REPORT - Search CIMMYT repository

To generate a maize population of maximum
variability, CIMMYT has begun to put
together, into a single composite, seed from
5,000 of the more than 10,000 collections
available in the germplasm bank in Mexico.
Six composites were made by taking 15 seeds
of each collection. These composites were
planted at ten-day intervals in a neutral environment
at Tlaltizapan in the State of Morelos
Plants of the third and fourth sowing date~
were detasseled. Natural selection was important
in eliminating some of the collections
insofar as seed production was concerned but
some plants, which did not produce seed,
probably produced pollen which was represented
in the seed harvested. The extent of
differential loss is not subject to determination.
Reaction to soil temperature and climate
probably resulted in variable elimination of
several of the collections, also.
Approximately 20,300 ears of the de-tasseled
rows from this first cycle of synthesis
were used to prepare two balance composites.
One composite was formed by taking ten
seeds from each ear harvested. Twenty seeds
were taken for the second composite.
The first composite was subdivided by taking
87,000 seeds to plant the male rows at
three different sowing dates for the second
cycle of intercrossing during the winter of
1969-1970. The remaining 116,000 seeds were
LJse to plant the female rows, which were
de-tasseled. This arrangement permitted maximum
recombination among early, intermediate,
and 'late genotypes.
The second composite will be developed
wHh the remaining collections. Then, the two
resulting composites will be planted together
to form the complete "world composite". Following
t~is compositing, the material will be
divided, and recombination and selection will
be continued at low, intermediate and high
elevations.
All potentially useful genes and/or gene
combinations are not likely to be present in
any single gene pool. Hence it is unlikely that
such a utopian population will be created
through the formation of a "world composite".
It is likely, however, that a far broader mixture
of genetic materials is being made than has
existed before, and this may be of considerable
value to maize improvement efforts everywhere.
Plant Height Studies
Most tropical corn plants grow too tall and
are vulnerable to lodging. Even moderate
winds exert tremendous force on very tall
plants. In order to maintain plant standability,
and to consider higher fertilizer levels aimed
at higher production, it appears that varieties
with shorter plants must be developed. Two
separate avenues of approach are being followed
to achieve shorter plants. Genetic
dwarfs are being studied as one possibility,
while another attempt is through recurrent selection
of ear-to-row plantings and hand pollination
among selected rows.
The genetic dwarfs being used are brachytic
1, brachytic 2, brachytic 3, anther ear,
d] short, d 1 tall, and pygmy. Others, as yet
not identified, are being studied. The brachytic
2 has been used to convert several diverse
germplasm pools to dwarf plants. Modified
ear-to-row mass selection for yield is being
carried out in Tuxpeno Crema I-brachytic 2.
Problems of irregularity of plant height, husk
cover, ear position, and other difficulties remain,
but the materials appear promising. The
other dwarf populations have not progressed
as far, but some show real promise.
CIMMYT has begun to put together, into a single
composite, seed from 5,000 of the more than
10,000 collections available in the germ plasm
bank in Mexico. The aim is to generate a maize
population of maximum variability... a "World
Composite".
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