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

straight and select for good combining lines extracted from tropical germplasm?
A Yes, I would certainly continue to adapt temperate lines to tropical conditions. Widely
used temperate lines have had extensive selection for high, stable performance by the recycling
of lines in elite by elite line crosses. At Iowa State University, we have adopted tropical
lowland germplasm to temperate conditions with intense mass selection (500 plants) in large
populations. Everett Gerrish, however, also was effective in adapting tropical germplasm to
temperate areas by crossing and backcrossing to elite lines. I feel the reverse situation can be
done (and should be done to capture the superior alleles of temperate lines), but it may be more
difficult because of the disease and insect pressures of the tropical environments. The disease
pressures experienced by tropical germplasm enhances their potential value in temperate areas.
Because temperate lines have not been exposed to the disease and insect pressures of the tropics,
more intercrossing between cycles of backcrosses may be needed to pyramid the necessary levels of
resistance. But it can be done.
FROM: E. R. Bautista
Q In hybrid maize development, which should be given more emphasis, heterotic pattern or
combining ability values? Is there any relationship between the two which could be used as a
good basis in hybrid maize development?
A Heterotic patterns vs general combining ability are two different issues. Assume we are
interested in identifying a heterotic pattern among 10 populations of maize. We could produce a
diallel set of 45 crosses among the 10 POPUlations. We evaluate the 10 populations and 45
populations crosses in replicated experiments conducted in different environments, analyze the
data, and estimate the general (GCA) and specific (SCA) combining ability effects for the populations
and population crosses, respectively. GCA effects are the average performance of a parent
in crosses and are due to primarily additive genetic and additive x additive epistatic effects.
A significantly positive estimate of GCA for a parent indicates the parent had above average
performance in crosses. A significantly positive estimate of SCA effects indicates that the
specific cross exceeded the expected, based on the average performance of the parents, or GCA,
i.e. non additive effects were expressed in the cross. This does not mean that a parent with
good GCA will not have good SCA with one specific parent. Iowa Stiff Stalk Synthetic (BSSS) is a
good example: BSSS has good GCA with other populations but it also has good SCA with Lancaster
Sure Crop. A Significantly positive SCA effect suggests a possible heterotic pattern, but one
must be careful because the one parent may have poor performance per se. Generally, populations
with good GCA also can have good SCA with other populations; the reverse situation of high SCA
for a specific pair may not indicate good GCA with other parents.
FROM: S. J. Patil
Q In the population improvement or maintenance how often you advise the introgression of
new variability?
A I feel that populations under selection to develop germplasm for breeding programs should
be open-ended populations, but the introgression of new genetic materials in the populations has
to be considered very carefully. Presumably, selection has been effective for increasing the
frequency of desirable alleles in the populations for the trait(s) considered in selection.
Hence, it seems genetic variability was reduced. If we introgress other materials into the
selected population, we do not want to dilute the previous progress from selection. If other
germplasm sources are introduced into a selected population, we would want to ensure that the new
genetic variability will not regress the previous gains. If information from other studies
suggest that other germplasm could enhance the' selected population; then it would be appropriate
to introgress the other germplasm source into the selected population.
FROM: M. Dahlan
Q In recurrent selection, the progeny is evaluated say in 3 very different environments.
There is high genotype by environment interaction, therefore difficult to select families with
good yield in all environment. How do you select the families for intercrossing?
A You will need to identify your target environments. It seems that if selection and
177