Research article Evolution of herbicide resistance in weeds: vertically transmitted fungal endophytes as genetic entities MARTIN M. VILA-AIUB,  , * M. ALEJANDRA MARTINEZ-GHERSA and CLAUDIO M. GHERSA IFEVA, Departamento de Recursos Naturales y Ambiente, Facultad de Agronomı´a, Universidad de Buenos Aires (UBA), Av. San Martı´n 4453, C1417DSE, Buenos Aires, Argentina (Present address: Western Australian Herbicide Resistance Initiative (WAHRI), School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia (UWA), 35 Stirling Hwy, Crawley, WA 6009, Australia) (*author for correspondence, tel.: +61-8-93807980; fax: +61-8-93807834; e-mail: mvilaaiu@agric.uwa.edu.au) Received 16 July 2002; accepted 26 June 2003 Co-ordinating editor: I. Olivieri Abstract. The appearance of heritable resistance to herbicides in weeds is an evolutionary process driven by human selection. Assuming that spontaneous and random mutations originate herbicide resistance genes, which are selected by selection pressure imposed by herbicides, is the simplest model to understand how this phenomenon appears and increases in weed populations. However, the rate of herbicide resistance evolution is not only determined by the amount of genetic variation within the populations and the selection pressure exerted by herbicides, but also by factors related to genetics, biology and ecology of weeds. The inheritance of the resistance genes, the mating patterns of the populations, the relative fitness of susceptible and resistant phenotypes and gene flow processes also control the mentioned rate. Many cool season grasses are often infected by fungal symbiotic endophytes (Neotyphodium spp.). These organisms modify the physiology, ecol- ogy and reproductive biology of their hosts, conferring greater tolerance to biotic and abiotic stresses, greater competitive ability and the capacity of reducing ecosystem biodiversity. In this work, we present new empirical data and propose new theoretical support on how these microbial symbionts can modulate the evolution of herbicide resistance in weeds. Fungal endophytes are vertically transmitted, and may act as genetic entities altering the evolution of herbicide resistance by reducing herbicide efficacy (delaying effect on evolution). In addition, indirect evidence suggests that fungal endophytes might reduce the fitness penalty associated with the newly arisen resistant phenotypes. The importance and dynamic of these opposite effects is discussed. Key words: fitness, herbicide selection, Lolium multiflorum, Neotyphodium, resistance genes Introduction Herbicide resistance in weeds is the consequence of an evolutionary process driven by complex interactions among a series of selective factors. Such factors include those manipulated by humans (namely herbicides, but also other Evolutionary Ecology 17: 441–456, 2003. Ó 2003 Kluwer Academic Publishers. Printed in the Netherlands.