Genome organization and transmission genetics of arbuscular mycorrhizal fungi*

Understanding the organism's mode of reproduction is critical from the standpoint of practical applications because clonal and sexual organisms require dramatically different management regimes. The reproductive mode is also interesting from an evolutionary perspective. Molecular phylogeny and the fossil record date Glomeromycota to the Ordovician, and indicate that these fungi are one of the oldest terrestrial organisms. If the extant AM fungi are indeed exclusively clonal, and have been clonal throughout their evolutionary history, they would be one of the most ancient asexual multicellular lineages, and serve as a model for studying long-term evolutionary survival in the absence of sex. If, however, AM fungal populations turn out to undergo recombination, we will face a challenge of understanding how they accomplish genetic exchanges in the absence of sexual morphology.


Organization of within-individual genetic variation in AM fungi

In the absence of
sexual morphology, cryptic sexuality can be discovered using tools of population genetics and phylogenetics to detect a signature of recombination among polymorphic genetic markers in a population. However, such an analysis requires a good understanding of within-individual organization of genetic variation.
Cells and spores of AM fungi harbor hundreds of nuclei and contain uncommonly polymorphic copies of ribosomal RNA genes. This unusual combination of traits caused uncertainty how these rDNA polymorphisms are partitioned among AM fungal nuclei. There are two hypotheses: (1) AM fungi are heterokaryotic, i.e. contain many types of dissimilar nuclei within a single cell or spore, (2) AM fungi are homokaryotic, i.e. all within-individual genetic variation is contained in each nucleus.
To test the hypothesis of heterokaryosis, we studied the transmission of polymorphic genetic markers in natural isolates of an AM fungal species, Glomus etunicatum, in the field and in clonally propagated laboratory cultures. We also developed a technique to directly PCR amplify rDNA variants from individually microdissected nuclei. Our results showed that, contrary to the heterokaryosis hypothesis, nuclei populating AM fungal individuals are genetically uniform. Consequently, the within-individual rDNA polymorphism is contained in each nucleus. Such intranuclear rDNA polymorphism indicates a relaxation of concerted evolution, the recombination-driven process responsible for homogenization of rDNA repeats within an individual and among the individuals of a recombining population in most other organisms.


Genome organization in AM fungi

Intranuclear polymorphism of rDNA repeats is an anomaly indicating that the repeat homogenizing process of concerted evolution does not operate efficiently or that the process of concerted evolution has been disrupted by a recent hybridization event bringing together rDNA arrays from different lineages or species. To better understand the causes underlying the molecular pattern of relaxed concerted evolution in AM fungi, we are studying their genome structure.


Reproductive mode of AM fungi


Armed with the understanding of organization of within-individual genetic variation in AM fungi, we are studying their reproductive mode in nature using methods of population genetics and phylogenetics.


Publications

VanKuren, N.W., H.C. den Bakker, J.B. Morton and T.E. Pawlowska. 2012. Ribosomal RNA gene diversity, effective population size, and evolutionary longevity in asexual Glomeromycota. Evolution DOI: 10.1111/j.1558-5646.2012.01747.x. Abstract
den Bakker H.C., N.W. VanKuren, J.B. Morton and T.E. Pawlowska. 2010. Clonality and recombination in the life history of an asexual arbuscular mycorrhizal fungus. Molecular Biology and Evolution 27(11): 2474-2486. Abstract

Jany J.-L. and T.E. Pawlowska.  2010. Multinucleate spores contribute to evolutionary longevity of asexual Glomeromycota.  American Naturalist 175: 424–435. Abstract. Nature Research Highlights.

Pawlowska T.E.  2010.  Population genetics of fungal mutualists of plants.  In Microbial Population GeneticsEd. J. Xu. Horizon Scientific Press.

Pawlowska T.E.  2007.  How the genome is organized in the Glomeromycota.  In Sex in Fungi: Molecular Determination and Evolutionary Implications.  Eds. J. Heitman, J.W. Kronstad, J.W. Taylor and L.A. Casselton.  ASM Press.  Washington, D.C.  pp. 419-430.

Pawlowska, T.E. 2005. Genetic processes in arbuscular mycorrhizal fungi. Minireview. FEMS Microbiology Letters 251: 185-192. PDF


*Funded by NSF MCB-0538363.

Home Lab personnel Publications Contact us Joining the lab Links
  • Copyright © 2004-2010 Cornell University