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