PARASITE TRANSMISSION MODES AND THE EVOLUTION OF VIRULENCE

Transmission of insect-vectored pathogens: effects of vector fitness as a function of infectivity status

The transmission of insect-vectored pathogens is dependent on the population dynamics of the vector. Epidemiological models typically assume that birth and death rates of pathogen-free and inoculative vectors are equal, an assumption that is not true for all pathosystems. Here, a series of simple and general epidemiological models were used to explore how assumptions about birth and death rates of vectors based on their infectivity status influence disease incidence. With fixed death rate of pathogen-free vectors, increasing the death rate of inoculative vectors reduced vector density, the proportion of vectors that were inoculative, and the proportion of hosts infected. This effect was mediated by acquisition rate. Specifically, increasing the acquisition rate increased the proportion of vectors that were inoculative, thereby increasing the proportion of the vector population that experienced the increased death rate. With fixed birth rate of pathogen-free vectors, variation in birth rate of inoculative vectors had little influence on disease incidence provided that the birth rate of pathogen-free vectors was much greater than their death rate. However, when the birth rate of pathogen-free vectors was only slightly greater than their death rate, large increases in the birth rate of inoculative vectors increased total vector density and disease incidence. The results indicate that assumptions about birth and death rates of vectors based on infectivity status can have important effects on the vector population that in turn affects disease incidence.

Evolution of multiple components of virulence in Drosophila-nematode associations

Virulence is of central importance in host-parasite interactions, yet little is known about how it changes over extended evolutionary periods. In this study, all four species in the testacea species group of Drosophila were experimentally infected with sympatric and allopatric nematodes in the Howardula aoronymphium species complex, and the effect of parasite infection on three components of host fitness was determined. The Drosophila species show striking differences in their responses to infection, with reductions reaching 80% in adult lifespan, 100% in female fertility, and 90% in male fertility. Female sterility appears to be determined by the host; species that are sterilized by their local nematodes are also sterilized by the other allopatric nematodes in the H. aoronymphium complex. Host species that are not sterilized by their local parasite are not sterilized by other nematodes in the complex. In contrast, reductions in host adult lifespan and male fertility depend on both the host and the parasite. Whereas all nematodes reduced the survival of their local host species equally (about 40-45%), survival of two host species was drastically reduced (about 80%) when infected with an allopatric parasite. Thus, virulence is evolutionarily labile in associations between Drosophila testacea group species and their Howardula parasites. The data suggest that changes in the sterility component of virulence are due primarily to host evolution, whereas changes in the host mortality component are due in large part to parasite evolution.

The evolution of virulence in a plant virus

The evolution of virulence is a rapidly growing field of research, but few reports deal with the evolution of virulence in natural populations of parasites. We present here an observational and experimental analysis of the evolution of virulence of the plant virus Cucumber mosaic virus (CMV) during an epidemic on tomato in eastern Spain. Three types of CMV isolates were found that caused in tomato plants either a systemic necrosis (N isolates), stunting and a severe reduction of leaf lamina (Y isolates), or stunting and leaf curl (A isolates). These phenotypes were due to the presence of satellite RNAs (satRNAs) necrogenic (in N isolates) or attenuative (in A isolates) of the symptoms caused by CMV without satRNA (Y isolates). For these three types of isolates, parameters of virulence and transmission were estimated experimentally. For virulence the ranking of isolates was N > Y > A, for transmissibility, Y > A > N. The predictions of theoretical models for the evolution of virulence were analyzed with these parameters and compared with observations from the field. A single-infection model predicted adequately the observed long-term evolution of the CMV population to intermediate levels of virulence. A coinfection model that considered competition between isolates with an effect on transmission explained the invasion of the CMV population by N isolates at the beginning of the epidemic, and its predictions also agreed with field data on the long-term evolution of the CMV population. An important conclusion from both models was that the density of the aphid vector's population is a major factor in the evolution of CMV virulence. This may be relevant for the design of control strategies for CMV-induced diseases.