Combining ability of different versions of Tuxpeño-based maize germplasm developed at CIMMYT
--G. Srinivasan,* S. K. Vasal and F. Gonzalez C.
Tuxpeño is one of the most productive and successful Mexican races of maize (Zea mays L.) for the lowland tropics. CIMMYT has made use of this germplasm extensively in its breeding programs. Over the last 25 years, CIMMYT has developed and improved many Tuxpeño-based germplasms with different objectives using varied breeding methodologies, and selecting for different characters.
In this study, nine different Tuxpeño versions developed at CIMMYT were compared for their combining ability using four testers (two from Tuxpeño and two from ETO Blanco). The different Tuxpeño versions used were: 1) Population 21 (C0); 2) Population 21 (C6); 3) Population 21 (Streak Resistant); 4) Tuxpeño Sequía (C8) - (Drought Resistant version); 5) Population 21 (C2) - (Inbreeding Stress Tolerance -IST); 6) Population 21 (C1) - (Modified Reciprocal Recurrent Selection - MRRS); 7) Population 49 (C4); 8) Population 49 (Streak Resistant); and 9) Population 49 (C2) (Inbreeding Stress Tolerance). Thirty-six line x tester crosses and 13 parents were evaluated in two environments at Poza Rica and Tlaltizapan, México during 1991.
Table 1. Mean cross performance of nine Tuxpeño versions and
four testers for grain yield, plant height, and days to 50% silking in
two environments at Poza Rica (PR) and Tlaltizapan (TL) tested during 1991.
|Tuxpeño version||Grain yield t/ha||GCA for for yield t/ha||Plant height (cm)||Days to 50% silking|
For per se performance, Pop. 21 (MRRS) ranked high at Poza Rica (7.5 Mg ha-1) and Pop. 21 (SR) yielded the best at Tlaltizapan (7.59 Mg ha-1). For cross-performance, Pop. 21 (IST) at Poza Rica and Pop. 21 (MRRS) at Tlaltizapan ranked first, yielding 7.6 Mg ha-1 and 8.16 Mg ha-1, respectively (Table 1). The improved cross performance of the two hybrid-oriented Tuxpeño versions was due to an increase in their ear length and 100 kernel weight. Highly significant positive general combining ability (GCA) effects for yield were recorded by Pop. 21 (IST) and Pop. 21 (MRRS) at Poza Rica and Tlaltizapan respectively (Table 1). The three versions of Pop. 49 showed negative GCA for yield primarily because they were earlier in maturity than the rest and shorter in plant height and hence relatively lower yielding than the versions of Pop. 21. However, among the three Pop. 49 versions, Pop. 49 (IST) was the highest yielder in crosses at Poza Rica and Pop. 49 (C4) was the highest yielder at Tlaltizapan.
Figure 1. Grain yield (t/ha) of six Tuxpeño versions crossed to four testers.
Tuxpeño Sequía crossed to Tester 1 (Pop. 32 Syn.) was the highest yielder (9.4 Mg ha-1) at Tlaltizapan. At Poza Rica, Pop. 21 (IST-C2) crossed to Tester 1 was the highest yielder (Fig. 1). Results showed that, after six cycles of improvement, although Pop. 21 showed selection gains of 1.72% per cycle for per se performance, there was little change in the cross performance. However, once the breeding methodology was changed with emphasis on hybrid development, the cross performance improved considerably showing gains of 1.4% and 2.4% per year respectively in Pop. 21 (IST) and Pop. 21 (MRRS) (Table 2). Interpopulation improvement using modified reciprocal recurrent selection (MRRS) and breeding for tolerance to inbreeding stress (IST) through S3 recurrent selection has considerably improved Tuxpeño into a more ideal germplasm for hybrid development work at CIMMYT and elsewhere.
Table 2. Cross performance of five Tuxpeño versions for grain
yield averaged over two environments (Poza Rica and Tlaltizapan, México)
and selection gains realized per year.
|Tuxpeño versions||Yield (Mg ha-1)||Gain/year (%)|
|Pop.21 C6 *||7.13||0.00|
|Tuxpeño Sequía) C8 **||7.69||0.99|
|Pop.21 (IST) C2 ?||7.72||1.39|
|Pop.21 (MRRS) C1?||7.64||2.40|
** A cycle of selection takes one year
? A cycle of selection takes three years
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