Abstract
Cassava (Manihot esculenta) is a vital food security crop and staple in Africa, yet cassava brown streak disease (CBSD) and cassava mosaic disease result in substantial yield losses. The aim of this study was to identify genes that have undergone positive selection during adaptive evolution, from CBSD resistant, tolerant and susceptible M. esculenta varieties and inter-specific hybrids, as well as a wild cassava species. Transcriptomes of 13 genotypes were sequenced and three genes with strong positive selection were detected (designated as EG2771, EG964 and EG5651). Sequence variation for candidate genes in 18 different cassava genotypes was examined in relation to known response to CBSD and whitefly infection. Although, we cannot ascribe a selection pressure that was responsible for the observed positive selection with complete certainty at this stage, given the congruence of the pattern of particular alleles of our positively selected genes and the pattern of disease resistance of the cassava varieties we examined, it is likely that some protein variants coded by alleles of EG2771 and EG964 may be associated with CBSD and whitefly resistance responses. This warrants further investigation. Other alleles of our positively selected genes were likely influenced by domestication or some other unknown selective pressure. Key words: Cassava, cassava brown streak disease (CBSD), resistant, tolerant, susceptible.
Highlights
Identiffication of ge enes th hat hav ve und dergon ne adaptive e evolution in cassav c va (Manihot e escule enta) and tha at may er resis stance e to cassava brown n strea ak dise ease confe
Cassava was collected at the Sugarcane Research Station, Kibaha, Tanzania, ‘homestead’ cassava was collected from homesteads close to Dar-es-Salaam and M. glaziovii was collected from a wild population at Sirawi in Tanzania
Fifty-four potentially positively-selected candidate gene sequences were obtained. The majority of these sequences were eliminated as a result of targeted Sanger re-sequencing: nucleotide base-for-base accuracy is required for rigorous computation of Ka/Ks ratios
Summary
Identiffication of ge enes th hat hav ve und dergon ne adaptive e evolution in cassav c va (Manihot e escule enta) and tha at may er resis stance e to cassava brown n strea ak dise ease confe. We cannot c ascriibe a selecttion pressurre that was responsible e for the observed positive selec ction with co omplete certa ainty at this s stage, given n the congrue ence of the p pattern of particular alleles a of ourr positively selected s gen nes and the pattern of disease resis stance of the e cassava varieties we e examined, it is likely th hat some pro otein variants s coded by a alleles of EG. Other alleles of our positively y selected ge enes were lik kely influenc ced by domes stication or some other unknown selective prressure
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