Tuesday, July 31, 2012

A Dermatophytic Divergence

ResearchBlogging.orgYour skin is a vast and complex habitat. There are all kinds of bacteria and fungi competing for every inch of real estate. But like all homesteads, resources are limited. Bacteria lower surface area pH rendering it inhospitable for others, and secrete chemical compounds to kill invaders. Fungi have found ways to more efficiently mine the "land" for minerals such as iron, as well as producing antibiotics that are specifically active against skin bacteria.

One of the families of fungi capable of infecting skin (Dermatophytes) is Arthrodermataceae, A family that includes the genera Microsporum and Trichophyton. These two fungi groups colonize keratinized areas and together are the most common cause of superficial fungal skin infections.

 A study in the July 30, 2012 edition of PLoS One takes a look at some of the closely related genes, gene clusters, that allow the species from Microsporum and Trichophyton to take such good advantage of the terrain that is your skin.

Two Different Secondary Metabolism Gene Clusters Occupied the Same Ancestral Locus in Fungal Dermatophytes of the Arthrodermataceae

In essence gene clusters are several genes that are physically linked or clustered together that share a common effect, such as production of antibiotics, or metabolically important compounds. Gene clusters can allow for quicker adaptation to new sources of sustenance as well as large scale genome remodeling. Due to the closely knit nature of gene clusters they also open the possibility of acquiring new functions by gain and loss of entire gene pathways through horizontal transfer.

Arthrodermataceae have a lot of gene clusters that many think are involved in host specificity and pathogenicity, especially when compared to other dermatophytes. Focusing on variable loci nestled inside a stable portion of the genome of Microsporum canis, Microsporum gypseum, and Trichophyton spp.; the researchers discovered three distinct conformation forms.

This variable locus(VL) has a difference of both length and functions across the three species. M. canis VL (VLA) consists of 539 base pairs that lack any protein sequencing sequences, M. gypseum (VLB) only has 35.89 bp but codes 12 different proteins, and Trichophyton spp.'s VL (VLC) is 26.78 bp long coding for 10 proteins.

When looking at the evolutionary past of the gene region it becomes apparent that the studied clusters have a very different history than the areas surrounding them. Those flanking regions have a largely vertical inheritance while the VL genes have been shaped by several different processes, including gene duplications and gene transfers.

The VL's of M. gypseum and Trichophyton spp. contain genes to produce compounds that appear to target glycine, which is the largest amino acid in human skin. This means the VL may be involved in skin colonization and thus infection. The effects of this area along with the evidence of gene duplications and losses led the team conclude that the common ancestor of these dermatophytes shared either a polymorphic version of VLB and VLC or both of them separately.

If the first scenario is correct, the common ancestor originally contained a version of the VL that alternated between the VLB and VLC form. During the evolution following that point the entire area was deleted from M. canis and the alleles were separated into the stable forms in M. gypseum and Trichophyton spp.

If the secondary version is true than at different times the main lineages came into their current genomic distributions by separate deletions in and of the VL.

It would take additional genome sequencing of closely related species in Arthrodermataceae, to determine the more likely of these two scenarios.

In the end, the mere existence of variable loci, like those in this study, could help explain how closely related species could show a dramatic difference in ability to establish an epidermal foothold.

Awesome Researchers:
Han Zhang, Antonis Rokas, & Jason C. Slot (2012). Two Different Secondary Metabolism Gene Clusters Occupied the Same Ancestral Locus in Fungal Dermatophytes of the Arthrodermataceae PLoS One DOI: 10.1371/journal.pone.0041903

Photo creds: 
Robert J. Galindo & the CDC via Wikimedia

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