Should we be worried when two fungal pathogens are exchanging genes?
Time for something completely different. During my weekly search for papers on hybridization I came across a paper in Evolution Letters, entitled “Recent admixture between species of the fungal pathogen Histoplasma.” This spurred (not spored) my interest and I decided to dedicate a blog post to it. So, get ready for some fungi hybrids!
The Emergence of Virulence
First, a note on health. Studying hybridization in pathogens can help scientists understand the emergence of virulence (i.e. a pathogen’s ability to infect or damage a host). Exchanging genes through hybridization can influence virulence in two ways. First, disease-causing organisms can transfer their genes to non-pathogenic species which can consequently become virulent. Second, related species often have different strategies to infect their hosts. Hybrids can inherit both strategies, making them extra virulent. I guess the relevance of studying hybridization in pathogens is crystal clear…
The current study focused on Histoplasma, a fungus that causes the lung disease histoplasmosis. It is one of the most common pulmatory diseases and is particularly prevalent in HIV-patients, often resulting in death. There are at least four species of Histoplasma, two in South America (H. suramericanum and H. capsulatum) and two in North America (H. ohiense and H. mississippiense). Colin Maxwell (University of North Carolina) and his colleagues focused on the North American species, which use different strategies to infect their hosts*.
Previous work already indicated that these two species were exchanging genetic material. The current study confirmed these findings and explored which parts of the genome have been transferred. It turned out that intergenic sequences were over-represented in the exchanged material. These are DNA sequences that lie between functional genes. This suggests that most of the exchanged material is deleterious or lethal in the hybrids. It seems that we should not worry about the virulence of these hybrids. But the authors do provide a word of caution:
The introgressed regions in hybrid Histoplasma are enriched for those alleles that would seem least likely to affect virulence. However, gene exchange does occur, and our results do not rule out alleles that do affect virulence.
Maxwell, C.S., Sepulveda, V.E., Turissini, D.A., Goldman, W.E. & Matute, D.R. (2018) Recent admixture between species of the fungal pathogen Histoplasma. Evolution Letters 2(3), 210-220.
* For readers interested in the technical details: H. ohiense uses α-(1,3)-glucan in its cell wall to avoid immune recognition by the host, while H. mississippiense uses Yps3p.