Grzegorz Buczkowski

Nestmate discrimination in ants: effect of bioassay on aggressive behavior

Summary: Aggression assays are commonly used to study nestmate recognition in social insects. Methods range from detailed behavioral observations on small numbers of insects to counts of individuals fighting in group interactions. These assays vary in the equipment used and the intensity and duration of observations. We used the Argentine ant, Linepithema humile, to compare four aggression bioassays for consistency between replicates, similarity between assays, and ability to predict whole colony interactions. The assays included were 1 live – 1 dead ant interactions, live 1-1 battles, live 5-5 battles, and 1 ant introduced to a foreign colony. We tested six ant colonies in all pairwise combinations using four different assays and two to three scoring methods per assay. We also conducted a colony merging experiment to see which assays were capable of predicting this ecologically important event. We found that scoring methods within assays yielded very similar results, giving us no reason to favor observationally intense procedures, such as continuous scanning, over less observationally intense systems, such as snapshot surveys. Assays differed greatly in their consistency between replicates. No two replicates of the 1 live – 1 dead assay were significantly correlated. The live 5-5 and the colony introduction assays were the most consistent across replicates. The mean scores of the live 1-1, live 5-5 and colony introduction assays were all significantly correlated with each other; only the live 5-5 assay was significantly correlated with the 1 live – 1 dead assay. Assays that utilized the greatest number of live ants were the most likely to reveal high levels of aggression. The aggression scores of all but the 1 live – 1 dead assay were positively correlated with the number of ants that died during whole colony encounters and negatively associated with colony merging. We conclude that all live ant assays tested are useful tools for analyzing aggressive interactions between colonies, but that the pairing of a live and dead ant produced inconsistent results and generally lower levels of aggression. We found relatively low consistency between trials using the live 1-1 assay, but found that with sufficient replication its results were highly correlated with the assays using more interacting ants. We suggest that isolated aggressive acts in assays do not necessarily predict whole colony interactions : some colonies that fought in bioassays merged when the entire colonies were allowed to interact.

Transcriptomic profiles of Drosophila melanogaster third instar larval midgut and responses to oxidative stress

Oligoarray analysis was used to determine the number and nature of genes expressed in third instar Drosophila melanogaster larval midguts. The majority of transcripts were associated with protein synthesis and metabolism. Serine proteases were the main proteolytic enzymes detected. Some 40% of the cytochrome P450 genes and 74% of the glutathione S transferases (GSTs) in the genome of D. melanogaster were observed to be expressed in the midgut by oligoarray analysis. We also identified potential transcription factor binding motifs (TFBMs) of P450s, GSTs and carboxylesterases. Many of the midgut‐expressed GST genes contained candidate TFBMs homologous to TFBMs in mammals that have been associated with responses to oxidative stress. We also investigated the response of GSTs in the midgut to dietary H2O2, which showed a dosage‐based differential response.

The diminutive supercolony: the Argentine ants of the southeastern United States

Native to Argentina and Brazil, the Argentine ant (Linepithema humile) is an invasive species that has become established on six continents and many oceanic islands. In several parts of its introduced range, including the western United States, southern Europe and Chile, the Argentine ant is unicolonial, forming extensive supercolonies. We examined population genetic structure and intercolony aggression in two regions of the introduced range of this species in the United States: California and the southeastern United States. Our results show that the southeastern L. humile population has high genotypic variability and strong intercolony aggression relative to the California population. In the California population, intercolony aggression was absent and 23 alleles were found across seven polymorphic microsatellite loci. However, in the Southeast, aggression between colonies was high and 47 alleles were present across the same seven loci in an equal number of colonies. We suggest that distinctly different colonization patterns for California and the Southeast may be responsible for the striking disparity in the genetic diversity of introduced populations. Southeastern colonies may have descended from multiple, independent introductions from the native range, undergoing a bottleneck at each introduction. In contrast, the California supercolony may have originated from one or more colonies inhabiting the southeastern United States, thus experiencing a double bottleneck. The differences in present‐day distribution patterns between California and the Southeast may be due to the combined effect of two factors: lower winter temperatures in the Southeast and/or competition with another successful and widely distributed ant invader, the fire ant Solenopsis invicta.

Diet-Related Modification of Cuticular Hydrocarbon Profiles of the Argentine Ant, Linepithema humile, Diminishes Intercolony Aggression

Territorial boundaries between conspecific social insect colonies are maintained through a highly developed nestmate recognition system modulated by heritable and, in some instances, nonheritable cues. Argentine ants, Linepithema humile, use both genetic and environmentally derived cues to discriminate nestmates from nonnestmates. We explored the possibility that intraspecific aggression in the Argentine ant might diminish when colonies shared a common diet. After segregating recently field-collected colony pairs into high or moderate aggression categories, we examined the effect of one of three diets: two hydrocarbon-rich insect prey, Blattella germanica and Supella longipalpa, and an artificial (insect-free) diet, on the magnitude of aggression loss. Aggression diminished between colony pairs that were initially moderately aggressive. However, initially highly aggressive colony pairs maintained high levels of injurious aggression throughout the study, independent of diet type. Each diet altered the cuticular hydrocarbon profile by contributing unique, diet-specific cues. We suggest that acquisition of common exogenous nestmate recognition cues from shared food sources may diminish aggression and promote fusion in neighboring colonies of the Argentine ant.

Context-dependent nestmate discrimination and the effect of action thresholds on exogenous cue recognition in the Argentine ant

The optimal acceptance threshold model predicts that kin/nestmate discrimination is context dependent and that, in a fluctuating environment, the action component of nestmate discrimination is plastic, rather than static. We examined changes in intraspecific aggression among colonies of Argentine ants, Linepithema humile , in various discrimination contexts, and found that aggression occurred at higher rates when either nestmates or familiar territory indicated nest proximity, but not when social context was absent, thereby providing additional support for the optimal acceptance threshold model. Context-dependent aggression in the Argentine ant appears to result from a shift in acceptance threshold in response to fitness costs associated with accepting nonkin. The change in the action component of Argentine ant nestmate discrimination was explained to some degree by the hypothesis that the presence of nestmates indicates nest proximity and denotes a fitness payoff for active defence (nest indicator hypothesis) and by the hypothesis that nestmates share the cost of nest defence in groups, but not singly (cost minimizer hypothesis). Isolated nest referents (familiar territory, conspecific brood, or single familiar nestmates), however, had no effect on aggression thresholds. We provide mixed support for the hypothesis that workers from genetically less diverse colonies attack workers from more diverse colonies. We found that, in the context of nest defence, genetically more diverse colonies initiated attacks on colonies with lower genetic diversity. Therefore, the role of asymmetrical aggression in reducing genetic diversity within introduced populations of L. humile remains unknown and other extrinsic factors such as nest status and/or colony size may affect the outcome of aggressive interactions in the field. Finally, our finding that colonies reared under uniform conditions showed diminished intraspecific aggression only when assayed in a social and/or ecological context underscores the importance of using appropriate aggression assays for testing patterns of intercolony aggression in L. humile , and that the use of different rearing regimes, source colonies and collection times may produce contradictory results.

The Effect of Urbanization on Ant Abundance and Diversity: A Temporal Examination of Factors Affecting Biodiversity

Numerous studies have examined the effect of urbanization on species richness and most studies implicate urbanization as the major cause of biodiversity loss. However, no study has identified an explicit connection between urbanization and biodiversity loss as the impact of urbanization is typically inferred indirectly by comparing species diversity along urban-rural gradients at a single time point. A different approach is to focus on the temporal rather than the spatial aspect and perform “before and after” studies where species diversity is cataloged over time in the same sites. The current study examined changes in ant abundance and diversity associated with the conversion of natural habitats into urban habitats. Ant abundance and diversity were tracked in forested sites that became urbanized through construction and were examined at 3 time points - before, during, and after construction. On average, 4.3±1.2 unique species were detected in undisturbed plots prior to construction. Ant diversity decreased to 0.7±0.8 species in plots undergoing construction and 1.5±1.1 species in plots 1 year after construction was completed. With regard to species richness, urbanization resulted in the permanent loss of 17 of the 20 species initially present in the study plots. Recovery was slow and only 3 species were present right after construction was completed and 4 species were present 1 year after construction was completed. The second objective examined ant fauna recovery in developed residential lots based on time since construction, neighboring habitat quality, pesticide inputs, and the presence of invasive ants. Ant diversity was positively correlated with factors that promoted ecological recovery and negatively correlated with factors that promoted ecological degradation. Taken together, these results address a critical gap in our knowledge by characterizing the short- and long-term the effects of urbanization on the loss of ant biodiversity.

Genome-wide analysis of phenobarbital-inducible genes in Drosophila melanogaster

An oligoarray analysis was conducted to determine the differential expression of genes due to phenobarbital exposure in Drosophila melanogaster (w1118 strain) third instar larvae. Seventeen genes were observed to be induced with increased expression by a statistical analysis of microarrays approach with a q ≤ 0.05. At q ≤ 0.12, four more genes (Cyp12d1, DmGstd4, and two genes with unknown function) were found to be up‐regulated, and 11 genes with unknown function were found to be down‐regulated. Fifteen of these genes, Cyp4d14, Cyp6a2, Cyp6a8, Cyp12d1, Cyp6d5, Cyp6w1, CG2065, DmGstd6, DmGstd7, Amy‐p/Amy‐d, Ugt86Dd, GC5724, Jheh1, Jheh2 and CG11893, were verified using quantitative real time polymerase chain reaction. Some of these genes have been shown to be over‐transcribed in metabolically DDT‐resistant Drosophila strains.

Extreme life history plasticity and the evolution of invasive characteristics in a native ant

Disturbance resulting from urbanization is a leading cause of biotic homogenization worldwide. Native species are replaced with widespread non-native species and ants are among the world’s most notorious invaders. To date, all documented cases of ant invasions involve exotic introduced species that are spread around the world by human-mediated dispersal. I investigated the effect of urbanization on the evolution of invasive characteristics in a native ant species, the odorous house ant, Tapinoma sessile (Say). Colony social structure, life history traits, and the spatial pattern of nest distribution were compared by sampling T. sessile across a gradient of three distinct habitats: natural, semi-natural, and urban. Results demonstrate a remarkable transition in colony social and spatial structure and life history traits between natural and urban environments. In natural habitats, T. sessile colonies are comprised of small, monogyne (single queen), and monodomous (single nest) colonies. In urban areas, T. sessile often exhibit extreme polygyny and polydomy, form large supercolonies, and become a dominant pest. Results also suggest that urban T. sessile colonies may have a negative impact on native ant abundance and diversity. In the natural environment T. sessile coexisted with a wide array of other ant species, while very few ant species were present in the urban environment invaded by T. sessile. Habitat degradation and urbanization can lead to extreme changes in social and spatial colony structure and life history traits in a native ant species and can promote the evolution of invasive characteristics such as polygyny, polydomy, and supercolonial colony structure.

Geographical variation in Argentine ant aggression behaviour mediated by environmentally derived nestmate recognition cues

Social insects use a complex of recognition cues when discriminating nestmates from non-nestmate conspecifics. In the Argentine ant, Linepithema humile, recognition cues can be derived from exogenous sources, and L. humile acquires prey-derived hydrocarbons that are used in nestmate discrimination. We studied Argentine ant population-level distinctions in response to external recognition cues. Ants belonging to a California population were strongly affected by the imposition of prey-derived hydrocarbons, with spatially isolated colony fragments that had been fed different cockroach prey (Blattella germanica or Supella longipalpa) showing high and injurious intracolony aggression when reunited. In contrast, colonies of Argentine ants from the southeastern U.S. showed only moderate and noninjurious aggression when subjected to the same treatment. Field-collected colonies of L. humile had hydrocarbons in the range of those provided by S. longipalpa, and colonies from the southeastern U.S. had significantly higher initial levels of Supella-shared hydrocarbons. When fed cockroaches, Argentine ants from both regions acquired additional amounts of Supella- and Blattella-specific hydrocarbons, with a significant increase in levels of Blattella-specific hydrocarbons. Therefore, diet partitioning produced a greater change in the proportion of prey hydrocarbons in the California than in the southeastern U.S. populations, which may be responsible for the altered behaviour observed in the California population. Identifying factors underlying geographical variation in cue expression and/or perception may bring us closer to elucidating the selective forces driving nestmate recognition systems.

Seasonal polydomy in a polygynous supercolony of the odorous house ant, Tapinoma sessile

Abstract 1. The odorous house ant, Tapinoma sessile, is a native ant species common throughout North America. In its natural habitat, T. sessile is a low‐key species that consists of small colonies. In invaded urban areas, T. sessile exhibits extreme polygyny and polydomy and becomes a dominant invasive pest.