Ancestral state reconstruction illuminates ant venom evolution and need for natural history studies

Ants (Hymenoptera: Formicidae) provide a unique opportunity to explore the evolutionary history of novel traits essential for inter- and intraspecific interactions. A prime example of this are the alkaloid venoms found in the tribe Solenopsidini. Alkaloids are secondary metabolites containing a nitrogen atom as part of a larger carbon ring which can be grouped based on the number of rings present (monocyclic, bicyclic, and tricyclic). Each species possesses a unique composition of alkaloids. Broadly, these venoms are used in prey capture, resource monopolization, and social parasitism (where one social species parasitizes another). Social parasitism has evolved independently multiple times within Solenopsidini, with the most well-known examples being the genera Solenopsis, Monomorium, and Megalomyrmex. Earlier work suggested bicyclic alkaloids evolved with the origin of social parasitism prompting the hypothesis that venom profiles correlate with natural history traits, however, this has never been formally tested. Here we address this gap by reconstructing the ancestral states of venom alkaloids in Megalomyrmex. Megalomyrmex is a relatively small genus, composed of 46 species, approximately one quarter of which are social parasites of fungus-growing ants. Our results show that bicyclic alkaloids have evolved multiple times within Megalomyrmex and not all taxa possessing them are social parasites. This suggests that the evolution of alkaloid venom class and social parasitism are not connected, and perhaps other facets of natural history are important to alkaloid venom evolution. The biology of many Megalomyrmex beyond host associations is largely unknown, emphasizing the importance of natural history studies.