[en] The spread of Ophiostoma novo-ulmi across Europe, North America and central Asia, resulted in the current, highly destructive Dutch elm disease (DED) pandemic, replacing O. ulmi, responsible for the first DED pandemic in the early 1900s. This process has resulted in a series of remarkable evolutionary and adaptive developments. Studies of O. novo- ulmi populations in the 1980s, especially in Spain and Portugal, showed the following: 1) that O. novo-ulmi initially spread across Europe as a series of genetic clones; 2) that deleterious RNA viruses were transmitted within the O. novo-ulmi clones; 3) that natural hybrids between O. novo-ulmi subspecies americana and subsp. novo-ulmi, emerged widely across Europe; 4) that there has also been a widespread emergence, across Europe, of natural hybrids between O. novo-ulmi subspecies americana and also subsp. novo-ulmi. The factors driving these changes have been examined by molecular analysis. Results show: 1) that the rapid change from clonality to genetic variability involved the acquisition of 'useful' mating type, vegetative compatibility type and other genes by O. novo-ulmi from O. ulmi via lateral (or interspecies) gene transfer; whereas 'unuseful' O. ulmi genes were discarded; 2) that the RNA viruses occurring in the O. novo-ulmi populations probably originated from O. ulmi; and 3) and that where O. novo-ulmi subsp. americana and subsp. novo-ulmi co-exist, natural hybrids are occurring very freely; in some areas most O. novo-ulmi isolates are already complex subspp. americana x novo-ulmi hybrids. These phenomena features are unique, and have considerable implications for the invasion history, successful spread and future behaviour of O. novo-ulmi.

[es] La expansi&oacute;n de Ophiostoma novo-ulmi en Europa, Norteam&eacute;rica y Asia central provoc&oacute; la actual pandemia de grafiosis, altamente destructiva, y reemplaz&oacute; a O. ulmi, responsable de la primera pandemia de grafiosis a principios del siglo XX. Este proceso ha provocado una serie de destacables desarrollos evolutivos y adaptativos. Los estudios realizados en la d&eacute;cada de 1980 en poblaciones de O. novo-ulmi, especialmente en Espa&ntilde;a y Portugal, mostraron lo siguiente: 1) que inicialmente O. novo-ulmi se expandi&oacute; a trav&eacute;s de Europa como una serie de clones gen&eacute;ticos; 2) que virus delet&eacute;reos de RNA se trsmitieron dentro de los clones de O. novo-ulmi; 3) que h&iacute;bridos naturales entre las subespecies americana y novo-ulmi de O. novo-ulmi aparecieron en muchas zonas de Europa; 4) que en toda Europa est&aacute; surgiendo un gran n&uacute;mero de h&iacute;bridos naturales entre las subespecies americana y novo-ulmi de O. novo-ulmi. Los factores conducentes a estos cambios han sido examinados mediante an&aacute;lisis molecular. Los resultados son: 1) que el r&aacute;pido paso desde una situaci&oacute;n de clonalidad a otra con gran variabilidad gen&eacute;tica supuso la aparici&oacute;n de formas &laquo;utiles&raquo; para el apareamiento, formas con compatibilidad vegetativa, y otros genes de O. novo-ulmi por transferencia lateral (o interespec&iacute;fica) a partir de O. ulmi; mientras que se descart&oacute; la presencia de genes &laquo;in&uacute;tiles&raquo; de O. ulmi; 2) que los virus ARN presentes en las poblaciones de O. novo-ulmi se originaron probablemente a partir de O. ulmi; y 3) que en los lugares donde O. novo-ulmi subsp. americana y subsp. novo-ulmi coexisten, los h&iacute;bridos naturales se generan libremente; en algunas &aacute;reas la mayor parte de los O. novo-ulmi aislados son en realidad complejos h&iacute;bridos subsp. americana x subsp. novo-ulmi. Estas caracter&iacute;sticas son &uacute;nicas, y tiene considerable implicaciones para la historia invasora, la exitosa dispersi&oacute;n y el futuro comportamiento de O. novo-ulmi.