Genetic variation in 12S and 16S mitochondrial rDNA genes of four geographically isolated populations of Gulf Coast ticks (Acari: Ixodidae)

Established Population of the Gulf Coast Tick, Amblyomma maculatum (Acari: Ixodidae), Infected with Rickettsia parkeri (Rickettsiales: Rickettsiaceae), in Connecticut

We identified an established population of the Gulf Coast tick (Amblyomma maculatum Koch) infected with Rickettsia parkeri in Connecticut, representing the northernmost range limit of this medically relevant tick species. Our finding highlights the importance of tick surveillance and public health challenges posed by geographic expansion of tick vectors and their pathogens.

Ecological Niche Models of Four Hard Tick Genera (Ixodidae) in Mexico

Simple Summary

Vector-borne diseases currently represent a significant threat to public health, mainly due to the changes that humans are producing in ecosystems and climates. Analyzing the environmental conditions that allow the establishment and survival of ticks could help determine possible sites for the appearance of infectious outbreaks. In this study, nine ecological niche models were generated from different algorithms to determine the current potential distribution of four tick genera in Mexico. Temperature and moisture have been considered as the main factors limiting tick distribution. However, the analysis of the ecological niche models determined that the four genera exhibited different distribution patterns, which may be associated with their physiological and ecological differences. This type of analysis can improve our understanding of the dynamics of ticks and, therefore, can be very useful in monitoring programs of the diseases they transmit.

Abstract

Ticks are vectors of a large number of pathogens of medical and veterinary importance, and in recent years, they have participated in the rise of multiple infectious outbreaks around the world. Studies have proposed that temperature and precipitation are the main variables that limit the geographical distribution of ticks. The analysis of environmental constraints with ecological niche modeling (ENM) techniques can improve our ability to identify suitable areas for emergence events. Algorithms used in this study showed different distributional patterns for each tick genera; the environmental suitability for Amblyomma includes warm and humid localities below 1000 m above the sea level, while Ixodes is mainly associated with ecosystems with high vegetation cover. Dermacentor and Rhipicephalus genus presented wider distribution patterns; the first includes species that are well adapted to resist desiccation, whereas the latter includes generalist species that are mostly associated with domestic hosts in Mexico. Ecological niche models have proven to be useful in estimating the geographic distribution of many taxa of ticks. Despite our limited knowledge of tick’s diversity, ENM can improve our understanding of the dynamics of vector-borne diseases and can assist public health decision-making processes.

Spotted Fever Group Rickettsia Infection and Transmission Dynamics in Amblyomma maculatum

Tick vectors are capable of transmitting several rickettsial species to vertebrate hosts, resulting in various levels of disease. Studies have demonstrated the transmissibility of both rickettsial pathogens and novel Rickettsia species or strains with unknown pathogenicity to vertebrate hosts during tick blood meal acquisition; however, the quantitative nature of transmission remains unknown.

Tick vectors are capable of transmitting several rickettsial species to vertebrate hosts, resulting in various levels of disease. Studies have demonstrated the transmissibility of both rickettsial pathogens and novel Rickettsia species or strains with unknown pathogenicity to vertebrate hosts during tick blood meal acquisition; however, the quantitative nature of transmission remains unknown. We tested the hypothesis that if infection severity is a function of the rickettsial load delivered during tick transmission, then a more virulent spotted fever group (SFG) Rickettsia species is transmitted at higher levels during tick feeding. Using Amblyomma maculatum cohorts infected with Rickettsia parkeri or “Candidatus Rickettsia andeanae,” a quantitative PCR (qPCR) assay was employed to quantify rickettsiae in tick salivary glands and saliva, as well as in the vertebrate hosts at the tick attachment site over the duration of tick feeding. Significantly greater numbers of R. parkeri than of “Ca. Rickettsia andeanae” rickettsiae were present in tick saliva and salivary glands and in the vertebrate hosts at the feeding site during tick feeding. Microscopy demonstrated the presence of both rickettsial species in tick salivary glands, and immunohistochemical analysis of the attachment site identified localized R. parkeri, but not “Ca. Rickettsia andeanae,” in the vertebrate host. Lesions were also distinct and more severe in vertebrate hosts exposed to R. parkeri than in those exposed to “Ca. Rickettsia andeanae.” The specific factors that contribute to the generation of a sustained rickettsial infection and subsequent disease have yet to be elucidated, but the results of this study suggest that the rickettsial load in ticks and during transmission may be an important element.

Comparative analysis of the Liriomyza chinensis mitochondrial genome with other Agromyzids reveals conserved genome features

Liriomyza chinensis is a serious pest of onions in many countries, especially in East Asia. We sequenced the complete mitochondrial genome of this species and compared it with five other Agromyzidae species. The L. chinensis mitogenome is a double-stranded 16,175 bp circular molecule with an A + T content of 78.3%. It contains 37 genes and a control region as do the sequenced Liriomyza species. The mitogenomes of L. chinensis and other Agromyzidae species showed a clear bias in nucleotide composition with a positive AT-skew. Most PCGs used standard ATN as start codons, and TAN as termination codons. The tRNAs exhibited the typical clover-leaf structure, except for tRNASer^(AGN) and the two rRNA genes are conserved with those of other Agromyzids. The L. chinensis mitogenome control region included several conserved regions, including a poly-T, two (TA)n and one poly-A stretch, which are considered important replication and transcription. The 13 PCGs were used to study the phylogeny of L. chinensis and five related Agromyzids. Analysis by maximum likelihood, Bayesian inference and genetic distance suggest congruent phylogenetic relationships in Liriomyza spp. and provide a useful supplement to taxonomic classification by morphology.

Distribution and phylogeny of Hyalomma ticks (Acari: Ixodidae) in Turkey

The genus Hyalomma includes some of the most medically and veterinarily important tick species in the world. To clarify and identify the current distribution of the species of Hyalomma, field studies were conducted in 65 localities in Turkey and five localities in Cyprus. Additionally, using mitochondrial 12S and 16S ribosomal DNA, specimens of Hyalomma from Turkey, H. excavatum from Cyprus, H. marginatum from Spain and Italy were evaluated together with the available sequences obtained from Genbank. Morphological and molecular analyses demonstrated the presence of four species in Turkey: H. marginatum, H. excavatum, H. aegyptium and H. asiaticum. Hyalomma marginatum is the dominant species in the Central and Northern parts of Turkey, whereas H. excavatum distributes mostly in the Southern parts. Hyalomma asiaticum is restricted to the Southeastern Anatolia. However, some sympatric regions were observed for these species. Phylogenetic trees obtained with Maximum Likelihood method demonstrated five clades. Data supported previous conclusions, but placed H. asiaticum, H. scupense, H. dromedarii and H. aegyptium in different clades with high bootstrap values. Specimens of H. anatolicum group and H. marginatum complex are sister groups. Pairwise distance analyses of these groups showed 2.8 and 3% differences for 12S rDNA and 16S rDNA, respectively. Therefore, additional analyses with the samples from different locations using different markers need to evaluate the exact status of the species of these groups.

Analysis of Amblyomma sculptum haplotypes in an area endemic for Brazilian spotted fever

Amblyomma sculptum (Ixodida: Ixodidae) Berlese, 1888, a member of the Amblyomma cajennense complex, is the major vector of Brazilian spotted fever (BSF) in southeastern Brazil. In this study, the genetic diversity of A. sculptum populations in the state of Rio de Janeiro (RJ), Brazil, was investigated because genetic variability in tick populations may be related to vector competence. Samples of A. sculptum from 19 municipalities in 7 regions of RJ were subjected to DNA extraction, amplification and sequencing of D-loop, cytochrome oxidase II and 12S rDNA mitochondrial genes. These sequences were used to map the genetic diversity of this tick. Amblyomma sculptum populations are genetically diverse in RJ, especially in the South Centre and Highland regions. Few unique haplotypes were observed in all populations, and the majority of genetic variation found was among ticks within each population. Phylogenetic reconstruction reinforced the assumption that all the haplotypes identified in RJ belong to A. sculptum. However, some RJ haplotypes are closer to A. sculptum from Argentina than to A. sculptum from elsewhere in Brazil. In RJ, A. sculptum has high genetic diversity, although little genetic differentiation. Observations also indicated a high level of gene flow among the studied populations and no evidence of population structure according to region in RJ.

Comparative population genetics of two invading ticks: Evidence of the ecological mechanisms underlying tick range expansions

Two species of ixodid tick, Ixodes affinis Neumann and Amblyomma maculatum Koch, are simultaneously expanding their ranges throughout the mid-Atlantic region of the US. Although we have some understanding of the ecology and life history of these species, the ecological mechanisms governing where and how new populations establish and persist are unclear. To assess population connectivity and ancestry, we sequenced a fragment of the 16S mitochondrial rRNA gene from a representative sample of individuals of both species from populations throughout the eastern US. We found that despite overlapping host preferences throughout ontogeny, each species exhibited very different genetic and geographic patterns of population establishment and connectivity. I. affinis was of two distinct mitochondrial clades, with a clear geographic break separating northern and southern populations. Both I. affinis populations showed evidence of recent expansion, although the southern population was more genetically diverse, indicating a longer history of establishment. A. maculatum exhibited diverse haplotypes that showed no significant relationship with geographic patterns and little apparent connectivity between sites. Heteroplasmy was also observed in the 16S mitochondrial rRNA gene in 3.5% of A. maculatum individuals. Genetic evidence suggests that these species rely on different key life stages to successfully disperse into novel environments, and that host vagility, habitat stability and habitat connectivity all play critical roles in the establishment of new tick populations.

High prevalence of "Candidatus Rickettsia andeanae" and apparent exclusion of Rickettsia parkeri in adult Amblyomma maculatum (Acari: Ixodidae) from Kansas and Oklahoma

Amblyomma maculatum (the Gulf Coast tick), an aggressive, human-biting, Nearctic and Neotropical tick, is the principal vector of Rickettsia parkeri in the United States. This pathogenic spotted fever group Rickettsia species has been identified in 8-52% of questing adult Gulf Coast ticks in the southeastern United States. To our knowledge, R. parkeri has not been reported previously from adult specimens of A. maculatum collected in Kansas or Oklahoma. A total of 216 adult A. maculatum ticks were collected from 18 counties in Kansas and Oklahoma during 2011-2014 and evaluated by molecular methods for evidence of infection with R. parkeri. No infections with this agent were identified; however, 47% of 94 ticks collected from Kansas and 73% of 122 ticks from Oklahoma were infected with "Candidatus Rickettsia andeanae" a spotted fever group Rickettsia species of undetermined pathogenicity. These preliminary data suggest that "Ca. R. andeanae" is well-adapted to survival in populations of A. maculatum in Kansas and Oklahoma, and that its ubiquity in Gulf Coast ticks in these states may effectively exclude R. parkeri from their shared arthropod host, which could diminish markedly or preclude entirely the occurrence of R. parkeri rickettsiosis in this region of the United States.

The Evolving Medical and Veterinary Importance of the Gulf Coast tick (Acari: Ixodidae)

Amblyomma maculatum Koch (the Gulf Coast tick) is a three-host, ixodid tick that is distributed throughout much of the southeastern and south-central United States, as well as several countries throughout Central and South America. A considerable amount of scientific literature followed the original description of A. maculatum in 1844; nonetheless, the Gulf Coast tick was not recognized as a vector of any known pathogen of animals or humans for >150 years. It is now identified as the principal vector of Hepatozoon americanum, the agent responsible for American canine hepatozoonosis, and Rickettsia parkeri, the cause of an emerging, eschar-associated spotted fever group rickettsiosis identified throughout much of the Western Hemisphere. Coincident with these discoveries has been recognition that the geographical distribution of A. maculatum in the United States is far more extensive than described 70 yr ago, supporting the idea that range and abundance of certain tick species, particularly those with diverse host preferences, are not fixed in time or space, and may change over relatively short intervals. Renewed interest in the Gulf Coast tick reinforces the notion that the perceived importance of a particular tick species to human or animal health can be relatively fluid, and may shift dramatically with changes in the distribution and abundance of the arthropod, its vertebrate hosts, or the microbial agents that transit among these organisms.

Analysis of three leafminers' complete mitochondrial genomes

Liriomyza trifolii (Burgess), Liriomyza huidobrensis (Blanchard), and Liriomyza bryoniae (Kaltenbach), are three closely related and economically important leafminer pests in the world. This study examined the complete mitochondrial genomes of L. trifolii, L. huidobrensis and L. bryoniae, which were 16,141 bp, 16,236 bp and 16,183 bp in length, respectively. All of them displayed 37 typical animal mitochondrial genes and an A+T-rich region. The genomes were highly compact with only 60-68 bp of non-coding intergenic spacer. However, considerable differences in the A+T-rich region were detected among the three species. Results of this study also showed the two ribosomal RNA genes of the three species had very limited variable sites and thus should not provide much information in the study of population genetics of these species. Data generated from three leafminers' complete mitochondrial genomes should provide valuable information in studying phylogeny of Diptera, and developing genetic markers for species identification in leafminers.

Population analyses of Amblyomma maculatum ticks and Rickettsia parkeri using single-strand conformation polymorphism

Gulf Coast ticks, Amblyomma maculatum, and the zoonotic agents they transmit, Rickettsia parkeri, are expanding into areas in the United States where they were not previously reported, and are emerging threats for public and veterinary health. The dynamics of this tick-pathogen system and implications for disease transmission are still unclear. To assess genetic variation of tick and rickettsial populations, we collected adult A. maculatum from 10 sites in Mississippi, 4 in the northern, one in the central, and 5 in the southern part of the state. PCR amplicons from tick mitochondrial 16S rRNA and rickettsial ompA genes as well as 5 intergenic spacer regions were evaluated for genetic variation using single-strand conformation polymorphism analysis. Frequencies of the 4 tick 16S haplotypes were not significantly different among regions of Mississippi, but within sites there were differences in distribution that can be explained by high migration rates. Phylogenetically, one lineage of tick haplotypes was a species-poor sister group to remaining haplotypes in the species-rich sister group. No genetic variation was identified in any of the 6 selected gene targets of R. parkeri examined in the infected ticks, suggesting high levels of intermixing.

Crossbreeding between different geographical populations of the brown dog tick, Rhipicephalus sanguineus (Acari: Ixodidae)

Brown dog ticks are distributed world-wide, and their systematics and phylogeny are the subject of an ongoing debate. The present study evaluates the reproductive compatibility between Rhipicephalus sanguineus ticks from North America, Israel, and Africa. Female ticks of the parent generation were mated with males from the same and alternate colonies. Every pure and hybrid cohort was maintained separately into the F2 generation with F1 females being allowed to mate only with males from the same cohort. The following survival parameters were measured and recorded for every developmental stage: feeding duration and success; engorgement weight, fertility, and fecundity of females; molting and hatching success. Ticks from North American and Mediterranean populations hybridized successfully. The survival parameters of all their hybrid lines were similar to those in pure lines throughout the F1 generation, and F1 adults were fully fertile. Parent adult ticks from the African population hybridized with either North American or Mediterranean ticks and produced viable progenies whose survival parameters were also similar to those in pure lines throughout the F1 generation. However, F1 adults in the four hybrid lines that included African ancestry were infertile. No parthenogenesis was observed in any pure or hybrid lines as proportion of males in F1 generation ranged from 40 to 60 %. Phylogenetic analysis of the 12S rDNA gene sequences placed African ticks into a separate clade from those of the North American or Mediterranean origins. Our results demonstrate that Rh. sanguineus ticks from North America and Israel represent the same species, whereas the African population used in this study is significantly distant and probably represents a different taxon.

Discrimination between Haemaphysalis longicornis and H. qinghaiensis based on the partial 16S rDNA and the second internal transcribed spacer (ITS-2)

In the present study, two hard tick species, Haemaphysalis longicornis and H. qinghaiensis from North-western China were characterized genetically by the second internal transcribed spacer (ITS-2) of nuclear ribosomal DNA and partial 16S rDNA. Based on a fragment within the hypervariable region of 16S rDNA with the length of approximately 453 bp, the phylogenetic trees were constructed by Neighbor-Joining and Maximum-parsimony methods. The results indicated that the phylogenetic status of H. qinghaiensis was distant from that of H. longicornis and closer to H. flava. Furthermore, the ITS-2 rDNA was amplified by PCR and sequenced from individual ticks. The length of ITS-2 is 1,606 bp for H. longicornis and 1,162 bp for H. qinghaiensis. Although sequence variation between the immature stages of H. longicornis was 0.1-0.4%, nucleotide differences between the tested species ranged 2.1-23.2%, indicating that ITS-2 rDNA sequences are genetic markers for the differentiation of the two hard ticks in China. Hence, a PCR-linked restriction fragment length polymorphism (RFLP) approach was developed for their unequivocal differentiation based on ITS-2 rDNA, which provides the foundation for further studies on ticks in China and has implications for studying the population genetic structure of the ticks and for identification and differentiation of closely related ticks.