--C. W. Roane and S. A. Tolin
Maize plants infected with maize dwarf mosaic virus (MDMV) have a variety of symptoms some of which are difficult to interpret. The typical pattern of symptoms of completely susceptible plants is a mosaic extending from leaf blade margin to margin and sheath to blade tip. Usually, under conditions of natural infection by aphids transmitting the virus in areas where there is a severe infestation of johnsongrass, plants will have 100% of their leaf area showing mosaic. Lower leaves are sometimes free of symptoms because plants may escape early inoculation. Under conditions of artificial infection, it is difficult to achieve symptoms in more than 90% of the blade area. Usually the lowest leaves remain symptom free and only 70 to 80% of the blade area is symptomatic. When resistant inbred lines are crossed and progeny are artificially inoculated with MDMV and grown under conditions at Blacksburg, there is a range of responses observed among inbred lines having resistance genes. Some lines are apparently immune from MDMV as judged from the lack of symptoms appearing either under natural or artificial conditions, e.g. Oh7B, Pa405. Others are highly resistant in that a few plants become symptomatic with a minimum of leaf area (2 to 5%) exhibiting narrow stripes of mosaic, e.g. Va35. Other resistant inbred lines consistently display a range of symptoms and severity of mosaic in those plants that are infected. Attempts to stabilize these lines to give uniform symptom expression by strict inbreeding have been unsuccessful. For example, the majority of the plants of the resistant line T8 are symptomless; those plants showing symptoms are quite variable in symptom expression. We have been interested in the inheritance of reaction to MDMV, and where we tried to publish our interpretation of the genetics of T8, we were chided by journal editors for choosing poor parents and thus we chose to delete all reference to T8. We feel, however, that it would be better to air the data and have it challenged than to pretend such a situation does not exist.
Genetics of MDMV reactions have been a frustrating topic. Different results and different interpretations from different workers make one wonder why genetics of MDMV reactions is such a difficult topic. Several factors may be involved:
1) Inadequate genetic information about parental lines - Most parents used in MDMV maize genetics may not have been selected under conditions which assure their homozygosity for characters to be studied. In other words, most are breeding materials, not genetic materials.
2) Use of uniform inoculum - MDMV exists as a number of strains, A, B, etc. There has been no demonstration that there may be a further subdivision of strains, A, B, etc., into "races" in sensu Puccinia graminis on wheat. There is evidence for a gene-for-gene relationship between bean and bean common mosaic virus (Drifjhout, Agri. Res. Rep. 872, Wageningen, 1978) and soybean and soybean mosaic virus (Roane, Tolin and Buss, Soybean Genet. Newsl. 13:136-139, 1986). Since there is evidence for more than one gene conditioning reaction to MDMV (Scott and Rosenkranz, Crop Sci. 22:756-751, 1982; Mikel, D'Arcy, Rhodes and Ford, Phytopathology 74:467-473, 1984), there may also be a gene-for-gene relationship, yet to be demonstrated, for maize and MDMV. If true, MDMV-A from Missouri, Mississippi, Ohio or Virginia used as inoculum might not yield like results in genetic experiments. Rust workers are well aware that an isolate of Puccinia graminis from one area may key out to a particular race (a waning concept) but an isolate from another area keying out to that same race may not be genetically identical. Therefore, the importance of assuring virus strain identity and integrity must be recognized and a system of preserving and utilizing virus cultures must be invoked in future studies of genetics of maize-MDMV relationships.
3) Use of virus mixtures and strain mixtures - In genetics of pathogen-host relations, the genes of two entities are interacting. Unknown factors which interfere with arriving at correct interpretations cause geneticists enough problems when they are studying the characters of a single species. When two genotypes are interacting the problems are enhanced in a more than additive fashion. Thus, as demonstrated many times, for many diseases, use of a single, genetically pure pathogen is essential for successful analysis of host-pathogen genetics.
The items above can be controlled but if maize-MDMV genetic experiments
are to be conducted in the field, many factors are beyond control. From
year-to-year and location-to-location, temperature, light intensity, edaphic
factors and other influences affect the expression of MDMV symptoms and,
consequently, the genetic conclusions. Secondary genes in both host and
pathogen may unpredictably modify expression of major genes. Thus, a uniform
environment, a uniform pathogen, and a uniform system of recording of data
are needed if the genetics of this relationship is to be understood either
at the classical or molecular genetic level.
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