So far in this course we have dealt entirely with the evolution of characters that are controlled by simple Mendelian inheritance at a single locus. There are notes on the course website about gametic disequilibrium and how allele frequencies change at two loci simultaneously, but we didn’t discuss them. In every example we’ve considered we’ve imagined that we could understand something about evolution by examining the evolution of a single gene. That’s the domain of classical population genetics. For the next few weeks we’re going to be exploring a field that’s actually older than classical population genetics, although the approach we’ll be taking to it involves the use of population genetic machinery. If you know a little about the history of evolutionary biology, you may know that after the rediscovery of Mendel’s work in 1900 there was a heated debate between the “biometricians” (e.g., Galton and Pearson) and the “Mendelians” (e.g., de Vries, Correns, Bateson, and Morgan). Biometricians asserted that the really important variation in evolution didn’t follow Mendelian rules. Height, weight, skin color, and similar traits seemed to

Human biochemical genetics

A doubling in global food demand projected for the next 50 years poses huge challenges for the sustainability both of food production and of terrestrial and aquatic ecosystems and the services they provide to society. Agriculturalists are the principal managers of global useable lands and will shape, perhaps irreversibly, the surface of the Earth in the coming decades. New incentives and policies for ensuring the sustainability of agriculture and ecosystem services will be crucial if we are to meet the demands of improving yields without compromising environmental integrity or public health.

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Agricultural sustainability and intensive production practices

羟氨苄青霉素引起大疱性类天疱疮样疹

Cultivar evaluation and mega-environment identification are among the most important objectives of multi-environment trials (MET). Although the measured yield is a combined result of effects of genotype (G), environment (E), and genotype × environment interaction (GE), only G and GE are relevant to cultivar evaluation and mega-environment identification. This paper presents a GGE (i.e., G + GE) biplot, which is constructed by the first two symmetrically scaled principal components (PC1 and PC2) derived from singular value decomposition of environment-centered MET data. The GGE biplot graphically displays G plus GE of a MET in a way that facilitates visual cultivar evaluation and mega-environment identification. When applied to yield data of the 1989 through 1998 Ontario winter wheat (Triticum aestivum L.) performance trials, the GGE biplots clearly identified yearly winning genotypes and their winning niches. Collective analysis of the yearly biplots suggests two winter wheat mega-environments in Ontario: a minor mega-environment (eastern Ontario) and a major one (southern and western Ontario), the latter being traditionally divided into three subareas. There were frequent crossover GE interactions within the major mega-environment but the location groupings were variable across years. It therefore could not be further divided into meaningful subareas. It was revealed that in most years PC1 represents a proportional cultivar response across locations, which leads to noncrossover GE interactions, while PC2 represents a disproportional cultivar response across locations, which is responsible for any crossover GE interactions. Consequently, genotypes with large PC1 scores tend to give higher average yield, and locations with large PC1 scores and near-zero PC2 scores facilitates identification of such genotypes

Cultivar Evaluation and Mega-Environment Investigation Based on the GGE Biplot

The rhizosphere is a complex environment where roots interact with physical, chemical and biological properties of soil. Structural and functional characteristics of roots contribute to rhizosphere processes and both have significant influence on the capacity of roots to acquire nutrients. Roots also interact extensively with soil microorganisms which further impact on plant nutrition either directly, by influencing nutrient availability and uptake, or indirectly through plant (root) growth promotion. In this paper, features of the rhizosphere that are important for nutrient acquisition from soil are reviewed, with specific emphasis on the characteristics of roots that influence the availability and uptake of phosphorus and nitrogen. The interaction of roots with soil microorganisms, in particular with mycorrhizal fungi and non-symbiotic plant growth promoting rhizobacteria, is also considered in relation to nutrient availability and through the mechanisms that are associated with plant growth promotion.

Acquisition of phosphorus and nitrogen in the rhizosphere and plant growth promotion by microorganisms

Germplasm from the spring wheat (Trificum oesfivum L.) breeding program at the International Center for Improvement of Maize and Wheat (CIMMYT has had a major impact on the yield of irrigated spring wheats in most developing countries in the past 30 yr. The rate and nature of yield potential progress in this germplasm was measured cornparing eight outstanding short cultivars released in northwest Mexico between 1962 and 1988. They were grown under irrigation and optimal management, including disease and lodging protection, in each of six winter growing seasons (1989-1990 to 1994-1995) at the CIANO (Centro de Investigaciones Agricolas del Noroeste) experiment station in Sonora, Mexico. There were highly significant effects of cultivar on grain yield, and, although cultivar x year interaction was significant, there were few significant crossover interactions between pairs of genotypes and years in the grain yield data set. Yield averaged across the 6 yr increased linearly from 6680 kg ha -l for the earliest cultivar, Pitic 62, to 8475 kg ha -l for Bacanora 88, the latest. The rate of progress against year of release was 67 kg ha -1 yr -1 (r = 0.99, P< 0.001), or 0.88% per year. Grain yield progress was correlated with kernel number per square meter (r = 0.84, P < 0.01) and harvest index (r = 0.81, P < 0.02), but not with total biomass production, kernel weight, days to anthesis, spikes per square meter, or kernels per spike. Thus linear progress in yield within short germplasm has continued at least until the late 1980s, and the yield components studied did not indicate any clear direction for future progress, apart from that suggested by the strong relationships between grain yield and harvest index and grain yield and kernels per square meter, as has been seen in most studies of yield progress in cereals.

Yield Potential Progress in Short Bread Wheats in Northwest Mexico

Physiological and Genetic Changes of Irrigated Wheat in the Post–Green Revolution Period and Approaches for Meeting Projected Global Demand

Achieving near-immunity to leaf and stripe rusts in wheat by combining slow rusting resistance genes.

Multilocation trials are important for the CIMMYT Bread Wheat Program in producing high-yielding, adapted lines for a wide range of environments. This study investigated procedures for improving predictive success of a yield trial, grouping environments and genotypes into homogeneous subsets, and determining the yield stability of 18 CIMMYT bread wheats evaluated at 25 locations. Additive Main effects and Multiplicative Interaction (AMMI) analysis gave more precise estimates of genotypic yields within locations than means across replicates. This precision facilitated formation by cluster analysis of more cohesive groups of genotypes and locations for biological interpretation of interactions than occurred with unadjusted means. Locations were clustered into two subsets for which genotypes with positive interactions manifested in high, stable yields were identified. The analyses highlighted superior selections with both broad and specific adaptation.

AMMI adjustment for statistical analysis of an international wheat yield trial.

Phosphorus deficiency is a major yield limiting constraint in wheat cultivation on acid soils. The plant factors that influence P uptake efficiency (PUPE) are mainly associated with root characteristics. This study was conducted to analyze the genotypic differences and relationships between PUPE, root length density (RLD), colonization by vesicular arbuscular and arbuscular mycorrhizal (V)AM fungi and root excretion of phosphatases in a P-deficient Andisol in the Central Mexican Highlands. Forty-two semidwarf spring-bread-wheat (Triticum aestivumL.) genotypes from CIMMYT were grown without (−P) and with P fertilization (+P), and subsequently in subsets of 30 and 22 genotypes in replicated field trials over 2 and 3 years, respectively. Acid phosphatase activity at the root surface (APASE) was analyzed in accompanying greenhouse experiments in nutrient solution. In this environment, PUPE contributed more than P utilization efficiency, in one experiment almost completely, to the variation of grain yield among genotypes. Late-flowering genotypes were higher yielding, because the postanthesis period of wheat was extended due to the cold weather at the end of the crop cycles, and postanthesis P uptake accounted for 40–45% of total P uptake. PUPE was positively correlated with the numbers of days to anthesis (at −P r=0.57 and at +P r=0.73). The RLD in the upper soil layer (0–20 cm) of the wheat germplasm tested ranged from 0.5 to 2.4 cm cm-3 at –P and 0.7 to 7.7 at +P. RLD was the most important root trait for improved P absorption, and it was positively genetically correlated with PUPE (at –P r=0.42 and at +P r=0.63) and the number of spikes m-2 (at –P r=0.58 and at +P r=0.36). RLD in the upper soil layer was more important with P fertilizer application. Without P fertilization, root proliferation in the deeper soil profile secured access to residual, native P in the deeper soil layer. (V)AM-colonisation and APASE were to a lesser degree correlated with PUPE. Among genoptypes, the level of (V)AM-colonisation ranged from 14 to 32% of the RLD in the upper soil layer, and APASE from 0.5 to 1.1 nmol s-1 plant-1 10-2.

Sanjaya Rajaram

Papers

Yield Potential Progress in Short Bread Wheats in Northwest Mexico

Physiological and Genetic Changes of Irrigated Wheat in the Post–Green Revolution Period and Approaches for Meeting Projected Global Demand

Achieving near-immunity to leaf and stripe rusts in wheat by combining slow rusting resistance genes.

AMMI adjustment for statistical analysis of an international wheat yield trial.

Traits associated with improved P-uptake efficiency in CIMMYT's semidwarf spring bread wheat grown on an acid Andisol in Mexico.