Ribosomal Rna Gene Diversity, Effective Population Size, and Evolutionary Longevity in Asexual Glomeromycota

Abstract

Arbuscular mycorrhizal fungi (phylum Glomeromycota) are among the oldest and most successful symbionts of land plants. With no evidence of sexual reproduction, their evolutionary success is inconsistent with the prediction that asexual taxa are vulnerable to extinction due to accumulation of deleterious mutations. To explore why Glomeromycota defy this prediction, we studied ribosomal RNA (rRNA) gene evolution in the Claroideoglomus lineage and estimated effective population size, Ne, in C. etunicatum. We found that rRNA genes of these fungi exhibit unusual and complex patterns of molecular evolution. In C. etunicatum, these patterns can be collectively explained by an unexpectedly large Ne combined with imperfect genome‐wide and population‐level rRNA gene repeat homogenization. The mutations accumulated in rRNA gene sequences indicate that natural selection is effective at purging deleterious mutations in the Claroideoglomus lineage, which is also consistent with the large Ne of C. etunicatum. We propose that in the near absence of recombination, asexual reproduction involving massively multinucleate spores typical for Glomeromycota is responsible for the improved efficacy of selection relative to drift. We postulate that large effective population sizes contribute to the evolutionary longevity of Glomeromycota.

Phylogenetically Structured Differences in rRNA Gene Sequence Variation among Species of Arbuscular Mycorrhizal Fungi and Their Implications for Sequence Clustering

“…Previous studies also documented a lack of genetic distinction between Claroideoglomus species (27, 28) but did not include E. infrequens in their analyses, nor has the most comprehensive AM fungal rRNA gene tree (25). In Claroideoglomus species, previous work demonstrated the presence of two evolutionary lineages in LSU rRNA gene sequences ("L" and "S" variants) (28). The PCR primer set used here amplified only L sequence variants due to primer site mismatches with S variants.…”

“…Previous studies that sampled a limited amount of rRNA gene sequence variation found that sequences from morphologically defined species generally formed clades on the gene tree (24)(25)(26), which is an essential prerequisite for clustering sequences into species-level OTUs. However, it is unclear whether this correspondence will remain robust to the sampling of additional sequence variation in AM fungi, as other work has called it into question for species of the genus Claroideoglomus (27,28).…”

Evidence of non-tandemly repeated rDNAs and their intragenomic heterogeneity inRhizophagus irregularis

“…Thus, we present here an improved whole-genome sequence of R. irregularis DAOM-181602 to facilitate examination of the genomics underlying specific features of AMF. In this paper, we focused on rDNA which is a key component used in many AMF studies including heterokaryosity 6,12,26,27 , phylogeny 35-37 , ecology 8,37,38 , and evolution 39 .…”

A combination of morphology and 28S rRNA gene sequences provide grouping and ranking criteria to merge eight into three Ambispora species (Ambisporaceae, Glomeromycota)

“…Linked rRNA gene polymorphisms are present in C. etunicatum strains that form a distinct clade, but they do not disrupt monphyly of the species (VanKuren et al 2013). The polymorphisms appear to be maintained in disjunction nucleoli, with concerted evolution occurring within each localized rRNA gene array.…”

“…Some phenotypic differences (novelties), no matter how stable or discrete they might be, do not reflect speciation events at all but instead indicate fixation of character variants that evolved as disjunct clonal populations (Morton and Msiska 2010a, b;vanKuren et al 2013). One example is intercalary spore formation, which was considered unique to G. fecundisporum.…”