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

2. Improvement of combining ability.
Good general combining ability (GCA) of source populations and of progenitors derived from
them is extremely important for hybrid development. Topcross test helps to evaluate the GCA of
the lines (Davis, 1927). With exception of few methods, most standard interpopulation improvement
schemes do not permit improvement of this trait specifically. Appropriate modifications can
be made to practically every scheme to obtain some information on general combining ability. In
later sections of this paper some schemes have been proposed that permit integration of population
improvement and hybrid research efforts with particular emphasis on improving this trait.
3. Improvi ng cross performance.
In most CIMMYT populations, intrapopulation improvement has been emphasized in the past.
However, emphasis on hybrids in recent years would necessitate improving cross-bred performance
of already known heterotic populations on a continuous basis. Two well known and most widely
used populations in CIMMYT's lowland tropical program are already undergoing interpopulation
improvement. A modified version of reciprocal recurrent selection scheme is being used since
1987 and two cycles have already been completed. Modifications over standard scheme are aimed at
improving tolerance to inbreeding by saving topcross seed from those S1 families that look good;
which is not possible in standard scheme outlined by Comstock, Robinson, and Harvey in 1949. In
addition to cyclic improvement of two populations for increased heterosis, several by-products of
narrow genetic base synthetics can be generated to develop intersynthetic hybrids. Recycling of
superior S1s, or S2s can also be initiated to generate new series of inbred lines within each
heterotic group. National programs involved in testing half-sib progenies of these populations
can request seed of improved cycles, site specific synthetics and in addition can request seed of
early generation line(s) for further inbreeding work to develop inbred lines. Other subprograms
within CIMMYT also are now practicing similar schemes.
4. Development of new heterotic populations.
Inbred-based material serves not only in hybrid formation but also can be used to develop
hybrid-oriented source germplasm. Formation of new heterotic populations can be achieved by
identifying progenitors of known heterotic performance followed by genetic mixing of components
within each group for at least three cycles. The progenitors involved could be inbred, noninbred,
and even a combination of both types. Involvement of inbred progenitors would be
desirable provided vigorous and productive lines are used in their formation. The resulting
products would be more readily usable for hybrid work as they would possess essential traits
needed in hybrid-oriented maize germplasm. Systematic attempts were made to work out heterotic
patterns of lines that had survived inbreeding and also were good general combiners. Since there
was no previous experience in developing such popUlations, four instead of two inbred testers
were used, representing Tuxpeno and Eto types of germplasm. Based on heterotic patterns, two
inbred testers were finally chosen that had positive GCA and which provided more clearcut
separation of lines into one group or another. As a result of this kind of work, two tropical
heterotic groups (THG-A and THG-B) , and two subtropical heterotic groups (STHG-A and STHG-B) ,
were formed (Vasal et aT. 1992d, 1992e). OVer the years additional heterotic populations have
been formed by other subprograms at headquarters and in the CIMMYT regional maize programs at
Cali and Zimbabwe. The use of such heterotic populations would greatly facilitate hybrid
development work in the national programs as lines developed from one heterotic group would be
destined to cross well with lines of the opposite heterotic group. Also, when such populations
are subjected to ~ystematic interpopulation improvement schemes, the breeders in the national
programs would be able to request early generation lines exhibiting good combining ability. To
reduce the number of currently available populations, efforts may be directed in the future to
consolidate and broaden germplasm belonging to a similar heterotic group.
5. Inbred based populations.
As pointed out earlier, conscientious effort should be made to make the best use of lines
in various ways in addition to the development of conventional hybrids. Inbreds within the same
heterotic group resulting from one or more populations can be recombined to form synthetics for
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