Wu, Yen Hsuan (2009-05). Genetic Variation and Evolution of the Size of NBS-LRR-Encoding Gene Family in Cotton and Related Species (Gossypium L.). Master's Thesis.
Most of genes contained in a genome have been shown to exist in forms of families; however, little is known about their variation and evolution during the course of genome evolution. The present study shows that the numbers of the genes of the NBS-LRR-encoding gene family vary extremely significantly among different lines or cultivars of a species and among related species from the same genus. This suggests that plant genetics and evolution depend on not only gene sequence variation, but also the number of genes in multigene families. This study has further revealed that the variation of number of genes in the gene family in the Gossypium species is affected significantly not only by genome size variation, polyploidization and natural selection, but also by domestication/breeding. There is a positive correlation (P less than or equal to 0.05) between genome size and number of genes in the family, suggesting that species with larger genomes tend to have more NBS-LRR-encoding genes. It was observed that natural polyploids have significantly larger numbers of genes in the family and larger genomes than the artificial polyploids of their putative diploid ancestors. This indicates that polyploidization, perhaps post-polyploidization as well, either led to the loss of the genes in a gene family or slowed the process of gene number increase after polyploidization. It was shown that cultivated cottons have significantly more NBSLRR- encoding genes than wild species at both diploid and polyploidy levels. This result indicates that plant breeding likely allows accumulation of NBS-LRR-encoding genes that potentially provide resistance to pathogens. Therefore, plant breeders have selected not only for favorable alleles and favorable allele combinations, but also for the number of genes. Finally, difference (P less than or equal to 0.001) was found in number of genes in the NBS-LRR-encoding gene family among the species native to different geographical regions, suggesting that natural selection has played an important role in the variation in number of genes in the NBS-LRR-encoding gene family. The gene members that are favorable for fitness at the time are selected and accumulated in the genomes, but those that are not favorable for fitness at the time are lost in natural selection. As this is the first study in the field, further studies remain. These include, but not limited to, the universality of the findings in plants and animals, the universality of the findings in different gene families, genetics of the gene family size variation, relationship between the gene family size variation and phenotypic variation, gene family size variation and breeding, etc. Nevertheless, the findings obtained from this study are sufficient to shed light on many fundamental questions in biology, diversity and complexity of plants and animals.