Several laboratories including ours have documented extensive variation in maize genome size. Genome size has been correlated with latitude (Laurie and Bennett, Heredity 55:307-313, 1985; Rayburn et al., Am. J. Bot. 72:1610-1617, 1985; Tito et al., TAG 83:58-64, 1991), altitude (Rayburn and Auger, TAG 79:470-474, 1990; Rayburn, Evol. Trends Plants 4:53-57, 1990), effective growing season (Bullock and Rayburn, Maydica 36:247-250, 1991), flowering time (Rayburn et al., Plant Breed. 112:318-322, 1994), and overall agronomic performance (Biradar et al., TAG 88:557-560, 1994).
With respect to cold tolerance, McMurphy and Rayburn (Plant Breed. 106:190-195, 1991) noted no clear trend in genome size. This study concentrated on populations which had been artificially selected for cold tolerance. The purpose of this study was to determine if, under natural selection, a correlation between cold tolerance and genome size exists.
Lines used in this study were open-pollinated Southwestern US Indian maize populations. The populations were obtained from North Central Regional Plant Introduction Station at Ames, Iowa. Nine of these populations originated in New Mexico and fourteen populations were from Arizona. Five seeds from each of these populations were sown in two replications. The pots were then kept in a cold chamber at a constant temperature of 11.5 C. The number of kernels germinated was recorded. In order to insure that the kernels used were of good quality, germination was also performed at 32 C. The relative percentage cold germination was calculated by dividing the percent germination at 11.5 C by the percent germination at 32 C.
Nuclear DNA content was determined according to the procedure of Rayburn et al. (J. Exp. Bot. 40:1179-1185, 1989). Nuclear DNA content of the populations was expressed with respect to A619 (= 100.00 arbitrary units) which was used as an external standard. Correlation analysis was carried out by using the relative cold germination percentage and the estimated genome sizes of the populations used in this study.
A wide variation in both of these parameters was observed among the
populations studied. Genome size ranged from 95.5 to 116.2 AU, while the
corrected germination percentage ranged from 20.0 to 100.0. A significant
negative correlation (R = -0.44; p = 0.04) was observed between genome
size and germination percentage of all the populations studied (Fig.1).
Figure 1. Correlations between DNA content and cold germination percentage of different populations.
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