All studies to date of breeding structure in the Morsitans group tsetse flies have indicated highly structured populations among which there has been little detectable gene flow [16
]. Our results in G. pallidipes
are in agreement with the earlier findings. Populations in western Uganda were significantly differentiated from flies in the northeastern corner of Lake Victoria, and these populations were further differentiated from the population in Nguruman in south-central Kenya. Furthermore, tsetse populations were not homogeneous within the three regions.
Indices of differentiation inferred from mtDNA and microsatellites indicated that populations at Okame, Kapesur and Lambwe Valley form a genetically homogeneous group relative to the populations lying approximately 400 km to the east or west. Within this group, however, genetic diversity was less in Okame and Kapesur than in Lambwe Valley. In fact, mtDNA haplotypes recovered from Okame and Kapesur formed a subset of those found in the Lambwe Valley. Similarly, with the exception of one allele at one locus, microsatellite alleles in Okame and Kapesur were also a subset of those found in the Lambwe Valley (data not shown). Past control operations under the Farming in Tsetse Controlled Areas (FITCA) project http://www.au-ibar.org/index.php/en/projects/completed-projects/fitca/achievements
, are likely to be responsible for the genetic structuring. Historically, the three populations may have been part of a large, panmictic, and genetically diverse population, and control activities may have severely reduced population sizes in Okame and Kapesur leading to the observed reduction in genetic diversity. Once the FITCA project ended in the early 2000s, gene flow from Lambwe Valley could have led to increased genetic diversity and allelic homogenization. Alternatively, the Okame and Kapesur populations are not relicts of a larger population but originated from two recent colonizations from the Lambwe Valley. A priori
, both scenarios are equally likely. However, since earlier genetic studies indicated that the Lambwe Valley tsetse population is large and has been in residence for a long time [14
], it is most likely that the low genetic diversity observed in Okame and Kapesur flies is due to recent colonization rather than a past bottleneck.
As in the Lake Victoria region, populations in western Uganda differed significantly over the approximately 190 km separating Kabunkanga and Murchison Falls. Populations of G. pallidipes at Kabunkanga and Murchison Falls exhibited similar microsatellite frequencies, but extremely divergent mtDNA haplotypes.
Because of differences in evolutionary rates and inheritance patterns between bi-parentally inherited microsatellite loci and maternally-inherited mtDNA, direct comparisons between the results of these two types of molecular marker might be misleading. To investigate the possibility of sex-biased dispersal, in addition to comparing microsatellite and mtDNA results, we carried out an individual-based sex-specific analysis of the level of genetic differentiation and relatedness using only microsatellite data. If dispersal is sex-biased, we expect to encounter higher genetic differentiation within populations and more genetically similar individuals across populations in the better-dispersing sex, while the more philopatric sex will exhibit higher relatedness values between individuals within populations and increased genetic dissimilarity and lesser relatedness between populations relative to the more mobile sex [37
Our data can suggest that males disperse over longer distances than females (Table and Figure ). Despite the fact that females are believed to be highly mobile [38
] due to their relatively larger body size, males are active for longer periods of time [39
] and devote blood-meals exclusively to the production of fat, which is used as an energy reserve for flight [40
]. Additionally, the asymmetry in male versus female dispersal could be attributed to flight constraints imposed on females by carrying a larva, which can double the weight of a female at the peak of pregnancy [41
]. The male-biased dispersal recovered from microsatellite data needs further scrutiny as the small male sample sizes in this study did not allow for rigorous testing of hypothesis, as the study was not designed for this purpose.
The low level of microsatellite differentiation between tsetse at Kabunkanga and Murchison Falls is also hard to reconcile with the absolute divergence in COI
sequences observed between tsetse flies from these two sites. The net average nucleotide divergence was 2.7%, consistent with a divergence time of 1.8 million years, assuming a molecular clock ticking at 1.5% divergence per million years [33
]. Therefore, unequal dispersal rates would have to have been maintained for an extremely long period in order to generate the conflicting signals in microsatellites and mtDNA.
A mitochondrial sweep, due perhaps to Wolbachia infection favoring the amplification of a particular mitochondrial lineage in one population, could have shortened the time frame over which this apparent divergence accumulated. Even in this case, though, sufficient time has passed to allow the accumulation of mtDNA diversity in both populations without any concomitant exchange of haplotypes. Owing to the possibility of past bottlenecks and rare long-distance colonizations, as well as sex-biased dispersal, the phylogeographic history of G. pallidipes appears to be complex.
Aside from the seemingly contradictory signals from microsatellites and mtDNA in western Uganda, we also observed neighboring populations in the Lake Victoria region that shared two mtDNA lineages differing by about 2% without observing any of the intervening haplotypes. This would suggest that, G. pallidipes colonized the Lake Victoria region independently at least twice or a very large and diverse population of G. pallidipes underwent a severe bottleneck or series of lesser bottlenecks, leaving only remnants of the past diversity. A deeper understanding of the phylogeography of G. pallidipes will require greater context and range-wide relationships should be explored more thoroughly in the future.
The current study greatly enhances our understanding of G. pallidipes
population dynamics especially in Uganda, which has been a missing link in previous samplings. To the best of our knowledge, this is the first report on the population structure of this species in Uganda based on natural samples. In an earlier paper that described the population structure of G. pallidipes
at a macrogeographic scale covering almost its entire range, only a single sample from a laboratory colony of G. pallidipes
originating from Uganda nearly three decades ago was analyzed [15
]. In another study Ouma et al. [14
] discussed the relict G. pallidipes
populations in Lambwe and Nguruman, and demonstrated temporal and seasonal stability of G. pallidipes
populations in these areas. Such temporal stability has also been reported in G. fuscipes fuscipes
]. These previous studies were reviewed [16
] and suggested significant differentiation among natural populations of G. pallidipes
in eastern and southern Africa. However, in the absence of samples from Uganda, it was always difficult to put the data into perspective and understand the re-infestation of western Kenya including Lambwe Valley and Busia-Teso regions by G. pallidipes
The findings of this study have reaffirmed the importance of gathering genetic data prior to implementing area-wide tsetse vector control operations as recommended for creation of G.p. gambiensis
free zones in the Niayes region of Senegal [43
]. Genetic data should be generated as part of baseline data collection to provide the much needed scientific evidence upon which control measures can be effectively implemented.