Why do we see so much infection in some places, some hosts, some species, and much less in others? What are the ecological and genetic drivers of this variation?
I also asked these questions for the fungus Microbotryum violaceum, which causes anther-smut disease in the Caryophyllaceae family (learn more here). To briefly summarize here:
Microbotryum is absent from rare species in the Silene genus, and this seems to be due to limited transmission opportunities in small, fragmented populations. We find that this pattern is quite general: federally endangered plant taxa have lower fungal pathogen richness than non-endangered relatives (Gibson et al. 2010 Oikos).
Anther smut is also noticeably absent from annual species – 80% of perennial Silene host species are host to M. violaceum, while no annual Silene species is known to naturally host the disease (Thrall et al. 1993, Hood et al. 2010). We showed that annuals can acquire the disease via direct inoculation – in other words, they’re susceptible to infection. Their lack of infection in the wild is thus very likely to be a direct result of their ecology. The life cycle of Microbotryum is such that the annual habit prevents transmission. We also show that this “ecological protection” selects against other forms of costly resistance in annual taxa (Gibson et al. 2013 Ecol Evol).
Silene-Microbotryum is also a fascinating system to study host shifts, hybridization, and speciation. These are particularly important topics in fungal pathogens – hybridization has led to the formation of new fungal species that emerge to devastate novel hosts (e.g. strains of Batrachochytrium dendrobatidis causing amphibian chytrid epidemics). Gibson et al.(2012 Evolution) shows that host shifts, hybridization and speciation are limited by 1) selfing in M. violaceum and 2) competition between selfed and hybrid lineages (which the hybrid tends to lose). Gibson et al. (2014 Int J Plant Sci) shows that hybrids in M. violaceum don’t necessarily have novel host ranges – rather, there’s a clear host-parasite genotype interaction that determines hybrid success.
References
- Hood M.E., J.I. Mena-Alí, A.K. Gibson, B. Oxelman, T. Giraud, R. Yockteng R, M.T.K. Arroyo, F. Conti, A.B. Pedersen, P. Gladieux, and J. Antonovics. 2010. Distribution of the anther-smut pathogen Microbotryum on species of the Caryophyllaceae. New Phytol. 2010;187:217–229.
- Thrall P.H., A. Biere , and J. Antonovics J. 1993. Plant life-history and disease susceptibility–the occurrence of Ustilago violacea on different species within the Caryophyllaceae. J Ecol 81:489–498.
the Gibson Lab 