Nuclear and chromosomal DNA amounts in Zea mays spp. mays

Contributed 1995 by: Professor Michael D Bennett

Zea mays ssp. mays has ten chromosomes (= linkage groups) in its haploid nuclear complement. The DNA content of the unreplicated haploid complement (n = 10) is known as the 1C value, while that of the replicated diploid complement (2n = 20) is the 4C value. DNA amounts are usually expressed in picograms (pg), or in megabase pairs of nucleotides (Mb). (N.B. 1pg = 10 exp-12g ; 1Mb = 10exp6 nucleotide base pairs; 1pg = 965 Mb; Bennett & Smith, 1976).

Significant intraspecific variation in nuclear DNA amount occurs in Zea mays ssp. mays, so one DNA C-value cannot be assumed to be correct for all lines of this species (Laurie & Bennett 1985; Rayburn et al. 1985,1989; Tito, Poggio & Naranjo 1991; Biradar & Rayburn 1993). Laurie & Bennett (1985) estimated 1C total nuclear DNA values to range from about 2.45 pg (2,364 Mb) to about 3.35 pg (3,233 Mb).

Interesting correlations have been reported between DNA C-value and

In general, DNA amount tends to increase with increasing knob number, and to decrease with increasing latitude or altitude of cultivation (Rayburn et al. 1985; Rayburn & Auger 1990). Genome size and effective growing season length were significantly correlated (Bullock & Rayburn 1991), and growth and yield parameters were negatively correlated with DNA C-value (Biradar, Bullock & Rayburn 1994).

Assuming a 1C value in the range c. 2.5 pg (c. 2,400 Mb) to c. 3.0 pg (c. 3,200 Mb) is an acceptable approximation for some purposes. However, when greater accuracy is needed, citing (or assuming as a calibration standard) a 1C value for one of the following identified lines is preferable: Seneca 60 - 2.46 pg, Knobless Tama Flint 2.57 pg; Va35 - 2.58 pg; W64A - 2.73 pg, KYS - 2.80 pg (Laurie & Bennett 1985).

The relative sizes and absolute 1C DNA contents of the individual chromosomes and chromosome arms of Zea mays ssp. mays cv. Seneca 60 were estimated (Bennett & Laurie 1995) as: 
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Chr         Total        Short Arm       Long Arm     Arm ratio
        ____________    ___________     ___________   _________
         %        Mb	 %       Mb      %      Mb   (Long/Short)

 1	14.23	 337	6.48	153	7.76	184	1.20
 2	11.76	 278	5.22	123	6.54	155	1.25
 3	10.96	 259	3.86	 91	7.10	168	1.84
 4	11.48	 271	4.44	105	7.04	166	1.59
 5	10.56	 250	5.04	119	5.52	131	1.10
 6	 8.76	 208	3.00	 71	5.78	137	1.92
 7	 8.42	 199	2.66	 63	5.76	136	2.16
 8	 8.56	 202	2.38	 56	6.18	146	2.60
 9	 8.10	 191	3.26	 77	4.84	114	1.49
10	 7.20	 170	2.42	 57	4.78	113	1.99
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(Assuming total haploid complement = 100%; 1C = 2.46 pg)

These values cannot be assumed to be correct for all lines, as the DNA contents of the ten chromosomes and their arms can vary depending on factors such as the number and size of heterochromatic knobs which they contain. In 1994 over 120 estimates of DNA C-values from over 25 sources for more than 100 genotypes of Zea mays ssp. mays were known, many of which were listed by Bennett & Smith 1979, 1991. Seneca 60 has few (3) knobs in its haploid chromosome complement, while its total nuclear DNA amount is very low among estimates for lines in the species. Thus, the above values offer useful approximations of minimum 1C DNA amounts for the chromosomes and chromosome arms in other lines.

REFERENCES
Contributed 1995 by: Professor Michael D Bennett
                     Jodrell Laboratory,  Royal Botanic Gardens, Kew,
                     Richmond, Surrey TW9 3DS,  UK
                     Tel:         +44 - (0)181 - 332 5311
                     Email:     M.Bennett@rbgkew.org.uk