Nina Wauters

Deciphering the virus-to-prokaryote ratio (VPR): insights into virus–host relationships in a variety of ecosystems

The discovery of the numerical importance of viruses in a variety of (aquatic) ecosystems has changed our perception of their importance in microbial processes. Bacteria and Archaea undoubtedly represent the most abundant cellular life forms on Earth and past estimates of viral numbers (represented mainly by viruses infecting prokaryotes) have indicated abundances at least one order of magnitude higher than that of their cellular hosts. Such dominance has been reflected most often by the virus‐to‐prokaryote ratio (VPR), proposed as a proxy for the relationship between viral and prokaryotic communities. VPR values have been discussed in the literature to express viral numerical dominance (or absence of it) over their cellular hosts, but the ecological meaning and interpretation of this ratio has remained somewhat nebulous or contradictory. We gathered data from 210 publications (and additional unpublished data) on viral ecology with the aim of exploring VPR. The results are presented in three parts: the first consists of an overview of the minimal, maximal and calculated average VPR values in an extensive variety of different environments. Results indicate that VPR values fluctuate over six orders of magnitude, with variations observed within each ecosystem. The second part investigates the relationship between VPR and other indices, in order to assess whether VPR can provide insights into virus–host relationships. A positive relationship was found between VPR and viral abundance (VA), frequency of visibly infected cells (FVIC), burst size (BS), frequency of lysogenic cells (FLC) and chlorophyll a (Chl a) concentration. An inverse relationship was detected between VPR and prokaryotic abundance (PA) (in sediments), prokaryotic production (PP) and virus–host contact rates (VCR) as well as salinity and temperature. No significant relationship was found between VPR and viral production (VP), fraction of mortality from viral lysis (FMVL), viral decay rate (VDR), viral turnover (VT) or depth. Finally, we summarize our results by proposing two scenarios in two contrasting environments, based on current theories on viral ecology as well as the present results. We conclude that since VPR fluctuates in every habitat for different reasons, as it is linked to a multitude of factors related to virus–host dynamics, extreme caution should be used when inferring relationships between viruses and their hosts. Furthermore, we posit that the VPR is only useful in specific, controlled conditions, e.g. for the monitoring of fluctuations in viral and host abundance over time.

Distribution, behavioral dominance and potential impacts on endemic fauna of tropical fire ant Solenopsis geminata (Fabricius, 1804) (Hymenoptera: Formicidae: Myrmicinae) in the Galápagos archipelago

Abstract. Tropical fire ant (TFA), Solenopsis geminata (Fabricius, 1804), is considered as one of the most serious threats to the terrestrial fauna of Galápagos, yet little is known about its distribution and impact in the archipelago. We reviewed literature, studied museum specimens and sampled over 62 sites on ten islands and islets in order to update the distribution of TFA. Since the 1980s, species occurrences for TFA have increased from three to 115 localities, and the species is now recorded on seven islands and 11 islets. Sixty-six new records were registered since the last review in 2008, including the islets Bayas, Champion, Cuevas, and Eden. TFA seems to be present in a wide range of habitats with a clear predominance in human-disturbed zones. Our data, compiled with previous studies, suggest that TFA can behaviorally dominate other ants. TFA was collected on 28 nesting sites of endangered and/or endemic vertebrate species, where it constitutes a potential or proven threat. This study identifies potential invasion sites as well as endemic species that may further be impacted by this highly invasive ant. It also recommends regular monitoring to prevent further invasion.

Habitat association and coexistence of endemic and introduced ant species in the Galápagos Islands

1. We investigated ant communities in all main vegetation zones of the model island of Santa Cruz in the Galápagos archipelago (155 collection points, spread over 21 sites; 28 ant species collected), and evaluated the distribution, coexistence, and effect of environmental factors in a community composed of endemic, probably endemic, and introduced ants of the New World and exotic origin.

Introduction history and genetic diversity of the invasive ant Solenopsis geminata in the Galápagos Islands

The Galápagos Islands constitute one of the most pristine tropical systems on Earth. However, the complex and fragile equilibrium of native species is threatened by invasive species, among which is one of the most successful ants in the world, the tropical fire ant, Solenopsis geminata. We characterized the genetic structure and diversity of populations of S. geminata in the Galápagos Islands and unravelled the archipelago colonization by combining Bayesian clustering methods and coalescent-based scenario testing. Using 12 microsatellite markers and one mitochondrial DNA fragment (COI), we analysed individuals collected in all main invaded islands of the archipelago and from the native areas in Costa Rica and mainland Ecuador. We also used mitochondrial DNA to infer evolutionary relationships of samples collected in Galápagos Islands, Ecuador, Costa Rica and other Latin American countries. Our results showed that genetic diversity was significantly lower in Galápagos Islands and mainland Ecuador populations when compared to Costa Rican populations, and that samples from Galápagos Islands and mainland Ecuador (Guayaquil) clustered in a single group and all share a single mtDNA haplotype. Approximate Bayesian Computation favoured a scenario assuming that populations from Galápagos Islands diverged from mainland Ecuador. The city of Guyaquil, an obligatory hub for tourism and trade, could act as a bridgehead.

Do agricultural zones on Santa Cruz Island, Galápagos, host native and endemic arthropods?

Abstract. The insect fauna of the Galápagos Islands is considered among the most pristine in the world and is well-studied in the preserved areas. However, introduced species are numerous and damaging, particularly in disturbed areas such as agricultural zones. In this study, we examined the arthropod diversity in two agricultural sites on Santa Cruz Island, Galápagos; a shaded mixed coffee plantation and an abandoned avocado plantation. We combined three sampling techniques to obtain a detailed picture of the invertebrate fauna in the leaf litter and in the lower arbustive zone. We then compared the two sites to assess the variation in arthropod fauna depending on the degree of disturbance. During the study, we collected individuals belonging to 96 taxa within the classes Arachnida, Insecta, Malacostracea and Diplopoda. More than half of the species were either endemic or native, but introduced species constituted the great majority (83.5%) of the sampled specimens. The invasive fire ant Solenopsis geminata Fabricius, 1804 was present in the majority of the samples and constituted almost half of the specimens collected. Although the faunal composition and species richness was similar in both sites, the evenness of species distribution was lower in the more disturbed avocado plantation. The same site hosted more S. geminata, fewer native species, and fewer predatory species than the shaded coffee plantation. We discuss these results by stressing the potential importance of non-intensive agricultural zones for the conservation of biodiversity and the possible impacts of S. geminata on native invertebrates in the Galápagos Islands.