Proceedings of the Fifth Asian Regional Maize Workshop - Search ...

help of maize breeders from the national programs. The overwhelming support from the u.s
breeders in this task was unprecedented. Dr. Larry Darrah from the Univ. of Missouri was
particularly helpful in arranging to distribute the trials within the u.s. From this extensive
work, several parents exhibiting high GCA were identified. As expected, the heterosis among
materials within each group, with a few exceptions, was not very high (Table 3). These results
did not surprise us considering the broad genetic base of most CIMMYT germp1asm. Some clear cut
heterotic groupings emerged from this work and some patterns reported earlier by other workers
were reconfirmed (Table 4). Heterosis among potentially important tropical and subtropical
germplasm was also studied. Lowland tropical population 43 gave the best performance in combination
with subtropical populations 42 and 44. Population 32 also gave good cross performance with
population 44. It may also be pointed out that Su~an-1 was also included in one of the dia11e1s.
It exhibited good heterotic performance in crosses with populations 24, 27 and pool 26. Population
24 also showed high level of heterosis with population 36. Among the white endosperms
germp1asm, populations, 25 and'32 performed well in crosses with populations 21 and 29. In
early/intermediate germp1asm, the best crosses were population 23 x pool 20, and population 26 x
pool 21. The heterotic pattern among QPM germp1asm were not c1earcut. The best general combinnears
for yield were populations 62, 63 and EV PR 7737 in tropical germp1asm, and pool 32 QPM and
populations 68 and 69 in subtropical germp1asm. Detailed information for each dia11el has been
published in scientific journals (Beck et a7. 1990, 1991; Crossa et a7. 1990; Vasal et a7.
1986, 1992a, 1992b, 1992c, 1993a, 1993b). The above information has provided useful guide to
CIMMYT breeders and to breeders in the national programs to initiate hybrid breeding activities.
Periodically, additional studies of this kind will be planned to determine combining ability of
new maize germplasm emerging from different subprograms.
b. Development of hybrid-oriented source germp1asm.
Source populations constitute the building block of hybrid development work. While
improved as well as unimproved germp1asm can be used, there are greater possibilities of extracting
superior lines in higher frequencies from the improved populations. Unimproved germp1asm
particularly the germp1asm accessions rarely, if ever, give rise to usable and useful lines.
However, germp1asm accessions can provide germp1asm with diverse traits which could be used in
hybrid work. Such germp1asm, however, may have to be carefully tailored and "civilized" through
appropriate prebreeding activities. Source populations designed as hybrid-oriented should
possess additional attributes including tolerance to inbreeding, good general combining ability,
good heterotic relationships, cross-bred performance to at least one or more populations,
possessing characteristics that will be useful either as pollen or seed parent, and, most
importantly, in possessing desirable agronomic attributes. Source populations tropical as well
as subtropical being handled in the national programs are not necessarily the best choices as
these have been improved using breeding schemes of mass selection, full-sib, half-sib, modified
ear-to-row or various modifications of these which in fact would not necessarily make then
potentially useful for hybrid work. We have done some work in developir.g hybrid-oriented source
germp1asm at CIMMYT and would like to share this information with you. Also, we would like to
suggest some strategies for developing and improving such germp1asm.
1. Improving tolerance to inbreeding stress.
Inbreeding depression is of little significance in self-pollinated crops that are accustomed
to automat. In out-pollinating species like maize, inbreeding depression is of considerable
concern if hybrids involving inbred parents are to be developed. Inbreeding effects in maize
have long been known to maize breeders as a result of excellent researches conducted by East
(1908) and Shull (1908, 1909). Different materials behave differently for this trait. Some
materials may collapse following a few successive generations of selling while others may provide
some useful lines even though in much lesser frequency. Maize in general is considered moderately
tolerant to inbreeding depression. Though some early generation inbreds can be developed
rather easily, it is extremely difficult to develop inbreds with high level of inbreeding that
have acceptable seed yield and are self-sustaining without any reproduction abnormality either on
male or on female side. This is particularly true of tropical and subtropical germplasm which
so far have not involved schemes that involved much inbreeding.
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