the world. However, in such hybrids the other parent would essentially be a low-knob genotype adapted to the cool environments. The observation that highland races combined poorly at low and medium altitudes suggests that temperate material may not be useful for hybrid production in the tropics and subtropics.
The question is, how can we explain the relationship between knob condition and hybrid performance? Or in other words, why are knob heterozygotes adapted to the highland areas, and knob homozygotes to the lowland and midland areas? Our studies (S. R. Chughtai, Ph.D. Thesis, University of Illinois, 1988) showed that knob homozygosity delays plant development. The maturity of the hybrid coincides with the knobless genotype regardless of the knob number and maturity of the other parent. This indicated the absence of the delay effect of the knob in the heterozygous state. We hypothesized that knob DNA affects development by controlling the expression of the bracketing genes by a cis-acting position effect. Thus, knob heterozygotes, due to their early maturity (like the low-knob genotypes), are well adapted to the temperate environment where the growing seasons are short and the temperatures are low. However, they are not adapted to the tropical and subtropical environments since, because of their early maturity, they cannot avail themselves of the full length of the long growing season. Knob homozygotes, on the other hand, are adapted to such warmer climates due to their late maturity because they can avail themselves of the full length of the growing season. Due to their late maturity, they are, however, not adapted to the cooler climates where low temperatures delay plant development and knob homozygotes are unable to mature during the growing season.
In cooler climates, the early maturity of the knobless or low-knob genotypes can be coupled with the high yield of the late-maturing high-knob genotypes. Thus the linkage between maturity and yield is broken. Apparently, this is the secret behind the success story of hybrid maize production in the U.S.A. and other temperate regions. This may also explain the relatively limited success of hybrid maize in tropical and subtropical regions of the world. The tropical and subtropical maize varieties are generally intolerant to inbreeding. We believe that selection of early vigorous plants in the tropics is over-emphasized. This leads to selection of knob heterozygotes and ultimately of low-knob genotypes which are poorly adapted to the warm climates. In these areas, selections for acceptably late maturing plants should be emphasized to ensure knob homozygosity (Chughtai and Steffensen, SABRAO J. 21:21-26, 1989).
The new evidence strongly supports earlier contentions (T. A. Kato Y., TA, Mass. Agric. Exp. Sta. Bull. No. 635, 1976; S. R. Chughtai and D. M. Steffensen, Maydica 32:171-187, 1987) that knob heterochromatin plays an active (though indirect) role in the adaptation of maize to its environment. Thus, there is a great need for considering knob heterochromatin in breeding and improvement of maize adapted to different regions of the world.
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