Looking for papers and comments about horizontal transfer occurrence in aphids for a lecture at the University of Parma (Italy), I found a post in the blog I love insects that linked to the famous paper published in 2010 by Nancy Moran reporting that aphids are the first animals able to produce carotenoids.
Carotenoid synthesis in aphids is feasible owing to horizontal transfer of biosynthesis genes from fungi into the insect genome. Both pea aphids Acyrthosiphon pisum (in green in the photo) and green peach aphids Myzus persicae harbour horizontally transferred genes encoding carotenoid desaturases and carotenoid cyclase–carotenoid synthase. These pigments underlie pea aphids’ red and green body colour polymorphism, which affects interactions with aphid natural enemies, including wasps.
Recently, aphids lost their uniqueness since Altincicek and colleagues provided evidence that the spider mite Tetranychus urticae (in red in the photo) harbours at least one carotenoid desaturase and one carotenoid cyclase–carotenoid synthase gene of fungal origin. Both genes are more highly expressed.
As a whole, these two papers demonstrate that carotenoid biosynthesis genes have been transferred multiple times into the genomes of higher animals. Given the ubiquity of carotenoids in arthropods, and the protective benefits provided by carotenoids, it is likely that these genes are present in the genomes of other arthropods as well. The knowledge that even complex biosynthetic pathways can be gained by horizontal gene transfer changes our understanding of evolutionary processes underlying animal adaptation.
As reported by Altincicek and colleagues ”while transfer of these genes could have occurred from fungi into a single arthropod ancestor of both spider mites and aphids, this scenario would require subsequent loss of these genes in countless extant arthropod taxa. Alternatively, horizontal transfer could have occurred directly between aphids and spider mites, or through some microbial intermediary. Such a sequential transfer could involve organisms like Wolbachia bacteria, which are known to infect both animal groups and which are involved in diverse horizontal gene transfer events in arthropods. It is perhaps more likely that these arthropods acquired genes from fungal symbionts, either beneficial or pathogenic, independently; both aphids and spider mite populations are frequently infected with fungal pathogens. Interestingly, it has been suggested that the phoretic and parasitic feeding behaviours of many mite species, including spider mites, coupled with their frequent association with viruses, make them ideal horizontal gene transfer vectors, and may also result in the incorporation of mobile genes into their own genomes”.
The multiple transfer of carotenoid biosynthesis is undoubtedly the most parsimonious scenario, but the transfer of the same genes in multiple species in different times makes very intriguing not only the study of the host arthropod genomes, but also of the fungi in order to identify the possible mechanism that could favour such a transfer from fungi.
Moran N.A., Jarvik T. (2010) Lateral transfer of genes from fungi underlies carotenoid production in aphids. Science 328: 624-7.
Altincicek B., Kovacs J.L., Gerardo N.M. (2012) Horizontally transferred fungal carotenoid genes in the two-spotted spider mite Tetranychus urticae. Biol Lett. in press.
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