Global Impact of Sown Temperate Pastures on Productivity and Ecosystem Stability–What Progress Have We Made?

Twenty years ago, in 1993, we published one of the first two alfalfa genetic linkage maps. At the time, hopes ran high that genetic marker technologies would revolutionize selection, making the development of superior cultivars both easier and faster. The objective of this paper is to critically examine forage improvement since that time and to suggest ways to more fully capitalize on those initial hopes in the future. Marker studies have been conducted around the world, identifying quantitative trait loci (QTL) for the major agronomically important traits, including biomass yield, nutritive value, disease resistance, abiotic stress tolerance, and others. But progress has been slow and no cultivars on the market today have been bred using marker technology in a significant way. I will discuss reasons for the limited progress, including the lack of a critical mass of researchers, funding limitations, and genetic complexities integral to the crop. Despite the limitations, I suggest that the international community can do a better job integrating resources to achieve better genetic gain in breeding programs. I will discuss focused methods that could successfully integrate markers into breeding programs by manipulating individual QTL from unadapted germplasm and by applying genomic selection to accelerate breeding cycles. Even so, the real world value of these technologies needs to be carefully considered before they can be adopted in a commercial scale

Hybrid Alfalfa

One goal of the Iowa State University forage breeding program is to increase alfalfa yields. In many crops such as corn, hybrids have been used to increase yield. Hybrid alfalfa varieties could be a possible way to increase alfalfa forage yield. Commercially available alfalfa varieties are purple flowered and are of the sativa type. A second type of alfalfa, falcata, has yellow flowers and is more winter hardy and morphologically distinct from sativa alfalfa. We made sativa-falcata hybrids and tested them to determine if hybrids would outperform commercially available varieties and thus offer the possibility of creating higher yielding varieties for Iowa

Alfalfa Variety Testing

New varieties of alfalfa are released by commercial breeding companies each year. The Iowa State University forage breeding program, in conjunction with the Iowa Crop Improvement Association, tests commercially available varieties at five locations in Iowa, including at the Northeast Research Farm. Funding to conduct these tests is provided by entrants who pay a fee to have their varieties included. Our tests provide an unbiased comparison among cultivars deemed by the companies to be adapted to particular regions of the state

Selection Mapping Identifies Loci Underpinning Autumn Dormancy in Alfalfa (Medicago sativa).

Autumn dormancy in alfalfa (Medicago sativa) is associated with agronomically important traits including regrowth rate, maturity, and winter survival. Historical recurrent selection experiments have been able to manipulate the dormancy response. We hypothesized that artificial selection for dormancy phenotypes in these experiments had altered allele frequencies of dormancy-related genes. Here, we follow this hypothesis and analyze allele frequency changes using genome-wide polymorphisms in the pre- and postselection populations from one historical selection experiment. We screened the nondormant cultivar CUF 101 and populations developed by three cycles of recurrent phenotypic selection for taller and shorter plants in autumn with markers derived from genotyping-by-sequencing (GBS). We validated the robustness of our GBS-derived allele frequency estimates using an empirical approach. Our results suggest that selection mapping is a powerful means of identifying genomic regions associated with traits, and that it can be exploited to provide regions on which to focus further mapping and cloning projects

Narrow Sense Heritability and Additive Genetic Correlations in Alfalfa subsp. falcata

The complex genetics of autotetraploid alfalfa (Medicago sativa L.) make additive genetic variance component estimation difficult. Halfsib family variances often are used to estimate additive genetic variances and, by extension, narrow sense heritabilities and additive genetic correlations. These estimates contain a portion of the dominance variance. Using such calculations, in conjunction with parent-offspring covariance estimates, the dominance component can be separated from the additive genetic component. This is rarely done. This study reports average estimates across 30 populations, of both additive and dominance variance component estimates based on between halfsib family variance and parent-offspring covariance for biomass yield, plant height, regrowth, plant width, plant growth angle, vegetative density, and maturity during each of three harvests. We consistently found negative dominance variance estimates. Based on previous theory, this suggests epistatic interactions are a noticeable component of most traits measured. Assuming no epistasis leads to inflated narrow sense heritability estimates when compared with estimates based on parent-offspring regression. Assuming no epistasis and no dominance variance, weighted averages of additive genetic variance between halfsib family and parent-offspring effects revealed plant width and vegetative density additively correlated with biomass yield. Peak photoperiod maturity had a nonsignificant negative additive correlation with biomass yield. Plant height had no additive correlation with biomass, in contrast to the strong phenotypic correlation observed. Additive genetic correlations for the same traits measured during different harvests in most instances were highly correlated. On average, third harvest heritabilities were greatest. Our results suggest selecting plants based on later season performance (August - October) is most effective for Iowa environments

A saturated genetic linkage map of autotetraploid alfalfa (Medicago sativa L.) developed using genotyping-by-sequencing is highly syntenous with the Medicago truncatula genome.

A genetic linkage map is a valuable tool for quantitative trait locus mapping, map-based gene cloning, comparative mapping, and whole-genome assembly. Alfalfa, one of the most important forage crops in the world, is autotetraploid, allogamous, and highly heterozygous, characteristics that have impeded the construction of a high-density linkage map using traditional genetic marker systems. Using genotyping-by-sequencing (GBS), we constructed low-cost, reasonably high-density linkage maps for both maternal and paternal parental genomes of an autotetraploid alfalfa F1 population. The resulting maps contain 3591 single-nucleotide polymorphism markers on 64 linkage groups across both parents, with an average density of one marker per 1.5 and 1.0 cM for the maternal and paternal haplotype maps, respectively. Chromosome assignments were made based on homology of markers to the M. truncatula genome. Four linkage groups representing the four haplotypes of each alfalfa chromosome were assigned to each of the eight Medicago chromosomes in both the maternal and paternal parents. The alfalfa linkage groups were highly syntenous with M. truncatula, and clearly identified the known translocation between Chromosomes 4 and 8. In addition, a small inversion on Chromosome 1 was identified between M. truncatula and M. sativa. GBS enabled us to develop a saturated linkage map for alfalfa that greatly improved genome coverage relative to previous maps and that will facilitate investigation of genome structure. GBS could be used in breeding populations to accelerate molecular breeding in alfalfa

Determining the Mechanism of Yield Stability in Alfalfa

Year-to-year stability of crop yields is important for farmers and, hence, is an important goal of plant breeding programs. Especially in perennial crops like alfalfa, farmers need to know that they can count on consistent yields over a period of three or more years. Alfalfa varieties are composed of a population of many genetically distinct plants (or genotypes), unlike corn hybrids or soybean lines, which are genetically uniform

Determining the Mechanism of Yield Stability in Alfalfa

Year-to-year stability of crop yields is important for farmers, and hence is an important goal of plant breeding programs. Especially in perennial crops like alfalfa farmers need to know that they can count on consistent yields over a period of three or more years. Alfalfa varieties are composed of a population of many genetically distinct plants (or genotypes), unlike corn hybrids or soybean lines, which are genetically uniform. Our goal is to determine whether the entire population of plants or single plants within a population determine yield stability of alfalfa. If varietal stability is due to the stability of individual plants in the population, then breeders can select individual plants with stable performance under many environmental conditions to use in the development of stable varieties. Conversely, if yield stability of a variety results from the interaction of many different genotypes, each of which performs better under some conditions than others, then alternative methods for developing stable cultivars must be investigated