Maize Genetics Cooperation Newsletter vol 81 2007
Ten mutants in our Hi27 NIL series, one on each of the 10 chromosomes, were chosen for a diallel analysis of heterosis. Each mutant had been backcrossed at least six times to Hi27, hardy tropical flint inbred (see above). Our NILs are sibbed following backcrossing, allowing preservation of some heterozygosity (1.5625% >BC6, 0.0977% >BC10). However, there is much evidence of linkage drag in such conversions, linkage that could also be associated with inter-NIL heterosis. Linkage drag with loci na1^Hi27 (3L-101) and lg2^Hi27 (3L-113) led us (Ming et al., MNL 69:60) to the Mv/mv locus on chromosome 3L-80 (all temperate corn carries allele mv for susceptibility to the tropical maize mosaic virus). Current studies in Hawaii seek to use linkage drag in spotting other QTLs of importance to corn breeders.
It can be conjectured that QTLs for yield heterosis are often linked to mutant genes we�ve backcrossed into Hi27. To test our hypothesis, ten NIL (one per chromosome) were crossed in a diallel manner, including parent Hi27 (Griffing method 2). Mutants selected were located at 1S-55 (gt^Hi27), 2S-11 (lg^Hi27), 3L-149.0 (a^Hi27), 4S-(55) (bm3^Hi27), 5S-41 (bm^Hi27), 6L-17 (y^Hi27), 7S-16 (o2^Hi27), 8-(0) (rf4^Hi27), 9S-31 (bz^Hi27) and 10L-64 (R-nj^Hi27). Mutant rf4 had been advanced 12 backcrosses. The diallel entries were planted in single-row 5m plots in Field S1-4 at Waimanalo on May 23 and June 21, 2006. Data were taken from two samples per row of 5 plants, with months treated as replications.
Heterosis among the 53 hybrids (two were omitted due to poor stand) was universal for measured traits. Highly significant differences (P<0.001) were observed for yield (as gm. per plant), for ear length and ear diameters in cm. (Table 1), and also for plant heights (not shown). The Experimental Error interaction of NIL x �Reps� (months) was never significant when tested against sampling error, a reflection of the homogeneity of Waimanalo soils on which our breeding nurseries have been grown since the 1960�s.
Table 1.
ANOVA for yield, ear length and ear diameter.
|
Source |
df |
Yield |
EL |
ED |
|
Entries |
63 |
456.3** |
1.60** |
0.05** |
|
Reps |
1 |
420.9 ns |
2.41* |
0.09* |
|
NIL & Parent |
10 |
277.6* |
2.04** |
0.06** |
|
F1s |
52 |
375.3** |
1.10** |
0.04** |
|
Heterosis (NIL vs F1s) |
1 |
6,456.4** |
22.93** |
0.39** |
|
EE (Ent x Rep) |
63 |
115.7 ns |
0.39 ns |
0.01 ns |
|
SE |
128 |
584.4 |
3.47 |
0.13 |
**,*
- Significant at the 5% and 1% level of probability
Means and standard deviations of these three sets of data are summarized in Table 2. The NILs were similar to their parent inbred Hi27, while all hybrids were significantly higher in yield and ear traits. Relative homogeneity characterized all traits, as evident in the standard deviations and CV values.
Table 2.
Means and standard deviations for yield and ear traits.
|
|
Yield |
EL |
ED |
|
Hi27 |
92.5 � 16.9 |
13.8 � 1.4 |
3.95 � 0.17 |
|
NILs |
93.8 � 20.4 |
12.7 � 1.7 |
4.05 � 0.32 |
|
Hybrids |
121.1 � 17.8 |
14.4 � 1.3 |
4.25 � 0.27 |
Individual variations were seen in GCA (general combining ability) and mid-parent heterosis values, and these will be studied in greater detail following duplicate plantings in 2007. Mutant bz^Hi27 had the highest GCA for yield, but it is in a known linkage group with C, and would be expected to have greater linkage drag in our conversions to Hi27 (which is Bz C-I). GCA for hybrid yield minimized for rf4^Hi27 (97.6 gm/plant). However, the rf4 conversion represented BC12 (~ .0244% heterozygosity) and the male-sterile hybrids were grown as a block, both facts helping account for their reduced apparent heterosis for yield.
Heterosis among NIL hybrids clearly can reflect the remnant of heterozygosity among their very distinct temperate dent and tropical flint parents; indeed, we make much use of modified sister-single crosses in our supersweet tropical breeding to exploit this kind of heterosis. But it is similarly clear that linkage drag with QTLs affecting vigor and yield may play a role in this heterotic response. The dent x flint heterosis is very widely exploited in tropical corn breeding, and localization of significant QTLs may improve our genetic advance.
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