Estimation of Molecular Pairwise Relatedness in Autopolyploid Crops

A suitable pairwise relatedness estimation is key to genetic studies. Several methods are proposed to compute relatedness in autopolyploids based on molecular data. However, unlike diploids, autopolyploids still need further studies considering scenarios with many linked molecular markers with known dosage. In this study, we provide guidelines for plant geneticists and breeders to access trustworthy pairwise relatedness estimates. To this end, we simulated populations considering different ploidy levels, meiotic pairings patterns, number of loci and alleles, and inbreeding levels. Analysis were performed to access the accuracy of distinct methods and to demonstrate the usefulness of molecular marker in practical situations. Overall, our results suggest that at least 100 effective biallelic molecular markers are required to have good pairwise relatedness estimation if methods based on correlation is used. For this number of loci, current methods based on multiallelic markers show lower performance than biallelic ones. To estimate relatedness in cases of inbreeding or close relationships (as parent-offspring, full-sibs, or half-sibs) is more challenging. Methods to estimate pairwise relatedness based on molecular markers, for different ploidy levels or pedigrees were implemented in the AGHmatrix R package

Development of novel theory and methods for QTL analysis and inferring crossover interference in autotetraploids

Polyploidization widely occurs in the evolutionary history of eukaryotes, especially for flowering plants. Exploring the evolutionary and agricultural important role by polyploidy is mystery and challenging job. In the first chapter of this thesis, I developed a quantitative genetics model based on orthogonal contrast scales, which would provide theoretical basis for any further bi-allelic QTL analysis in autotetraploid species. In the next chapter, I developed an interval mapping method for QTL mapping in outbred population for autotetraploid, considering both bivalent and quadrivalent pairing during meiosis. Extensive simulation work has been implemented to demonstrate the reliability of this method. This work would provide practical tools for breeding the world’s third most important crop, the cultivated potato. To give some insight into the evolutionary important role played by polyploidization, I developed a statistical method for inferring crossover interference based on three-locus analysis in autotetraploids in the third chapter and a method to predict genome-wide crossover rate for autotetraploid yeast in the final chapter

Phenotypic and meiotic differences between diploid and polyploid plants

Polyploidy is present in a large number of crop plants and is considered as one of the driving forces in the evolution of angiosperms. Unlocking genetic variation in various autopolyploid crop plants is highly relevant to crop breeders. Homologous recombination, a tightly controlled cell process during the production of gametes in meiosis, is responsible for creation of genetic variation. Owing to the presence of more than two homologous chromosomes, polyploid meiosis faces a variety of challenges, such as multivalent formation and mis-segregation. Using a plant trial with more than 300 diploid and tetraploid Arabidopsis thaliana F2 individuals, significant differences were found in various traits between the two populations. Cytological analysis using FISH on diploid and tetraploid plants revealed an overall increase in meiotic recombination in tetraploids, although the per bivalent frequency was reduced. The process of meiotic recombination was further explored in potato (Solanum tuberosum), a globally important autotetraploid crop. Chiasma frequency and multivalent frequency for chromosomes 1 and 2 varied according to variety, where the diploid variety showed a reduced chiasma frequency compared with tetraploid varieties. Immunolocalisation of the axis and synaptonemal complex proteins, ASY1 and ZYP1, demonstrated the complexities that may arise during meiosis in an autotetraploid plant

Genetic mapping in polyploids

Many of our most important crop species are polyploid – an unusual phenomenon whereby each chromosome is present in multiple copies (more than the usual two copies). The most common such arrangement is tetraploidy, where each chromosome is present four times. Plant species can tolerate this condition quite well (the same cannot be said of animals or humans). In fact, polyploidy can confer certain advantages such as larger fruits and flowers, seedless fruits (useful for fruit growers) or improved tolerance to environmental stresses. However, carrying multiple copies of each chromosome complicates things, particularly when crop breeders would like to use DNA information to help inform selection decisions. This PhD project looked at how DNA information of polyploids should be best analysed, developing methods and new software tools to achieve this. We analysed DNA information from polyploid crops such as potato, rose and chrysanthemum, yielding many novel insights and important results.</p

The "Polyploid Hop": shifting challenges and opportunities over the evolutionary lifespan of genome duplications

The duplication of an entire genome is no small affair. Whole genome duplication (WGD) is a dramatic mutation with long-lasting effects, yet it occurs repeatedly in all eukaryotic kingdoms. Plants are particularly rich in documented WGDs, with recent and ancient polyploidization events in all major extant lineages. However, challenges immediately following WGD, such as the maintenance of stable chromosome segregation or detrimental ecological interactions with diploid progenitors, commonly do not permit establishment of nascent polyploids. Despite these immediate issues some lineages nevertheless persist and thrive. In fact, ecological modelling supports patterns of adaptive niche differentiation in polyploids, with young polyploids often invading new niches and leaving their diploid progenitors behind. In line with these observations of polyploid evolutionary success, recent work documents instant physiological consequences of WGD associated with increased dehydration stress tolerance in first-generation autotetraploids. Furthermore, population genetic theory predicts both short-and long-term benefits of polyploidy and new empirical data suggests that established polyploids may act as 'sponges' accumulating adaptive allelic diversity. In addition to their increased genetic variability, introgression with other tetraploid lineages, diploid progenitors, or even other species, further increases the available pool of genetic variants to polyploids. Despite this, the evolutionary advantages of polyploidy are still questioned, and the debate over the idea of polyploidy as an evolutionary dead-end carries on. Here we broadly synthesise the newest empirical data moving this debate forward. Altogether, evidence suggests that if early barriers are overcome, WGD can offer instantaneous fitness advantages opening the way to a transformed fitness landscape by sampling a higher diversity of alleles, including some already preadapted to their local environment. This occurs in the context of intragenomic, population genomic, and physiological modifications that can, on occasion, offer an evolutionary edge. Yet in the long run, early advantages can turn into long-term hindrances, and without ecological drivers such as novel ecological niche availability or agricultural propagation, a restabilization of the genome via diploidization will begin the cycle anew

Genetic analysis of complex traits in alfalfa (Medicago sativa L)

The genetic structure of complex agronomic traits in alfalfa ( Medicago sativa) is not well understood. By crossing the subspecies M. sativa subsp. falcata and M. sativa subsp. sativa, a fullsib F1 population was created from which a genetic linkage map of each parental genome was developed using RFLP and SSR markers. These maps include simplex, duplex, and simplex-simplex alleles along with a number of alleles exhibiting segregation distortion. The inclusion of these more complicated segregation ratios resulted in greater saturation of the genome, a better convergence to eight consensus linkage groups, and a more realistic view of regions of the genome that may not behave normally due to segregation distortion than would have been possible by only using simplex alleles as has been done previously.;The population was clonally propagated and grown at three field locations with phenotypic data collected over three years for various agronomic traits, including biomass production, forage height, and forage regrowth. Combining the marker data with the phenotypic data, markers were identified from each parental genome that were associated with these traits, suggesting that both major germplasm sources of cultivated alfalfa contain alleles that may contribute to improved alfalfa cultivars. These results provide a much better understanding of the genomic regions underlying these traits and are an important start in efforts aimed at the use of marker-assisted selection for the improvement of alfalfa cultivars.*;*This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Microsoft Office