Diversité et structuration génétique des populations émergentes d’aleurodes vecteurs de maladies sur manioc en Afrique de l’Est (Malawi, Tanzanie, et Uganda)

High population of the whitefly, Bemisia tabaci Gennadius, a cryptic species complex had been associated with the vectoring and spread of viruses causing two diseases of cassava in East Africa: the cassava mosaic disease (CMD) and cassava brown streak disease (CBSD). Among the B. tabaci species, sub-Saharan Africa 2 (SSA2) was the vector associated with an epidemic of CMD since the 1990s in Uganda. However, this species is now replaced by the SSA1 and led to development of another epidemic by CBSD since the mid 2000s. The spread of both diseases toward South and West Africa is feared with this new supposed invader. In my thesis I have used ecological data and molecular approaches (mitochondrial and nuclear markers) to better understand the factors driving the presence of the superabundant whitefly populations on cassava in East Africa. We have analyzed: i) species abundance, diversity and distribution (geographic and host plants) along a transect survey over three East African countries: Uganda, Tanzania, Malawi, ii) the genetic diversity and structure of current populations of B. tabaci species, and iii) comparing genetic changes between the old and new populations collected in 1997 and 2017, respectively.This study involving large number of samples provided insights of a more complex picture than expected. SSA1 was found to be the source of the some observed outbreaks although other species, notably IO and sub-group 3 of SSA1 (SSA1-SG3) have also shown this capability. The observed outbreaks are therefore not just related to a single species in East Africa. In addition, we showed that the species community and its genetic diversity differ from one country to another, involving different epidemiological situations, without any clear pattern of invasion detected between the countries. Analysis of old samples did not show the involvement of a new species or the emergence of a new population in 20 years, although the dynamics within the whitefly genetic groups was observed over time. Our results contributed new knowledge on the super abundant populations on cassava in Eastern Africa and help develop targeted control measures for the local populations.Des pullulations d’aleurodes du complexe d'espèces cryptiques de Bemisia tabaci ont été associées à la propagation de deux maladies frappant le manioc en Afrique orientale: la maladie de la mosaïque du manioc (CMD) et, plus récemment (2000), la maladie de la striure brune du manioc (CBSD). Parmi les espèces d’aleurodes de ce complexe, l’espèce SSA2 a été associée à la première épidémie de CMD au cours des années 1990 en Ouganda. Cependant, SSA2 aurait été remplacée par SSA1 dans les années 2000, provoquant une recrudescence de CMD et de CBSD, participant à leur propagation dans plusieurs pays voisins. L’hypothèse défendue à ce jour expliquant la propagation de ces maladies vers le sud et l'ouest de l'Afrique incrimine cette nouvelle espèce considérée comme émergente dans certains de ces pays. Dans ma thèse, j’ai utilisé des données écologiques et des approches moléculaires afin de mieux comprendre les facteurs à l'origine des pullulations de vecteurs en Afrique de l'Est. Nous avons ainsi analysé : i) l’abondance, la diversité et la répartition des espèces sur un transect comprenant : Ouganda, Tanzanie et Malawi, ii) la diversité génétique et la structure des populations actuelles des espèces de B. tabaci, iii) des échantillons des années 90 comparés aux populations actuelles (2017). Cette étude nous a permis d’avoir une image d’une situation plus complexe qu’attendue, en effet, l’espèce SSA1 a été détectée comme à l’origine dans certaines des pullulations observées mais également d’autres espèces, notamment IO et SSA1-SG3 ont aussi montrées cette capacité. Les pullulations observées ne sont donc pas uniquement liées à une seule espèce en Afrique de l’Est. En outre, nous avons pu montrer que la communauté d'espèces et sa diversité génétique diffère d'un pays à l'autre, impliquant des situations épidémiologiques différentes, sans aucun schéma d'invasion détecté entre pays. En outre, l’analyse des anciens échantillons n’a pas montré l’implication d’une nouvelle espèce ou population en 20 ans, toutefois, nous avons observé un changement de dynamique au sein des groupes génétiques représentés au cours du temps

Evaluation of sweet potato genotypes for drought tolerance in the central zone of Tanzania

Drought is the most significant factor restricting crop growth and productivity in most agricultural fields of the world. The purpose of this study was to identify sweet potato genotypes tolerant for drought that could be used in breeding programs for improving sweet potato production in Tanzania. A field experiment involving 17 sweet potato genotypes was conducted at the Agriculture Research Institute (ARI) Hombolo, Dodoma between April and September 2012 (dry season). The experiment was laid out in a randomized complete block design (RCBD) with three replications. The genotypes were evaluated under well watered (WW) and water stress (WS) conditions where moisture level was maintained at field capacity until 60 days after planting (DAP). In the WW plots, supplementary irrigation was applied up to maturity at two days interval to maintain moisture level at field capacity while for WS irrigation was stopped at 60 DAP. Data was collected on yield, morphological parameters and level of antioxidant activity (catalase and peroxidase) and analyzed by GENSTAT, 13th Edition. Under WS conditions, the highest yield observed was 2.8 t ha-1 compared to 15 t ha-1 for WW. The study revealed that WS caused yield reduction of up to 97%. The five genotypes performed better under WS were SPKBH06/216, Simama, D12-02, SPKBH01/264 and Polista. On morphological parameters, the study revealed a reduction in plant size under WS. The level of antioxidant enzyme activities catalase and peroxidase showed to be higher under WS. Implying accumulation of reactive oxygen species. Thus such high levels could be used as an indicator for drought tolerant genotypes in sweet potato. From farmer’s participatory evaluation for culinary characteristics it was revealed that genotypes Polista, Simama, Pipi and SPKBH06/216 were the most preferred in terms of root taste, color and dry matter. The above genotypes could be used as breeding materials.Commission for Science and Technology (COSTECH) and SIDA-SARE

Evaluation of sweet potato genotypes for drought tolerance in the central zone of Tanzania

Drought is the most significant factor restricting crop growth and productivity in most agricultural fields of the world. The purpose of this study was to identify sweet potato genotypes tolerant for drought that could be used in breeding programs for improving sweet potato production in Tanzania. A field experiment involving 17 sweet potato genotypes was conducted at the Agriculture Research Institute (ARI) Hombolo, Dodoma between April and September 2012 (dry season). The experiment was laid out in a randomized complete block design (RCBD) with three replications. The genotypes were evaluated under well watered (WW) and water stress (WS) conditions where moisture level was maintained at field capacity until 60 days after planting (DAP). In the WW plots, supplementary irrigation was applied up to maturity at two days interval to maintain moisture level at field capacity while for WS irrigation was stopped at 60 DAP. Data was collected on yield, morphological parameters and level of antioxidant activity (catalase and peroxidase) and analyzed by GENSTAT, 13th Edition. Under WS conditions, the highest yield observed was 2.8 t ha-1 compared to 15 t ha-1 for WW. The study revealed that WS caused yield reduction of up to 97%. The five genotypes performed better under WS were SPKBH06/216, Simama, D12-02, SPKBH01/264 and Polista. On morphological parameters, the study revealed a reduction in plant size under WS. The level of antioxidant enzyme activities catalase and peroxidase showed to be higher under WS. Implying accumulation of reactive oxygen species. Thus such high levels could be used as an indicator for drought tolerant genotypes in sweet potato. From farmer’s participatory evaluation for culinary characteristics it was revealed that genotypes Polista, Simama, Pipi and SPKBH06/216 were the most preferred in terms of root taste, color and dry matter. The above genotypes could be used as breeding materials.Commission for Science and Technology (COSTECH) and SIDA-SARE

Genetic diversity, distribution, and structure of Bemisia tabaci whitefly species in potential invasion and hybridization regions of East Africa.

Outbreaks of whitefly, Bemisia tabaci species in East and Central Africa, have become increasingly prevalent during the previous 25 years and are responsible for driving the spread of plant-virus diseases, such as cassava mosaic disease and cassava brown steak disease. Epidemics of these diseases have expanded their ranges over the same period, spreading from Uganda into other sub-Saharan African countries. It was hypothesised that a highly abundant 'invader' population of B. tabaci was responsible for spreading these diseases from Uganda to neighbouring countries and potentially hybridising with the resident cassava B. tabaci populations. Here, we test this hypothesis by investigating the molecular identities of the highly abundant cassava B. tabaci populations from their supposed origin in Uganda, to the northern, central, eastern and coastal regions of Tanzania. Partial mitochondrial cytochrome oxidase I (mtCOI) barcoding sequences and nuclear microsatellite markers were used to analyse the population genetic diversity and structure of 2734 B. tabaci collected from both countries and in different agroecological zones. The results revealed that: (i) the putative SSA1 species is structured according to countries, so differ between them. (ii) Restricted gene flow occurred between SSA1-SG3 and both other SSA1 subgroups (SG1 and SG2), even in sympatry, demonstrating strong barriers to hybridization between those genotypes. (iii) Not only B. tabaci SSA1-(SG1 and SG2) was found in highly abundant (outbreak) numbers, but B. tabaci SSA1-SG3 and the Indian Ocean (IO) species were also recorded in high numbers in several sites in Tanzania. (iv) The SSA1-(SG1 and SG2) species was distributed in both countries, but in Tanzania, the B. tabaci IO and SSA1-SG3 species predominated. These data confirm that multiple, local Tanzanian B. tabaci species produce highly abundant populations, independent of the spread of the putative invasive B. tabaci SSA1-(SG1 and SG2) populations

Spatio-temporal changes in endosymbiont diversity and composition in the African cassava whitefly, Bemisia tabaci SSA1

The data presented in the study are deposited and available in the GenBank repository, accession number from OP160970 to OP161013.International audienceSap-sucking insects, including whiteflies, are amongst the most devastating and widely distributed organisms on the planet. They are often highly invasive and endosymbiont communities within these insects help them adapt to new or changing environments. Bemisia tabaci (Gennadius; Hemiptera: Aleyrodidae) whitefly species are vectors of more than 500 known plantviruses and harbour highly diverse endosymbionts communities. To date, however, whitefly-endosymbiont interactions, community structure and their spatio-temporal changes are still poorly understood. In this study, we investigated the spatio-temporal changes in the composition and diversity of bacterial endosymbionts in the agricultural crop pest whitefly species, Bemisia tabaci sub-Saharan Africa 1-subgroup 1 and 2 (SSA1-SG1 and SSA1-SG2). 16S rRNA amplicon sequencing analysis was carried out to characterise endosymbiont compositionsin field-collected SSA1 (SSA1-SG1 and SSA1-SG2) populations infesting cassava in Uganda in 1997 and 2017. We detected Portiera, Arsenophonus, Wolbachia, Hamiltonella and Hemipteriphilus, with Arsenophonus and Wolbachia infections being predominant. Hemipteriphilus and Hamiltonella frequencies were very low and were detected in seven and two samples, respectively. Bacterial diversity based on three independent parameters including Simpson index, number of haplotypes and Bray-Curtis dissimilarity matrix was significantly higher in 1997 than in 2017. This period also coincided with the advent of super-abundant cassava-whitefly populations on cassava crops in Uganda. We discuss how endosymbionts may influence the biology and behaviour of whiteflies leading to population explosions

Spatio-temporal changes in endosymbiont diversity and composition in the African cassava whitefly, Bemisia tabaci SSA1

The data presented in the study are deposited and available in the GenBank repository, accession number from OP160970 to OP161013.International audienceSap-sucking insects, including whiteflies, are amongst the most devastating and widely distributed organisms on the planet. They are often highly invasive and endosymbiont communities within these insects help them adapt to new or changing environments. Bemisia tabaci (Gennadius; Hemiptera: Aleyrodidae) whitefly species are vectors of more than 500 known plantviruses and harbour highly diverse endosymbionts communities. To date, however, whitefly-endosymbiont interactions, community structure and their spatio-temporal changes are still poorly understood. In this study, we investigated the spatio-temporal changes in the composition and diversity of bacterial endosymbionts in the agricultural crop pest whitefly species, Bemisia tabaci sub-Saharan Africa 1-subgroup 1 and 2 (SSA1-SG1 and SSA1-SG2). 16S rRNA amplicon sequencing analysis was carried out to characterise endosymbiont compositionsin field-collected SSA1 (SSA1-SG1 and SSA1-SG2) populations infesting cassava in Uganda in 1997 and 2017. We detected Portiera, Arsenophonus, Wolbachia, Hamiltonella and Hemipteriphilus, with Arsenophonus and Wolbachia infections being predominant. Hemipteriphilus and Hamiltonella frequencies were very low and were detected in seven and two samples, respectively. Bacterial diversity based on three independent parameters including Simpson index, number of haplotypes and Bray-Curtis dissimilarity matrix was significantly higher in 1997 than in 2017. This period also coincided with the advent of super-abundant cassava-whitefly populations on cassava crops in Uganda. We discuss how endosymbionts may influence the biology and behaviour of whiteflies leading to population explosions

Spatio-temporal changes in endosymbiont diversity and composition in the African cassava whitefly, Bemisia tabaci SSA1

Sap-sucking insects, including whiteflies, are amongst the most devastating and widely distributed organisms on the planet. They are often highly invasive and endosymbiont communities within these insects help them adapt to new or changing environments. Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) whitefly species are vectors of more than 500 known plant-viruses and harbour highly diverse endosymbionts communities. To date, however, whitefly-endosymbiont interactions, community structure and their spatio-temporal changes are still poorly understood. In this study, we investigated the spatio-temporal changes in the composition and diversity of bacterial endosymbionts in the agricultural crop pest whitefly species, B. tabaci Sub-Saharan Africa 1-sub group 1 (SSA1-SG1). 16S rRNA amplicon sequencing analysis was carried out to characterise endosymbiont compositions in field-collected SSA1 populations infesting cassava in Uganda in 1997 and 2017. We detected Portiera, Arsenophonus, Wolbachia, Hamiltonella and Hemipteriphilus, with Arsenophonus and Wolbachia infections being predominant. Hemipteriphilus and Hamiltonella frequencies were very low and were detected in seven and 2 samples, respectively. Bacterial diversity based on three independent parameters including Simpson index, number of haplotypes and Bray-Curtis Dissimilarity matrix were significantly higher in 1997 than in 2017. This period also coincided with the advent of super-abundant cassava-whitefly populations on cassava crops in Uganda. We discuss how endosymbionts may influence the biology and behaviour of whiteflies leading to population explosions