Fungus growing ants

De Souza et al. (2007) - The fungus-growing ants are a New World group of > 200 species, all obligate symbionts with a fungus they use for food. The These ants are found across the American continents and the West Indies and comprise at present > 200 described species. Two genera, Atta and Acromyrmex, are called leaf-cutting ants because most species culture their fungus on freshly cut foliage and flowers (Müeller et al., 2001). This particular foraging behavior makes these ants major agricultural pests.

See this chapter of The Ants for a general overview of these intriguing ants.

Ant-Fungus mutualism
The evolution and maintenance of the complex mutualism between fungus growing ants and the fungi they cultivate is an intriguing area of study. While much has been discovered about the ants, the fungi and their coevolution there remain many interesting questions.

Here are some examples:

Fungal Ploidy
Flowering plants are rife with species that exhibit polyploidy, i.e., possess more than two sets of chromosomes. The evolutionary basis and potential fitness advantages of this condition have been explored for many decades. Kooij et al. (2015) discovered analogous duplications of genetic material in attine fungi. They examined fungi samples that were representatives of the different evolutionary stages of fungus farming, i.e., higher attine ants with domesticated gongylidia-bearing fungi and lower- and paleo-attine ants growing fungi without obvious adaptations to being crops. They found: Domesticated symbionts of higher attine ants are polykaryotic with 7–17 nuclei per cell, whereas nonspecialized crops of lower attines are dikaryotic similar to most free-living basidiomycete fungi. Our opposite ploidy models indicated that the symbionts of Trachymyrmex and Sericomyrmex are likely to be lowly and facultatively polyploid (just over two haplotypes on average), whereas Atta and Acromyrmex symbionts are highly and obligatorily polyploid (ca. 5–7 haplotypes on average). Genetic variation, as shown by examining variation in microsatellite loci revealed the polykaryotic symbionts of the basal higher attine genera Trachymyrmex and Sericomyrmex was only slightly enhanced, but the evolutionarily derived crop fungi of Atta and Acromyrmex leaf-cutting ants had much higher genetic variation. This stepwise transition appears analogous to ploidy variation in plants and fungi domesticated by humans and in fungi domesticated by termites and plants, where gene or genome duplications were typically associated with selection for higher productivity but allopolyploid chimerism was incompatible with sexual reproduction.

Social Parasitism
De Souza et al. (2007) - Social parasitism, the exploitation of the nest of another species without contributing to colony maintenance, for example the cultivation of a fungus garden, has been reported occasionally in the attine ants. Megalomyrmex species can coexist as social parasites in attine colonies, consuming the fungus garden (Brandão, 1990; Adams et al., 2000). Gnamptogenys hartmani is also a specialized agro-predator of Trachymyrmex and Sericomyrmex fungus-growing ants in Panama (Dijkstra & Boomsma, 2003). Five taxa are known to be social parasites of other Acromyrmex species, living in and feeding on their fungus gardens, but not contributing to its maintenance: Pseudoatta argentina and Pseudoatta argentina platensis (parasites of Acromyrmex lundii, Acromyrmex heyeri and possibly Acromyrmex balzani), and Pseudoatta sp. (a parasite of Acromyrmex rugosus) produce no worker caste (Santschi, 1926; Bruch, 1928; Gallardo, 1929; Delabie et al., 1993). Rabeling and Bacci (2010) have discovered a workerless inquiline species Mycocepurus castrator that parasitizes Mycocepurus goeldii. In contrast, the recently discovered Acromyrmex insinuator (a parasite of Acromyrmex echinatior) does produce workers (Schultz et al., 1998). Sumner et al. (2004) found that Pseudoatta sp. was not closely related to its host, but A. insinuator was closely related to its host, A. echinatior. Acromyrmex ameliae, a social parasite of Acromyrmex subterraneus and Acromyrmex subterraneus brunneus in Minas Gerais, Brasil. Like A. insinuator, it produces workers and appears to be closely related to its hosts.

Genera

 * Acromyrmex
 * Apterostigma
 * Atta
 * Cyatta
 * Cyphomyrmex
 * Kalathomyrmex
 * Mycetagroicus
 * Mycetarotes
 * Mycetophylax
 * Mycetosoritis
 * Mycocepurus
 * Myrmicocrypta
 * Paramycetophylax
 * Sericomyrmex 
 * Trachymyrmex