Ecological and evolutionary context of the new phytoplasma-host associations
None of the leafhopper species that tested positive for presence of phytoplasmas in the present study were previously reported as hosts or vectors of phytoplasmas (Trivellone, 2019). These 6 leafhopper specimens were collected from native grassy vegetation in four countries: South Africa, Kyrgyzstan, Australia, and China (Table 1 and Figure 1).Leofa dispar (LH078) and P. exquadratus (LH082) were collected from native grassland and fynbos vegetation in two different provinces in South Africa (KwaZulu-Natal and Western Cape) in 2004. The distance between these two sampling sites is about 1,120 km. In South Africa only four 16Sr phytoplasma groups were previously recorded (16SrI, 16SrII, 16SrIII, 16SrXII) (for an overview see Trivellone, 2019), and only two species of leafhoppers were recorded as potential vectors and competent vectors of phytoplasmas in the 16SrI group:Austroagallia sp. (subfamily Megophthalminae) and Mgenia fuscovaria (Stål) (Coelidiinae) (Kruger et al., 2015). Thus, this is the first record of phytoplasma strains in the clade 16SrXI/16SrXIV in South Africa. The leafhopper fauna of Africa is diverse but remains poorly known, with new genera and species continuing to be discovered (e.g., Stiller, 2019; 2020). Pravistylus exquadratus and other members of the same genus have never been reported as pests, except for single records of this species on Korog wheat cultivar and on rye grass (Stiller, 2010). The species is mainly associated with native grassland and fynbos vegetation, and it is always macropterous with high potential for dispersal. Leofa dispar also occurs in native grassland and has not been reported from crops. Both,P. exquadratus and L. dispar are restricted to South Africa.
Macrosteles sordidipennis (LH102) specimens were collected in a riparian sedge meadow, on a river bank in Kyrgyzstan (Jeti-Ögüz District) in 1999. The single tested specimen LH102 was infected by a strain of group 16SrI related to aster yellows phytoplasma strains. Based on a recent review (see Trivellone, 2019), this appears to be the first record of this phytoplasma group in Kyrgyzstan. The only previous phytoplasma record from this country was the potato stolbur disease, associated with the 16SrXII phytoplasma group. Interestingly, other species in the genus Macrosteles have been reported as competent vectors of phytoplasmas (Trivellone, 2019). In particular, competent and potential vectors of Macrosteles show a strong cophylogenetic signal with the 16SrI phytoplasma group (Trivellone, unpublished data) suggesting that these two lineages have been associated for a long time. Nine additional species of Macrosteles have been documented in Kyrgyzstan (Novikov et al., 2000), including four that are competent vectors of 16SrI phytoplasmas in Europe, although 16SrI phytoplasmas have not been previously recorded from this country (Trivellone, 2018). Our discovery of a new association between a Macrosteles species not previously recorded as a phytoplasma host and a new 16SrI group strain or host suggest that further surveys and phytoplasma screening in Kyrgyzstan may be important for assessing the potential threat of emerging phytoplasma diseases in this region of Central Asia.
Mayawa capitata (LH133) and M. affinifacialis (LH139) were collected at lights from two different nature reserves in Australia in 2010 and 2009, respectively. The two sampling sites are about 3,597 km away from each other. Mayawa capitata belongs to the grass-specialist leafhopper tribe Paralimnini and reportedly occurs on grasses and Sida acuta (Malvaceae) (Fletcher, 2000). Mayawa affinifacialis has been recently described (Dietrich, in review) and little is known about its ecology; however, the species was collected in grassland is likely a grass-feeding species. A specimen of the first species (LH133) was infected with a phytoplasma strain closely related to 16SrXV group and the second one (LH139) with a phytoplasma strain closely related to 16SrXI. Although the last group was never detected in Australia, only 3 competent vectors for phytoplasma strains in group 16SrII are known for this country, two species of Orosius, tribe Opsiini (Deltocephalinae), and Batracomorphus angustatus (Osborn) in the subfamily Iassinae (for an overview see Trivellone, 2019). A recent review of Australian phytoplasma pathosystems revealed an important gap of knowledge, with several recorded phytoplasma strains not yet assigned to 16Sr groups and subgroups (Liu et al., 2017). Moreover, information on competent vectors is scarce with many species still undescribed, hampering the understanding of epidemiological cycles. Our results expand the spectrum of potential vectors recorded in Australia to include species from the tribe Paralimnini, and reveal new possible epidemiological routes that require further investigation.
Acharis ussuriensis (LH143) was collected in China from grasses on a dry hillside at Zhouzhi Nature Reserve (Zhouzhi county, Shaanxi Province). The sampling location is entirely surrounded by forest with the nearest farming settlement about 10 Km away. The specimen testing positive was infected with a strain closely related to strains in the 16SrXI/16SrXIV groups (Figs 4, cluster A). Although both phytoplasma groups were previously detected in China, further investigation on the pattern of transmission and host plants involved in this pristine area will provide useful insights into the characterization of phytoplasma-host relationships in natural areas.
Underestimated phytoplasma diversity