Coupling of bacterial endosymbiont and host mitochondrial genomes in the hydrothermal vent clam Calyptogena magnifica

The hydrothermal vent clam Calyptogena magnifica (Bivalvia: Vesicomyidae) depends for its nutrition on sulfur-oxidizing symbiotic bacteria housed in its gill tissues. This symbiont is transmitted vertically between generations via the clam's eggs; however, it remains uncertain whether occasionally symbionts are horizontally transmitted or acquired from the environment. If symbionts are transmitted strictly vertically through the egg cytoplasm, inheritance of symbiont lineages should behave as if coupled to the host's maternally inherited mitochondrial DNA. This coupling would be obscured, however, with low rates of horizontal or environmental transfers, the equivalent of recombination between host lineages. Population genetic analyses of C. magnifica clams and associated symbionts from eastern Pacific hydrothermal vents clearly supported the hypothesis of strictly maternal cotransmission. Host mitochondrial and symbiont DNA sequences were coupled in a clam population that was polymorphic for both genetic markers. These markers were not similarly coupled with sequence variation at a nuclear gene locus, as expected for a randomly mating sexual population. Phylogenetic analysis of the two cytoplasmic genes also revealed no evidence for recombination. The tight association between vesicomyid clams and their vertically transmitted bacterial endosymbionts is phylogenetically very young (<50 million years) and may serve as a model for the origin and evolution of eukaryotic organelles.     

Host hybridization as a potential mechanism of lateral symbiont transfer in deep‐sea vesicomyid clams

Deep‐sea vesicomyid clams live in mutualistic symbiosis with chemosynthetic bacteria that are inherited through the maternal germ line. On evolutionary timescales, strictly vertical transmission should lead to cospeciation of host mitochondrial and symbiont lineages; nonetheless, examples of incongruent phylogenies have been reported, suggesting that symbionts are occasionally horizontally transmitted between host species. The current paradigm for vesicomyid clams holds that direct transfers cause host shifts or mixtures of symbionts. An alternative hypothesis suggests that hybridization between host species might explain symbiont transfers. Two clam species, Archivesica gigas and Phreagena soyoae, frequently co‐occur at deep‐sea hydrocarbon seeps in the eastern Pacific Ocean. Although the two species typically host gammaproteobacterial symbiont lineages marked by divergent 16S rRNA phylotypes, we identified a number of clams with the A. gigas mitotype that hosted symbionts with the P. soyoae phylotype. Demographic inference models based on genome‐wide SNP data and three Sanger sequenced gene markers provided evidence that A. gigas and P. soyoae hybridized in the past, supporting the hypothesis that hybridization might be a viable mechanism of interspecific symbiont transfer. These findings provide new perspectives on the evolution of vertically transmitted symbionts and their hosts in deep‐sea chemosynthetic environments.     

Symbiodinium diversity among host clionaid sponges from Caribbean and Pacific reefs: Evidence of heteroplasmy and putative host-specific symbiont lineages

Among the Porifera, symbiosis with Symbiodinium spp. (i.e., zooxanthellae) is largely restricted to members of the family Clionaidae. We surveyed the diversity of zooxanthellae associated with sponges from the Caribbean and greater Indo-Pacific regions using chloroplast large subunit (cp23S) domain V sequences. We provide the first report of Clade C Symbiodinium harbored by a sponge (Cliona caesia), and the first report of Clade A Symbiodinium from an Indo-Pacific sponge (C. jullieni). Clade A zooxanthellae were also identified in sponges from the Caribbean, which has been reported previously. Sponges that we examined from the Florida Keys all harbored Clade G Symbiodinium as did C. orientalis from the Indo-Pacific, which also supports earlier work with sponges. Two distinct Clade G lineages were identified in our phylogenetic analysis; Symbiodinium extracted from clionaid sponges formed a monophyletic group sister to Symbiodinium found in foraminiferans. Truncated and 'normal' length variants of 23S rDNA sequences were detected simultaneously in all three morphotypes of C. varians providing the first evidence of chloroplast-based heteroplasmy in a sponge. None of the other sponge species examined showed evidence of heteroplasmy. As in previous work, length variation in cp23S domain V sequences was found to correspond in a highly precise manner to finer resolution of phylogenetic topology among Symbiodinium clades. On a global scale, existing data indicate that members of the family Clionaidae that host zooxanthellae can form symbiotic associations with at least four Symbiodinium clades. The majority of sponge hosts appear to harbor only one cladal type of symbiont, but some species can harbor more than one clade of zooxanthellae concurrently. The observed differences in the number of partners harbored by sponges raise important questions about the degree of coevolutionary integration and specificity of these symbioses. Although our sample sizes are small, we propose that one of the Clade G lineages identified in this study is comprised of sponge-specialist zooxanthellae. These zooxanthellae are common in Caribbean sponges, but additional work in other geographic regions is necessary to test this idea. Sponges from the Indo-Pacific region harbor zooxanthellae from Clades A, C, and G, but more sponges from this region should be examined.     

Cellular growth of host and symbiont in a cnidarian-zooxanthellar symbiosis

The hydroid Myrionema ambionense, a fast-growing cnidarian (doubling time = 8 days) found in shallow water on tropical back-reefs, lives in symbiosis with symbiotic dinoflagellates of the genus Symbiodinium (hereafter also referred to as zooxanthellae). The symbionts live in vacuoles near the base of host digestive cells, whereas unhealthy looking zooxanthellae are generally located closer to the apical end of the host cell. Cytokinesis of zooxanthellae occurred at night, with a peak in number of symbionts with division furrows (mitotic index, MI = 12%-20%) observed at dawn. The MI of zooxanthellae decreased to near zero by the middle of the afternoon and remained there until the middle of the next night. Densities of live zooxanthellae living inside of host digestive cells peaked following cytokinesis, whereas densities of unhealthy looking symbionts were highest just before the division peak. Mitosis of host digestive cells was highest in the evening, also preceding the peak in zooxanthellar MI. This is the first study relating phased host cell division to diel zooxanthellar division in marine cnidarians. Food vacuoles were prevalent inside of digestive cells of field-collected hydroids within a few hours after sunset and throughout the night, coinciding with digestion of captured demersal plankton. Laboratory experiments showed that food vacuoles appeared in digestive cell cytoplasm within 2 h of feeding with nauplii of Artemia. The number and size of food vacuoles per digestive cell and the percentage of digestive cells with food vacuoles all decreased 5-7 h following feeding in laboratory experiments, and by mid-day in field-collected hydroids. Light and external food supply were important in maintaining phased division of the symbionts, with a lag in response time to both parameters of 11-36 h. Altering light and feeding during the night did not influence the level of the peak MI the next morning, though in one experiment the absence of light slowed final separation of daughter cells at the end of cytokinesis. In another experiment, hydroids starved for 3-7 d and "pulse-fed" Artemia nauplii for 1 h at the beginning of the dark period showed continued low symbiont division (< 5%) after 11 h, whether maintained in constant light or darkness, implying that most algal division is set more than 24 h prior to actual cytokinesis. Transferred to a 14:10 h light:dark cycle for another 24 h (36 h after feeding), the same hydroids exhibited a "normal" peak MI (ca. 15%) at dawn, but zooxanthellae from hydroids kept in constant darkness still showed a low MI. These results show that mitosis of symbiotic dinoflagellates requires three factors: external food; a minimum period of time following feeding (11-36 h), presumably for digestion; and a period of light following feeding, presumably to provide carbon skeletons necessary for completing cytokinesis.     

Reduced genome of the thioautotrophic intracellular symbiont in a deep-sea clam, Calyptogena okutanii

Although dense animal communities at hydrothermal vents and cold seeps rely on symbioses with chemoautotrophic bacteria [1, 2], knowledge of the mechanisms underlying these chemosynthetic symbioses is still fragmentary because of the difficulty in culturing the symbionts and the hosts in the laboratory. Deep-sea Calyptogena clams harbor thioautotrophic bacterial symbionts in their gill epithelial cells [1, 2]. They have vestigial digestive tracts and nutritionally depend on their symbionts [3], which are vertically transmitted via eggs [4]. To clarify the symbionts' metabolic roles in the symbiosis and adaptations to intracellular conditions, we present the complete genome sequence of the symbiont of Calyptogena okutanii. The genome is a circular chromosome of 1,022,154 bp with 31.6% guanine + cytosine (G + C) content, and is the smallest reported genome in autotrophic bacteria. It encodes 939 protein-coding genes, including those for thioautotrophy and for the syntheses of almost all amino acids and various cofactors. However, transporters for these substances to the host cell are apparently absent. Genes that are unnecessary for an intracellular lifestyle, as well as some essential genes (e.g., ftsZ for cytokinesis), appear to have been lost from the symbiont genome. Reductive evolution of the genome might be ongoing in the vertically transmitted Calyptogena symbionts.     

European Y-chromosomal lineages in Polynesians: a contrast to the population structure revealed by mtDNA.

We have used Y-chromosomal polymorphisms to trace paternal lineages in Polynesians by use of samples previously typed for mtDNA variants. A genealogical approach utilizing hierarchical analysis of eight rare-event biallelic polymorphisms, seven microsatellite loci, and internal structural analysis of the hypervariable minisatellite, MSY1, has been used to define three major paternal-lineage clusters in Polynesians. Two of these clusters, both defined by novel MSY1 modular structures and representing 55% of the Polynesians studied, are also found in coastal Papua New Guinea. Reduced Polynesian diversity, relative to that in Melanesians, is illustrated by the presence of several examples of identical MSY1 codes and microsatellite haplotypes within these lineage clusters in Polynesians. The complete lack of Y chromosomes having the M4 base substitution in Polynesians, despite their prevalence (64%) in Melanesians, may also be a result of the multiple bottleneck events during the colonization of this region of the world. The origin of the M4 mutation has been dated by use of two independent methods based on microsatellite-haplotype and minisatellite-code diversity. Because of the wide confidence limits on the mutation rates of these loci, the M4 mutation cannot be conclusively dated relative to the colonization of Polynesia, 3,000 years ago. The other major lineage cluster found in Polynesians, defined by a base substitution at the 92R7 locus, represents 27% of the Polynesians studied and, most probably, originates in Europe. This is the first Y-chromosomal evidence of major European admixture with indigenous Polynesian populations and contrasts sharply with the picture given by mtDNA evidence.     

Phylogeographical analysis of Ligia oceanica (Crustacea: Isopoda) reveals two deeply divergent mitochondrial lineages

Isopods of the species Ligia oceanica are typical inhabitants of the rocky intertidal of the northern European coastline. The aim of this study was to assess the genetic structure of this species using mitochondrial and nuclear sequence data. We analysed partial mitochondrial cytochrome c oxidase subunit I (CO1) and 16S rRNA gene sequence data of 161 specimens collected from ten sites ranging from Spain to Norway. For selected specimens, we also sequenced the hypervariable V7 expansion segment of the nuclear 18S rRNA gene as a supplementary marker. Furthermore, we studied the infection rate of all analysed specimens by the alphaproteobacterium Wolbachia. Our analyses revealed two deeply divergent mitochondrial lineages for Ligia oceanica that probably diverged in the late Pliocene to mid Pleistocene. One lineage comprised specimens from northern populations (‘lineage N’) and one primarily those from France and Spain (‘lineage S’). Distribution patterns of the haplotypes and the genetic distances between both lineages revealed two populations that diverged before the Last Glacial Maximum. Given that we found no homogenization of mitochondrial haplotypes, our present results also reject any influence of Wolbachia on the observed mtDNA variability. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112 , 16–30.     

Genome-wide analysis of single nucleotide polymorphisms uncovers population structure in Northern Europe

Background

Genome-wide data provide a powerful tool for inferring patterns of genetic variation and structure of human populations.

Principal Findings

In this study, we analysed almost 250,000 SNPs from a total of 945 samples from Eastern and Western Finland, Sweden, Northern Germany and Great Britain complemented with HapMap data. Small but statistically significant differences were observed between the European populations (F_ST = 0.0040, p<10⁻⁴), also between Eastern and Western Finland (F_ST = 0.0032, p<10⁻³). The latter indicated the existence of a relatively strong autosomal substructure within the country, similar to that observed earlier with smaller numbers of markers. The Germans and British were less differentiated than the Swedes, Western Finns and especially the Eastern Finns who also showed other signs of genetic drift. This is likely caused by the later founding of the northern populations, together with subsequent founder and bottleneck effects, and a smaller population size. Furthermore, our data suggest a small eastern contribution among the Finns, consistent with the historical and linguistic background of the population.

Significance

Our results warn against a priori assumptions of homogeneity among Finns and other seemingly isolated populations. Thus, in association studies in such populations, additional caution for population structure may be necessary. Our results illustrate that population history is often important for patterns of genetic variation, and that the analysis of hundreds of thousands of SNPs provides high resolution also for population genetics.     

Adaptation in a spider mite population after long-term evolution on a single host plant

Evolution in a single environment is expected to erode genetic variability, thereby precluding adaptation to novel environments. To test this, a large population of spider mites kept on cucumber for approximately 300 generations was used to establish populations on novel host plants (tomato or pepper), and changes in traits associated to adaptation were measured after 15 generations. Using a half-sib design, we investigated whether trait changes were related to genetic variation in the base population. Juvenile survival and fecundity exhibited genetic variation and increased in experimental populations on novel hosts. Conversely, no variation was detected for host choice and developmental time and these traits did not evolve. Longevity remained unchanged on novel hosts despite the presence of genetic variation, suggesting weak selection for this trait. Hence, patterns of evolutionary changes generally matched those of genetic variation, and changes in some traits were not hindered by long-term evolution in a constant environment.     

Anaerobic methanotrophic community of a 5346‐m‐deep vesicomyid clam colony in the Japan Trench

Vesicomyidae clams harbor sulfide‐oxidizing endosymbionts and are typical members of cold seep communities where active venting of fluids and gases takes place. We investigated the central biogeochemical processes that supported a vesicomyid clam colony as part of a locally restricted seep community in the Japan Trench at 5346 m water depth, one of the deepest seep settings studied to date. An integrated approach of biogeochemical and molecular ecological techniques was used combining in situ and ex situ measurements. In sediment of the clam colony, low sulfate reduction rates (maximum 128 nmol mL^?1 day^?1) were coupled to the anaerobic oxidation of methane. They were observed over a depth range of 15 cm, caused by active transport of sulfate due to bioturbation of the vesicomyid clams. A distinct separation between the seep and the surrounding seafloor was shown by steep horizontal geochemical gradients and pronounced microbial community shifts. The sediment below the clam colony was dominated by anaerobic methanotrophic archaea (ANME‐2c) and sulfate‐reducing Desulfobulbaceae (SEEP‐SRB‐3, SEEP‐SRB‐4). Aerobic methanotrophic bacteria were not detected in the sediment, and the oxidation of sulfide seemed to be carried out chemolithoautotrophically by Sulfurovum species. Thus, major redox processes were mediated by distinct subgroups of seep‐related microorganisms that might have been selected by this specific abyssal seep environment. Fluid flow and microbial activity were low but sufficient to support the clam community over decades and to build up high biomasses. Hence, the clams and their microbial communities adapted successfully to a low‐energy regime and may represent widespread chemosynthetic communities in the Japan Trench. In this regard, they contributed to the restricted deep‐sea trench biodiversity as well as to the organic carbon availability, also for non‐seep organisms, in such oligotrophic benthic environment of the dark deep ocean.     

Classical MHC class I genes composed of highly divergent sequence lineages share a single locus in rainbow trout (Oncorhynchus mykiss).

The classical MHC class I genes have been known to be highly polymorphic in various vertebrates. To date, putative allelic sequences of the classical MHC class I genes in teleost fish have been reported in several studies. However, the establishment of their allelic status has been hampered in most cases by the lack of appropriate genomic information. In the present study, using heterozygous and homozygous fish, we obtained classical-type MHC class I sequences of rainbow trout (Oncorhynchus mykiss) and investigated their allelic relationship by gene amplification and Southern and Northern hybridization analyses. The results indicated that all MHC class I sequences we obtained were derived from a single locus. Based on this, a unique polymorphic nature of the MHC class I locus of rainbow trout has been revealed. The mosaic combination of highly divergent ancient sequences in the peptide-binding domains is notable, and the variable nature around the boundary between the alpha3 and transmembrane domains is unprecedented.     

Sequential evolution of a symbiont inferred from the host: Wolbachia and Drosophila simulans

This study aims to unravel the biogeography of a model symbiont/host system by exploiting the prediction that a symbiont will leave a signature of infection on the host. Specifically, a global sample of 1,442 Drosophila simulans from 33 countries and 64 sampling localities was employed to infer the phylogeography of the maternally inherited alpha-proteobacteria Wolbachia. Phylogenetic analyses, from three symbiont genes and 24 mtDNA genomes (excluding the A + T-rich region), showed that each of four Wolbachia strains infected D. simulans once. The global distribution and abundance of the Wolbachia strains and the three mtDNA haplogroups (D. simulans siI, siII and siIII) was then determined. Finally, network analyses of variable regions within siI (584 bp from seven additional lines) and siII (1,701 bp from 383 lines) facilitated a detailed biogeographic discussion. There is little variation in siIII and the haplogroup is restricted in its distribution. These data show how the history of an infection can be mapped by combining data from the symbiont and the host. They say little about the organismal history of the host because the mtDNA genome is a biased representation of the whole genome.     

SOP3: a web-based tool for selection of oligonucleotide primers for single nucleotide polymorphism analysis by Pyrosequencing

SOP3 is a web-based software tool for designing oligonucleotide primers for use in the analysis of single nucleotide polymorphisms (SNPs). Accessible via the Internet, the application is optimized for developing the PCR and sequencing primers that are necessary for Pyrosequencing. The application accepts as input gene name, SNP reference sequence number, or chromosomal nucleotide location. Output can be parsed by gene name, SNP reference number, heterozygosity value, location, chromosome, or function. The location of an individual polymorphism, such as an intron, exon, or 5' or 3' untranslated region is indicated, as are whether nucleotide changes in an exon are associated with a change in an amino acid sequence. SOP3 presents for each entry a set of forward and biotinylated reverse PCR primers as well as a sequencing primer for use during the analysis of SNPs by Pyrosequencing. Theoretical pyrograms for each allele are calculated and presented graphically. The method has been tested in the development of Pyrosequencing assays for determining SNPs and for deletion/insertion polymorphisms in the human genome. Of the SOP3-designed primer sets that were tested, a large majority of the primer sets have successfully produced PCR products and Pyrosequencing data.     

Cryptic speciation in the field vole: a multilocus approach confirms three highly divergent lineages in Eurasia

Species are generally described from morphological features, but there is growing recognition of sister forms that show substantial genetic differentiation without obvious morphological variation and may therefore be considered ‘cryptic species’. Here, we investigate the field vole (Microtus agrestis), a Eurasian mammal with little apparent morphological differentiation but which, on the basis of previous sex‐linked nuclear and mitochondrial DNA (mtDNA) analyses, is subdivided into a Northern and a Southern lineage, sufficiently divergent that they may represent two cryptic species. These earlier studies also provided limited evidence for two major mtDNA lineages within Iberia. In our present study, we extend these findings through a multilocus approach. We sampled 163 individuals from 46 localities, mainly in Iberia, and sequenced seven loci, maternally, paternally and biparentally inherited. Our results show that the mtDNA lineage identified in Portugal is indeed a distinct third lineage on the basis of other markers as well. In fact, multilocus coalescent‐based methods clearly support three separate evolutionary units that may represent cryptic species: Northern, Southern and Portuguese. Divergence among these units was inferred to have occurred during the last glacial period; the Portuguese lineage split occurred first (estimated at c. 70 000 bp ), and the Northern and Southern lineages separated at around the last glacial maximum (estimated at c. 18 500 bp ). Such recent formation of evolutionary units that might be considered species has repercussions in terms of understanding evolutionary processes and the diversity of small mammals in a European context.     

The evolution of haplodiploidy by male‐killing endosymbionts: importance of population structure and endosymbiont mutualisms

Haplodiploid inheritance systems, characterized by male transmission of only their maternally inherited genomic elements, have evolved more than 20 times within the animal kingdom. A number of theoretical studies have argued that infection with certain male‐killing endosymbionts can potentially lead to the evolution of haplodiploidy. By explicitly investigating the coevolutionary dynamics between host and endosymbiont, we show that the assumptions of current models cannot explain the evolution of haplodiploidy very well, as the endosymbiont will often go extinct in the long term. Here, we provide two additional mechanisms that can explain the stable evolution of haplodiploidy by male‐killing endosymbionts. First of all, a spatially structured population can facilitate the long‐term persistence of haplodiploidy, but this applies only when levels of inbreeding are very high. By contrast, endosymbionts that are mutualistic with their hosts provide a much more general and promising route to the stable evolution of haplodiploidy. This model is the first to provide a formal explanation of the supposed association between the evolution of haplodiploidy and the highly inbred lifestyles of some ancestors, while it also provides a hypothesis for the evolution of haplodiploidy in more outbred ancestors.     

DNA barcoding of Scandinavian birds reveals divergent lineages in trans-Atlantic species

Birds are a taxonomically well-described group of animals, yet DNA barcoding, i.e., the molecular characterization of species using a standardized genetic marker, has revealed unexpected patterns of genetic divergences among North American birds. We performed a comprehensive COI (cytochrome c oxidase subunit I) barcode survey of 296 species of Scandinavian birds, and compared genetic divergences among 78 trans-Atlantic species whose breeding ranges include both Scandinavia and North America. Ninety-four percent of the Scandinavian species showed unique barcode clusters; the remaining 6% had overlapping barcodes with one or more congeneric species, which may reflect incomplete lineage sorting or a single gene pool. Four species showed large intra-specific divergences within Scandinavia, despite no apparent morphological differentiation or indications of reproductive isolation. These cases may reflect admixture of previously isolated lineages, and may thus warrant more comprehensive phylogeographic analyses. Nineteen (24%) of 78 trans-Atlantic species exhibited divergent genetic clusters which correspond with regional subspecies. Three of these trans-Atlantic divergences were paraphyletic. Our study demonstrates the effectiveness of COI barcodes for identifying Scandinavian birds and highlights taxa for taxonomic review. The standardized DNA barcoding approach amplified the power of our regional studies by enabling independently obtained datasets to be merged with the established avian barcode library.     

Embryonic development and endosymbiont transmission mode in the symbiotic clam Lucinoma aequizonata (Bivalvia: Lucinidae)

Summary

Lucinoma aequizonata is a large lucinid clam which lives in reducing mud around 500 m deep. Adults harbor intracellular chemoautotrophic sulfur-oxidizing bacteria in specialized gill cells called bacteriocytes. The embryonic and early larval development of L. aequizonata is described by using light and scanning electron microscopy. Gametes were obtained by injection of 0.2ml of 4 mM serotonin solution in seawater into the posterior adductor muscle. The oocytes, 200 μm in diameter, are surrounded by a glycoprotein capsule which gives to the egg a total diameter of 500μm. The development which occurs at 10°C is slow. The first polar body is detected 2.5 h after contact between sperm and oocytes (T_o+2.5 h), and the first cleavage begins 10 h later (T_o+12.5 h). The following successive cleavages produce a nonciliated morula, then a ciliated gastrula which begins to rotate within the egg-capsule at T_o+4.5 days. At this stage, the first shell pellicle appears on the dorsal side of the embryo. At T₀+8 days, the trochophore larvae develop discrete ciliary bands which constitute the prototroch. Typical straight-hinge veligers, D-shaped larvae, hatch from the egg-capsule 12 days after fertilization. The newly hatched larvae are 240 μm in length and 200μm in height, and the straight hinge 150μm long. To elucidate the symbiont transmission mode, two symbiont-specific primers were designed and used in amplifications by PCR. This primer set was unsuccessful in amplifying symbiont DNA targets from mature gonads, spawned oocytes, eggs, and veligers whereas successful amplifications were obtained from symbiont-containing gill tissues. These data rule out the vertical transmission mode and strongly suggest that the symbionts are environmentally transmitted to the new host generation in L. aequizonata as for all tropical lucinids examined to date.     

Parasitoids as drivers of symbiont diversity in an insect host

Immune systems have repeatedly diversified in response to parasite diversity. Many animals have outsourced part of their immune defence to defensive symbionts, which should be affected by similar evolutionary pressures as the host’s own immune system. Protective symbionts provide efficient and specific protection and respond to changing selection pressure by parasites. Here we use the aphid Aphis fabae, its protective symbiont Hamiltonella defensa, and its parasitoid Lysiphlebus fabarum to test whether parasite diversity can maintain diversity in protective symbionts. We exposed aphid populations with the same initial symbiont composition to parasitoid populations that differed in their diversity. As expected, single parasitoid genotypes mostly favoured a single symbiont that was most protective against that particular parasitoid, while multiple symbionts persisted in aphids exposed to more diverse parasitoid populations, which in turn affected aphid population density and rates of parasitism. Parasite diversity may be crucial to maintaining symbiont diversity in nature.     

Cellular tropism, population dynamics, host range and taxonomic status of an aphid secondary symbiont, SMLS (Sitobion miscanthi L type symbiont)

SMLS (Sitobion miscanthi L type symbiont) is a newly reported aphid secondary symbiont. Phylogenetic evidence from molecular markers indicates that SMLS belongs to the Rickettsiaceae and has a sibling relationship with Orientia tsutsugamushi. A comparative analysis of coxA nucleotide sequences further supports recognition of SMLS as a new genus in the Rickettsiaceae. In situ hybridization reveals that SMLS is housed in both sheath cells and secondary bacteriocytes and it is also detected in aphid hemolymph. The population dynamics of SMLS differ from those of Buchnera aphidicola and titer levels of SMLS increase in older aphids. A survey of 13 other aphids reveals that SMLS only occurs in wheat-associated species.     

Genetic structure of an aphid studied using microsatellites: cyclic parthenogenesis, differentiated lineages and host specialization

In a previous study, samples of the grain aphid Sitobion avenae (F.) were collected from wheat and adjacent cocksfoot hosts in a population thought to be primarily parthenogenetic, and DNA from individual aphids was analysed with a multilocus technique. Here we have applied single-locus microsatellites and a mitochondrial DNA marker to a subset of the same DNA extracts, and have made several additional inferences about important genetic and population processes in S. avenae . Microsatellite analysis indicated very high levels of genic and genotypic variation. S. avenae fell into three genotypic groups inferred to be almost noninterbreeding, while analysis of linkage and Hardy-Weinberg equilibria suggested high levels of sexual recombination within each genotypic group. Host specialization was evident: one lineage was found only on wheat, and one (bearing many alleles inferred to be introgressed from the blackberry-grass aphid S. fragariae (Walker)) was found only on cocksfoot. The third group of interrelated genotypes was found commonly on both hosts. Although most genotypes were found only once, some were much more numerous in the sample than expected from the frequency of the alleles they contained. This, and rapid temporal changes in genotypic composition of samples, indicates strong selective differences between genotypes and lineages. In the major genotypic group, the commonest genotypes were significantly more homozygous than were rare ones: thus these data may help to explain the frequent observation of homozygous excess in aphid allozymes. The genotype group showing S. avenae -like as well as S. fragariae -like alleles also carried S. fragariae -like mitochondrial DNA in at least 25/31 cases, indicating gender-asymmetrical hybridization.