Evolutionary Transitions in Enzyme Activity of Ant Fungus Gardens

Abstract

Fungus-growing (attine) ants and their fungal symbionts passed through several evolutionary transitions during their 50 millionyear old evolutionary history. The basal attine lineages often shifted between two main cultivar clades, whereas the derived higher-attine lineages maintained an association with a monophyletic clade of specialized symbionts. In conjunction with the transition to specialized symbionts, the ants advanced in colony size and social complexity. Here we provide a comparative study of the functional specialization in extracellular enzyme activities in fungus gardens across the attine phylogeny. We show that, relative to sister clades, gardens of higher-attine ants have enhanced activity of protein-digesting enzymes, whereas gardens of leaf-cutting ants also have increased activity of starch-digesting enzymes. However, the enzyme activities of lower-attine fungus gardens are targeted primarily toward partial degradation of plant cell walls, reflecting a plesiomorphic state of nondomesticated fungi. The enzyme profiles of the higher-attine and leaf-cutting gardens appear particularly suited to digest fresh plant materials and to access nutrients from live cells without major breakdown of cell walls. The adaptive significance of the lower-attine symbiont shifts remains unclear. One of these shifts was obligate, but digestive advantages remained ambiguous, whereas the other remained facultative despite providing greater digestive efficiency.

Leucoagaricus gongylophorus uses leaf-cutting ants to vector proteolytic enzymes towards new plant substrate

“…In the 1970s, leaf-cutting ant fecal fluid was shown to contain active proteases (Martin, 1970(Martin, , 1974Martin, 1970a, b, 1971) with similar chemical properties as enzymes originating from the fungal symbiont (Boyd and Martin, 1975a, b), and recent work has shown that fungus garden endo-protease activity in evolutionarily derived leaf-cutting ants is much higher than in sister ant lineages that do not use fresh leaves to make their gardens grow (De Fine Licht et al, 2010), activity that could subsequently be assigned to metalloproteases and serine proteases (Semenova et al, 2011).…”

“…Particularly, the latter transition allowed the ants to evolve much larger and complex societies (Mueller et al, 1998;Villesen et al, 2002;Schultz and Brady, 2008;Fernandez-Marin et al, 2009;Mehdiabadi and Schultz, 2010) after they had managed to overcome a series of challenges that are associated with a herbivorous life-style (Schiøtt et al, 2008De Fine Licht et al, 2010. Although depending on live vegetation rather than dead leaf litter undoubtedly implied better access to the proteins that normally limit insect growth, an almost exclusive fungal diet is generally poorer in protein than the carnivorous diets of many hunter-gatherer species in the Myrmicinae subfamily of ants (Davidson, 2004).…”

“…In terms of biomass conversion efficiency, the functional complementarity between attine ants and their fungus garden symbionts achieved spectacular progression, moving from initial stages of ant-driven litter-based decomposition farming that evolved 50 million years ago (MYA) to advanced farming of irreversibly domesticated crop fungi 20 MYA (Mueller et al, 1998;Schultz and Brady, 2008;Mehdiabadi and Schultz, 2010), culminating in aggressive herbivorous leaf-cutting farming 10 MYA (De Fine Licht et al, 2010;Schiøtt et al, 2010). Particularly, the latter transition allowed the ants to evolve much larger and complex societies (Mueller et al, 1998;Villesen et al, 2002;Schultz and Brady, 2008;Fernandez-Marin et al, 2009;Mehdiabadi and Schultz, 2010) after they had managed to overcome a series of challenges that are associated with a herbivorous life-style (Schiøtt et al, 2008De Fine Licht et al, 2010.…”

Metagenomic and metaproteomic insights into bacterial communities in leaf-cutter ant fungus gardens

“…The ants presumably gain indirect access to the carbon stored in plant cell walls through the metabolic activities of their fungus gardens, which act as an ancillary digestive system (Pinto-Tomas et al, 2009). Despite being a critical aspect of leaf-cutter ant biology, the process through which fungus gardens degrade plant forage has only recently been intensely investigated (De Fine Licht et al, 2010;Schiott et al, 2010;Suen et al, 2010;Semenova et al, 2011). Originally it was thought that the fungal cultivar primarily degraded cellulose, and that this was the main polymer converted into nutrients for the ants (Martin and Weber, 1969).…”

Symbiotic adaptations in the fungal cultivar of leaf-cutting ants

“…Relative to fungal hyphae, the gongylidia have high concentrations of lipids and carbohydrates (free sugars and polysaccharides) and provide all food for the ant larvae and most food for the adult ants 13 . Based on comparisons of extant species, the evolution of gongylidia coincided with distinct changes in fungus garden enzyme activity 14 , proportional shifts in plant material used as fungal substrate 15 and the cultivated fungi loosing the ability to live independently without the ants 10 .…”