Chemical tools to distinguish the fire ant species Solenopsis invicta and S. saevissima (Formicidae: Myrmicinae) in Southeast Brazil

Links between prey assemblages and poison frog toxins: A landscape ecology approach to assess how biotic interactions affect species phenotypes

Ecological studies of species pairs showed that biotic interactions promote phenotypic change and eco-evolutionary feedbacks. However, it is unclear how phenotypes respond to synergistic interactions with multiple taxa. We investigate whether interactions with multiple prey species explain spatially structured variation in the skin toxins of the neotropical poison frog Oophaga pumilio. Specifically, we assess how dissimilarity (i.e., beta diversity) of alkaloid-bearing arthropod prey assemblages (68 ant species) and evolutionary divergence between frog populations (from a neutral genetic marker) contribute to frog poison dissimilarity (toxin profiles composed of 230 different lipophilic alkaloids sampled from 934 frogs at 46 sites). We find that models that incorporate spatial turnover in the composition of ant assemblages explain part of the frog alkaloid variation, and we infer unique alkaloid combinations across the range of O. pumilio. Moreover, we find that alkaloid variation increases weakly with the evolutionary divergence between frog populations. Our results pose two hypotheses: First, the distribution of only a few prey species may explain most of the geographic variation in poison frog alkaloids; second, different codistributed prey species may be redundant alkaloid sources. The analytical framework proposed here can be extended to other multitrophic systems, coevolutionary mosaics, microbial assemblages, and ecosystem services.

Cuticular hydrocarbon chemistry, an important factor shaping the current distribution pattern of the imported fire ants in the USA

Two sibling species, Solenopsis richteri and S. invicta, were both introduced into the southern USA from South America in the early 20th century. Today, S. richteri occupies higher latitudes and colder areas, while S. invicta occupies lower latitudes. Between the distributions of the two species, there is a large area of viable hybrid (S. richteri × S. invicta) populations. This study aimed to characterize the forces driving this distribution pattern and the underlying mechanisms. Cuticular hydrocarbons (CHCs) of freshly killed workers of S. invicta, hybrids, and S. richteri were removed using hexane. Both intact and CHCs-extracted workers were subjected to a constant rate of increasing temperature from 10 to 60 °C to obtain relative water loss and the water loss transition temperature (T_c-ant). Mass loss and T_c-ant were both significantly increased with CHCs removal. We then examined the CHC composition of three species. CHC profiles of S. richteri are characterized by significant amounts of short-chain (C₂₃-C₂₇) saturated and unsaturated hydrocarbons. In contrast, profiles of S. invicta consist primarily of long-chain (C₂₇-C₂₉) saturated hydrocarbons; unsaturated alkenes are completely lacking. Hybrid fire ants show intermediate profiles of the two parent species. We measured the melting point (T_m) and water-loss transition temperature of CHC blends (T_c-CHC) of different ant species colonies using differential scanning calorimetry (DSC) and an artificial membrane system, respectively. There were 3-5 T_ms of each CHCs sample of different ant colonies due to their complex chemistry. The highest T_ms (T_m-maxs) of CHCs samples from S. invicta and the hybrid were significantly higher than that from S. richteri. The correlation between T_c-CHC and T_m-max obtained from the same CHCs sample was highly significant. These results reveal that species having higher T_c and T_m-max retain more water under relatively higher temperature, and consequently are able to occupy warmer environments. We conclude that CHC chemistry plays a role in shaping current distribution patterns of S. richteri, S. invicta and their hybrid in the United States.

The Guianese population of the fire ant Solenopsis saevissima is unicolonial

In this study, conducted in French Guiana, a part of the native range of the fire ant Solenopsis saevissima, we compared the cuticular hydrocarbon profiles of media workers with previous results based on intraspecific aggressiveness tests. We noted a strong congruence between the two studies permitting us to delimit 2 supercolonies extending over large distances (up to 54 km), a phenomenon known as unicoloniality. Solenopsis geminata workers, taken as an out-group for cluster analyses, have a very different cuticular hydrocarbon profile. Because S. saevissima has been reported outside its native range, our conclusion is that this species has the potential to become invasive because unicoloniality (i.e., the main attribute for ants to become invasive) was shown at least for the Guianese population.

Differential necrophoric behaviour of the ant Solenopsis invicta towards fungal-infected corpses of workers and pupae

Necrophoric behaviour is critical sanitation behaviour in social insects. However, little is known about the necrophoric responses of workers towards different developmental stages in a colony as well as its underlying mechanism. Here, we show that Solenopsis invicta workers display distinct necrophoric responses to corpses of workers and pupae. Corpses of workers killed by freezing (dead for <1 h) were carried to a refuse pile, but pupal corpses would take at least 1 day to elicit workers' necrophoric response. Metarhizium anisopliae-infected pupal corpses accelerated the necrophoric behaviour of resident workers, with 47.5% of unaffected corpses and 73.8% infected corpses discarded by 1 day post-treatment). We found that fungus-infected pupal corpses had a higher concentration of fatty acids (palmitic acid, oleic acid and linoleic acid) on their surface. We experimentally confirmed that linoleic and oleic acids would elicit a necrophoric response in workers. The appearance of linoleic and oleic acids appeared to be chemical signals involved in recognition of pupal corpses, and M. anisopliae infection could promote the accumulation of fatty acids on surface of pupal corpses resulting in accelerated necrophoric responses of workers.

Cuticular hydrocarbons discriminate cryptic Macrolophus species (Hemiptera: Miridae)

Macrolophus pygmaeus is commercially employed in the biological control of greenhouse and field vegetable pests. It is morphologically undistinguishable from the cryptic species M. melanotoma, and this interferes with the evaluation of the biological control activity of M. pygmaeus. We analysed the potential of cuticular hydrocarbon composition as a method to discriminate the two Macrolophus species. A third species, M. costalis, which is different from the other two species by having a dark spot at the tip of the scutellum, served as a control. Sex, diet and species, all had significant effects in the cuticular hydrocarbon profiles, but the variability associated to sex or diet was smaller than among species. Discriminant quadratic analysis of cuticular hydrocarbons confirmed the results of previous molecular genetic studies and showed, using cross-validation methods, that M. pygmaeus can be discriminated from M. costalis and M. melanotoma with prediction errors of 6.75% and 0%, respectively. Therefore, cuticular hydrocarbons can be used to separate M. pygmaeus from M. melanotoma reliably.

Venom alkaloid and cuticular hydrocarbon profiles are associated with social organization, queen fertility status, and queen genotype in the fire ant Solenopsis invicta

Queens in social insect colonies advertise their presence in the colony to: a) attract workers' attention and care; b) gain acceptance by workers as replacement or supplemental reproductives; c) prevent reproductive development in nestmates. We analyzed the chemical content of whole body surface extracts of adult queens of different developmental and reproductive stages, and of adult workers from monogyne (single colony queen) and polygyne (multiple colony queens) forms of the fire ant Solenopsis invicta. We found that the composition of the most abundant components, venom alkaloids, differed between queens and workers, as well as between reproductive and non-reproductive queens. Additionally, workers of the two forms could be distinguished by alkaloid composition. Finally, sexually mature, non-reproductive queens from polygyne colonies differed in their proportions of cis-piperidine alkaloids, depending on their Gp-9 genotype, although the difference disappeared once they became functional reproductives. Among the unsaturated cuticular hydrocarbons characteristic of queens, there were differences in amounts of alkenes/alkadienes between non-reproductive polygyne queens of different Gp-9 genotypes, between non-reproductive and reproductive queens, and between polygyne and monogyne reproductive queens, with the amounts increasing at a relatively higher rate through reproductive ontogeny in queens bearing the Gp-9 b allele. Given that the genotype-specific piperidine differences reflect differences in rates of reproductive maturation between queens, we speculate that these abundant and unique compounds have been co-opted to serve in fertility signaling, while the cuticular hydrocarbons now play a complementary role in regulation of social organization by signaling queen Gp-9 genotype.