Molecular Characterization of Bean – Infecting Geminiviruses and Antiviral Strategies:
DIVERSITY OF BEAN-INFECTING GEMINIVIRUSES
The variation in biological properties, such as sap transmissibility, among the different golden mosaic-inciting geminiviruses indicated that there were likely to be major sequence differences among isolates of geminiviruses as well as for the dwarf symptom-inducing isolates. This was confirmed by cloning and sequencing five isolates that represented the potential differences among the bean-infecting geminiviruses (Gilbertson et al., 1988; Loniello et al., 1992). Three distinct geminiviruses were found among those that caused golden mosaic symptoms, and each one was placed in a unique phylogenetic cluster (Faria et al., 1994). The isolate from Brazil formed its own unique phylogenetic cluster (Gilbertson et al., 1993), whereas the isolates from the Caribbean Islands, Jamaica, Central America and Southern Mexico were placed together in a cluster with the isolate from Puerto Rico (AF173555, Faria et al., 1994; McLaughlin et al., 1994). The golden mosaic-like virus, bean calico mosaic virus (BCMoV) (Brown et al., 1990; Brown et al., 1999; Loniello et al., 1992), was in a phylogenetic cluster with squash leaf curl virus (SqLCV) (Faria et al., 1994). The bean dwarf mosaic virus (BDMV) isolate from Columbia was associated with the abutilon mosaic virus (AbMV) cluster (Faria et al., 1994).
These phylogenetic studies supported the renaming of distinctly different bean-infecting geminiviruses. Since BCMoV and BDMV had unique names, these were retained; however, for the distinct geminiviruses that caused golden mosaic symptoms, there was a dilemma. At the recent 2nd International Workshop on Whiteflies and Geminiviruses in 1998, a consensus was reached that the isolates similar to the BGMV isolate from Brazil (type I) would retain the name BGMV and that those isolates similar to the BGMV isolates from the Caribbean and Central America (type II) would be designated bean golden yellow mosaic virus (BGYMV). The name BGYMV was first used Dr. Julio Bird and adopted by Goodman et al. (1977).
In addition to these studies on phylogenetic analyses and sequence divergence among isolates, the data with pseudorecombinants between components of BGMV and BGYMV also support the separation of these two viruses into distinct populations (Faria et al., 1994). It was noted by Faria et al. (1994) that there are conserved, 5-7 nucleotide repeats immediately 5' of the rep gene TATA-box for geminiviruses in the same phylogenetic cluster. These were shown by L. Hanley-Bowdoin and associates (see Hanley-Bowdoin et al., 1999) to be the site for the binding of the Rep protein. And, as expected, pseudorecombinants between BGMV and BGYMV were not infectious on beans, and pseudorecombinants between the DR isolate and the GA isolate of BGYMV were infectious (Faria et al., 1994). These data support the separation of the isolates of bean-infecting geminiviruses from Brazil and the Caribbean Basin into two distinct species with different ancestors. Unfortunately, anomalies exist for the Florida isolate (Homestead isolate) of BGYMV (Blair et al., 1995; Hiebert et al., 1996). Phylogenetic analysis based on the hypervariable region of DNA-B and the intergenic region of DNA-A by Hiebert et al. (1996) clearly show that this isolate is closely related to the BGYMV cluster isolates. However, a detailed analysis of the DNA-A ori region sequence provided evidence that this isolate was not a direct introduction of a BGYMV isolate from the Caribbean Basin (Potter and Maxwell, unpublished data; the DNA-A sequence provided by E. Hiebert). Sequence analyses are consistent with the Florida isolate being a recombinant between BGYMV and another virus, perhaps, a sida-infecting virus (Potter and Maxwell, unpublished data).
There is always the concern that exotic geminiviruses from other regions might be introduced into a country. This is the case with the monopartite Middle Eastern geminivirus, tomato yellow leaf curl virus (TYLCV), which has been introduced into the Western Hemisphere (Nakhla et al., 1994), and detected in several Caribbean islands and, most recently, in Florida on tomatoes and beans (Polston, 1998).
These studies on diversity indicate that for most geographical regions in the Western Hemisphere it is reasonable to develop geminivirus-resistant germplasm by either classical breeding methods or genetic engineering.
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