David L. Price and Dr. Mikel R. Stevens, Plant and Wildlife Sciences
The tomato is susceptible to more than 200 diseases. Tomato spotted wilt virus is one of the most damaging pathogens in tomatoes. Due to its wide spread, TSWV has become an important limiting factor in tomato production worldwide. Most often the virus is spread by Frankliniella occidentalis (commonly known as thrips), tiny insects that act as a vector carrying the virus. After the plant is infected, the virus will spread throughout the plant. Tomatoes showing symptoms of the disease are unmarketable due to severe discolorations. In areas of high infection, the economic losses for farmers are substantial. By developing lines of tomato that have a host resistance to TSWV, it is anticipated that the negative effects of TSWV can be minimized and that economic losses from TSWV will be reduced.
Due to the huge impact of TSWV, many researchers have sought methods to control the disease. A variety of methods have been employed to stop the spread of TSWV. Basic methods have included the physical containment and control of the virus and its vectors (thrips).Unfortunately these methods have been largely unsuccessful. Many researchers agree that the most effective long-term solution is to breed for host resistance. In 1992 Mikel Stevens identified Sw-5 which conferred dominant resistance to TSWV in tomato. Sw-5 has shown a very good resistance, but recently it has been overcome by isolates in some parts of the world. This is a continuing problem because, due to the high variability of the pathogen, new isolates that overcome existing resistance will continue to develop. As a hedge against these identified isolates and as a part of a preventative plant breeding strategy, new sources of resistance are needed. For this purpose continued research has been conducted in order to identify new resistance genes to TSWV. If successful, this research will have many implications for both tomato breeders and growers whose crops are affected by this disease
A plan was created in order to identify DNA markers linked to a new TSWV resistance gene (preliminary data suggest it to be a single gene) in lines of tomato identified in the plant genetics program at Brigham Young University (BYU). These lines have been developed from wide crossing with wild relatives of tomato, Solanum chilense. From these crosses a new source of TSWV resistance has been incorporated into cultivated tomato. Resistance from S. chilense has been shown and has been proven valuable under field conditions; however, linked molecular markers need to be identified to implement its full use in coordination with an additional TSWV resistance gene Sw-5.
Thirty-seven sister families of tomato lines that purportedly contain this new source of TSWV resistance as well as variation of many other characteristics in the tomato genome, have been developed. Because this new source of resistance has been difficult to manipulate in the artificial environment of the greenhouse, a number of plants were planted from each of these families along with the resistant and susceptible parent controls, for replication’s sake. These plants were inoculated with TSWV isolates (strains of virus) to confirm the presence of the TSWV resistance gene in every family. Additionally, we have collected DNA from both the parents and the 37 sister lines. These DNA samples were analyzed using the AFLP (amplified fragment length polymorphism) technique.
AFLP analysis was conducted using the LI-COR 4300 DNA analyzer. This was a huge asset to the project as it greatly increased the speed and accuracy at which DNA polymorphisms were identified. By comparing phenotypic data (physical observation of resistance or susceptibility) collected from the 37 sister lines and data from AFLP analysis (genotypic data, genetics of the plant), it was determined which lines carry the TSWV resistance genes of interest. In the future this data will result in a molecular marker linked to the new TSWV resistance gene that can easily be used in the development of new tomato cultivars.
AFLP analysis was conducted on 31 unique lines of tomato progeny. Included in the analysis were the two original parents CK and 7482 as positive and negative controls respectively; seven elite lines of tomatoes were also included as further controls. Four of these elite lines had previously demonstrated resistance to TSWV and three, which had demonstrated susceptibility to TSWV. These controls proved very valuable in confirming the presence of a marker linked to TSWV resistance.
AFLP analysis yielded significant data suggesting that a resistance gene has been integrated into the cultivated tomato. By comparing AFLP analysis of both the resistant and susceptible parents, with progeny 14 markers were identified. Of the 14 one of these markers produced an especially promising result. This marker is of special importance since similar matches correlate with the AFLP analysis of the seven elite control lines of tomato that have demonstrated resistance to TSWV in the past. From this we can conclude that this marker is potentially linked to the Sw-7 gene.
Having found a marker that is potentially linked to the Sw-7 gene will be the first step in developing a breeding strategy that will allow this gene to be integrated into other varieties of cultivated tomato. Due to the great number of isolates of TSWV it is expected that this gene will have greatest functionality when combined with the Sw-5 gene that is currently in many tomato varieties. Currently efforts are being made to clone and sequence bands of interest in order to develop a PCR based marker.
Results of this research were presented at the 2007 American Society for Horticultural Sciences annual conference in Scottsdale, AZ. The presentation was awarded first place in the undergraduate student oral competition. Results are also being prepared for the future submission in a peer reviewed journal.