Co-evolution of phytoplasma, insect vectors and host plants
Phytoplasma are wall-less, non-helical prokaryotes that colonise plant phloem and depend of phloem-feeding insect vectors (leafhoppers, planthoppers, and psyllids) for transmission. Phytoplasma are important pathogens of many agricultural and ornamental plants. The dissemination of vector-transmitted plant symbionts/pathogens depends on several key factors, which include: survival and dispersal of the vector, the vector’s ability to acquire and transmit the pathogen, while the host-plant specificity (host range) of vector and pathogen determine the breath of transmission possibilities. Being dependent on the vector for dispersal (i.e. colonisation of new plants), a key question in the study of vector-borne plant endosymbionts is whether they interact with the vector to influence the vector’s specialization to and/or preference for specific plants. Of epidemiological significance is the host range: narrow host and feeding ranges may lead to specialised transmission cycles whereas polyphagous vectors may introduce different pathogens to diverse end-hosts.
In a series of papers, we have studied the emergence of the grapevine yellows pathogen Candidatus Phytoplasma solani (SP) in Western Europe. SP epidemiology is highly or completely determined by pathogen reservoirs in perennial herbaceous plants. SP have two major strains, tuf-a and -b. Stinging nettle (Urtica dioica) seems to be the only reservoir plant of tuf-a, while tuf-b is associated predominantly with field bindweed (Convolvulus arvensis). Being both nymphal host plants for Hyalesthes obsoletus (Cixiidae) and reservoir plants of SP, differential use of C. arvensis and U. dioica by H. obsoletus severely impacts the epidemiology of SP diseases of agricultural crops by creating separate vector-based epidemiological cycles. Our research has taken several routes and combines evolutionary and applied biology. First, we have quantified the ecology and behaviour of the vector to study the degree and evolution of host-plant specialisation, i.e. host-race evolution, and the influence of phytoplasma on decision making in the vector. Second, using different bacterial endosymbionts as "parasites as proxy", I investigate the origin and sequence of new host plants used by H. obsoletus and SP. Because the relationship between H. obsoletus and SP are intertwined, the incidence and diversity of different endosymbionts and their geographic distribution of infection are mapped as traits on the Hyalesthes phylogeny (broad sense evolution) and H. obsoletus’ plant use (narrow sense evolution). Thirdly, we have established the historical epidemiology of the tuf-a strain in Western Europe using genetic co-dispersal analysis between vector and pathogen.
Dr. Jes Johannesen, Dr. Michael Maixner (Julius-Kühn-Institut, Dossenheim)