Joseph E. Ironside

Multiple losses of sex within a single genus of Microsporidia

BackgroundMost asexual eukaryotic lineages have arisen recently from sexual ancestors and contain few ecologically distinct species, providing evidence for long-term advantages of sex. Ancient asexual lineages provide rare exceptions to this rule and so can yield valuable information relating to the evolutionary forces underlying the maintenance of sex. Microsporidia are parasitic, unicellular fungi. They include many asexual species which have traditionally been grouped together into large, presumably ancient taxonomic groups. However, these putative ancient asexual lineages have been identified on the basis of morphology, life cycles and small subunit ribosomal RNA (16S rRNA) gene sequences, all of which hold questionable value in accurately inferring phylogenetic relationships among microsporidia.ResultsThe hypothesis of a single, ancient loss of sex within the Nosema/Vairimorpha group of microsporidia was tested using phylogenetic analyses based on alignments of rRNA and RPB1 gene sequences from sexual and asexual species. Neither set of gene trees supported ancient asexuality, instead indicating at least two, recent losses of sex.ConclusionSex has been lost on multiple, independent occasions within the Nosema/Vairimorpha group of microsporidia and there is no evidence for ancient asexual lineages. It appears therefore that sex confers important long-term advantages even upon highly simplified eukaryotes such as microsporidia. The rapid evolution of microsporidian life cycles indicated by this study also suggests that even closely related microsporidia cannot be assumed to have similar life cycles and the life cycle of each newly discovered species must therefore be completely described. These findings are relevant to the use of microsporidia as biological control agents, since several species under consideration as potential agents have life cycles that have been incompletely described.

Phylogeography and environmental diversification of a highly adaptable marine amphipod, Gammarus duebeni.

Genetic diversity and phylogeographic population structure in the gammarid amphipod, Gammarus duebeni, were investigated across its broad latitudinal distribution in the NE and NW Atlantic by analysis of mitochondrial DNA sequence. Gammarus duebeni has exceptional tolerance of salinity change and inhabits environments ranging from marine to freshwater. The longstanding debate on whether there are distinct marine and freshwater subspecies was assessed by sampling populations from sites characterized by different salinities. Our sequence data demonstrates that there are two major lineages, with little internal geographic structuring. Evidence is provided to suggest a pre-glacial divergence of these two clades, involving segregation between a region historically associated with the freshwater form and the majority of the marine localities on both sides of the Atlantic. A modern contact zone between the marine and freshwater forms is proposed in western Britain.

Diversity and recombination of dispersed ribosomal DNA and protein coding genes in microsporidia.

Microsporidian strains are usually classified on the basis of their ribosomal DNA (rDNA) sequences. Although rDNA occurs as multiple copies, in most non-microsporidian species copies within a genome occur as tandem arrays and are homogenised by concerted evolution. In contrast, microsporidian rDNA units are dispersed throughout the genome in some species, and on this basis are predicted to undergo reduced concerted evolution. Furthermore many microsporidian species appear to be asexual and should therefore exhibit reduced genetic diversity due to a lack of recombination. Here, DNA sequences are compared between microsporidia with different life cycles in order to determine the effects of concerted evolution and sexual reproduction upon the diversity of rDNA and protein coding genes. Comparisons of cloned rDNA sequences between microsporidia of the genus Nosema with different life cycles provide evidence of intragenomic variability coupled with strong purifying selection. This suggests a birth and death process of evolution. However, some concerted evolution is suggested by clustering of rDNA sequences within species. Variability of protein-coding sequences indicates that considerable intergenomic variation also occurs between microsporidian cells within a single host. Patterns of variation in microsporidian DNA sequences indicate that additional diversity is generated by intragenomic and/or intergenomic recombination between sequence variants. The discovery of intragenomic variability coupled with strong purifying selection in microsporidian rRNA sequences supports the hypothesis that concerted evolution is reduced when copies of a gene are dispersed rather than repeated tandemly. The presence of intragenomic variability also renders the use of rDNA sequences for barcoding microsporidia questionable. Evidence of recombination in the single-copy genes of putatively asexual microsporidia suggests that these species may undergo cryptic sexual reproduction, a possibility with profound implications for the evolution of virulence, host range and drug resistance in these species.

Cucumispora dikerogammari n. gen. (Fungi: Microsporidia) infecting the invasive amphipod Dikerogammarus villosus: a potential emerging disease in European rivers.

Dikerogammarus villosus is an invasive amphipod that recently colonized the main rivers of Central and Western Europe. Two frequent microsporidian parasites were previously detected in this species, but their taxonomic status was unclear. Here we present ultrastructural and molecular data indicating that these two parasites are in fact a single microsporidian species. This parasite shares numerous characteristics of Nosema spp. It forms elongate spores (cucumiform), developing in direct contact with host cell cytoplasm; all developmental stages are diplokaryotic and the life cycle is monomorphic with disporoblastic sporogony. Initially this parasite was described as Nosema dikerogammari Ovcharenko and Kurandina 1987. However, phylogenetic analysis based on the complete sequence of SSU rDNA places the parasite outside the genus Nosema and it is therefore ascribed to a new genus Cucumispora. The key features characteristic to this genus are: presence of a very well-developed, umbrella-shape anchoring disk covering the anterior part of polaroplast; arrangement of isofilar polar filament into 6-8 coils convoluted with different angles, voluminous diplokaryon, thin spore wall and relatively small posterior vacuole containing posterosome. The parasite infects most host tissues but mainly muscles. It showed high rates of horizontal trophic transmission and lower rates of vertical transmission.

No amicable divorce? Challenging the notion that sexual antagonism drives sex chromosome evolution

Although sexual antagonism may have played a role in forming some sex chromosome systems, there appears to be little empirical or theoretical justification in assuming that it is the driving force in all cases of sex chromosome evolution. In many species, sex chromosomes have diverged in size and shape through the accumulation of mutations in regions of suppressed recombination. It is commonly assumed that recombination is suppressed in sex chromosomes due to selection to resolve sexually antagonistic pleiotropy. However, the requirement for a sex chromosome‐specific mechanism for suppressing recombination is questionable, since more general models of recombination suppression on autosomes also appear to be applicable to sex chromosomes. Direct tests of the predictions of the sexual antagonism hypothesis offer only limited support in specific sex chromosome systems and circumstantial evidence remains open to interpretation.

Modelled larval dispersal and measured gene flow: seascape genetics of the common cockle Cerastoderma edule in the southern Irish Sea

The role of marine currents in shaping population connectivity in the common cockle Cerastoderma edule was investigated in the southern Irish Sea. C. edule is one of the most valuable and exploited shellfish species in the area, yet very little is known about its population dynamics. In the present study, coupled hydrodynamic and particle tracking models are used in conjunction with genetic data collected at twelve microsatellite loci to estimate the influence of the Celtic Sea front on larval transport between the coasts of Britain and Ireland. Genetic analysis highlights the presence of at least three genetic clusters partitioned within locations, suggesting a contact zone between separate subpopulations. Samples collected from the Irish coast are most similar to each other. On the British coast, the Burry Inlet appears genetically isolated while samples collected from the coast of Pembrokeshire show evidence of connectivity between Britain and Ireland. These results agree with the model’s predictions: away from the coastal zone, residual baroclinic currents develop along tidal mixing fronts and act as conduit systems, transporting larvae great distances. Larvae spawned in south Wales are capable of travelling west towards Ireland due to the Celtic Sea front residual current, confirming the action of the Celtic Sea front on larval transport. Sheltered, flood-dominant estuaries such as the Burry Inlet promote self-recruitment. The validation of the model using genetic data represents progress towards a sustainable future for the common cockle, and paves the way for a more effective approach to management of all Irish Sea shellfisheries.

Genetic diversity of the feminising microsporidian parasite Dictyocoela: New insights into host-specificity, sex and phylogeography

Microsporidia of the genus Dictyocoela are parasites of gammarid amphipod Crustacea. They typically exhibit low virulence and efficient vertical transmission and at least some strains are capable of feminising their hosts. Sequencing of a region of the 16S rDNA of Dictyocoela spp. from various gammarid host species and localities in Europe and northern Asia indicates that Dictyocoela is genetically diverse and that different strains predominate in different host species. However, the presence of intermediate sequences casts doubt upon previous attempts to describe Dictyocoela spp. on the basis of rDNA divergence alone. Phylogenetic analysis provides little support for coevolution between gammarids and Dictyocoela. Furthermore, observations of heavily infected individuals, together with genetic evidence of recombination, suggest that some strains of Dictyocoela may be horizontally transmitted and are sexually reproducing. These findings suggest that Dictyocoela may be phenotypically, as well as genotypically, diverse, with the potential to exhibit a range of different interactions with its host populations.

Enemy release and genetic founder effects in invasive killer shrimp populations of Great Britain

The predatory “killer shrimp” Dikerogammarus villosus invaded Britain from mainland Europe in 2010. Originating in the Ponto-Caspian region, this invader has caused significant degradation of European freshwater ecosystems by predating and competitively excluding native invertebrate species. In contrast to continental Europe, in which invasions occurred through the migration of large numbers of individuals along rivers and canals, the invasion of Great Britain must have involved long distance dispersal across the sea. This makes the loss of genetic diversity and of debilitating parasites more likely. Analysis of nuclear microsatellite loci and mitochondrial DNA sequences of D. villosus samples from the four known populations in Britain reveal loss of rare alleles, in comparison to reference populations from the west coast of continental Europe. Screening of the British D. villosus populations by PCR detected no microsporidian parasites, in contrast with continental populations of D. villosus and native amphipod populations, most of which are infected with microsporidia. These findings suggest that the initial colonisation of Great Britain and subsequent long distance dispersal within Britain were associated with genetic founder effects and enemy release due to loss of parasites. Such effects are also likely to occur during future long-distance dispersal events of D. villosus to Ireland or North America.

The current state of sustainability in bioscience laboratories: a statistical examination of a UK tertiary institute

Purpose – This study aims to identify the current barriers to sustainability in the bioscience laboratory setting and to determine which mechanisms are likely to increase sustainable behaviours in this specialised environment.Design/methodology/approach – The study gathers qualitative data from a sample of laboratory researchers presently conducting experimentation in the biological sciences. A questionnaire, regarding sustainability in the laboratory, was developed and distributed to all bioscience researchers at Aberystwyth University.Findings – Although the majority of respondents had favourable attitudes to sustainability, almost three‐quarters (71 per cent) stated that they were not conducting their research in the most sustainable way possible. The factors most likely to hinder sustainable behaviour were lack of support, lack of information and time constraints. However, monetary costs and benefits, closely followed by “other” costs and benefits, were most likely to encourage sustainable behaviour i...

Distribution and host range of the microsporidian Pleistophora mulleri

ABSTRACT. Microsporidia of the genus Pleistophora are important parasites of fish and crustacea. Pleistophora mulleri has been described previously as a parasite of the gammarid amphipod crustacean Gammarus duebeni celticus in Irish freshwater habitats. Through a survey of European G. duebeni populations, P. mulleri was found to be widely distributed in the western British Isles (Wales, Scotland, and the Isle of Man), and populations of the subspecies Gammarus duebeni duebeni as well as G. d. celticus were infected. Pleistophora infections were also detected in G. d. duebeni sampled from the Bay of Gdansk on Polands Baltic coast, indicating a wide distribution of Pleistophora in European G. duebeni. Sequencing and phylogenetic analysis of the 16S rRNA, 18S rRNA, and Rpb1 genes of P. mulleri suggest that this species may be synonymous with P. typicalis, a parasite of fish. These findings suggest that amphipod crustaceans may act as intermediate or reservoir hosts for microsporidian parasites of fish.