Neurospora speciation through experimental evolution

ResearchBlogging.orgDettman, Anderson, and Kohn recently published a paper in BMC Evolutionary Biology on reproductive experimental evolution in two Neurospora crassa populations evolved under different selective conditions. This is a great study that complements work published last year in Nature on experimental evolution in Saccharomyces cerevisiae populations. Neurospora populations were evolved under high salt and low temperature and were started from either high diversity (interspecific crosses, N. crassa vs N. intermedia) or low diversity (intraspecific cross, two N. crassa isolates D143 (Louisiana, USA)and D69 (Ivory Coast)) as described in Figure 1. The experimentally evolved populations were then tested for asexual and sexual fitness (they were taken through complete meiotic cycle throughout the experiment to avoid insure there was selection on the sexual reproduction pathway.

D143 and D69 are both N. crassa NcA clade species so they should be quite similar to each other, hence the low diversity cross. While the N. intermedia cross represents two distinct phylogenetic species. and they are both See Dettman et al 2003 for the relationship between these strains.

Dettman BMC EvolBiol 2008, Figure 1

The authors were able to show that divergent selection through adaptation to different environments can lead to specialization that produced Dobzhansky-Muller genetic interactions. This is an example of ecological speciation. In other words, the adaptation to different environments can lead to new species purely on the basis of fixing alleles necessary for survival in the environments and not on the phenotype of reproductive isolation at all. When evolved individuals interact, the hybrids are less fit or even sterile even though the selected trait was only on a high salt or low temperature regime. Ecological speciation can arise from the decreased fitness of hybrids from differential selection of the environment. So this reproductive isolation is happening even though reproductive isolation was not selected for during the experiment suggesting that interactions between alleles which did not arise in the same genetic background are influencing the poor hybrid fitness (negative or antagonistic epistasis).

This work represents the first experimental evolution system for ecological speciation in a filamentous fungus and only the second in a fungus (the other fungal is the S. cerevisiae Dettman et al 2007 experiment). According to the authors, the only other previous work on experimental evolution to test ecological speciation models has been done in Drosophila.

I really like how this study uses the power of fungi as model systems for evolutionary and cellular biology and produces some clear support for ecological speciation model. In addition the ability to map mutations and sequence these strains will hopefully lead to an understanding of the molecular basis of the adaptations and the negative epistasis.

Dettman, J.R., Anderson, J.B., Kohn, L.M. (2008). Divergent adaptation promotes reproductive isolation among experimental populations of the filamentous fungus Neurospora. BMC Evolutionary Biology, 8(1), 35. DOI: 10.1186/1471-2148-8-35
Dettman, J.R., Sirjusingh, C., Kohn, L.M., Anderson, J.B. (2007). Incipient speciation by divergent adaptation and antagonistic epistasis in yeast. Nature, 447(7144), 585-588. DOI: 10.1038/nature05856

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