Evaluating the factors that promote invasive ant abundance is critical to assess their ecological impact and inform their management. Many invasive ant species show reduced nestmate recognition and an absence of boundaries between unrelated nests, which allow populations to achieve greater densities due to reduced intraspecific competition. We examined nestmate discrimination and colony boundaries in introduced populations of the red imported fire ant (Solenopsis invicta; hereafter, fire ant). Fire ants occur in two social forms: monogyne (colonies with a single egg-laying queen) and polygyne (colonies with multiple egg-laying queens). In contrast with monogyne nests, polygyne nests are thought to be interconnected due to the reduced antagonism between non-nestmate polygyne workers, perhaps because polygyne workers habituate the colony to an odor unique to Gp-9b-carrying adults. However, colony boundaries and nestmate discrimination are poorly documented, particularly for worker-brood interactions. To delimit boundaries between field colonies, we correlated the exchange of a 15N-glycine tracer dissolved in a sucrose solution with social form. We also evaluated nestmate discrimination between polygyne workers and larvae in the laboratory. Counter to our expectations, polygyne colonies behaved identically to monogyne colonies, suggesting both social forms maintain strict colony boundaries. Polygyne workers also preferentially fed larval nestmates and may have selectively cannibalized non-nestmates. The levels of relatedness among workers in polygyne colonies was higher than those previously reported in North America (mean ±SE: 0.269 ± 0.037). Our study highlights the importance of combining genetic analyses with direct quantification of resource exchange to better understand the factors influencing ant invasions.

The level of gene flow between diverging lineages ultimately determines the outcome of a speciation event. If secondary contact occurs before this process is complete, reproductive isolation barriers must exist or evolve to prevent hybridization. The selective pressures facilitating and maintaining genetic divergence do not always involve an observable phenotypic response, thus cryptic species form. The inability to distinguish between sibling species can be a particularly serious problem in groups responsible for pathogen transmission. Culicoides biting midges occur almost world-wide and vector many disease-causing pathogens that affect wildlife and livestock. In North America, the C. variipennis species complex contains three currently recognized species, only one of which is a vector, and limited molecular and morphological differences have hindered vector surveillance. Here, genomic methods were used to investigate speciation and genetic structure within this complex. Single nucleotide polymorphism (SNP) data were generated using ddRAD sequencing for 206 individuals originating from 17 locations throughout the United States and Canada. Clustering analyses consistently suggest the occurrence of five putative species with significant differentiation occurring in both sympatric and allopatric populations. Evidence of hybridization was detected in three different species pairings, indicating a lack of pre-zygotic reproductive isolation within the complex. Mitochondrial genes were used to trace the hybrid parentage of these individuals, which illuminated discordance with the SNP data. In this study, we highlight the potential role of geographic, ecological, and behavioral isolation in speciation and in maintaining species boundaries, despite hybridization and long range dispersal.

As native ranges are often geographically structured, invasive species originating from a single source population only carry a fraction of the genetic diversity present in their native range. This invasion pathway is thus often associated with a drastic loss of genetic diversity resulting from a founder event. However, the fraction of diversity brought to the invasive range may vary under different invasion histories, increasing with the size of the propagule, the number of re-introduction events, and/or the total genetic diversity represented by the various source populations in a multiple-introduction scenario. In this study, we generated a SNP dataset for the invasive termite Reticulitermes flavipes from 23 native populations in the eastern United States and six introduced populations throughout the world. Using population genetic analyses and approximate Bayesian computation (ABC), we investigated its worldwide invasion history. We found a complex invasion pathway with multiple events out of the native range and bridgehead introductions from the introduced population in France. Our data suggest that extensive long-distance jump dispersal appears common in both the native and introduced ranges of this species, likely through human transportation. Overall, our results show that similar to multiple introduction events into the invasive range, admixture in the native range prior to invasion can potentially favor invasion success by increasing the genetic diversity that is later transferred to the introduced range.

Unicoloniality, or the absence of behavioral boundaries between nests, is thought to promote ant abundance due to reduced intraspecific competition. Workers within unicolonial populations may increase their own inclusive fitness by preferentially caring for more related individuals (nepotism), but nepotism has only rarely been documented in ants. We tested for unicoloniality and nepotism in polygyne red imported fire ants (Solenopsis invicta; hereafter fire ants). Fire ants occur in two social forms: monogyne (i.e., colonies with a single egg-laying queen) and polygyne (i.e., colonies with multiple egg-laying queens). Introduced populations of polygyne fire ants are commonly referred to as unicolonial, but cooperation between and within colonies is poorly documented. To delimit boundaries between colonies in the field, we quantified the exchange of a 15N-glycine tracer dissolved in a sucrose solution and correlated this exchange with colony genetic structure. We also quantified within-colony conflict between workers and larvae using close siblings (i.e., from the same mother) and non-siblings (i.e., from a different mother). Counter to our expectations, polygyne colonies did not exchange resources or workers, indicating distinct colony boundaries. Polygyne workers also preferentially fed larval sibling and may have preferentially cannibalized non-siblings. Polygyne colony behavior was correlated with higher levels of within-mound relatedness between workers in the field than those previously reported in North America (mean ± SE: 0.269 ± 0.037). Our study challenges fundamental assumptions about introduced populations of polygyne fire ants and suggests that polygyne colonies are multicolonial and likely engage in high levels of intraspecific competition.