Date of Award
Doctor of Philosophy (PhD)
Molecular, Cellular, and Developmental Biology
One of the greatest challenges to human civilization in the 21st century will be to provide global food security to a growing population while reducing the environmental footprint of agriculture. Despite increasing demand, the fundamental issue of limited genetic diversity in domesticated crops provides windows of opportunity for emerging pandemics and the insufficient ability of modern crops to respond to a changing global environment. The wild relatives of crop plants, with large reservoirs of untapped genetic diversity, offer great potential to improve the resilience of elite cultivars. Utilizing this diversity requires advanced technologies to comprehensively identify genetic diversity and understand the genetic architecture of beneficial traits. The primary focus of the dissertation is developing computational tools to facilitate variant discovery and trait mapping for plant genomics. In Chapter 1, I benchmarked the performance of variant discovery algorithms based on simulated and diverse plant datasets. The comparison of sequence aligners found that BWA-MEM consistently aligned the most plant reads with high accuracy, whereas Bowtie2 had a slightly higher overall accuracy. Variant callers, such as GATK HaplotypCaller and SAMtools mpileup, were shown to significantly differ in their ability to minimize the frequency of false negatives and maximize the discovery of true positives. A cross-reference experiment of Solanum lycopersicum and Solanum pennellii reference genomes revealed significant limitations of using a single reference genome for variant discovery. Next, I demonstrated that a machine-learning-based variant filtering strategy outperformed the traditional hard-cutoff filtering strategy, resulting in a significantly higher number of true positive and fewer false-positive variants. Finally, I developed a 2-step imputation method resulted in up to 60% higher accuracy than direct LD-based imputation methods. In Chapter 2, I focused on developing a trait mapping algorithm tailored for plants considering the high levels of diversity found in plant datasets. This novel trait mapping framework, HapFM, had the ability to incorporate biological priors into the mapping model to identify casual haplotypes for traits of interest. Compared to conventional GWAS analyses, the haplotype-based approach significantly reduced the number of variables while aggregating small effect SNPs to increase mapping power. HapFM could account for LD between haplotype segments to infer the causal haplotypes directly. Furthermore, HapFM could systemically incorporate biological priors into the probability function during the mapping process resulting in greater mapping resolution. Overall, HapFM achieves a balance between powerfulness, interpretability, and verifiability. In Chapter 3, I developed a computational algorithm to select a pan-genome cohort to maximize the haplotype representativeness of the cohort. Increasing evidence suggest that a single reference genome is often inadequate for plant diversity studies due to extensive sequence and structural rearrangements found in many plant genomes. HapPS was developed to utilize local haplotype information to select the reference cohort. There are three steps in HapPS, including genome-wide block partition, representative haplotype identification, and genetic algorithm for reference cohort selection. The comparison of HapPS with global-distance-based selection showed that HapPS resulted in significantly higher block coverage in the highly diverse genic regions. The GO-term enrichment analysis of the highly diverse genic region identified by HapPS showed enrichment for genes involved in defense pathways and abiotic stress, which might identify genomic regions involved in local adaptation. In summary, HapPS provides a systemic and objective solution to pan-genome cohort selection.
Wu, Xing, "Uncovering Hidden Diversity in Plants" (2022). Yale Graduate School of Arts and Sciences Dissertations. 677.