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

Scarcity of water is a severe environmental constraint to plant productivity. Drought-induced loss in crop yield probably exceeds losses from all other causes, since both the severity and duration of the stress are critical. Here, we have reviewed the effects of drought stress on the growth, phenology, water and nutrient relations, photosynthesis, assimilate partitioning, and respiration in plants. This article also describes the mechanism of drought resistance in plants on a morphological, physiological and molecular basis. Various management strategies have been proposed to cope with drought stress. Drought stress reduces leaf size, stem extension and root proliferation, disturbs plant water relations and reduces water-use efficiency. Plants display a variety of physiological and biochemical responses at cellular and whole-organism levels towards prevailing drought stress, thus making it a complex phenomenon. CO2 assimilation by leaves is reduced mainly by stomatal closure, membrane damage and disturbed activity of various enzymes, especially those of CO2 fixation and adenosine triphosphate synthesis. Enhanced metabolite flux through the photorespiratory pathway increases the oxidative load on the tissues as both processes generate reactive oxygen species. Injury caused by reactive oxygen species to biological macromolecules under drought stress is among the major deterrents to growth. Plants display a range of mechanisms to withstand drought stress. The major mechanisms include curtailed water loss by increased diffusive resistance, enhanced water uptake with prolific and deep root systems and its efficient use, and smaller and succulent leaves to reduce the transpirational loss. Among the nutrients, potassium ions help in osmotic adjustment; silicon increases root endodermal silicification and improves the cell water balance. Low-molecular-weight osmolytes, including glycinebetaine, proline and other amino acids, organic acids, and polyols, are crucial to sustain cellular functions under drought. Plant growth substances such as salicylic acid, auxins, gibberrellins, cytokinin and abscisic acid modulate the plant responses towards drought. Polyamines, citrulline and several enzymes act as antioxidants and reduce the adverse effects of water deficit. At molecular levels several drought-responsive genes and transcription factors have been identified, such as the dehydration-responsive element-binding gene, aquaporin, late embryogenesis abundant proteins and dehydrins. Plant drought tolerance can be managed by adopting strategies such as mass screening and breeding, marker-assisted selection and exogenous application of hormones and osmoprotectants to seed or growing plants, as well as engineering for drought resistance.

https://hal.archives-ouvertes.fr/hal-00886451/document

Plant drought stress: effects, mechanisms and management

Crop productivity relies heavily on nitrogen (N) fertilization. Production and application of N fertilizers consume huge amounts of energy, and excess is detrimental to the environment; therefore, increasing plant N use efficiency (NUE) is essential for the development of sustainable agriculture. Plant NUE is inherently complex, as each step—including N uptake, translocation, assimilation, and remobilization—is governed by multiple interacting genetic and environmental factors. The limiting factors in plant metabolism for maximizing NUE are different at high and low N supplies, indicating great potential for improving the NUE of current cultivars, which were bred in well-fertilized soil. Decreasing environmental losses and increasing the productivity of crop-acquired N requires the coordination of carbohydrate and N metabolism to give high yields. Increasing both the grain and N harvest index to drive N acquisition and utilization are important approaches for breeding future high-NUE cultivars.

/pdf/plant-nitrogen-assimilation-and-use-efficiency-35cshj4xrf.pdf

Plant nitrogen assimilation and use efficiency.

The Report, Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply (generally referred to as the Billion-Ton Study or 2005 BTS), was an estimate of “potential” biomass within the contiguous United States based on numerous assumptions about current and
future inventory and production capacity, availability, and technology. In the 2005 BTS, a strategic analysis was undertaken to determine if U.S. agriculture and forest resources have the capability to potentially produce at least one billion dry tons of biomass annually, in a sustainable manner—enough to displace approximately
30% of the country’s present petroleum consumption. To ensure reasonable confidence in the study results, an effort was made to use relatively conservative assumptions. However, for both agriculture and forestry, the resource potential was not restricted by price. That is, all identified biomass was potentially available, even though some potential feedstock would more than likely be too expensive to actually be economically available.
In addition to updating the 2005 study, this report attempts to address a number of its shortcomings

/pdf/u-s-billion-ton-update-biomass-supply-for-a-bioenergy-and-39t7zcevg2.pdf

U.S. Billion-ton Update: Biomass Supply for a Bioenergy and Bioproducts Industry

A reviewer for the Journal of the Royal Statistical Society of England comments \"This is the first book covering in one volume all important topics in genetical statistics.\" Written to provide basic probability ideas in terms of genetic situations, since the theory of genetics is a probability theory; to give a definitive treatment of applications of these ideas to genetic theory; and to describe statistical methods appropriate to the data models that are developed.

An Introduction to Genetic Statistics

Use of drought response index for identification of drought tolerant genotypes in rainfed lowland rice

Genotype differences in nutrient uptake and utilisation for grain yield production of rainfed lowland rice under fertilised and non-fertilised conditions

Water limitation is a major production constraint for sugarcane worldwide. However, to date, there has been little investigation of patterns of genetic variation in the response to water stress in sugarcane. Field experiments were conducted over 3 years under fully irrigated and managed water stress conditions at two locations in Northern Queensland in Australia. Eighty-nine genetically diverse clones were evaluated for their yield performance and sugar attributes. Water stress treatments reduced cane yield [tonnes of cane per hectare (TCH)] and total dry matter (TDM) by 17-52% and 20-56%, respectively, compared with irrigated treatments in the same experiments. Nevertheless, there was little genotype×environment interaction variation for TCH, TDM, or commercial cane sugar (CCS), and hence high genetic correlations between the irrigated and water stress treatments across environments. Both commercial and unselected clones performed poorly under severe stress environments, while the commercial clones outperformed the unselected clones under mild and moderate stress conditions. The results presented here highlight the contribution of intrinsic potential yields (yield under well-irrigated conditions) of some selected and unselected clones to maintain relatively high productivity in a range of moderate stress conditions imposed. The physiological basis for the high genetic correlations is at present unclear, but some explanations are hypothesized. The choice of stress levels in selection trials would not appear to be a critical issue for sugarcane breeding programmes, at least for the early phases of selection, where similar ranking clones across a range of moderate water stresses may be expected.

Sugarcane for water-limited environments. Genetic variation in cane yield and sugar content in response to water stress

Low temperature is a common production constraint in rice cultivation in temperate zones and high-elevation environments, with the potential to affect growth and development from germination to grain filling. There is a wide range of genotype-based differences in cold tolerance among rice varieties, these differences often reflecting growth conditions in the place of origin, as well as breeding history. However, improving low temperature tolerance of varieties has been difficult, due to a lack of clarity of the genetic basis to low temperature tolerance for different growth stages of the rice plant. Seeds or plants of 17 rice varieties of different origins were exposed to low temperature during germination (15°C), seedling, booting, and flowering stages (18.5°C), to assess their cold tolerance at different growth stages. Low temperature at the germination stage reduced both the percentage and speed of germination. Varieties from China (B55, Banjiemang, and Lijianghegu) and Hungary (HSC55) were more tolerant of low temperature than other varieties. Most of the varieties showed moderate levels of low temperature tolerance during the seedling stage, the exceptions being some varieties from Australia (Pelde, YRL39, and YRM64) and Africa (WAB160 and WAB38), which were susceptible to low temperature at the seedling stage. Low temperature at booting and flowering stages reduced plant growth and caused a significant decline in spikelet fertility. Some varieties from China (B55, Bangjiemang, Lijiangheigu), Japan (Jyoudeki), the USA (M103, M104), and Australia (Quest) were tolerant or moderately tolerant, while the remaining varieties were susceptible or moderately susceptible to low temperature at booting and flowering stages. Three varieties from China (B55, Lijianghegu, Banjiemang) and one from Hungary (HSC55) showed consistent tolerance to low temperature at all growth stages. These varieties are potentially important gene donors for breeding and genetic studies. The cold tolerance of the 17 rice varieties assessed at different growth stages was correlated. Screening for cold tolerance during early growth stages can therefore potentially be an effective way for assessing cold tolerance in breeding programs.

J. Basnayake

Papers

Use of drought response index for identification of drought tolerant genotypes in rainfed lowland rice

Genotype differences in nutrient uptake and utilisation for grain yield production of rainfed lowland rice under fertilised and non-fertilised conditions

Sugarcane for water-limited environments. Genetic variation in cane yield and sugar content in response to water stress

Cold tolerance in rice varieties at different growth stages

Toposequential effects on water balance and productivity in rainfed lowland rice ecosystem in Southern Laos