Quantitative Trait Loci for Ruminal Starch Degradability of Opaque2 Maize (Zea mays L.)

Ntjapa G. Lebaka¹, James G. Coors¹, Andres Gutierrez², Monica A. Menz², Javier F. Betran²

1. Dept. of Agronomy, University of Wisconsin–Madison, WI;

2. Dept. of Soil & Crop Sciences, Texas A&M Univ., College Station, TX.

The opaque2 (o2) gene that alters protein composition in maize also influences starch digestibility in ruminants. The softer, less dense kernel texture of o2 grain improves starch digestibility. Unfortunately, the softer kernels of o2 maize also adversely affect agronomic performance (Vasal, Specialty Corns. 2nd Ed., 2001). Breeding efforts were initiated to improve the yield and kernel characteristics of o2 genotypes as part of the Quality Protein Maize (QPM) project at the International Center for Maize and Wheat Improvement. Ruminal starch degradability of o2 maize has been reported (Phillipeau et al., J. Sci. Food Agric. 80:404-408, 2000). Most of the published research results show that ruminal digestibility is inversely related to kernel hardness (Philippeau et al., J. Anim. Sci. 77:238-243, 1999; Philippeau et al., J. Sci. Food Agric. 80:404-408, 2000 and Correa et al, J. Dairy Sci. 85:3008-3012, 2002). To our knowledge there are no published research studies on quantitative trait loci (QTL) that model ruminal starch digestibility in o2 maize.

One-hundred and thirty-six opaque2 recombinant inbred lines (RILs) spanning a wide range of kernel hardness were evaluated for in situ ruminal degradabilities. The RILs were derived from the cross CML161 o2/o2 (hard kernels) x B73 o2/o2 (soft kernels). The inbreds were raised, genotyped and rated for kernel hardness (light box) by Dr. J. Betran’s lab at Texas A & M University. A 2.0 g sample (90% dry matter) of kernels ground through a 6 mm Wiley mill screen was used for measurement of in-situ ruminal dry matter degradation (RDMD) at 0 and 14-hr incubation (1.5 g per bag by 2 bag replicates per corn sample in 5 cm x 5 cm dacron bags of 50micron pore size) in 3 mid to late lactation dairy cows fitted with ruminal cannulae and fed ad libitum a total mixed ration comprised of 60% forage (60% corn silage to 40% haylage mix) and 40% concentrate (DM basis).

Correlation analysis was done to determine the relationship between kernel hardness and in situ starch disappearance. Kernel hardness was determined using the score of 1 to 5 (where 1 – hard and 5 – soft). QTL analysis was done using composite interval mapping (Liu, Statistical Genomics, 1998) of QTL cartographer (version 2.5) using Kosambi mapping function and assuming no gene interaction with the threshold LOD score of 2.5.

Dry matter disappearance was positively correlated (r=0.73, p<0.05) with kernel hardness. Results for composite interval mapping analysis are shown in Table 1. The analysis revealed significant QTLs on chromosomes 1, 6 and 7 for 14-h dry matter disappearance. The QTLs on chromosome 7 occupy about the same position with the opaque-5 locus located near the centromere of the long arm of chromosome 7 (Gibbon and Larkins, Trends in Genet. 21(4):227-233, 2005). This suggests the effect of particle size on starch digestibility. The results indicate the positive contribution of B73 (soft endosperm) to improved ruminal starch degradability. For 0-hr DM disappearance, QTLs were detected on chromosomes 3 and 6. Three QTLs were detected for the difference between 14-h and 0-h DM disappearance.

Table 1. Detected QTLs for 0, 14 hours and (14 – 0 hr) starch disappearance in opaque2 maize RILs

Chromosome	Position (cM)	LOD	Additive (%)	R-square
14 hrs incubation
1	131	2.94	-2.7	0.08
6	96	4.30	-3.9	0.18
7	75	3.40	3.0	0.11
7	83	3.00	2.7	0.08
0-hr incubation
3	114	3.23	2.0	0.08
6	98	5.07	-3.0	0.19
(14-hrs - 0-hr) incubation
7	41	4.37	2.8	0.04
7	56	3.55	1.6	0.11
7	61	3.44	1.6	0.11

14-h DM disappearance includes both 0-h and 14-h minus 0-h disappearance. 0-h DM disappearance represents an instantaneously soluble part of the total DM that dissolves instantly in the ruminal fluid. The 6-mm ground softer endosperm generally has a higher component of finer particles (instantaneously soluble particles) than harder endosperm. The difference between 0-h and 14-h disappearance represents the DM that is degraded in the rumen and therefore the actual ruminal DM degradability. One to three QTLs for the difference were detected on the shorter arm near the centromere of chromosome 7. This position coincides with the position of one of the modifiers located between the locus for opaque 2 and the centromere on the shorter arm of chromosome 7 (Lopes et al., Mol. Gen. Genet. 247:603-613, 1995 and Gibbon and Larkins, Trends in Genet. 21(4):227-233, 2005). QTLs for the difference did not overlap with QTLs for the other traits suggesting that the true DM degradability can probably be selected for independent of the other traits. Alleles for improved digestibility came from B73 and those reducing digestibility were contributed by CML161. However some RILs performed better or worse than the best or worse parent respectively indicating the presence of transgressive segregation and possible additive by additive gene interaction.