Coexistence of genetically different Rhizophagus irregularis isolates induces genes involved in a putative fungal mating response

Abstract

Arbuscular mycorrhizal fungi (AMF) are of great ecological importance because of their effects on plant growth. Closely related genotypes of the same AMF species coexist in plant roots. However, almost nothing is known about the molecular interactions occurring during such coexistence. We compared in planta AMF gene transcription in single and coinoculation treatments with two genetically different isolates of Rhizophagus irregularis in symbiosis independently on three genetically different cassava genotypes. Remarkably few genes were specifically upregulated when the two fungi coexisted. Strikingly, almost all of the genes with an identifiable putative function were known to be involved in mating in other fungal species. Several genes were consistent across host plant genotypes but more upregulated genes involved in putative mating were observed in host genotype (COL2215) compared with the two other host genotypes. The AMF genes that we observed to be specifically upregulated during coexistence were either involved in the mating pheromone response, in meiosis, sexual sporulation or were homologs of MAT-locus genes known in other fungal species. We did not observe the upregulation of the expected homeodomain genes contained in a putative AMF MAT-locus, but observed upregulation of HMG-box genes similar to those known to be involved in mating in Mucoromycotina species. Finally, we demonstrated that coexistence between the two fungal genotypes in the coinoculation treatments explained the number of putative mating response genes activated in the different plant host genotypes. This study demonstrates experimentally the activation of genes involved in a putative mating response and represents an important step towards the understanding of coexistence and sexual reproduction in these important plant symbionts.

Reciprocal recombination reflects sexual reproduction in symbiotic arbuscular mycorrhizal fungi

“…In contrast, in parasexual recombination, we would expect non-symmetrical recombination patterns leading to loss of heterogeneity between the two different nucleotypes (Forche et al ., 2008). This result, coupled with the experimental evidence of molecular activation of genes related to mating in AMF (Mateus et al ., 2020), further suggests sexual reproduction in AMF.…”

“…It could be plausible that parasexual recombination could lead to a reciprocal recombination pattern by fusion of two monokaryons, mitotic reciprocal recombination, followed by reciprocal chromosome loss. However, the presence of a complete meiotic machinery, the presence of compatible putative MAT-types, and the upregulation of genes involved in a putative mating response when two isolates co-exist (Mateus et al , 2020, 2021), suggest that a plausible hypothesis explaining the results observed in this study is a reciprocal recombination pattern detected in dikaryons, the result of sexual reproduction.…”

“…However, to date there is no direct experimental evidence that different putative MAT-types could be compatible. An indirect evidence of compatibility between different putative MAT-types was found, when two different isolates harboring different putative MAT-types elicited a putative fungal mating response (Mateus et al , 2020, 2021). However, Mateus et al, did not test if the mating of two co-existing isolates did took place (Mateus et al , 2020).…”

“…An indirect evidence of compatibility between different putative MAT-types was found, when two different isolates harboring different putative MAT-types elicited a putative fungal mating response (Mateus et al , 2020, 2021). However, Mateus et al, did not test if the mating of two co-existing isolates did took place (Mateus et al , 2020). In consequence, in order to experimentally identify mating and MAT-type compatibility, crossing experiments between strains harboring different putative MAT-types, including transcriptome and recombinant progeny analyses should be performed.…”

“…However, evidence suggests that sexual reproduction could be possible in AMF because these fungi contain a complete meiosis machinery (Halary et al ., 2011). Furthermore, a putative MAT-locus has been proposed (Ropars et al ., 2016), population genetic data suggests the existence of recombination in AMF populations (Croll & Sanders, 2009) and activation of genes related to mating has been detected when different isolates of the same species co-exist in plant roots (Mateus et al ., 2020). Inter-nucleus recombination has been reported (Chen et al ., 2018a), although the robustness of the analysis has been questioned.…”