Allelic Differences within and among Sister Spores of the Arbuscular Mycorrhizal Fungus Glomus etunicatum Suggest Segregation at Sporulation

Abstract

Arbuscular mycorrhizal fungi (AMF) are root-inhabiting fungi that form mutualistic symbioses with their host plants. AMF are made up of coenocytic networks of hyphae through which nuclei and organelles can freely migrate. In this study, we investigated the possibility of a genetic bottleneck and segregation of allelic variation at sporulation for a low-copy Polymerase1-like gene, PLS. Specifically, our objectives were (1) to estimate what allelic diversity is passed on to a single spore (2) to determine whether this diversity is less than the total amount of variation found in all spores (3) to investigate whether there is any differential segregation of allelic variation. We inoculated three tomato plants with a single spore of Glomus etunicatum each and after six months sampled between two and three daughter spores per tomato plant. Pyrosequencing PLS amplicons in eight spores revealed high levels of allelic diversity; between 43 and 152 alleles per spore. We corroborated the spore pyrosequencing results with Sanger- and pyrosequenced allele distributions from the original parent isolate. Both sequencing methods retrieved the most abundant alleles from the offspring spore allele distributions. Our results indicate that individual spores contain only a subset of the total allelic variation from the pooled spores and parent isolate. Patterns of allele diversity between spores suggest the possibility for segregation of PLS alleles among spores. We conclude that a genetic bottleneck could potentially occur during sporulation in AMF, with resulting differences in genetic variation among sister spores. We suggest that the effects of this bottleneck may be countered by anastomosis (hyphal fusion) between related hyphae.

Evolutionary maintenance of genomic diversity within arbuscular mycorrhizal fungi

“…Fusion ( anastomosis ) promotes the (proximate) spread of new genomes through populations of individuals (Croll et al, ; de Novais et al, ), but destabilizes genomic diversity in evolutionary time by reducing variation between individuals for selection to act on. Stochasticity in sporulation can result in the (proximate) loss of genomic diversity over a generation (Angelard et al, ; Boon et al, ; Marleau et al, ; Masclaux et al, ), though it stabilizes genomic diversity in evolutionary time by increasing variation between individuals. Other possible proximate influencers of genomic diversity include the following: de novo mutations; the restriction of fusion to close kin ( allorecognition ) (Czárán et al, ); and genetic exchange between nuclei (Chen et al, ; Croll & Sanders, ).…”

“…In other words, we would expect within-individual selection to lead to genomic purity (Gilbert, Foster, Mehdiabadi, Strassmann, & Queller, 2007;Inglis, Ryu, Asikhia, Strassmann, & Queller, 2017;Kooij, Aanen, Schiøtt, & Boomsma, 2015;Meunier, Hosseini, Heidari, Maryush, & Johannesson, 2018;Vreeburg, Nygren, & Aanen, 2016). Within-individual evolution would eventually lead to genomic purity even if nuclei are equally competitive, through drift, because not all nuclei migrate from parent hyphal networks into daughter cells (Angelard et al, 2010;Boon, Zimmerman, St-Arnaud, & Hijri, 2013;Marleau, Dalpé, St-Arnaud, & Hijri, 2011;Masclaux, Wyss, Mateus-Gonzalez, Aletti, & Sanders, 2018). Secondly, we would expect genomic variation within individuals to lead to conflict among different genomic (nuclear) lineages and hence reduce the fitness of that individual.…”

Considering Intra-individual Genetic Heterogeneity to Understand Biodiversity

“…Arbuscular mycorrhizal fungi (AMF) are an ancient phylum of heterotrophs that form symbioses with the majority of land plants (Wang and Qiu 2006). AMF were reported to contain hundreds or thousands of genetically differentiated nuclei within the same cytoplasm (Kuhn et al 2001;Hijri and Sanders 2005;Boon et al 2010Boon et al , 2013bBoon et al , 2015. The exact number of genetically differentiated nuclei is not clear, since genetic variation has never been exhaustively sampled for any locus in these fungi.…”

“…For example, spores of Rhizophagus irregularis do not germinate under a certain volume, which is positively correlated with the number of nuclei within the spore (Marleau et al 2011). Some authors have proposed that genetic differentiation between nuclei within the AMF cytoplasm is maintained through the fusion of related hyphae, or anastomosis (Giovannetti et al 2015 and references therein), and is lost at sporulation (Boon et al 2013b). This means that AMF are both mosaic individuals in the sense that their nuclei share the same genealogy, and chimeras in the sense that at least some of this variation is the result of hyphal fusion between related hyphal systems.…”

Genome diversity in arbuscular mycorrhizal fungi

“…By contrast, most of the studies of nuclear diversity have involved other isolates, which may be very different. Indeed, many studies [20][21][22]30] concern Claroideoglomus, which has been diverging from Rhizophagus for almost 500 million years [8], so may differ in fundamental ways. The studies of diversity in R. irregularis have not used DAOM197198 but other, potentially less 'domesticated', isolates, and in some cases have deliberately enhanced variation through anastomosis [23,25 ,31].…”