Genetics of continuous storage root formation and bulking in sweetpotato
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This dissertation draws attention to the need to breed for “continuous storage root formation and bulking” (CSRFAB) in sweetpotato which is an important trait for many small scale farmers who practice sequential or piecemeal harvesting. Many studies have been carried out in the areas of storage root formation but there has been a shortage of information about CSRFAB and about genotypes with this trait. This study, therefore, contributed to the understanding of CSRFAB traits in sweetpotato and identified suitable genotypes from within the Ugandan sweetpotato germplasm collection. The study determined the genetic variability of CSRFAB in the sweetpotato germplasm collection in Uganda, as well as genetic markers associated with putative functional genes for CSRFAB traits in sweetpotato. The study identified the mode of gene action through the analysis of the general and specific combining ability of CSRFAB in sweetpotato and suggested appropriate breeding methods for the trait. The study identified also sweetpotato genotypes that were most distinct for CSRFAB, including SPK004, Kala, BSH740, KML872, NASPOT 9 O, Mayai, Ukerewe, NASPOT 7, APA352 and RAK819 and genotypes that were most distinct for discontinuous storage root formation and bulking (DCSRFAB) as MPG 1146, Dimbuka-Bukulula, NASPOT 1, Otada, MPG 1128, MSK 1040, RAK786, KM188 and KBL648. This gives farmers the opportunity to select genotypes from the two categories knowing that use of CSRFAB genotypes are suitable for sequential harvesting with a yield advantage of 117% over the DCSRFAB genotypes. The study gives opportunities for farmers interested in other traits such as early and synchronized maturity to select DCSRFAB genotypes. A number of single nucleotide polymorphism (SNP) markers and their putative functional related genes were identified in this study and this will go a long way in speed breeding for the CSRFAB trait i.e. 34 SNPs were significantly associated with the CSRFAB phenotype at 150 DAP and eight SNPs were significantly associated with DCSRFAB at 90 DAP. Nineteen novel genes, including 12 candidate genes were associated with CSRFAB markers and five genes were putative for DCSRFAB markers. These candidate genes deepen our understanding of molecular mechanisms underlying CSRFAB in sweetpotato. Crosses made between CSRFAB and DCSRFAB generated transgressive segregants (F1s whose performance is superior to both parents) with a yield gain of over 2 t/ha. The crosses also revealed that additive gene effects (general combining ability) were greater than the non-additive gene effects (specific combining ability - SCA) for most of the traits (storage root number, storage root yield, and vine yield). However, SCA effects were greater for CSRFAB suggesting that dominance and epistatic gene interaction were more important in controlling the trait. However, for adaptive traits like CSRFAB, many small additive genetic effects may contribute leading to low GCA compared to SCA. Understanding the gene action will enable breeders to decide on the crop breeding method for the CSRFAB trait improvement.