Reproductive Manipulators in the Bark Beetle Pityogenes chalcographus (Coleoptera: Curculionidae)-The Role of Cardinium, Rickettsia, Spiroplasma, and Wolbachia.

Heritable bacterial endosymbionts can alter the biology of numerous arthropods. They can influence the reproductive outcome of infected hosts, thus affecting the ecology and evolution of various arthropod species. The spruce bark beetle Pityogenes chalcographus (L.) (Coleoptera: Curculionidae: Scolytinae) was reported to express partial, unidirectional crossing incompatibilities among certain European populations. Knowledge on the background of these findings is lacking; however, bacterial endosymbionts have been assumed to manipulate the reproduction of this beetle. Previous work reported low-density and low-frequency Wolbachia infections of P. chalcographus but found it unlikely that this infection results in reproductive alterations. The aim of this study was to test the hypothesis of an endosymbiont-driven incompatibility, other than Wolbachia, reflected by an infection pattern on a wide geographic scale. We performed a polymerase chain reaction (PCR) screening of 226 individuals from 18 European populations for the presence of the endosymbionts Cardinium, Rickettsia, and Spiroplasma, and additionally screened these individuals for Wolbachia. Positive PCR products were sequenced to characterize these bacteria. Our study shows a low prevalence of these four endosymbionts in P. chalcographus. We detected a yet undescribed Spiroplasma strain in a single individual from Greece. This is the first time that this endosymbiont has been found in a bark beetle. Further, Wolbachia was detected in three beetles from two Scandinavian populations and two new Wolbachia strains were described. None of the individuals analyzed were infected with Cardinium and Rickettsia. The low prevalence of bacteria found here does not support the hypothesis of an endosymbiont-driven reproductive incompatibility in P. chalcographus.

The hitchhiker's guide to Europe: the infection dynamics of an ongoing Wolbachia invasion and mitochondrial selective sweep in Rhagoletis cerasi.

Wolbachia is a maternally inherited and ubiquitous endosymbiont of insects. It can hijack host reproduction by manipulations such as cytoplasmic incompatibility (CI) to enhance vertical transmission. Horizontal transmission of Wolbachia can also result in the colonization of new mitochondrial lineages. In this study, we present a 15-year-long survey of Wolbachia in the cherry fruit fly Rhagoletis cerasi across Europe and the spatiotemporal distribution of two prevalent strains, wCer1 and wCer2, and associated mitochondrial haplotypes in Germany. Across most of Europe, populations consisted of either 100% singly (wCer1) infected individuals with haplotype HT1, or 100% doubly (wCer1&2) infected individuals with haplotype HT2, differentiated only by a single nucleotide polymorphism. In central Germany, singly infected populations were surrounded by transitional populations, consisting of both singly and doubly infected individuals, sandwiched between populations fixed for wCer1&2. Populations with fixed infection status showed perfect association of infection and mitochondria, suggesting a recent CI-driven selective sweep of wCer2 linked with HT2. Spatial analysis revealed a range expansion for wCer2 and a large transition zone in which wCer2 splashes appeared to coalesce into doubly infected populations. Unexpectedly, the transition zone contained a large proportion (22%) of wCer1&2 individuals with HT1, suggesting frequent intraspecific horizontal transmission. However, this horizontal transmission did not break the strict association between infection types and haplotypes in populations outside the transition zone, suggesting that this horizontally acquired Wolbachia infection may be transient. Our study provides new insights into the rarely studied Wolbachia invasion dynamics in field populations.

Divergent evolutionary histories of two sympatric spruce bark beetle species.

Ips typographus and Pityogenes chalcographus are two sympatric Palearctic bark beetle species with wide distribution ranges. As both species are comparable in biology, life history, and habitat, including sharing the same host, Picea abies, they provide excellent models for applying a comparative approach in which to identify common historical patterns of population differentiation and the influence of species-specific ecological characteristics. We analysed patterns of genetic diversity, genetic structure and demographic history of ten I. typographus and P. chalcographus populations co-distributed across Europe using both COI and ITS2 markers. Rather than similarities, our results revealed striking differences. Ips typographus was characterised by low genetic diversity, shallow population structure and strong evidence that all extant haplogroups arose via a single Holocene population expansion event. In contrast, genetic variation and structuring were high in P. chalcographus indicating a longer and more complex evolutionary history. This was estimated to be five times older than I. typographus, beginning during the last Pleistocene glacial maximum over 100 000 years ago. Although the expansions of P. chalcographus haplogroups also date to the Holocene or just prior to its onset, we show that these occurred from at least three geographically separated glacial refugia. Overall, these results suggest that the much longer evolutionary history of P. chalcographus greatly influenced the levels of phylogeographic subdivision among lineages and may have led to the evolution of different life-history traits which in turn have affected genetic structure and resulted in an advantage over the more aggressive I. typographus.

Hit or miss in phylogeographic analyses: the case of the cryptic NUMTs.

Phylogeographic studies call for attention as nuclear copies of mitochondrial DNA (NUMT) may generate erroneous results. Here, we report the presence of NUMTs differing only by 1-3 bp from authentic mitochondrial haplotypes, consequently named cryptic NUMTs. In contrast to traditional NUMTs, for which reliable tools for detection are established, cryptic NUMTs question the validity of phylogeographic analyses based solely on mitochondrial DNA, like the one presented here on the European bark beetle Ips typographus. Caution is called as cryptic NUMTs might be responsible for haplotype richness found in several species, and the necessity of refined methods for NUMT detection is highlighted.