Abstract
The complexity of tospovirus–vector–host plant interaction is linked to a range of factors influencing vector's efficacy in virus transmission, leading to high variability in the transmission efficiency within vector populations. Main shortcomings of most studies are the missing information on the intrinsic potential of individual insects to serve as efficient vectors, both at phenotypic and at genotypic levels. Moreover, detailed analysis of vector competence heredity and monitoring the splitting of both genotypes and phenotypes in filial generations has not been reported. In this study, using the model system Frankliniella occidentalis and Tomato spotted wilt virus, we evaluated the inheritance and stability of the trait vector competence in a population through basic crossings of individually characterized partners, as well as virgin reproduction. We hypothesized that the trait is heritable in F. occidentalis and is controlled by a recessive allele. From the results, 83% and 94% of competent and noncompetent males respectively, inherited their status from their mothers. The trait was only expressed when females were homozygous for the corresponding allele. Furthermore, the allele frequencies were different between males and females, and the competent allele had the highest frequency in the population. These suggest that the trait vector competence is inherited in single recessive gene in F. occidentalis, for which the phenotype is determined by the haplodiploid mechanism. These findings are fundamental for our understanding of the temporal and spatial variability within vector populations with respect to the trait vector competence and at the same time offer an essential basis for further molecular studies.
Highlights
The order Thysanoptera encompasses about 7,700 described species (CSIRO Australia, 2005), out of which only about 14 species have been identified as vectors of tospoviruses (Riley, Joseph, Srinivasan,& Diffie, 2011), indicative of the specificity of these virus–vector interactions
Not obvious yet, whether this mechanism of inheritance found in C. clarathris and T. tabaci is common in all thrips–tospovirus relationships
Sex determination in F. occidentalis is by haplodiploid mechanism, with females laying two kinds of eggs; fertilized eggs that have diploid sets of chromosomes from both parents developing to females, whereas unfertilized eggs with only one copy of the mother’s chromosomes produce males (Crespi, Evolution, & Mar, 1991; Hedrick & Parker, 1997)
Summary
The order Thysanoptera encompasses about 7,700 described species (CSIRO Australia, 2005), out of which only about 14 species have been identified as vectors of tospoviruses Transovarial transmission of TSWV is not possible in F. occidentalis, so each generation must re-acquire the virus for the disease epidemic to continue (Nagata et al, 1999) This specificity in the virus transmission cycle can neither explain the always high individual variability in vector competence when same aged L1 thrips are subjected to the same virus sources (tospovirus infected host plants), nor the dynamic change of relative amounts of vector competent individuals in populations of different sizes, or in isolation (inbreeding). Under inbreeding conditions in an isolated colony, the ratio of competent versus noncompetent individuals strongly declined with increasing homozygosity, suggesting that vector competence in C. claratris is controlled by a recessive allele (Halaweh & Poehling, 2009) It is, not obvious yet, whether this mechanism of inheritance found in C. clarathris and T. tabaci is common in all thrips–tospovirus relationships. The focus on individual crossings is based on our hypotheses that the available inheritance studies on the trait vector competence in C. clarathris and T. tabaci (Cabrera-La Rosa & Kennedy, 2007; Halaweh & Poehling, 2009) crossing experiments were performed only at population levels, which might have obscured the contribution of individual’s genetic constitution
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