Carlson Etal Mycologia 2017
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MYCOLOGIA 2017, VOL. 109, NO. 5, 832–846 https://doi.org/10.1080/00275514.2017.1400304 Nuclear populations of the multinucleate fungus of leafcutter ants can be dekaryotized and recombined to manipulate growth of nutritive hyphal nodules harvested by the ants Alexis L. Carlsona, Heather D. Ishaka, James Kurian a, Alexander S. Mikheyev b, Isaac Gifforda, and Ulrich G. Mueller a aDepartment of Integrative Biology, University of Texas at Austin, Austin, Texas 78712; bOkinawa Institute of Science and Technology, 1919-1 Tancha, Onna-son, Kunigami, Okinawa 904-2234, Japan ABSTRACT ARTICLE HISTORY We dekaryotized the multinucleate fungus Leucocoprinus gongylophorus, a symbiotic fungus Received 9 March 2017 cultivated vegetatively by leafcutter ants as their food. To track genetic changes resulting from Accepted 31 October 2017 dekaryotization (elimination of some nuclei from the multinuclear population), we developed two KEYWORDS multiplex microsatellite fingerprinting panels (15 loci total), then characterized the allele profiles Attamyces bromatificus; of 129 accessions generated by dekaryotization treatment. Genotype profiles of the 129 acces- Attini; fungus-growing ant; sions confirmed allele loss expected by dekaryotization of the multinucleate fungus. We found no Leucoagaricus evidence for haploid and single-nucleus strains among the 129 accessions. Microscopy of fluor- gongylophorus; escently stained dekaryotized accessions revealed great variation in nuclei number between cells Leucocoprinus of the same vegetative mycelium, with cells containing typically between 3 and 15 nuclei/cell gongylophorus; (average = 9.4 nuclei/cell; mode = 8). We distinguish four mycelial morphotypes among the multinucleate; multiplex dekaryotized accessions; some of these morphotypes had lost the full competence to produce microsatellite markers; polyploidy gongylidia (nutritive hyphal-tip swellings consumed by leafcutter ants as food). In mycelial growth confrontations between different gongylidia-incompetent accessions, allele profiles suggest exchange of nuclei between dekaryotized accessions, restoring full gongylidia competence in some of these strains. The restoration of gongylidia competence after genetic exchange between dekaryotized strains suggests the hypothesis that complementary nuclei interact, or nuclear and cytoplasmic factors interact, to promote or enable gongylidia competence. INTRODUCTION Atta leafcutter ants grow their fungi in single-clone Leafcutter ants in the genera Atta and Acromyrmex culti- monoculture (Mueller et al. 2010) and vertically trans- vate as their main food source monocultures of fungi, mit their fungal strain from one generation to the next. called Leucocoprinus gongylophorus (Leucocoprini, When a virgin leafcutter queen leaves her natal nest for Agaricales) as sexual morph (Heim 1957; Mueller et al. her mating flight and dispersal, she maintains an inocu- 2017)orAttamyces bromatificus as asexual morph lum of her natal fungal cultivar in a storage pocket in (Kreisel 1972). The ants cultivate their fungus in sheltered her mouth, then uses this inoculum as a starter culture gardens (typically underground) and sustain fungal to initiate the garden in her newly founded nest. Such growth in these gardens by supplying their cultivars vertical transmission of clonally propagated fungal with substrate for growth (typically finely minced leaves) strains is typical for all leafcutter ants (Weber 1972; and nourishment through fecal manuring (Weber 1972; Della Lucia 2011; Marti et al. 2015; Meirelles et al. De Fine Licht and Boomsma 2010). Because the ants 2016), and fungal exchange between attine ant nests depend on their fungal garden as primary food source, appears to be constrained by incompatibilities or low and because L. gongylophorus fungi have so far not been fitness of specific ant-fungus combinations (Seal et al. found growing independently of leafcutter ants (Mueller 2012, 2014a, 2014b). However, population-genetic stu- et al. 1998;Pagnoccaetal.2001; Mueller 2002; Bacci et al. dies revealed occasional horizontal exchange of fungal 2009;Voetal.2009), the leafcutter ant–fungus association cultivars between sympatric leafcutter ant species, as appears to be an obligate symbiosis for leafcutter fungi. well as rare cases of genetic admixture between L. CONTACT Ulrich Mueller [email protected] Alexis L. Carlson and Heather D. Ishak contributed equally to this work. Supplemental data for this article can be accessed on the publisher’s Web site. © 2018 The Mycological Society of America Published online 04 Jan 2018 MYCOLOGIA 833 gongylophorus strains (Adams et al. 2000; Mueller 2002; per cell in fungal symbionts of five species of Panamanian Mikheyev et al. 2006, 2007, 2010; Mueller et al. 2011a, leafcutter ants and that L. gongylophorus cells are highly and 2011b, 2017). obligatorily polyploid, with about 5–7different,heterokar- Although common and well studied in plants and some yotic nuclei per cell. Polyploidy and heterokaryosis in leaf- animal lineages, polyploidy—the coexistence of multiplied cutter fungi may boost metabolism (Kooij et al. 2015)or genomes in the same nucleus—is much less frequent and help compensate for evolutionary disadvantages stemming understudied in fungi (Campbell et al. 2016). Of the known from the apparent absence or rarity of sexual reproduction polyploid fungi, most derived from evolutionarily recent via meiospore production in leafcutter fungi (Mueller 2002; transitions to polyploidy, and only two fungal polyploids of Mikheyev et al. 2006;Scottetal.2009;Muelleretal.2011b, ancient evolutionary origin are known (Saccharomyces, 2017;Kooijetal.2015). However, leafcutter fungi actually Rhizopus)(Maetal.2009;AlbertinandMarullo2012; do exhibit low levels of population-genetic admixture Shelest and Voigt 2014;Campbelletal.2016). Among the (Mueller et al. 2011b), most likely through the occasional Basidiomycota, the known cases of polyploidy appear to be exchange of nuclei between anastomosing mycelia (Mueller of recent evolutionary origins, including both autopoly- et al. 2011b), but true recombinatorial exchange of genetic ploid origins (e.g., Ustilago, Armillaria, Cryptococcus)and material between nuclei also seems possible. Theory pre- allopolyploid origins (e.g., Heterobasidion, Armillaria) dicts that such rare facultative genetic exchange between (Campbell et al. 2016). Some Basidiomycota can also differentiated mycelia, rare recombination, or rare ploidy have diverse populations of differentiated nuclei in cycling can generate almost the same evolutionary advan- multinucleate cells, a state called heterokaryosis. Such mul- tages of obligatory sexual reproduction (Kondrashov 1994; tinucleate cells have been observed only a few times in Otto and Lenormand 2002). species such as Phanerochaete chrysosporium (Alic et al. The multiallelic microsatellite DNA genotypes reported 1987), Heterobasidion parviporum (James et al. 2008), and for L. gongylophorus (3–5allelestypicalperlocus,upto8 several species of Agaricus and Termitomyces (Saksena et al. alleles per locus; Scott et al. 2009;Muelleretal.2011b;Kooij 1976;DeFineLichtetal.2005;Diasetal.2008), as well as L. et al. 2015)likelyunderestimatethetruenumberofcoex- gongylophorus (Scott et al. 2009;Kooijetal.2015). The isting nuclei per cell, for two reasons. First, some nuclei may number of nuclei present in the vegetative mycelium of be genetically similar or identical to each other; and differ- these species varies greatly. For example, cytological studies ences between such nuclei would not be readily apparent in of Agaricus brasiliensis determined an average of 5.8 nuclei the microsatellite marker profiles, as reported by Kooij et al. per cell (Dias et al. 2008), whereas studies of Agaricus (2015). Second, microsatellite loci may contain null alleles bisporus revealed as many as 25 nuclei per cell (Saksena (nonamplifiable alleles lacking an appropriate primer site), et al. 1976). Both polyploidy and heterokaryosis appear as leading to an inherent underestimate of the true number of multiallele genotypes in genotyping analyses (e.g., genotyp- coexisting nuclei. Despite these potential complications, ing indicates more than two alleles per locus for a single- microsatellite genotyping of L. gongylophorus has been strain mycelium), and it is therefore a challenge to deter- useful to advance understanding of population-genetic mine whether the reason for multiallele genotypes derives structure (Mikheyev et al. 2007;Muelleretal.2011b, from polyploidy (multiple genomes per nucleus), from 2017), single-strain cultivation per nest (i.e., fungal mono- heterokaryosis (multiple, genetically differentiated nuclei culture; Mueller et al. 2010), symbiont choice (Sen et al. coexist in the same cell), or from both polyploidy and 2010), polyploidy and heterokaryon states (Kooij et al. heterokaryosis. 2015), quantitative genetics of cold tolerance (Mueller The earliest histological studies of L. gongylophorus et al. 2011a), as well as genetic admixture between closely mycelium revealed that these fungi lack clamp connections related L. gongylophorus strains (Sen et al. 2010;Mueller (Möller 1893; Hervey et al. 1977)andaremultinucleate, et al. 2011b). Although the evidence for occasional sexual or with more than two nuclei per cell (Hervey et al. 1977; parasexual reproduction in natural L. gongylophorus popu- Mohali 1998). Subsequent microsatellite marker analyses lations is compelling (Mueller 2002; Mikheyev et al. 2006; of L. gongylophorus from