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A Comparisonof of Two Methods
of of Stratified Mass Selection
Stratified Mass Selection as as usually practiced
by by maize breeders involves the time
consuming operation of of physically weighing
the dry grain producedon on eachof of the selected
plants.
Selection for prolificacy has been suggested
as as an an effective alternative.
The study
described below attempts to to compare the effectiveness
of of these two systems.
Two populations adapted to to the inter­
inter~
mediate altitudes of of Mexico (Composite II II
Celaya and Puebla GroupI) I) were subjectedtoto
three cycles of of Stratified Mass Selection by by
grain weight and by prolificacy.
The resulting
populations were studied in in yield trial for a
period of of three year and the data are summarized
in in Table 17.
It It is is apparent that the two methods were
substantially equal in in their effectivenessinin increasing
yield and that the yields after two
and three cycles in in Composite Celaya II II
and
Puebla Group I, I, respectively, were equal to to
or or better than the best adapted hybrid for the
region. It It should be be noted, however, that the
Puebla GroupI, I, Cycle22 population, could not
be be recommended to to farmers because of of its its
lodging susceptibility even thoughitit ly ly outyields the recommended hybrids.
Another fact is is worth noting. These data
do not show the clearcut tendency for the accompanying
undesirable side effects -- in­
increaseinin
daystoto flower norinin percent lodging
apparent­
as as was reportedinin the studyof of Stratified Mass
Selection versus Ear-to-row selection in in the
two Chalquerio populations.
GENETICS AND CYTOLOGY
Studieson on the Chromosome
Morphologyof of Maize
Extensive data on on chromosome knobs of of
maize have been accumulated during the last
decade, beginning with the workof of Dr. Barbara
McClintockinin 1958.
These studies have been
extremely useful in in determining the origin,
relationships and migration of of the different
racesof of maizeinin Latin America.
Through detailed analysis of of these data,
CIMMYT scientists have localized several
geographical centers of of origin of of the present
day racesof of maize.
Eachof of these centers has
been located on on the basis of of aa specific combinationof
of basic knobbed positions and sizes
called "chromosome knob complexes".
These complexes include: (1) (1) Mesa Central
Complex, characterizedby by the absenceof
of
knobsininall all the knob forming positions, center
of of originof of this complexisisinin the highlandsof
of
Mexico. (2) Eastern Mexico Complex, characterized
by by the presence of of medium-sized
knobs in in manyof of the knob forming positions.
The centerof of origin of of this complex is is in in the
eastern coastof of Mexico. (3) Zapalote Complex,
having a very special knob combination of of
several sizes in in several of of the knob forming
positions. This complex originated in in southwestern
Mexico, in in the states of of Oaxaca and
Chiapas.
(4) Small Knob Complex, characterized
by the presence of of small knobs in in many
of of the knob forming positions.
Originisisinin the
highlands of of Guatemala. (5) (5) Southern Guatemala
Complex, characterized by by several large
and some small knobs. Center of of origin is is
probably in in the lowlands of of southern Guatemala.
(6) (6) Venezuelan Complex, possesses
large knobsinin several knob forming positions,
primarilyon on the long armsof of the chromosomes.
Geographical center of of origin is is in in some
regionof
of Venezuela.
(7) Andean Complex, characterized
by by the presence of of only two small
knobs; oneon on the long armof of chromosome 7
and the other on on the distal position of of the
long arm of of chromosome 6. 6.
Center of of origin
of of this complexisis locatedinin the highlandsof
of
South America.
Studyof of the Chromosome Knob
Constitutionof of Teosinte
To study the chromosome constitution of of
maize and its its relatives, 14 14 new collections
of of Teosinte were obtained from the States of of
Guanajuato, Mexico, Guerrero and Michoacan
in in Mexico, making aa total of of 39 39 Teosinte collections
among CIMMYT's seed stocks.
In In an an investigation of of the frequency with
which different maize and teosinte races hybridizeinin
nature, several maize races and the
teosintes Chalco (Td and Guerrero (T2)
2 )
were
planted at at the Tepalcingo Experiment Station
in in Mexico. At At flowering time, five pollinations
were made on on shoots of of each maize variety,
using aa mixtureof of equal volumesof of pollenof
of
the variety and Tlo T i • Mixing equal volumes of of
maize and teosinte pollen simulated aa hypothetical
situation in in which such populations
would grow together and coincide in in their
flowering periods. The same procedure was
used in in the case of of T 2 2 to to pollinate the maize
races.
No attempt was made to to use teosinte
populations as as females.
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