Comment on “Conserved meiotic machinery in Glomus spp., a putatively ancient asexual fungal lineage” by S. Halary et al. 2011.
The origin and significance of sex remains one of the most intriguing topics in evolutionary biology. Sexual reproduction requires a big deal of complexity and energy expense at all levels, from the intricate molecular gymnastics needed to achieve meiotic recombination between homologous DNA strands in the soon-to-be gamete cells, to the other, more mundane kind of gymnastics. Asexual reproduction is simpler and less costly, and its efficiency at making more individuals is more than proven by the myriads of bacteria that have reigned over the Earth for eons. But it appears that, experts say, that all that energy is well spent. Lineages of organisms engaged in sexual reproduction are, in addition to happier, less prone to accumulate deleterious mutations in their genomes, which ultimately would lead to an unbearable burden of genetic defects and extinction by boredom. But in spite of the well-known benefits of this ancient habit (Costa & Brody 2011), several eukaryotic lineages have managed to survive over long evolutionary periods of time without sex. Living examples of asexual lineages are bdelloid rotifers (tiny aquatic animals), some mycorrhizal fungi and various kinds of protists. Explanations for this aberrant, excessively chaste lifestyle have been attempted, in some cases appealing to some sort of “parasexual” processes resulting in degrees of recombination that alleviate the accumulation of deleterious mutations over time. Another possibility is that those apparently asexual organisms do have the ability to undergo meiotic recombination, and when nobody is looking, have sex. In some cases, the effects of meiotic recombination can be observed in populations of allegedly asexual organisms. Moreover, the genomes of several parasitic protists long believed to be asexual have been found to contain the battery of genes needed for the meiotic process.
One of the lineages believed to have gone through asexual reproduction for millions of years are the arbuscular mycorrhizal fungi. These fungi constitute
a poorly recognized but extremely important component of terrestrial ecosystems. They live in close association with the roots of many land plants in a symbiotic relationship that results absolutely essential for both, the plant and the fungus. In this study, led by Nicolas Corradi from the University of Ottawa, heaps of genomic data from four species of the mycorrhizal fungi Glomus were carefully sieved and analysed for evidence of meiotic machinery. By comparing the Glomus data with sequences of known meiotic genes from the baker’s yeast Saccharomyces cerevisiae, 51 genes encoding proteins involved in meiosis were found. This set of genes would be sufficient to account for the entire process of meiotic recombination. Both the completeness of the pool of genes, and the degree of conservation of the encoded proteins strongly suggest that these proteins conform and active, yet so far undetected by direct means, meiotic or meiotic-like stage in the life cycle of Glomus. In addition, the genome sequences of the Glomus species exhibit numerous transposons and also genetic evidence of recombination in populations. These features, combined with the suite of conserved meiotic recombination genes, are compelling indicators of sexual reproduction.
Glomus mycorrhizal fungi appear to have become yet another organism that, rather than asexual, are just shy.
The authors of the study are Sébastien Halary and Mohamed Hijri from the Université de Montréal, Shehre-Banoo Malik and Claudio H. Slamovits from Dalhousie University and Levannia Lildhar and Nicolas Corradi from the University of Ottawa.
The paper was published in Genome Biology and Evolution and can be downloaded here.