Maize Genetics Cooperation Newsletter vol 81 2007
University of
Wisconsin-Madison
Texas A&M
University
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 50 micron 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-hr dry matter disappearance. The QTLs on
chromosome 7 occupy about the same position with the opaque5 locus
located near the centromere of the long arm of
chromosome 7 (Gibbon and Larkins, Trends 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-hr and 0-hr 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-hr 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-hr - 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-hr DM
disappearance includes both 0-hr and 14-hr minus 0 h disappearance. 0-hr 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-hr and 14-hr
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 opaque2 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 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.
Please Note: Notes submitted to the Maize Genetics Cooperation
Newsletter may be cited only with consent of authors.