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Mass Selection
In the lowland tropical research program
at the Poza Rica Station (Mexico), mass selection
for yield is being conducted on several
populations. These include Tuxpeno Crema 1,
Mix 1 x Colima Group 1, [(Mix 1 x Colima
Group 1) x Eto Blanco] and Eto Blanco. The
Tuxpeno population is being studied to determine
the effects of two cycles of selection per
year, compared to the effect of a single cycle.
The others are being selected individually as
varieties, and as cross composites, to obtain
data on the most desirable type of population
base with which to begin mass selection. All
are high-yielding, relatively good agronomically,
and could be used directly as improved
varieties.
One of CIMMYT's studies in lowland areas
of Mexico involves a comparison of the yields
of flint varieties with dent varieties. Several
cycles of mixing and separation into flints and
dents have been conducted and yield comparisons
will begin this year. Samples of
these materials have been provided to maize
breeders in several areas, and some have
been put directly into commercial use. The
Santa Cruz lowland area of Bolivia is now
growing more than 1,000 hectares of a yellow
flint variety from this study, only two years
after receiving a sample of seed.
Improved Populations
This work involves the formation and exposure
of an heterozygous and heterogeneous
population to the action of natural and artificial
selection under varying environments.
Objectives are: (1) development of an
improved population, per se; as well as an
improved source of breeding material for development
of high yielding varieties and/or
hybrids for a broad area, (2) development of
a germplasm pool with a high degree of resistance
to crop hazards, (3) improvement of food
quality through incorporation of genetic factors
contributing to high lysine content, and (4)
strengthening of cooperative research activities
among interested maize research workers.
The method used in this projecf (for the
formation of a widely adapted variety from a
Tuxpeno germplasm complex) is referred to
as convergent-divergent selection. A modification
of the ear-to-row selection method is
being used to insure a high degree of recombination.
The convergence phase of the project involves
the synthesis of a composite containing
most of the Tuxpeno material in CIMMYT's
germplasm bank adapted to altitudes ranging
from a to 1,000 meters above sea level. Seven
hundred and ninety two varieties of the Tuxpeno
race have been grown under isolation
for four generations of synthesis. The composite
should reach a workable level of genetic
equilibrium in the fifth generation.
The divergence phase of the project will
be started in 1971. Sublots of the basic composite
then will be sent to collaborators around
the world. Mass selection for prolific, highly
productive, and healthy plants of moderate
height will be made at harvest. After two or
three generations of mass selection, a sample
from each collaborator is to be sent to CIMMYT
where compositing will be done to start the
next cycle of convergence. After one generation
of recombination, the new population
will again be divided and a sample sent to the
same collaborators for planting and resumption
of the divergence phase. The plan can
be continued as long as it is productive. Each
collaborator may continue to select in his own
population for future improvement at his own
location.
This method seems useful in adaptation
of a population to a wide area. Each subgroup
will become characterized by a particular combination
of traits to the extent that different
environmental effects exist in the several locations
involved. Through successful selection,
resistance to different crop hazards would
become a characteristic of the population.
Development of a World Composite
Local gene pools have in recent years
been enriched through use of exotic germplasm
sources.
A number of maize scientists have suggested
that the formation of a world composite
in maize could be a worthwhile long-range approach
to the effective use of available genetic
materials and to the selective improvement of
more complex traits. Exotic stocks generally
have been used to search for single genes
responsible for simply inherited traits, such as
disease resistance. The value of these stocks
for breeding greatly exceeds that commonly
attributed to them, but a different approach is
needed in their management and use.
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