Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a cryptic species complex
comprising at least 35 morphologically indistinguishable species that exhibit high genetic variation. B. tabaci is the vector of cassava mosaic geminiviruses (CMGs) causing cassava mosaic disease (CMD) to cassava (Manihot esculenta Crantz) – one of the most important food security crops in sub-Saharan Africa. After the outbreak and spread of a severe form of CMD from Uganda to several East and African countries, the production of cassava has been impaired causing severe yield loss resulting in food shortages and forcing some farmers to abandon the cultivation of the crop. The severe CMD pandemic is mainly characterized by high severity and incidence of the disease dominated by whitefly-borne infection and super-abundant populations of B. tabaci. All Bemisia tabaci individuals harbour a primary bacterial symbiont (Portiera aleyrodidarum) essential for their survival, and many also harbour non-essential secondary symbionts.
In an attempt to understand the cause of the super-abundant populations, it was suggested that a distinct genotype cluster is associated with the pandemic, however, there was no definitive proof for this or other potential causes of super-abundance. Moreover, these super-abundant populations, that occur in pandemic areas, remain unstudied in terms of the endosymbiotic bacteria they harbour, their population structure, and the potential for gene flow and hybridization. Therefore, this research through the use of field survey and molecular approaches aimed to fill the knowledge gap to this effect and sought (i) to assess the genetic diversity and distribution of cassava-associated B. tabaci in CMD pandemicaffected areas in Tanzania and to establish the association of a distinct B. tabaci genotype with the pandemic; (ii) to establish the prevalence of endosymbionts among B. tabaci populations on cassava and nearby plants, to assess if there is a unique  endosymbiont that is associated with super-abundant whitefly populations and to provide baseline information on endosymbionts of whiteflies for Tanzania; and (iii) to investigate the population structure of and the possibilities of gene flow between B. tabaci populations found in CMD pandemic affected and not yet affected parts of Tanzania. Hence, several field surveys were conducted in CMD pandemic-affected and unaffected areas in Tanzania and whiteflies were collected from cassava plants, intercrops and weeds in and surrounding cassava fields. Insects collected in this way were subjected to molecular analyses using mitochondrial and nuclear markers.
The four B. tabaci putative species found to be present in the study area, based on mitochondrial data, were Sub-Saharan Africa 1 (SSA1), Mediterranean (MED), Indian Ocean (IO) and East Africa 1 (EA1). Moreover, a group of unknown whitefly species was also identified. There were four sub-groups under SSA1 which included SSA1-SG1, SSA1-SG2, SSA1-SG1/2 and SSA1-SG3.
The findings of this study also established SSA1-SG1 to be the pandemic-associated B. tabaci in Tanzania, being found predominantly in the pandemic affected north-western part of the country. B. tabaci abundance and mean CMD severity values were highest for north-western Tanzania, matching the characteristic of the pandemic. The SSA1-SG2 was shown to be present in the pandemic-unaffected central part of Tanzania whereas the
coastal sub-group that is SSA1-SG3 was exclusively found confined to the coastal parts and was absent from the central and north-western parts.
Furthermore, microsatellite data also identified SSA1, MED and IO putative species from the B. tabaci species complex as well as B. afer. It further revealed two sub-clusters within SSA1, which were each dominating different geographic areas across the sampling zones.
One type predominated in the north-western pandemic affected part of Tanzania but
diminished in frequency towards the south-east, whilst the other became increasingly
common until it becomes almost the only type present in the coastal area. The central part was a mix of the two clusters. Some evidence of gene flow and hybridization was observed between the two sub-clusters where several hybrids were prevalent with no strong assignment to any of the clusters.
The CMD pandemic front was estimated to lie in Geita Region located in north-western
Tanzania. The pandemic is spreading from the north-west to south-east of the country at an approximate rate of 26 km per year. The pandemic-associated SSA1-SG1 was also found extending up to 180 km to the south east of the pandemic front and very recently also in the central parts of Tanzania – an area not yet affected by the pandemic.
In this study cassava was noted to be colonized only by SSA1 putative species and was
never seen to be colonized by the other putative species suggesting that cassava in Africa
may only be colonized by cassava types.
The different sub-groups and putative species in the study exhibited contrasting
endosymbionts profiles. Arsenophonus was the most commonly infecting secondary
symbiont among the pandemic-associated SSA1-SG1 populations. While multiple infection
was infrequent in this group, Arsenophonus/Cardinium was the most common double
infection type. Although no particular endosymbiont was found to be associated only with
the pandemic-associated SSA1-SG1, the most remarkable feature was the absence of
secondary symbiont infection for more than a third of the population and being the only
sub-group comprising a significant number of individuals with no secondary symbiont
infection. Multiple infection was very common in the remaining SSA1 sub-groups; SG2 and
SG1/2, mainly occurring in the non-pandemic central parts of Tanzania, frequently
harboured Arsenophonus/Cardinium or Hamiltonella/Arsenophonus/Cardinium infections.
The coastal type sub-group, SSA1-SG3, often showed multiple infection by
Rickettsia/Hamiltonella/Arsenophonus/Cardinium. Multiple infections were common in
whiteflies from MED which harboured combinations of Rickettsia/Hamiltonella/Wolbachia
with Arsenophonus or Cardinium. Similarly, double and multiple infections were also
common in the IO and EA1 putative species where EA1 individuals commonly harboured
Cardinium/Wolbachia with several additional combinations of Rickettsia, Hamiltonella or
Arsenophonus. The secondary endosymbiont Fritschea was never recorded in this study.
Finally, the extended presence of the pandemic-associated SSA1-SG1 in areas of up to 180 km to the south-east of the pandemic front is not entirely surprising, as changes in whitefly populations precede changes in disease status. However, the advancement of this
pandemic-associated sub-group further into the non-pandemic central part coupled with
the possibilities of gene flow and hybridization with other sub-groups poses an on-going
threat to cassava production in the region as it implicates a further spread of the pandemic to previously unaffected areas. This pandemic-associated sub-group is also distinct from the others in terms of its secondary endosymbiont profile, being the only sub-group in this
study exhibiting no infection in more than one third of the population.
Therefore, future studies should focus towards understanding further the B. tabaci species
complex in the area in general and in understanding this super-abundant pandemicassociated population in particular. Specific studies to assess the presence of other secondary endosymbionts, antibiotic treatments to eliminate certain endosymbionts or to artificially introduce endosymbionts are paramount to fully understand their exact effects on these insects in the wider region of East and Central Africa. Moreover, mating studies involving the pandemic-associated and the other sub-groups are necessary to exactly determine if the pandemic-associated type is taking over through hybridization and if the hybrids are conferred with increased fitness. Only by understanding the vector better can we design integrated pest management strategies to stop the spread of the severe CMD pandemic that continues to threaten the food security of several millions of people in sub-Saharan Africa.
This work was the first study that confirmed the association of a distinct genetic sub group
to the severe CMD pandemic in Tanzania more than a decade after similar associations
were first made in Uganda in 2002. Moreover, the current study showed for the first time
some evidence of gene flow and hybridization among the sub-clusters of the SSA1 putative species. The study was also the first work that provided baseline information on the contrasting endosymbiont patterns of whiteflies for a part of East Africa which until now had not been studied.