Taxonomic revision, phylogenomic analyses, and natural history of the fungus-farming ant genus Sericomyrmex (Hymenoptera: Formicidae)

Abstract

Fungus-farming ("attine") ants, a New World clade of nearly 300 species in 16 genera, are model systems for studies of symbiosis, coevolution, and advanced eusociality. All attine ants cultivate fungal symbionts for food. "Lower" attine ants cultivate facultatively symbiotic fungi, but in "higher" attine ants both ants and fungi are obligate, highly coevolved symbionts. Sericomyrmex is a poorly studied higher-attine genus known for its problematic taxonomy. This work represents the first comprehensive study of Sericomyrmex species and their fungal cultivars.

For Chapter One I sequenced, assembled, and analyzed transcriptomes of three species of Sericomyrmex and a lower-attine-ant species, Apterostigma megacephala. I characterize the transcriptomes, conduct phylogenetic analyses, and search for genes of interest, most importantly arginine biosynthesis pathway genes, the absence of which in A. megacephala strongly suggests that attine ants became dependent on their fungi early in their evolution.

In Chapter Two I address the phylogenomics of Sericomyrmex, sequencing ~990 ultra-conserved-element loci for 88 Sericomyrmex samples. Maximum-likelihood and species-tree phylogenetic methods recover nearly identical topologies across data sets, identifying nine species-level lineages. Divergence-dating analyses indicate that Sericomyrmex is the product of a recent, rapid radiation, with a crown-group age estimate of 4.3 million years. I sequence two nuclear ribosomal regions for 32 Sericomyrmex fungi. The fungal phylogeny indicates that Sericomyrmex cultivars are generalized higher-attine cultivars, which, rather than forming a separate clade, are interspersed with Trachymyrmex-associated fungi, indicating cultivar sharing and horizontal transfer between those genera.

In Chapter Three, guided by my phylogenomic results, I study the morphology of Sericomyrmex workers, males, queens, and larvae and conduct a comprehensive taxonomic revision, including a key to the worker caste and species distribution maps. Sericomyrmex comprises 11 species, including three new species. The number of recognized species is lower than the previously recognized 19 species and three subspecies.

In Chapter Four I review the literature of and report newly acquired data on the natural history of Sericomyrmex, with a focus on nesting biology, presenting data for 19 nests of seven Sericomyrmex species.