Evaluación del intercambio de genes entre micovirus y sus huéspedes: el caso de la proteína de la envuelta viral

Abstract

[ES] Los micovirus son virus que infectan a hongos y son una de las clases menos conocidas de virus hasta la fecha. La gran mayoría de los micovirus identificados son virus de RNA y dentro de estos predominan aquellos de clase III, con un genoma dsRNA. El interés de conocer los micovirus radica en su uso como agentes de biocontrol de plagas fúngicas que pueden afectar a los cultivos o precisamente para mantener buenas medidas de higiene en cultivos de champiñones y otros hongos comestibles. A nivel básico, el estudio filogenético de los micovirus, como del resto de virus, nos permite hacernos una idea de su ecología y de cómo han coevolucionado con sus huéspedes.

Este estudio persigue comprobar la hipótesis de que el genoma de los micovirus es un mosaico compuesto de una región propiamente viral, donde se encuentra el gen de la RNA replicasa dependiente de RNA (RdRP), y otra de origen eucariota, que contendría el gen de la proteína de la envuelta (CP), que en algún momento de la evolución pudo haberse transferido horizontalmente desde un hospedador fúngico a un virus ancestral.

Para ello, se reconstruyeron árboles filogenéticos con las secuencias proteicas RdRP y CP de distintos grupos de micovirus de dsRNA, haciendo coincidir las especies de RdRP y CP para poder hacer una evaluación de la cofilogenia, es decir, en qué medida ambos loci explican la misma historia evolutiva. Los árboles separaban los mismos grupos, pero en el árbol de CP no se consiguen establecer relaciones entre grupos que sí se observan con RdRP.

La búsqueda PSI-BLAST de la CP de un grupo de micovirus no clasificados de dsRNA reveló una gran similitud de secuencia con una proteína hipotética de hongos presente en numerosas especies, mayor a la que presenta la CP de este grupo con la de otros micovirus. La comparación entre las estructuras simuladas in silico de estas proteínas fúngicas y la CP de este grupo de virus no clasificados mostró una importante homología. Estos datos, en conjunto, sugieren que este gen eucariota, de función desconocida en el hongo, se transfirió a un antepasado de estos virus, adoptando una nueva función como CP. Eventos de transferencia de este tipo pueden ser responsables de que no se puedan establecer las mismas relaciones con los árboles de CP y RdRP.

[EN] Viruses that infect fungi are known as mycovirus. Recently, more and more mycovirus species are being identified and having their genomes sequenced. Nevertheless, the majority of mycoviruses are yet to be discovered. By now, most of known mycoviruses have RNA-based genomes, being most of these dsRNA viruses or class III viruses, according to the Baltimore classification. Main applications of having an integrated knowledge of mycoviruses and their environment comprehend their use as biocontrol agents against crop¿s fungal pathogens and their exclusion from edible mushroom cultivations. A phylogenetic approach to mycoviruses would yield many information at a fundamental level involving mycoviral evolution and their relation to eukaryotic species ecologically related i.e. mainly the fungi that serve them as hosts.

This study¿s main hypothesis is that of the mycoviral genome having a mosaic structure, this means the genome having two origins, one viral and other eukaryotic, the viral part comprising the RNA-dependent RNA polymerase (RdRP) gene and the eukaryotic part being composed of the coat protein (CP) gene, which might have been transferred to an ancestral virus from its fungal host.

For this purpose, two phylogenetic trees were reconstructed for RdRP and CP, using sequences of the same virus species on each side to compare the results and address the question of both loci explaining the same evolutionary history. The CP tree was not able to find relationships between virus groups as the RdRP tree did, although the same final groups were segregated with both trees.

A PSI-BLAST search with the CP sequence of the different mycovirus groups included in the study was able to find an hypothetical protein with many orthologues in fungal genomes which showed a very significant sequence homology with the CP of a unclassified dsRNA mycovirus group. Some structures of different gruoups¿ CP and this hypothetical protein were predicted in silico to establish relationships that could not be seen working only with sequences. The CP of this unclassified mycovirus group was distinct to others¿ and more similar to this fungal hypothetical protein. Both results suggest that this eukaryotic gene is a strong candidate to have been transferred to this viruses¿ ancestor and to have replaced the native viral CP of this group. This and other plausible horizontal gene transfers between mycoviruses and their hosts that were not seen in this study could explain at least partly the high divergence of groups observed in the CP tree for viruses related in RdRP.