57 research outputs found
La importancia del intervalo de la floración en el mejoramiento para la resistencia a sequía en maíz tropical
La longitud de intervalo entre la aparición de estigmas y antesis se incrementa cuando la sequía coincide con la época de floración del maíz (Zea mays L.). Cuatro poblaciones élite de maíz tropical del CIMMYT están siendo mejoradas para resistencia a sequía por esquemas de selección recurrente (S1 o hermanos completos) para rendimiento de grano y varias otras características, tanto bajo estrés de sequía, como bajo condiciones de buena humedad. Los datos recolectados de más de 2,000 famillas por población evaluadas en parcelas de un solo surco, bajo tres niveles de estrés de humedad de suelo (1. estrés severo durante el período de pre y postfloración; 2. estrés intermedio durante el llenado de grano; y 3. irrigación normal, todos en ausencia de lluvia), mostraron correlación débil o ausencia de ésta entre el rendimiento de grano y otras características relacionadas al balance hídrico de la planta, tales como: enrollamiento foliar y senescencia, foto-oxidación, concentración foliar de clorofila, tasa de elongación vegetativa, temperatura foliar y potencial hídrico matutino. El rendimiento bajo todos los niveles de estrés fue correlacionado negativamente con el intervalo de floración y el intervalo de floración se incrementó debido a la sequía. Asimismo, los granos y mazorcas por plantase redujeron significativamente. En todas las poblaciones el rendimiento disminuyó aproximadamente 10% por día de incremento en el intervalo de floración y hasta 8 días. En varias situaciones de estrés, la heredabilidad de sentido amplio para el intervalo de floración fue mayor que aquella del rendimiento de grano y la correlación genética entre rendimiento de grano y el intervalo de floración aproximadamente -1.00. Los sintéticos formados a partir de una de las poblaciones, seguidos de selección bidireccional y evaluados bajo sequía, demostraron ventajas adaptativas como baja temperatura foliar, baja senescencia foliar, un intervalo de floración reducido y hojas erectas, especialmente, cuando todas estas características fueron combinadas en un índice de selección. La selección por intervalo de floración corto y alto rendimiento de grano puede ser un medio eficaz de mejorar la tolerancia a la sequía en maíz tropical
Interpreting genotype × environment interaction in tropical maize using linked molecular markers and environmental covariables
An understanding of the genetic and environmental
basis of genotype´environment interaction (GEI)
is of fundamental importance in plant breeding. In mapping
quantitative trait loci (QTLs), suitable genetic populations
are grown in different environments causing
QTLs´environment interaction (QEI). The main objective
of the present study is to show how Partial Least
Squares (PLS) regression and Factorial Regression (FR)
models using genetic markers and environmental covariables
can be used for studying QEI related to GEI. Biomass
data were analyzed from a multi-environment trial
consisting of 161 lines from a F3:4 maize segregating
population originally created with the purpose of mapping
QTLs loci and investigating adaptation differences
between highland and lowland tropical maize. PLS and
FR methods detected 30 genetic markers (out of 86) that
explained a sizeable proportion of the interaction of
maize lines over four contrasting environments involving
two low-altitude sites, one intermediate-altitude site, and
one high-altitude site for biomass production. Based on a
previous study, most of the 30 markers were associated
with QTLs for biomass and exhibited significant QEI. It
was found that marker loci in lines with positive GEI for
the highland environments contained more highland alleles,
whereas marker loci in lines with positive GEI for
intermediate and lowland environments contained more
lowland alleles. In addition, PLS and FR models identified maximum temperature as the most-important environmental
covariable for GEI. Using a stepwise variable
selection procedure, a FR model was constructed for
GEI and QEI that exclusively included cross products
between genetic markers and environmental covariables.
Higher maximum temperature in low- and intermediatealtitude
sites affected the expression of some QTLs,
while minimum temperature affected the expression of
other QTLs
Genetic analysis of adaptation differences between highland and lowland tropical maize using molecular markers
Molecular-marker loci were used to investigate
the adaptation differences between highland and
lowland tropical maize. An F2 population from the cross
of two inbred lines independently derived from highland
and lowland maize germplasm was developed, and extracted
F3:4 lines were phenotype in replicated field trials
at four thermally diverse tropical testing sites, ranging
from lowland to extreme highland (mean growing season
temperature range 13.2–24.6°C). Traits closely related
with adaptation, such as biomass and grain yield, yield
components, days from sowing to male and female flowering,
total leaf number, plant height and number of primary
tassel branches (TBN), were analyzed. A large line
´ environment interaction was observed for most traits.
The genetic basis of this interaction was reflected by significant,
but systematic, changes from lowland to highland
sites in the correlation between the trait value and
genomic composition (designated by the proportion of
marker alleles with the same origin). Joint analysis of
quantitative trait loci (QTLs) over sites detected 5–8
QTLs for each trait (except disease scores, with data only
from one site). With the exception of one QTL for
TBN, none of these accounted for more than 15% of the
total phenotypic variation. In total, detected QTLs accounted
for 24–61% of the variation at each site on average.
For yield, yield components and disease scores, alleles
generally favored the site of origin. Highland-derived
alleles had little effect at lowland sites, while lowland-
derived alleles showed relatively broader adaptation.
Gradual changes in the estimated QTL effects with
increasing mean site temperature were observed, and
paralleled the observed patterns of adaptation in high land and lowland germplasm. Several clusters of QTLs
for different traits reflected the relative importance in the
adaptation differences between the two germplasm types,
and pleiotropy is suggested as the main cause for the
clustering. Breeding for broad thermal adaptation should
be possible by pooling genes showing adaptation to specific
thermal regimes, though perhaps at the expense of
reduced progress for adaptation to a specific site. Molecular
marker-assisted selection would be an ideal tool for
this task, since it could greatly reduce the linkage drag
caused by the unintentional transfer of undesirable trait
Selection improves drought tolerance in tropical maize populations: II. Direct and correlated responses among secondary traits
Recurrent selection for drought tolerance for three to eight cycles has increased grain yield (GY) under drought at flowering by 30 to 50% in three lowland tropical maize (Zea mays L.) populations. The relationships among secondary traits as a result of this selection have not been determined, however. The objectives of this study were to measure direct and correlated changes due to selection in secondary traits by evaluating cycles of selection and appropriate check entries at five water-stressed (mean yield 2.35, range 1.01-4.48 Mg ha) and five well-watered environments (mean yield 7.96, range 5.81-10.40 Mg ha). Under drought, changes per cycle with recurrent S selection (P < 0.05) were as follows: GY 12.6%, fertile ears per plant (EPP) 8.9%, grains per fertile ear (GPE) 6.3%, grain number per square meter 12.2%, 1000 grain weight no change, anthesis-silking interval (ASI) -22.0%, days from sowing to 50% anthesis -0.7%, plant height -2.0%, primary tassel branch number -5.9%, and senesced leaf area 2.7%. Responses under well-watered conditions were smaller but generally of the same sign. Grain yield was strongly associated with grain number per square meter in both water-stressed and well-watered environments (r = 0. 96; r = 0.87; P < 0.001). Grain yield, EPP, and GPE were strongly correlated with ASI across entries under drought (r = -0.89, -0.93, -0.90; P < 0.001), though not when water was plentiful. The use of managed stress environments that consistently reveal genetic variation for these traits at specific times during crop development is endorsed for selection purposes
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