Two hypotheses have been proposed to explain the abundance-occupancy relationship (AOR) in parasites. The niche breadth hypothesis suggests that host generalists are more abundant and efficient at colonizing different host communities than specialists. The trade-off hypothesis argues that host specialists achieve high density across their hosts' ranges, whereas generalists incur the high cost of adaptation to diverse immuno-defence systems. We tested these hypotheses using 386 haemosporidian cytochrome-b lineages (1894 sequences) recovered from 2318 birds of 103 species sampled in NW Africa, NW Iberia, W Greater Caucasus and Transcaucasia. The number of regions occupied by lineages was associated with their frequency suggesting the presence of AOR in avian Haemosporidia. However, neither hypothesis provided a better explanation for the AOR. Although the host generalist Plasmodium SGS1 was over three times more abundant than other widespread lineages, both host specialists and generalists were successful in colonizing all study regions and achieved high overall prevalence.
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The massasauga Sistrurus catenatus was historically divided into three subspecies, but this long-standing taxonomy has recently been called into question. Genetic research now recognizes a split of the species into the eastern massasauga S. catenatus and western massasauga S. tergeminus, with the latter split into two subspecies, the desert massasauga S. t. edwardsii and the prairie massasauga S. t. tergeminus. Although the distinction between geographically isolated populations of S. catenatus and S. tergeminus is well-supported genetically, the geographic relationships among populations of S. t. tergeminus and S. t. edwardsii remain unresolved because of incomplete sampling throughout the species’ range. This poses a difficult challenge for conservation and management of this species. Sistrurus t. tergeminus does not have state or federal conservation status, but S. t. edwardsii has been petitioned for listing under the U.S. Endangered Species Act. In this study, we used nuclear and mitochondrial DNA from 52 individuals from 7 states to explore the taxonomic and geographic relationships between S. t. tergeminus and S. t. edwardsii populations. Maximum likelihood and Bayesian inference frameworks for both nuclear and mtDNA genes indicated that S. t. tergeminus and S. t. edwardsii populations were genetically indistinguishable. However, at the species level, we did find eight well-supported mtDNA clades within S. tergeminus, including individuals from five peripheral populations in 1) Arizona and western New Mexico, 2) Colorado and Kansas, 3) Missouri, 4) Oklahoma, and 5) southern Texas. These peripherally isolated populations surrounded a larger population of individuals from north-central Texas, New Mexico, and Oklahoma that was contiguous with three additional genetically distinct populations. We conclude that the putative subspecies S. t. tergeminus and S. t. edwardsii, as currently defined, most likely represent polytypic phenotypes of S. tergeminus rather than discrete taxonomic entities. Instead, we suggest that S. tergeminus existed historically as a large, contiguous collection of populations that only recently became fragmented into several, as opposed to two, potentially discrete taxonomic entities.
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