About the Author
Howard Judelson received his Ph.D. in Molecular Biology from the University of Wisconsin in 1985. He was a postdoctoral fellow in the Department of Vegetable Crops at the University of California-Davis from 1986 to 1990 and Assistant Geneticist at the National Science Foundation Center for Engineering Plants for Resistance to Pathogens at the UC-Davis from 1991 to 1994. He was appointed Assistant Professor of Plant Pathology at the UC-Riverside in 1994. Judelson has worked with the genetics of Bremia lactucae (lettuce downy mildew) and since 1991 has been involved in the genetics, developmental biology, and pathology of oomycete fungi, particularly Phytophthora infestans.
Genetics of Phytophthora infestans
Howard S. Judelson
Department of Plant Pathology
University of California Riverside, California
Genetic explorations of Phytophthora infestans are required to understand its biology, population genetics, and pathology, and to assist in the development of strategies for controlling the late blight diseases. For example, controlled laboratory crosses are required for dissecting the genetic basis of traits underlying host specificity and pathogenicity, and methods for the DNA-mediated transformation of P. infestans are needed to test the function of cloned genes.
The classical genetics of P. infestans is now reasonably well understood. The sexual cycle has been exploited to study the genetic basis of traits such as race-cultivar specificity, mating type, and fungicide resistance although such studies would be aided by further improvements in crossing methods. A genetic map has also been developed based on DNA markers, and these will be useful for population genetics studies and for cloning genes by positional cloning. Positional cloning techniques, using DNA markers from the map and others obtained by bulked segregant analysis, will play an important role in P. infestans; this is because gene isolation methods often used for other fungi, such as shotgun cloning, will be difficult to apply to P. infestans due to its large genome and the modest rates of DNA transformation now possible. Current transformation rates are adequate for many applications, however, as hundreds of transformants can be recovered per experiment. All isolates tested (>12) have so far proved amenable to the procedure, including both A1 and A2 types. Three selectable marker genes have been developed, enabling multiple rounds of transformation. Visible markers such as beta-glucuronidase (GUS) are also available. Technologies such as high-efficiency cotransformation and antisense RNA-mediated inhibition of gene expression have been demonstrated. Vectors have been constructed for expressing sequences such as cDNAs that incorporate a variety of promoters enabling expression over a range of levels. The generally stable maintenance of expression of nonselected genes has been documented. Further improvements in transformation and the demonstration of additional techniques such as gene disruption will be useful. Nevertheless, based on the current status of techniques for classical and molecular analysis, the potential now exists to make rapid advances in our understanding of P. infestans.
GENETICA DE PHYTOPHTHORA INFESTANS
HOWARD S. JUDELSON
La exploracion genetica de Phytophthora infestans es un requesito para entender su biologia, genetica de poblaciones, patologia, y para auxiliar en el desarrollo de estrategias de controlal de la enfermedad de tizon tardio. Por ejemplo, cruzamientos en laboratorios controlados son requeridos para analizar los rasgos de la base genetica, subrayar la especificidad del hospedero y patogenecidad, y metodos para mediar en la transformacion de DNA de P. infestans son necesarios para las pruebas de la funcion de los genes clonados.
La genetica clasica de P. infestans es ahora rasonablemente bien entendido. El ciclo sexual ha estado explotado dando lugar ha estudiar la base genetica de rasgos tales como la especificidad de las razas de los cultivares, typos de apareamientos, y resistencia a fungicidas, aunque estos estudios podrian ser una ayuda para mejorar los metodos de crusamientos. Un mapa genetico ha estado siendo desarrollado basado en marcadores de DNA, y estos trabajos podrian ser muy usuales para estudios de genetica de poblaciones y para clonar los genes por ubicacion de clones. La tecnica de ubicacion de clones, usando marcadores de DNA de los mapas y otros obtenidos por medio de analices de la masa de segregantes, van ha jugar un rol muy importante en P. infestans; esto porque los metodos de aislar genes frecuentemente usados para otros hongos, tales como multiple (shotgun) clonamiento, va ha ser dificil de aplicar a P. infestans debido a su largo genomio y las modestas proporciones de transformacion del DNA ahora posible. La proporciones de transformacion actual son adecuados apara muchas aplicaciones, sin embargo, como cientos de transformaciones pueden ser recuperados para un experimento. Todos los aislamientos (>12) estan hasta ahora sujeto a procedimientos de probacion, incluyendo los dos typos A1 y A2. Tres marcadores selecionables han estado siendo desarrollados, permitiendo multiples secuencias de transformaciones. Marcadores visibles tales como B-glucuronidase (GUS) tambien estan disponibles. Tecnologias tales como los cotransformadores de mucha eficiencia y los antisense RNA-intermediarios de inhibicion de expresion de genes han sido demostrados. Han estado siendo construidos vectores para expresar secuencias tales como cDNAs que incorporan una variedad de promotores permitiendo la expresion sobre una gama de niveles. Generalmente el matenimiento estable de la expresion de genes ha estado siendo documentado. Futuras mejoras de las transformacion y la demostracion de tecnicas adicionales tales como la separacion de genes va ha ser de mucha utilidad. Sin embargo, basados en el estado actual de las tecnicas clasicas y de analices molecular, permitira ahora potencialmente realizar avances rapidos sobre nuestros conocientos de P. infestans.
