Are local plague endemicity and ecological characteristics of vectors and reservoirs related? A case study in north-east Tanzania

The pattern of plague endemicity in Tanzania is characterized by continuous re-appearance of the disease in some locations,while in other neighbouring villages the disease has never or seldom been observed.To understand the reasons for this pattern,we studied small mammal and flea species composition,diversity and relative abundances in two plague-endemic and two plague-free locations.We asked(a)whether fleas more abundant in plague-endemic locations differ in their characteristic abundance and the degree of their host specificity from fleas more abundant in plague-free locations and(b)whether hosts most abundant in plague-endemic locations differ in the diversity of their flea assemblages from hosts most abundant in plague-free locations.We characterized(a)each host species by species richness and degree of taxonomic relatedness of its flea assemblages and(b)each flea species by its mean abundance and size,and degree of taxonomic relatedness of its host spectrum and compared their relative abundances between locations.No significant difference between plague-endemic and plague-free locations in either host density or any variable related to flea abundance or diversity was found.However,there was marginally significant effect of taxonomic distinctness of a flea assemblage harboured by a host on its probability to be more abundant in either plague-endemic or plague-free locations.Furthermore,hosts more abundant in plague-endemic locations tended to harbour closely-related fleas.Finally,while opportunistic and specialist fleas were equally distributed in both sets of locations,fleas exploiting distantly-related hosts were found mainly in plague-free locations during the rainy season.We suggest that the observed patterns might arise due to seasonal and spatial differences in local microclimatic conditions and landscape connectivity.

Latitudinal gradients in body size and sexual size dimorphism in fleas:males drive Bergmann’s pattern

We tested for the effects of latitude and geographic range size(GRS)on body size,leg length,and sexual size dimorphism(SSD)across 103 species offleas,taking into account phylogenetic between-species relationships.When the data on body size were combined for males and females,the positive correlation between body size and latitude,but not GRS,was revealed.When the analysis was restricted to one sex only,the effect of latitude appeared to be non-significant for females,whereas male body size increased with an increase in latitude.Intraspecific body size variation was not associated with either the latitude or the latitudinal span of the geographic range,independently of which data subset was analyzed.No evidence of association between size-independent tibia length and latitude was found for either females,males,or both sexes combined.The degree of SSD decreased with a decrease in latitude but was not affected by GRS.We conclude that macroecological patterns might be manifested differently in males and females.This should be kept in mind while searching for these patterns.

Adaptation to a novel host and performance trade-off in host-generalist and host-specific insect ectoparasites

We investigated the performance trade-offs of fleas(Siphonaptera)while adapting to a novel host using two host generalists(Xenopsylla conformis and Xenopsylla ramesis)and one host specialist(Parapulex chephrenis)maintained on their principal hosts(Meriones crassus for Xenopsylla and Acomys cahirinus for P.chephrenis).We asked whether,over generations,(i)a host generalist may become a specialist by evolving the ability to exploit a novel host and losing the ability to exploit an original host and(ii)a host specialist can become a generalist by evolving the ability to exploit a novel host without losing the ability to exploit an original host.We established an experimental line of each species on a novel host(Acomys russatus for Xenopsylla and M.crassus for P.chephrenis)and maintained this line on this host during 23 generations.We compared reproductive performance of progenitors of each line and their descendants when they exploited either original or novel host in terms of egg number and size,hatching success,offspring production,and offspring size.We found changes in performance over generations in female offspring size only.Xenopsylla conformis demonstrated a tendency to become a host specialist(increased performance on the novel host with a concomitant decreased performance on the original host),whereas P.chephrenis demonstrated a tendency to become a host generalist(increased performance on the novel host without a concomitant decreased performance on the original host).We conclude that the probability of generalist to specialist transition,and vice versa,is context-dependent and varies between species.