Multiple introgressions shape mitochondrial evolutionary history in Drosophila paulistorum and the Drosophila willistoni group

Hybridization and the consequent introgression of genomic elements is an important source of genetic diversity for biological lineages. This is particularly evident in young clades in which hybrid incompatibilities are still incomplete and mixing between species is more likely to occur. Drosophila paulistorum, a representative of the Neotropical Drosophila willistoni subgroup, is a classic model of incipient speciation. The species is divided into six semispecies that show varying degrees of pre- and post-mating incompatibility with each other. In the present study, we investigate the mitochondrial evolutionary history of D. paulistorum and the willistoni subgroup. For that, we perform phylogenetic and comparative analyses of the complete mitochondrial genomes and draft nuclear assemblies of 25 Drosophila lines of the willistoni and saltans species groups. Our results show that the mitochondria of D. paulistorum are polyphyletic and form two non-sister clades that we name α and β. Identification and analyses of nuclear mitochondrial insertions further reveal that the willistoni subgroup has an α-like mitochondrial ancestor and strongly suggest that both the α and β mitochondria of D. paulistorum were acquired through introgression from unknown fly lineages of the willistoni subgroup. We also uncover multiple mitochondrial introgressions across D. paulistorum semispecies and generate novel insight into the evolution of the species.

Complex effects of environment and Wolbachia infections on the life history of Drosophila melanogaster hosts

Wolbachia bacteria are common endosymbionts of many arthropods found in gonads and various somatic tissues. They manipulate host reproduction to enhance their transmission and confer complex effects on fitness-related traits. Some of these effects can serve to increase the survival and transmission efficiency of Wolbachia in the host population. The Wolbachia-Drosophila melanogaster system represents a powerful model to study the evolutionary dynamics of host-microbe interactions and infections. Over the past decades, there has been a replacement of the ancestral wMelCS Wolbachia variant by the more recent wMel variant in worldwide D. melanogaster populations, but the reasons remain unknown. To investigate how environmental change and genetic variation of the symbiont affect host developmental and adult life-history traits, we compared effects of both Wolbachia variants and uninfected controls in wild-caught D. melanogaster strains at three developmental temperatures. While Wolbachia did not influence any developmental life-history traits, we found that both lifespan and fecundity of host females were increased without apparent fitness trade-offs. Interestingly, wMelCS-infected flies were more fecund than uninfected and wMel-infected flies. By contrast, males infected with wMel died sooner, indicating sex-specific effects of infection that are specific to the Wolbachia variant. Our study uncovered complex temperature-specific effects of Wolbachia infections, which suggests that symbiont-host interactions in nature are strongly dependent on the genotypes of both partners and the thermal environment.

Restriction of Wolbachia Bacteria in Early Embryogenesis of Neotropical Drosophila Species via Endoplasmic Reticulum-Mediated Autophagy

Wolbachia are maternally transmitted intracellular bacteria that are not only restricted to the reproductive organs but also found in various somatic tissues of their native hosts. The abundance of the endosymbiont in the soma, usually a dead end for vertically transmitted bacteria, causes a multitude of effects on life history traits of their hosts, which are still not well understood. Thus, deciphering the host-symbiont interactions on a cellular level throughout a host's life cycle is of great importance to understand their homeostatic nature, persistence, and spreading success. Using fluorescent and transmission electron microscopy, we conducted a comprehensive analysis of Wolbachia tropism in soma and germ line of six Drosophila species at the intracellular level during host development. Our data uncovered diagnostic patterns of infections to embryonic primordial germ cells and to particular cells of the soma in three different neotropical Drosophila species that have apparently evolved independently. We further found that restricted patterns of Wolbachia tropism are determined in early embryogenesis via selective autophagy, and their spatially restricted infection patterns are preserved in adult flies. We observed tight interactions of Wolbachia with membranes of the endoplasmic reticulum, which might play a scaffolding role for autophagosome formation and subsequent elimination of the endosymbiont. Finally, by analyzing D. simulans lines transinfected with nonnative Wolbachia, we uncovered that the host genetic background regulates tissue tropism of infection. Our data demonstrate a novel and peculiar mechanism to limit and spatially restrict bacterial infection in the soma during a very early stage of host development. IMPORTANCE All organisms are living in close and intimate interactions with microbes that cause conflicts but also cooperation between both unequal genetic partners due to their different innate interests of primarily enhancing their own fitness. However, stable symbioses often result in homeostatic interaction, named mutualism, by balancing costs and benefits, where both partners profit. Mechanisms that have evolved to balance and stably maintain homeostasis in mutualistic relationships are still quite understudied; one strategy is to "domesticate" potentially beneficial symbionts by actively controlling their replication rate below a critical and, hence, costly threshold, and/or to spatially and temporally restrict their localization in the host organism, which, in the latter case, in its most extreme form, is the formation of a specialized housing organ for the microbe (bacteriome). However, questions remain: how do these mutualistic associations become established in their first place, and what are the mechanisms for symbiont control and restriction in their early stages? Here, we have uncovered an unprecedented symbiont control mechanism in neotropical Drosophila species during early embryogenesis. The fruit fly evolved selective autophagy to restrict and control the proliferation of its intracellular endosymbiont Wolbachia in a defined subset of the stem cells as soon as the host's zygotic genome is activated.

Characterization and Tissue Tropism of Newly Identified Iflavirus and Negeviruses in Glossina morsitans morsitans Tsetse Flies

Tsetse flies cause major health and economic problems as they transmit trypanosomes causing sleeping sickness in humans (Human African Trypanosomosis, HAT) and nagana in animals (African Animal Trypanosomosis, AAT). A solution to control the spread of these flies and their associated diseases is the implementation of the Sterile Insect Technique (SIT). For successful application of SIT, it is important to establish and maintain healthy insect colonies and produce flies with competitive fitness. However, mass production of tsetse is threatened by covert virus infections, such as the Glossina pallidipes salivary gland hypertrophy virus (GpSGHV). This virus infection can switch from a covert asymptomatic to an overt symptomatic state and cause the collapse of an entire fly colony. Although the effects of GpSGHV infections can be mitigated, the presence of other covert viruses threaten tsetse mass production. Here we demonstrated the presence of two single-stranded RNA viruses isolated from Glossina morsitans morsitans originating from a colony at the Seibersdorf rearing facility. The genome organization and the phylogenetic analysis based on the RNA-dependent RNA polymerase (RdRp) revealed that the two viruses belong to the genera Iflavirus and Negevirus, respectively. The names proposed for the two viruses are Glossina morsitans morsitans iflavirus (GmmIV) and Glossina morsitans morsitans negevirus (GmmNegeV). The GmmIV genome is 9685 nucleotides long with a poly(A) tail and encodes a single polyprotein processed into structural and non-structural viral proteins. The GmmNegeV genome consists of 8140 nucleotides and contains two major overlapping open reading frames (ORF1 and ORF2). ORF1 encodes the largest protein which includes a methyltransferase domain, a ribosomal RNA methyltransferase domain, a helicase domain and a RdRp domain. In this study, a selective RT-qPCR assay to detect the presence of the negative RNA strand for both GmmIV and GmmNegeV viruses proved that both viruses replicate in G. m. morsitans. We analyzed the tissue tropism of these viruses in G. m. morsitans by RNA-FISH to decipher their mode of transmission. Our results demonstrate that both viruses can be found not only in the host’s brain and fat bodies but also in their reproductive organs, and in milk and salivary glands. These findings suggest a potential horizontal viral transmission during feeding and/or a vertically viral transmission from parent to offspring. Although the impact of GmmIV and GmmNegeV in tsetse rearing facilities is still unknown, none of the currently infected tsetse species show any signs of disease from these viruses.

Parallel Sequencing of Wolbachia wCer2 from Donor and Novel Hosts Reveals Multiple Incompatibility Factors and Genome Stability after Host Transfers

The application of Wolbachia in insect pest and vector control requires the establishment of genotypically stable host associations. The cytoplasmic incompatibility (CI) inducing Wolbachia strain wCer2 naturally occurs in the cherry fruit fly Rhagoletis cerasi as co-infection with other strains and was transferred to other fruit fly species by embryonic microinjections. We obtained wCer2 genome data from its native and three novel hosts, Drosophila simulans, Drosophila melanogaster, and Ceratitis capitata and assessed its genome stability, characteristics, and CI factor (cif) genes. De novo assembly was successful from Wolbachia cell-enriched singly infected D. simulans embryos, with minimal host and other bacterial genome traces. The low yield of Wolbachia sequence reads from total genomic extracts of one multiply infected R. cerasi pupa and one singly infected C. capitata adult limited de novo assemblies but was sufficient for comparative analyses. Across hosts wCer2 was stable in genome synteny and content. Polymorphic nucleotide sites were found in wCer2 of each host; however, only one nucleotide was different between R. cerasi and C. capitata, and none between replicated D. simulans lines. The wCer2 genome is highly similar to wAu (D. simulans), wMel (D. melanogaster), and wRec (Drosophila recens). In contrast to wMel and wRec (each with one cif gene pair) and wAu (without any cif genes), wCer2 has three pairs of Type I cif genes, and one Type V cifB gene without a cifA complement. This may explain previously reported CI patterns of wCer2, including incomplete rescue of its own CI modification in three novel host species.

Integrative taxonomy and a new species description in the sturtevanti subgroup of the Drosophila saltans group (Diptera: Drosophilidae)

Although the biological concept of species is well established in animals, sometimes the decision about the specific status of a new species is difficult and hence requires support of an integrative analysis of several character sets. To date, the species Drosophila sturtevanti, D. magalhaesi, D. milleri and D. dacunhai, belonging to the sturtevanti subgroup of the Neotropical saltans species group, are identified mainly by the aedeagus morphology, but also present some differences in spot coloration and patterning of the female sixth tergite and in the shape and size of the spermathecae, parallel to a pattern of reproductive isolation. In the present study, we describe a novel saltans group species from French Guiana belonging to the sturtevanti subgroup. Our species designation is based on an integrative approach covering (i) aedeagi and spermathecae morphology by scanning electron microscopy, (ii) analysis of female sixth-tergite color, (iii) morphometrical analysis of aedeagi and wings, (iv) analysis of partial sequence of the COI, COII and ND4 mitochondrial genes as well as (v) intercrosses for analysis of reproductive isolation. The comparative analysis of the results on these markers with those of D. sturtevanti, D. milleri and D. dacunhai supports that this line belongs to a new species of the sturtevanti subgroup that we name Drosophila lehrmanae sp. nov. in honor of Prof. Lee Ehrman´s 85th birthday.

Age, tissue, genotype and virus infection regulate Wolbachia levels in Drosophila

The bacterial symbiont Wolbachia can protect insects against viral pathogens, and the varying levels of antiviral protection are correlated with the endosymbiont load within the insects. To understand why Wolbachia strains differ in their antiviral effects, we investigated the factors controlling Wolbachia density in five closely related strains in their natural Drosophila hosts. We found that Wolbachia density varied greatly across different tissues and between flies of different ages, and these effects depended on the host-symbiont association. Some endosymbionts maintained largely stable densities as flies aged while others increased, and these effects in turn depended on the tissue being examined. Measuring Wolbachia rRNA levels in response to viral infection, we found that viral infection itself also altered Wolbachia levels, with Flock House virus causing substantial reductions in symbiont loads late in the infection. This effect, however, was virus-specific as Drosophila C virus had little impact on Wolbachia in all of the five host systems. Because viruses have strong tissue tropisms and antiviral protection is thought to be cell-autonomous, these effects are likely to affect the virus-blocking phenomenon. However, we were unable to find any evidence of a correlation between Wolbachia and viral titres within the same tissues. We conclude that Wolbachia levels within flies are regulated in a complex host-symbiont-virus-dependent manner and this trinity is likely to influence the antiviral effects of Wolbachia.

Comparative genomic analysis of six Glossina genomes, vectors of African trypanosomes

BACKGROUND

Tsetse flies (Glossina sp.) are the vectors of human and animal trypanosomiasis throughout sub-Saharan Africa. Tsetse flies are distinguished from other Diptera by unique adaptations, including lactation and the birthing of live young (obligate viviparity), a vertebrate blood-specific diet by both sexes, and obligate bacterial symbiosis. This work describes the comparative analysis of six Glossina genomes representing three sub-genera: Morsitans (G. morsitans morsitans, G. pallidipes, G. austeni), Palpalis (G. palpalis, G. fuscipes), and Fusca (G. brevipalpis) which represent different habitats, host preferences, and vectorial capacity.

RESULTS

Genomic analyses validate established evolutionary relationships and sub-genera. Syntenic analysis of Glossina relative to Drosophila melanogaster shows reduced structural conservation across the sex-linked X chromosome. Sex-linked scaffolds show increased rates of female-specific gene expression and lower evolutionary rates relative to autosome associated genes. Tsetse-specific genes are enriched in protease, odorant-binding, and helicase activities. Lactation-associated genes are conserved across all Glossina species while male seminal proteins are rapidly evolving. Olfactory and gustatory genes are reduced across the genus relative to other insects. Vision-associated Rhodopsin genes show conservation of motion detection/tracking functions and variance in the Rhodopsin detecting colors in the blue wavelength ranges.

CONCLUSIONS

Expanded genomic discoveries reveal the genetics underlying Glossina biology and provide a rich body of knowledge for basic science and disease control. They also provide insight into the evolutionary biology underlying novel adaptations and are relevant to applied aspects of vector control such as trap design and discovery of novel pest and disease control strategies.

Comparative genomic analysis of six Glossina genomes, vectors of African trypanosomes

BACKGROUND

Tsetse flies (Glossina sp.) are the vectors of human and animal trypanosomiasis throughout sub-Saharan Africa. Tsetse flies are distinguished from other Diptera by unique adaptations, including lactation and the birthing of live young (obligate viviparity), a vertebrate blood-specific diet by both sexes, and obligate bacterial symbiosis. This work describes the comparative analysis of six Glossina genomes representing three sub-genera: Morsitans (G. morsitans morsitans, G. pallidipes, G. austeni), Palpalis (G. palpalis, G. fuscipes), and Fusca (G. brevipalpis) which represent different habitats, host preferences, and vectorial capacity.

RESULTS

Genomic analyses validate established evolutionary relationships and sub-genera. Syntenic analysis of Glossina relative to Drosophila melanogaster shows reduced structural conservation across the sex-linked X chromosome. Sex-linked scaffolds show increased rates of female-specific gene expression and lower evolutionary rates relative to autosome associated genes. Tsetse-specific genes are enriched in protease, odorant-binding, and helicase activities. Lactation-associated genes are conserved across all Glossina species while male seminal proteins are rapidly evolving. Olfactory and gustatory genes are reduced across the genus relative to other insects. Vision-associated Rhodopsin genes show conservation of motion detection/tracking functions and variance in the Rhodopsin detecting colors in the blue wavelength ranges.

CONCLUSIONS

Expanded genomic discoveries reveal the genetics underlying Glossina biology and provide a rich body of knowledge for basic science and disease control. They also provide insight into the evolutionary biology underlying novel adaptations and are relevant to applied aspects of vector control such as trap design and discovery of novel pest and disease control strategies.

The effect of Wolbachia on gene expression in Drosophila paulistorum and its implications for symbiont-induced host speciation

Background

The Neotropical fruit fly Drosophila paulistorum (Diptera: Drosophilidae) is a species complex in statu nascendi comprising six reproductively isolated semispecies, each harboring mutualistic Wolbachia strains. Although wild type flies of each semispecies are isolated from the others by both pre- and postmating incompatibilities, mating between semispecies and successful offspring development can be achieved once flies are treated with antibiotics to reduce Wolbachia titer. Here we use RNA-seq to study the impact of Wolbachia on D. paulistorum and investigate the hypothesis that the symbiont may play a role in host speciation. For that goal, we analyze samples of heads and abdomens of both sexes of the Amazonian, Centro American and Orinocan semispecies of D. paulistorum.

Results

We identify between 175 and 1192 differentially expressed genes associated with a variety of biological processes that respond either globally or according to tissue, sex or condition in the three semispecies. Some of the functions associated with differentially expressed genes are known to be affected by Wolbachia in other species, such as metabolism and immunity, whereas others represent putative novel phenotypes involving muscular functions, pheromone signaling, and visual perception.

Conclusions

Our results show that Wolbachia affect a large number of biological functions in D. paulistorum, particularly when present in high titer. We suggest that the significant metabolic impact of the infection on the host may cause several of the other putative and observed phenotypes. We also speculate that the observed differential expression of genes associated with chemical communication and reproduction may be associated with the emergence of pre- and postmating barriers between semispecies, which supports a role for Wolbachia in the speciation of D. paulistorum.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5816-9) contains supplementary material, which is available to authorized users.

High-sensitivity detection of cryptic Wolbachia in the African tsetse fly (Glossina spp.)

BACKGROUND

In African tsetse flies Glossina, spp. detection of bacterial symbionts such as Wolbachia is challenging since their prevalence and distribution are patchy, and natural symbiont titers can range at levels far below detection limit of standard molecular techniques. Reliable estimation of symbiont infection frequency, especially with regard to interrelations between symbionts and their potential impact on host biology, is of pivotal interest in the context of future applications for the control and eradication of Glossina-vectored African trypanosomosis. The presence or absence of symbionts is routinely screened with endpoint polymerase chain reaction (PCR), which has numerous advantages, but reaches its limits, when detecting infections at natural low titer. To not only determine presence of native tsetse symbionts but also to localize them to specific host tissues, fluorescence in situ hybridization (FISH) can be applied. However, classic FISH assays may not detect low-titer infections due to limitations in sensitivity.

RESULTS

We have compared classic endpoint PCR with high-sensitivity blot-PCR. We demonstrate that the latter technique allows for clear detection of low-titer Wolbachia in the morsitans and palpalis groups while classic endpoint PCR does not. In order to localize Wolbachia in situ in high and low-titer Glossina species, we applied high-end Stellaris® rRNA-FISH. We show that with this high sensitivity method, even low amounts of Wolbachia can be traced in specific tissues. Furthermore, we highlight that more tissues and organs than previously recorded are infested with Wolbachia in subspecies of the morsitans and palpalis groups.

CONCLUSIONS

Our results demonstrate that overall symbiont infection frequencies as well as the presence in specific host tissues may be underestimated when using low-sensitivity methods. To better understand the complex interrelation of tsetse flies and their native symbionts plus the pathogenic trypanosomes, it is important to consider application of a broader range of high-sensitivity detection tools.

Effect of antibiotic treatment and gamma-irradiation on cuticular hydrocarbon profiles and mate choice in tsetse flies (Glossina m. morsitans)

BACKGROUND

Symbiotic microbes represent a driving force of evolutionary innovation by conferring novel ecological traits to their hosts. Many insects are associated with microbial symbionts that contribute to their host's nutrition, digestion, detoxification, reproduction, immune homeostasis, and defense. In addition, recent studies suggest a microbial involvement in chemical communication and mating behavior, which can ultimately impact reproductive isolation and, hence, speciation. Here we investigated whether a disruption of the microbiota through antibiotic treatment or irradiation affects cuticular hydrocarbon profiles, and possibly mate choice behavior in the tsetse fly, Glossina morsitans morsitans. Four independent experiments that differentially knock down the multiple bacterial symbionts of tsetse flies were conducted by subjecting tsetse flies to ampicillin, tetracycline, or gamma-irradiation and analyzing their cuticular hydrocarbon profiles in comparison to untreated controls by gas chromatography - mass spectrometry. In two of the antibiotic experiments, flies were mass-reared, while individual rearing was done for the third experiment to avoid possible chemical cross-contamination between individual flies.

RESULTS

All three antibiotic experiments yielded significant effects of antibiotic treatment (particularly tetracycline) on cuticular hydrocarbon profiles in both female and male G. m. morsitans, while irradiation itself had no effect on the CHC profiles. Importantly, tetracycline treatment reduced relative amounts of 15,19,23-trimethyl-heptatriacontane, a known compound of the female contact sex pheromone, in two of the three experiments, suggesting a possible implication of microbiota disturbance on mate choice decisions. Concordantly, both female and male flies preferred non-treated over tetracycline-treated flies in direct choice assays.

CONCLUSIONS

While we cannot exclude the possibility that antibiotic treatment had a directly detrimental effect on fly vigor as we are unable to recolonize antibiotic treated flies with individual symbiont taxa, our results are consistent with an effect of the microbiota, particularly the obligate nutritional endosymbiont Wigglesworthia, on CHC profiles and mate choice behavior. These findings highlight the importance of considering host-microbiota interactions when studying chemical communication and mate choice in insects.

Symbiont-Driven Male Mating Success in the Neotropical Drosophila paulistorum Superspecies

Microbial symbionts are ubiquitous associates of living organisms but their role in mediating reproductive isolation (RI) remains controversial. We addressed this knowledge gap by employing the Drosophila paulistorum-Wolbachia model system. Semispecies in the D. paulistorum species complex exhibit strong RI between each other and knockdown of obligate mutualistic Wolbachia bacteria in female D. paulistorum flies triggers loss of assortative mating behavior against males carrying incompatible Wolbachia strains. Here we set out to determine whether de novo RI can be introduced by Wolbachia-knockdown in D. paulistorum males. We show that Wolbachia-knockdown D. paulistorum males (i) are rejected as mates by wild type females, (ii) express altered sexual pheromone profiles, and (iii) are devoid of the endosymbiont in pheromone producing cells. Our findings suggest that changes in Wolbachia titer and tissue tropism can induce de novo premating isolation by directly or indirectly modulating sexual behavior of their native D. paulistorum hosts.

Wolbachia modifies thermal preference in Drosophila melanogaster

Environmental variation can have profound and direct effects on fitness, fecundity, and host-symbiont interactions. Replication rates of microbes within arthropod hosts, for example, are correlated with incubation temperature but less is known about the influence of host-symbiont dynamics on environmental preference. Hence, we conducted thermal preference (Tp ) assays and tested if infection status and genetic variation in endosymbiont bacterium Wolbachia affected temperature choice of Drosophila melanogaster. We demonstrate that isogenic flies infected with Wolbachia preferred lower temperatures compared with uninfected Drosophila. Moreover, Tp varied with respect to three investigated Wolbachia variants (wMel, wMelCS, and wMelPop). While uninfected individuals preferred 24.4°C, we found significant shifts of -1.2°C in wMel- and -4°C in flies infected either with wMelCS or wMelPop. We, therefore, postulate that Wolbachia-associated Tp variation within a host species might represent a behavioural accommodation to host-symbiont interactions and trigger behavioural self-medication and bacterial titre regulation by the host.

Short and long-term evolutionary dynamics of subtelomeric piRNA clusters in Drosophila

Two Telomeric Associated Sequences, TAS-R and TAS-L, form the principal subtelomeric repeat families identified in Drosophila melanogaster. They are PIWI-interacting RNA (piRNA) clusters involved in repression of Transposable Elements. In this study, we revisited TAS structural and functional dynamics in D. melanogaster and in related species. In silico analysis revealed that TAS-R family members are composed of previously uncharacterized domains. This analysis also showed that TAS-L repeats are composed of arrays of a region we have named “TAS-L like” (TLL) identified specifically in one TAS-R family member, X-TAS. TLL were also present in other species of the melanogaster subgroup. Therefore, it is possible that TLL represents an ancestral subtelomeric piRNA core-cluster. Furthermore, all D. melanogaster genomes tested possessed at least one TAS-R locus, whereas TAS-L can be absent. A screen of 110 D. melanogaster lines showed that X-TAS is always present in flies living in the wild, but often absent in long-term laboratory stocks and that natural populations frequently lost their X-TAS within 2 years upon lab conditioning. Therefore, the unexpected structural and temporal dynamics of subtelomeric piRNA clusters demonstrated here suggests that genome organization is subjected to distinct selective pressures in the wild and upon domestication in the laboratory.

Insertion sequence polymorphism and genomic rearrangements uncover hidden Wolbachia diversity in Drosophila suzukii and D. subpulchrella

Ability to distinguish between closely related Wolbachia strains is crucial for understanding the evolution of Wolbachia-host interactions and the diversity of Wolbachia-induced phenotypes. A useful model to tackle these issues is the Drosophila suzukii - Wolbachia association. D. suzukii, a destructive insect pest, harbor a non-CI inducing Wolbachia 'wSuz' closely related to the strong CI-inducing wRi strain. Multi locus sequence typing (MLST) suggests presence of genetic homogeneity across wSuz strains infecting European and American D. suzukii populations, although different Wolbachia infection frequencies and host fecundity levels have been observed in both populations. Currently, it is not clear if these differences are due to cryptic wSuz polymorphism, host background, geographical factors or a combination of all of them. Here, we have identified geographical diversity in wSuz in D. suzukii populations from different continents using a highly diagnostic set of markers based on insertion sequence (IS) site polymorphism and genomic rearrangements (GR). We further identified inter-strain diversity between Wolbachia infecting D. suzukii and its sister species D. subpulchrella (wSpc). Based on our results, we speculate that discernible wSuz variants may associate with different observed host phenotypes, a hypothesis that demands future investigation. More generally, our results demonstrate the utility of IS and GRs in discriminating closely related Wolbachia strains.

Restricted distribution and lateralization of mutualistic Wolbachia in the Drosophila brain

Microbial symbionts are universal entities of all living organisms that can significantly affect host fitness traits in manifold ways but, even more fascinating, also their behaviour. Although better known from parasitic symbionts, we currently lack any cases where 'neurotrophic' symbionts have co-evolved mutualistic behavioural interactions from which both partners profit. By theory, most mutualistic associations have originated from ancestral parasitic ones during their long-term co-evolution towards a cost-benefit equilibrium. To manipulate host behaviour in a way where both partners benefit in a reciprocal manner, the symbiont has to target and remain restricted to defined host brain regions to minimize unnecessary fitness costs. By using the classic Drosophila paulistorum model system we demonstrate that (i) mutualistic Wolbachia are restricted to various Drosophila brain areas, (ii) form bacteriocyte-like structures within the brain, (iii) exhibit strictly lateral tropism, and (iv) finally propose that their selective neuronal infection affects host sexual behaviour adaptively.

More than fishing in the dark: PCR of a dispersed sequence produces simple but ultrasensitive Wolbachia detection

BACKGROUND

Detecting intracellular bacterial symbionts can be challenging when they persist at very low densities. Wolbachia, a widespread bacterial endosymbiont of invertebrates, is particularly challenging. Although it persists at high titers in many species, in others its densities are far below the detection limit of classic end-point Polymerase Chain Reaction (PCR). These low-titer infections can be reliably detected by combining PCR with DNA hybridization, but less elaborate strategies based on end-point PCR alone have proven less sensitive or less general.

RESULTS

We introduce a multicopy PCR target that allows fast and reliable detection of A-supergroup Wolbachia--even at low infection titers--with standard end-point PCR. The target is a multicopy motif (designated ARM: A-supergroup repeat motif) discovered in the genome of wMel (the Wolbachia in Drosophila melanogaster). ARM is found in at least seven other Wolbachia A-supergroup strains infecting various Drosophila, the wasp Muscidifurax and the tsetse fly Glossina. We demonstrate that end-point PCR targeting ARM can reliably detect both high- and low-titer Wolbachia infections in Drosophila, Glossina and interspecific hybrids.

CONCLUSIONS

Simple end-point PCR of ARM facilitates detection of low-titer Wolbachia A-supergroup infections. Detecting these infections previously required more elaborate procedures. Our ARM target seems to be a general feature of Wolbachia A-supergroup genomes, unlike other multicopy markers such as insertion sequences (IS).

Uncovering Wolbachia diversity upon artificial host transfer

The common endosymbiotic Wolbachia bacteria influence arthropod hosts in multiple ways. They are mostly recognized for their manipulations of host reproduction, yet, more recent studies demonstrate that Wolbachia also impact host behavior, metabolic pathways and immunity. Besides their biological and evolutionary roles, Wolbachia are new potential biological control agents for pest and vector management. Importantly, Wolbachia-based control strategies require controlled symbiont transfer between host species and predictable outcomes of novel Wolbachia-host associations. Theoretically, this artificial horizontal transfer could inflict genetic changes within transferred Wolbachia populations. This could be facilitated through de novo mutations in the novel recipient host or changes of haplotype frequencies of polymorphic Wolbachia populations when transferred from donor to recipient hosts. Here we show that Wolbachia resident in the European cherry fruit fly, Rhagoletis cerasi, exhibit ancestral and cryptic sequence polymorphism in three symbiont genes, which are exposed upon microinjection into the new hosts Drosophila simulans and Ceratitis capitata. Our analyses of Wolbachia in microinjected D. simulans over 150 generations after microinjection uncovered infections with multiple Wolbachia strains in trans-infected lines that had previously been typed as single infections. This confirms the persistence of low-titer Wolbachia strains in microinjection experiments that had previously escaped standard detection techniques. Our study demonstrates that infections by multiple Wolbachia strains can shift in prevalence after artificial host transfer driven by either stochastic or selective processes. Trans-infection of Wolbachia can claim fitness costs in new hosts and we speculate that these costs may have driven the shifts of Wolbachia strains that we saw in our model system.

Transgenerational transmission of the Glossina pallidipes hytrosavirus depends on the presence of a functional symbiome

The vertically transmitted endosymbionts (Sodalis glossinidius and Wigglesworthia glossinidia) of the tsetse fly (Diptera: Glossinidae) are known to supplement dietary deficiencies and modulate the reproductive fitness and the defense system of the fly. Some tsetse fly species are also infected with the bacterium, Wolbachia and with the Glossina hytrosavirus (GpSGHV). Laboratory-bred G. pallidipes exhibit chronic asymptomatic and acute symptomatic GpSGHV infection, with the former being the most common in these colonies. However, under as yet undefined conditions, the asymptomatic state can convert to the symptomatic state, leading to detectable salivary gland hypertrophy (SGH(+)) syndrome. In this study, we investigated the interplay between the bacterial symbiome and GpSGHV during development of G. pallidipes by knocking down the symbionts with antibiotic. Intrahaemocoelic injection of GpSGHV led to high virus titre (10(9) virus copies), but was not accompanied by either the onset of detectable SGH(+), or release of detectable virus particles into the blood meals during feeding events. When the F1 generations of GpSGHV-challenged mothers were dissected within 24 h post-eclosion, SGH(+) was observed to increase from 4.5% in the first larviposition cycle to >95% in the fourth cycle. Despite being sterile, these F1 SGH(+) progeny mated readily. Removal of the tsetse symbiome, however, suppressed transgenerational transfer of the virus via milk secretions and blocked the ability of GpSGHV to infect salivary glands of the F1 progeny. Whereas GpSGHV infects and replicates in salivary glands of developing pupa, the virus is unable to induce SGH(+) within fully differentiated adult salivary glands. The F1 SGH(+) adults are responsible for the GpSGHV-induced colony collapse in tsetse factories. Our data suggest that GpSGHV has co-evolved with the tsetse symbiome and that the symbionts play key roles in the virus transmission from mother to progeny.

Global Wolbachia prevalence, titer fluctuations and their potential of causing cytoplasmic incompatibilities in tsetse flies and hybrids of Glossina morsitans subgroup species

We demonstrate the high applicability of a novel VNTR-based (Variable-Number-Tandem-Repeat) molecular screening tool for fingerprinting Wolbachia-infections in tsetse flies. The VNTR-141 locus provides reliable and concise differentiation between Wolbachia strains deriving from Glossina morsitans morsitans, Glossina morsitans centralis, and Glossina brevipalpis. Moreover, we show that certain Wolbachia-infections in Glossina spp. are capable of escaping standard PCR screening methods by 'hiding' as low-titer infections below the detection threshold. By applying a highly sensitive PCR-blot technique to our Glossina specimen, we were able to enhance the symbiont detection limit substantially and, consequently, trace unequivocally Wolbachia-infections at high prevalence in laboratory-reared G. swynnertoni individuals. To our knowledge, Wolbachia-persistence was reported exclusively for field-collected samples, and at low prevalence only. Finally, we highlight the substantially higher Wolbachia titer levels found in hybrid Glossina compared to non-hybrid hosts and the possible impact of these titers on hybrid host fitness that potentially trigger incipient speciation in tsetse flies.

Tandem repeat markers as novel diagnostic tools for high resolution fingerprinting of Wolbachia

Background

Strains of the endosymbiotic bacterium Wolbachia pipientis are extremely diverse both genotypically and in terms of their induced phenotypes in invertebrate hosts. Despite extensive molecular characterisation of Wolbachia diversity, little is known about the actual genomic diversity within or between closely related strains that group tightly on the basis of existing gene marker systems, including Multiple Locus Sequence Typing (MLST). There is an urgent need for higher resolution fingerprinting markers of Wolbachia for studies of population genetics, horizontal transmission and experimental evolution.

Results

The genome of the wMel Wolbachia strain that infects Drosophila melanogaster contains inter- and intragenic tandem repeats that may evolve through expansion or contraction. We identified hypervariable regions in wMel, including intergenic Variable Number Tandem Repeats (VNTRs), and genes encoding ankyrin (ANK) repeat domains. We amplified these markers from 14 related Wolbachia strains belonging to supergroup A and were successful in differentiating size polymorphic alleles. Because of their tandemly repeated structure and length polymorphism, the markers can be used in a PCR-diagnostic multilocus typing approach, analogous to the Multiple Locus VNTR Analysis (MLVA) established for many other bacteria and organisms. The isolated markers are highly specific for supergroup A and not informative for other supergroups. However, in silico analysis of completed genomes from other supergroups revealed the presence of tandem repeats that are variable and could therefore be useful for typing target strains.

Conclusions

Wolbachia genomes contain inter- and intragenic tandem repeats that evolve through expansion or contraction. A selection of polymorphic tandem repeats is a novel and useful PCR diagnostic extension to the existing MLST typing system of Wolbachia, as it allows rapid and inexpensive high-throughput fingerprinting of closely related strains for which polymorphic markers were previously lacking.

Allele intersection analysis: a novel tool for multi locus sequence assignment in multiply infected hosts

Wolbachia are wide-spread, endogenous α-Proteobacteria of arthropods and filarial nematodes. 15-75% of all insect species are infected with these endosymbionts that alter their host's reproduction to facilitate their spread. In recent years, many insect species infected with multiple Wolbachia strains have been identified. As the endosymbionts are not cultivable outside living cells, strain typing relies on molecular methods. A Multi Locus Sequence Typing (MLST) system was established for standardizing Wolbachia strain identification. However, MLST requires hosts to harbour individual and not multiple strains of supergroups without recombination. This study revisits the applicability of the current MLST protocols and introduces Allele Intersection Analysis (AIA) as a novel approach. AIA utilizes natural variations in infection patterns and allows correct strain assignment of MLST alleles in multiply infected host species without the need of artificial strain segregation. AIA identifies pairs of multiply infected individuals that share Wolbachia and differ in only one strain. In such pairs, the shared MLST sequences can be used to assign alleles to distinct strains. Furthermore, AIA is a powerful tool to detect recombination events. The underlying principle of AIA may easily be adopted for MLST approaches in other uncultivable bacterial genera that occur as multiple strain infections and the concept may find application in metagenomic high-throughput parallel sequencing projects.

Infectious speciation revisited: impact of symbiont-depletion on female fitness and mating behavior of Drosophila paulistorum

The neotropical Drosophila paulistorum superspecies, consisting of at least six geographically overlapping but reproductively isolated semispecies, has been the object of extensive research since at least 1955, when it was initially trapped mid-evolution in flagrant statu nascendi. In this classic system females express strong premating isolation patterns against mates belonging to any other semispecies, and yet uncharacterized microbial reproductive tract symbionts were described triggering hybrid inviability and male sterility. Based on theoretical models and limited experimental data, prime candidates fostering symbiont-driven speciation in arthropods are intracellular bacteria belonging to the genus Wolbachia. They are maternally inherited symbionts of many arthropods capable of manipulating host reproductive biology for their own benefits. However, it is an ongoing debate as to whether or not reproductive symbionts are capable of driving host speciation in nature and if so, to what extent. Here we have reevaluated this classic case of infectious speciation by means of present day molecular approaches and artificial symbiont depletion experiments. We have isolated the α-proteobacteria Wolbachia as the maternally transmitted core endosymbionts of all D. paulistorum semispecies that have coevolved towards obligate mutualism with their respective native hosts. In hybrids, however, these mutualists transform into pathogens by overreplication causing embryonic inviability and male sterility. We show that experimental reduction in native Wolbachia titer causes alterations in sex ratio, fecundity, and mate discrimination. Our results indicate that formerly designated Mycoplasma-like organisms are most likely Wolbachia that have evolved by becoming essential mutualistic symbionts in their respective natural hosts; they have the potential to trigger pre- and postmating isolation. Furthermore, in light of our new findings, we revisit the concept of infectious speciation and discuss potential mechanisms that can restrict or promote symbiont-induced speciation at post- and prezygotic levels in nature and under artificial laboratory conditions.

Epigenetic regulation of a murine retrotransposon by a dual histone modification mark

Large fractions of eukaryotic genomes contain repetitive sequences of which the vast majority is derived from transposable elements (TEs). In order to inactivate those potentially harmful elements, host organisms silence TEs via methylation of transposon DNA and packaging into chromatin associated with repressive histone marks. The contribution of individual histone modifications in this process is not completely resolved. Therefore, we aimed to define the role of reversible histone acetylation, a modification commonly associated with transcriptional activity, in transcriptional regulation of murine TEs. We surveyed histone acetylation patterns and expression levels of ten different murine TEs in mouse fibroblasts with altered histone acetylation levels, which was achieved via chemical HDAC inhibition with trichostatin A (TSA), or genetic inactivation of the major deacetylase HDAC1. We found that one LTR retrotransposon family encompassing virus-like 30S elements (VL30) showed significant histone H3 hyperacetylation and strong transcriptional activation in response to TSA treatment. Analysis of VL30 transcripts revealed that increased VL30 transcription is due to enhanced expression of a limited number of genomic elements, with one locus being particularly responsive to HDAC inhibition. Importantly, transcriptional induction of VL30 was entirely dependent on the activation of MAP kinase pathways, resulting in serine 10 phosphorylation at histone H3. Stimulation of MAP kinase cascades together with HDAC inhibition led to simultaneous phosphorylation and acetylation (phosphoacetylation) of histone H3 at the VL30 regulatory region. The presence of the phosphoacetylation mark at VL30 LTRs was linked with full transcriptional activation of the mobile element. Our data indicate that the activity of different TEs is controlled by distinct chromatin modifications. We show that activation of a specific mobile element is linked to a dual epigenetic mark and propose a model whereby phosphoacetylation of histone H3 is crucial for full transcriptional activation of VL30 elements.

The majority of genomic sequences in higher eukaryotes do not contain protein coding genes. Large fractions are covered by repetitive sequences, many of which are derived from transposable elements (TEs). These selfish genes, only containing sequences necessary for self-propagation, can multiply and change their location within the genome, threatening host genome integrity and provoking mutational bursts. Therefore host organisms have evolved a diverse repertoire of defence mechanisms to counteract and silence these genomic parasites. One way is to package DNA sequences containing TEs into transcriptionally inert heterochromatin, which is partly achieved via chemical modification of the packaging proteins associated with DNA, the histones. To better understand the contribution of histone acetylation in the activation of TEs, we treated mouse fibroblasts with a specific histone deacetylase inhibitor. By monitoring the expression of ten different types of murine mobile elements, we identified a defined subset of VL30 transposons specifically reactivated upon increased histone acetylation. Importantly, phosphorylation of histone H3, a modification that is triggered by stress, is required for acetylation-dependent activation of VL30 elements. We present a model where concomitant histone phosphorylation and acetylation cooperate in the transcriptional induction of VL30 elements.

Applying mobile genetic elements for genome analysis and evolution

Transposable elements (TEs) are ubiquitous components of all living organisms, and in the course of their coexistence with their respective host genomes, these parasitc DNAs have played important roles in the evolution of complex genetic networks. The interaction between mobile DNAs and their host genomes are quite diverse, ranging from modifications of gene structure and regulation to alterations in general genome architecture. Thus during evolutionary time these elements can be regarded as natural molecular tools in shaping the organization, structure, and function of eukaryotic genes and genomes. Based on their intrinsic properties and features, mobile DNAs are widely applied at present as a technical "toolbox," essential for studying a diverse spectrum of biological questions. In this review, we aim to summarize both the evolutionary impact of TEs on genome evolution and their valuable and diverse methodological applications as molecular tools.

Evidence for a global Wolbachia replacement in Drosophila melanogaster

Wolbachia are maternally inherited intracellular alpha-Proteobacteria found in numerous arthropod and filarial nematode species. They influence the biology of their hosts in many ways. In some cases, they act as obligate mutualists and are required for the normal development and reproduction of the host. They are best known, however, for the various reproductive parasitism traits that they can generate in infected hosts. These include cytoplasmic incompatibility (CI) between individuals of different infection status, the parthenogenetic production of females, the selective killing of male embryos, and the feminization of genetic males. Wolbachia infections of Drosophila melanogaster are extremely common in both wild populations and long-term laboratory stocks. Utilizing the newly completed genome sequence of Wolbachia pipientis wMel, we have identified a number of polymorphic markers that can be used to discriminate among five different Wolbachia variants within what was previously thought to be the single clonal infection of D. melanogaster. Analysis of long-term lab stocks together with wild-caught flies indicates that one of these variants has replaced the others globally within the last century. This is the first report of a global replacement of a Wolbachia strain in an insect host species. The sweep is at odds with current theory that cannot explain how Wolbachia can invade this host species given the observed cytoplasmic incompatibility characteristics of Wolbachia infections in D. melanogaster in the field.

Evolutionary dynamics of wAu-like Wolbachia variants in neotropical Drosophila spp

Wolbachia bacteria are common intracellular symbionts of arthropods and have been extensively studied in Drosophila. Most research focuses on two Old Word hosts, Drosophila melanogaster and Drosophila simulans, and does not take into account that some of the Wolbachia associations in these species may have evolved only after their fast global expansion and after the exposure to Wolbachia of previously isolated habitats. Here we looked at Wolbachia of Neotropical Drosophila species. Seventy-one lines of 16 Neotropical Drosophila species sampled in different regions and at different time points were analyzed. Wolbachia is absent in lines of Drosophila willistoni collected before the 1970s, but more recent samples are infected with a strain designated wWil. Wolbachia is absent in all other species of the willistoni group. Polymorphic wWil-related strains were detected in some saltans group species, with D. septentriosaltans being coinfected with at least four variants. Based on wsp and ftsZ sequence data, wWil of D. willistoni is identical to wAu, a strain isolated from D. simulans, but can be discriminated when using a polymorphic minisatellite marker. In contrast to wAu, which infects both germ line and somatic tissues of D. simulans, wWil is found exclusively in the primordial germ line cells of D. willistoni embryos. We report on a pool of closely related Wolbachia strains in Neotropical Drosophila species as a potential source for the wAu strain in D. simulans. Possible evolutionary scenarios reconstructing the infection history of wAu-like Wolbachia in Neotropical Drosophila species and the Old World species D. simulans are discussed.

Mobile genetic elements as natural tools for genome evolution

Transposable elements (TEs) are ubiquitous components of all living organisms, and in the course of their coexistence with their respective host genomes, these parasitic DNAs have played important roles in the evolution of complex genetic networks. The interaction between mobile DNAs and their host genomes are quite diverse, ranging from modifications of gene structure and regulation to alterations in general genome architecture. Thus over evolutionary time these elements can be regarded as natural molecular tools in shaping the organization, structure, and function of eukaryotic genes and genomes. Based on their intrinsic properties and features, mobile DNAs are widely applied at present as a technical "toolbox," essential for studying a diverse spectrum of biological questions. In this chapter we aim to review both the evolutionary impact of TEs on genome evolution and their valuable and diverse methodological applications as the molecular tools presented in this book.

Randomized clinical trial of thalidomide, cyclosporine, and prednisone versus cyclosporine and prednisone as initial therapy for chronic graft-versus-host disease

Chronic graft-versus-host disease (CGVHD) is a major cause of morbidity following allogeneic bone marrow transplantation. Thalidomide is active in salvage therapy for high-risk or resistant CGVHD. In a prospective randomized trial, we tested initial therapy with thalidomide. Patients with extensive CGVHD were randomized to receive either cyclosporine and alternate-day prednisone (n = 27, no-thalidomide [no-thal] group) or cyclosporine, prednisone, and thalidomide (200-800 mg/day; n = 27, thal group). Although most patients responded, initial therapy with thalidomide did not improve control of CGVHD. Response rates were 83% versus 89% at 2 months (P = .7), 88% versus 84% at 6 months (P > .8) and 85% versus 73% at 1 year (P = .5) in the thal and no-thal groups, respectively. Multivariate analysis revealed related donor transplant (odds ratio [OR] = 11.3; P =.03) and de novo or quiescent onset of CGVHD (OR = 7.7; P =.04) to be significant predictors of good early response, whereas a platelet count of > or =100,000/microL was a significant predictor of good response (OR = 10.4; P =.04) at 1 year. Survival for the thal and no-thal groups was similar at 1 year (66% versus 74%) and 2 years (66% versus 54%, P = .85). Multivariate analysis revealed progressive onset CGVHD (relative risk [RR] = 4.2; P =.01), unrelated donor (RR = 5.7; P < .01), sex mismatch (RR = 7.9; P < .01), and platelet counts of <100,000/microL (RR = 3.8; P = .01) as significant predictors of poorer survival. These data suggest that despite a high response rate (79% response and 53% complete response) and encouraging survival rates (70% at 1 year and 60% at 2 years), thalidomide offers no clinical benefit when incorporated into initial therapy for CGVHD. The value of thalidomide as salvage therapy requires further study.

The evolutionary life history of P transposons: from horizontal invaders to domesticated neogenes

P elements, a family of DNA transposons, are known as aggressive intruders into the hitherto uninfected gene pool of Drosophila melanogaster. Invading through horizontal transmission from an external source they managed to spread rapidly through natural populations within a few decades. Owing to their propensity for rapid propagation within genomes as well as within populations, they are considered as the classic example of selfish DNA, causing havoc in a genomic environment permissive for transpositional activity. Tracing the fate of P transposons on an evolutionary scale we describe different stages in their evolutionary life history. Starting from horizontal transfer events, which now appear to be rather a common phenomenon, the initial transpositional burst in the new host is slowed down by the accumulation of defective copies as well as host-directed epigenetic silencing. This leads to the loss of mobility and, finally, to molecular erosion by random mutations. Possible escape routes from genomic extinction are the reactivation within the original host genome by recombination or suspension of the repressing regime, horizontal emigration to a virgin gene pool, or genomic integration and acquisition of a novel function as a domesticated host gene.

Evolutionary dynamics of the SGM transposon family in the Drosophila obscura species group

SGM (Drosophila subobscura, Drosophila guanche, and Drosophila madeirensis) transposons are a family of transposable elements (TEs) in Drosophila with some functional and structural similarities to miniature inverted-repeat transposable elements (MITEs). These elements were recently active in D. subobscura and D. madeirensis (1-2 MYA), but in D. guanche (3-4 MYA), they gave rise to a species-specifically amplified satellite DNA making up approximately 10% of its genome. SGM elements were already active in the common ancestor of all three species, giving rise to the A-type specific promoter section of the P:-related neogene cluster. SGM sequences are similar to elements found in other obscura group species, such as the ISY elements in D. miranda and the ISamb elements in Drosophila ambigua. SGM elements are composed of different sequence modules, and some of them, i.e., LS and LS-core, are found throughout the Drosophila and Sophophora radiation with similarity to more distantly related TEs. The LS-core module is highly enriched in the noncoding sections of the Drosophila melanogaster genome, suggesting potential regulatory host gene functions. The SGM elements can be considered as a model system elucidating the evolutionary dynamics of mobile elements in their arms race with host-directed silencing mechanisms and their evolutionary impact on the structure and composition of their respective host genomes.

Response to thalidomide therapy in refractory chronic graft-versus-host disease

Chronic graft-versus-host disease (GVHD) refractory to standard immunosuppressive therapy remains a major cause of morbidity and mortality after allogeneic bone marrow transplantation (BMT). Thalidomide may be effective in some patients with high-risk or refractory chronic GVHD. We report a single-institution study of thalidomide in 37 BMT patients with extensive chronic GVHD refractory to standard immunosuppressive therapy. Acute GVHD occurred in 34 (91%) of patients and evolved progressively into chronic GVHD in 23 (62%) patients. Thalidomide was added to standard immunosuppressive therapy a median of 11 months (range 0-105 months) after the diagnosis of chronic GVHD. Fourteen of 37 (38%) patients responded after introduction of thalidomide (one complete, 13 partial). Ten of 21 (46%) children and four of 16 (25%) adults responded. Responses were seen in eight of 17 (47%) recipients of related donor marrow and six of 20 (30%) recipients of unrelated donor marrow. Eight of 23 (34%) patients with progressive onset of chronic GVHD showed a response. There were no deaths among the responders. The remaining 23 patients (62%) did not respond and of these only two survive, one with progressive scleroderma, and the other with bronchiolitis obliterans. Chronic GVHD with associated infection (most commonly disseminated fungal infection) was a major contributor to mortality in all cases. Overall, after initiation of thalidomide, the 2-year Kaplan-Meier survival was 41% (95% C.I. 24%-59%). We conclude that thalidomide is a useful and well-tolerated therapy for patients with previously treated refractory chronic GVHD, including those with progressive onset of chronic GVHD, recipients of unrelated donor marrow, and children. Earlier introduction of thalidomide as an adjunct to standard immunosuppressive therapy may lead to more frequent responses and possible better survival.

Molecular domestication--more than a sporadic episode in evolution

Transposable elements are short but complex pieces of DNA or RNA containing a streamlined minimal-genome with the capacity for its selfish replication in a foreign genomic environment. Cis-regulatory sections within the elements orchestrate tempo and mode of TE expression. Proteins encoded by TEs mainly direct their own propagation within the genome by recruitment of host-encoded factors. On the other hand, TE-encoded proteins harbor a very attractive repertoire of functional abilities for a cell. These proteins mediate excision, replication and integration of defined DNA fragments. Furthermore, some of these proteins are able to manipulate important host factors by altering their original function. Thus, if the host genome succeeds in domesticating such TE-encoded proteins by taming their 'anarchistic behavior,' such an event can be considered as an important evolutionary innovation for its own benefit. In fact, the domestication of TE-derived cis-regulatory modules and protein coding sections took place repeatedly in the course of genome evolution. We will present prominent cases that impressively demonstrate the beneficial impact of TEs on host biology over evolutionary time. Furthermore, we will propose that molecular domestication might be considered as a resumption of the same evolutionary process that drove the transition from 'primitive genomes' to 'modern' ones at the early dawn of life, that is, the adaptive integration of a short piece of autonomous DNA into a complex regulatory network.

Localization of the fragile X mental retardation 2 (FMR2) protein in mammalian brain

The transcriptional silencing of the FMR2 gene has been implicated in FRAXE mental retardation. FRAXE individuals have been shown to exhibit learning deficits, including speech delay, reading and writing problems. FMR2 encodes a large protein of 1311 amino acids and is a member of a gene family encoding proline-serine-rich proteins that have properties of nuclear transcription factors. To characterize the expression of the fragile X mental retardation 2 (FMR2) protein, polyclonal antibodies were raised against two regions of the human FMR2 protein and used in immunofluorescence experiments on mouse brain cryosections. Our results demonstrate for the first time that the FMR2 protein is localized in neurons of the neocortex, Purkinje cells of the cerebellum and the granule cell layer of the hippocampus. FMR2 staining is shown to colocalize with the nuclear stain 4,6-diamidino-2-phenylindole (DAPI) confirming that FMR2 is a nuclear protein. The localization of FMR2 protein to the mammalian hippocampus and other brain structures involved with cognitive function is consistent with the learning deficits seen in FRAXE individuals.

Re-organising emergency medical admitting: the Stobhill experience, 1992-1997

OBJECTIVE

To examine patterns of acute medical emergency admissions and the effect of reorganisation on their management.

DESIGN

Examination of statistics for emergency medical admissions from 1992 to 1997, a period that included a major reorganisation of the emergency admitting system within the hospital.

SETTING

General hospital in a Scottish conurbation.

RESULTS

There was a slow annual increase in numbers of admissions during the period of study with very considerable variations in daily and weekly numbers of admissions. Reorganisation achieved a reduction in average length of stay from seven to 4.5 days permitting reduction of the bed complement from 223 to 161. Following reorganisation, 31% of admissions were discharged home within 48 hours directly from the acute medical receiving ward, 18% of admissions were transferred directly to care of the elderly, and 33% of admissions were transferred to medical wards. Patient and staff satisfaction surveys indicated preference for the new system over the old. Admission peaks over the winter months of the last three years occurred at different weeks in the year.

CONCLUSIONS

Reorganisation of the medical admitting system can improve efficiency and allow reductions in staffed beds. The considerable [table: see text] variation in daily demands in the system makes it important to retain flexibility. There may be scope for dealing with the large numbers of short-term admissions in other ways.

Expression and purification of recombinant tick anticoagulant peptide (Y1W/D10R) double mutant secreted by Saccharomyces cerevisiae

A double mutant of tick anticoagulant peptide (TAP) was cloned as a chimeric fusion with the yeast alpha-mating factor pre-proleader peptide. Expression in yeast (Saccharomyces cerevisiae) resulted in the secretion of the TAP mutein into the culture medium. An HPLC-based assay was used to screen yeast strains to find those giving highest expression levels. Efficiency of cleavage at the junction of the leader-TAP mutein varied from strain to strain, and a rapid purification method followed by N-terminal sequence analysis was used to identify a host strain that minimized undesirable cleavage products. A purification scheme was developed which separated the TAP mutein from improperly processed peptides present in the medium. This scheme employed cation-exchange chromatography and reversed-phase HPLC. Scale-up of the process was successful and produced 100 mg of fully functional TAP mutein of >96% homogeneity from a 50-L yeast culture.

Ganciclovir susceptibilities and analysis of UL97 region in cytomegalovirus (CMV) isolates from bone marrow recipients with CMV disease after antiviral prophylaxis

Ganciclovir susceptibilities and UL97 sequences were analyzed in 20 cytomegalovirus (CMV) isolates recovered from 15 bone marrow transplant recipients with active CMV infection after prophylaxis with acyclovir (group I; 12 isolates) or after acyclovir prophylaxis followed by ganciclovir therapy (group II; 8 isolates). All group I isolates were susceptible to ganciclovir. Five group II isolates were susceptible to ganciclovir, and 3 isolates (all from the same person) were resistant to ganciclovir (IC50 > 12 microM). Ganciclovir resistance UL97 mutations were found in 4 group II isolates, including a ganciclovir-susceptible isolate obtained from 1 patient after 41 days of therapy with ganciclovir and 3 ganciclovir-resistant isolates obtained from another patient after 73, 116, and 132 days of treatment with ganciclovir. Ganciclovir-resistant CMV isolates may emerge rapidly in bone marrow transplant recipients who are treated with ganciclovir after receiving prophylaxis with acyclovir.

Nested insertions of short mobile sequences in Drosophila P elements

A potentially full-sized P element isolated from the genome of Drosophila ambigua by polymerase chain reaction amplification was completely sequenced. It has a length of 3329 bp and the termini are formed by 33 bp inverted repeats. Sequence comparisons show that it can be classified as a member of the T-type P element subfamily. The translational reading frames of all four exons are interrupted by stop codons and frameshift mutations. At the 3' end of exon 3 a 687 bp insertion sequence (IS-amb-P) is found that also occurs in the form of dispersed copies (IS-amb) in the genome in D. ambigua. At the interspecific level it shows homology to mobile sequences of other species of the obscura group. Although variable in length, these IS elements are characterized by conserved sections without coding function and by 14 bp inverted repeats, one at a terminal, the other at a subterminal position. In situ hybridization revealed that P elements in D. ambigua are restricted to only two euchromatic sites on chromosome elements A and E. This situation resembles that found in Drosophila guanche and Drosophila subobscura where P homologs are clustered at a single site on chromosome element E and where the section corresponding to exon 3 of P elements carries an IS element. The gene sik-hom, which is located at the 5' side of the D. guanche cluster of P homologs, was used as a marker to examine whether the P element sites on chromosome element E of D. guanche and D. ambigua are homologous. The results suggest that the nested insertions of IS elements into P elements must have occurred independently in the two different lineages.

LTR retrotransposons and the evolution of eukaryotic enhancers

Since LTR retrotransposons and retroviruses are especially prone to regional duplications and recombination events, these viral-like systems may be especially conducive to the evolution of closely spaced combinatorial regulatory motifs. Using the Drosophila copia LTR retrotransposon as a model, we show that a regulatory region contained within the element's untranslated leader region (ULR) consists of multiple copies of an 8 bp motif (TTGTGAAA) with similarity to the core sequence of the SV40 enhancer. Naturally occurring variation in the number of these motifs is correlated with the enhancer strength of the ULR. Our results indicate that inter-element selection may favor the evolution of more active enhancers within permissive genetic backgrounds. We propose that LTR retroelements and perhaps other retrotransposons constitute drive mechanisms for the evolution of eukaryotic enhancers which can be subsequently distributed throughout host genomes to play a role in regulatory evolution.

Molecular domestication of mobile elements

Transposable elements are ubiquitous in all organisms and represent a dynamic component of their genomes, causing mutations and thereby genetic variation. Because of their independent and expansive replication strategy, these elements are called selfish and were thought to have no impact on the adaptive evolution of their host organisms. Although most TE-induced mutations seem to exert only negative effects on the fitness of their carrier, recent evidence indicates that in the course of evolution at least some TE-mediated changes have become established features of the host genome. For example, the insertion of TEs may provide novel cis-regulatory regions to preexisting host genes or TE-derived trans-acting factors may undergo a molecular transition into novel host genes through a process described as molecular domestication. The stationary P element related gene clusters of D. guanche, D. madeirensis and D. subobscura provide an excellent model system to study the evolutionary impact of TEs on genome evolution. Each cluster unit consists of a cis-regulating section composed of different insertion sequences followed by the first three exons of a P element that are coding for a 66 kDa 'repressor-like' protein.

Evaluation of the portal venous system: complementary roles of invasive and noninvasive imaging strategies

Evaluation of the portal venous system is required in several clinical circumstances, including before and after liver transplantation, before creation of a transjugular intrahepatic portosystemic shunt, in the clinical setting of bowel ischemia, or to evaluate varices. Several noninvasive modalities (magnetic resonance [MR] imaging and MR angiography, computed tomography [CT], and ultrasound [US]) are available for evaluation of the portal venous system in addition to the invasive angiographic methods. In most clinical circumstances, either CT or MR imaging and MR angiography in combination with US of the liver vasculature will allow complete evaluation of the portal venous system. Invasive evaluation of the portal venous system is necessary when results of the noninvasive tests disagree or are inconclusive. Angiography may also be indicated whenever noninvasive tests indicate occlusion of the portal venous system, as this is often a crucial clinical question and false-positive results can occur with the noninvasive tests.

Autologous bone marrow versus non-mobilized peripheral blood stem cell transplantation for lymphoid malignancies: a prospective, comparative trial

Autologous transplantation using bone marrow stem cells (BMSC) or peripheral blood stem cells (PBSC) is widely used for non-Hodgkin's lymphoma (NHL) and Hodgkin's disease (HD). We report a randomized, comparative trial comparing BMSC vs. non-mobilized PBSC for responsive NHL or HD. Patients randomized to BMSC (n = 13) vs. PBSC (n = 15) had more rapid neutrophil recovery (median 23 vs. 30 days), RBC independence (25 vs. 62 days), platelet independence (24 vs. 54 days), and shorter hospital stay. However, neither relapse, overall survival, nor relapse-free survival were different receiving BMSC vs. PBSC (all P > .7). Concurrently, 54 others (34 BMSC, 20 PBSC) were assigned non-randomly because of resistant disease or marrow unsuitable for harvest and similar patterns of engraftment favoring BMSC over PBSC were observed. In the entire group, BMSC transplantation (n = 47) led to quicker neutrophil recovery (P = .02), RBC (P = .06), and platelet independence (P =.04) and earlier hospital discharge (P = .02) vs. PBSC (n = 35). No difference in relapse, overall, or relapse-free survival were observed using BMSC vs. PBSC. These data suggest that non-mobilized PBSC are a satisfactory alternative to BMSC in patients with unsuitable marrow; however, transplantation with non-mobilized PBSC was associated with slower hematologic recovery, and longer hospital stay. No difference in tumor recurrence rates was observed between the PBSC or BMSC recipients. Unprimed PBSC transplantation offered no clinical advantage to BMSC.