Cuticular hydrocarbon chemistry, an important factor shaping the current distribution pattern of the imported fire...

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.