Botany
Subdividing the spectrum—quantifying host specialization in mistletoes
Journal: Botany
Manuscript ID cjb-2019-0207.R1
Manuscript Type: Article
Date Submitted by the 17-Mar-2020 Author:
Complete List of Authors: Milner, Kirsty; University of Technology Sydney Leigh, Andrea; University of Technology Sydney Gladstone, William; University of Technology Sydney Watson, David;Draft Charles Sturt University, Keyword: host preference, host spectrum, host turn-over, parasitic plant, mistletoe
Is the invited manuscript for consideration in a Special IUFRO 2019 Dwarf Mistletoes Symposium Issue? :
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Subdividing the spectrum—quantifying host specialization in mistletoes
Kirsty V. Milner1, Andrea Leigh1, William Gladstone1, David M. Watson2,3
1 School of Life Sciences, University of Technology Sydney, Ultimo NSW 2007 Australia;
([email protected], [email protected], [email protected] )
2 Institute of Land, Water and Society, Charles Sturt University, Albury NSW 2640 Australia
3 Corresponding author:
David M. Watson, School of Environmental Sciences, Charles Sturt University, P.O. Box 789,
Albury, 2640 Australia E: [email protected] Draft P: +61 2 6051 9621
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Abstract
Parasites necessarily depend on their hosts, but the number of host species used by a parasite varies from one to hundreds. Estimating host range and identifying preferred host species that influence distributional boundaries and confer greater advantage to the parasite has proven elusive. As well as the confounding effects of sampling effort, characterising host specificity and preference has been hindered by considering host use without accounting for availability. We selected three mistletoe species—Lysiana exocarpi, Amyema quandang, and Amyema lucasii— and sampled mistletoe-host interactions and host availability free from sampling bias. To quantify host specificity and identify preferred host species we applied specialist/generalist scores (G) and resource selection ratios (ω) respectively. Host specificity and preference were assessed at four scales. The generalist L. exocarpi was foundDraft to parasitise 31 plant species. Even at small scales G values and host species turnover were high, with eight preferred hosts identified.
Amyema quandang had a low G score with significant preference for half of its Acacia hosts.
Amyema lucasii significantly preferred one host, consequently having low G value at all scales.
By collecting potential host data and applying G scores and ω, parasite host spectrum can be quantitatively estimated rather than qualitatively described.
Keywords host preference, host spectrum, host turn-over, parasitic plant, mistletoe
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Introduction
Parasites have a pervasive influence on ecosystems (Hudson et al. 2006) and, while their
combined effects extend well beyond pairwise interactions with individual hosts, the magnitude
and extent of their influence is often tied to the number of host species they infect (i.e., their host
range). The degree to which parasites are able to use different host species—host spectrum (Poulin
2011)—ranges between two divergent strategies: high specificity, the reliance upon very few hosts
vs generalism, the use of many hosts, often in proportion to availability. Although a primary
determinant of many aspects of parasite life history, demarcating any groupings along this
continuum is challenging.
Three aspects have confounded estimation of host range, and constrained our
understanding of the actual variation in hostDraft use among parasites. The first is sampling bias, where
greater sampling effort necessarily identifies greater numbers of hosts (Walther et al. 1995;
Downey 1998; Grenfell and Burns 2009;). Although ecological parasitologists have long
recognised that estimates of host spectrum are susceptible to sampling bias (Poulin 1992; Walther
et al. 1995; Guégan and Kennedy 1996; Walther and Morand 1998), recent work with mistletoes
has successfully applied results-based stopping rules to standardize sampling and generate
comparable estimates (Watson et al. 2017). The second issue relates to the quantification of host
spectrum. Simply listing all recorded host species for a parasite assumes that all hosts are equally
important (Krasnov et al. 2011; Poulin et al. 2011), the accidental infection of one host individual
just as relevant as the most frequently infected host species that confers reproductive advantages.
Host preference, on the other hand, considers the relative ecological importance of each host
species (Poulin et al. 2011) and the evolutionary time over which host-dependent fitness has
formed (Giorgi et al. 2004), rather than simply consider whether a host is used by a parasite. To
date, however, variation in sampling effort has confounded estimation of host preference,
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4 preventing comparative studies. Finally, since spatial variation and relative abundance of hosts in the environment controls host specificity via encounter rate (Jaenike 1990; Norton and Carpenter
1998; Norton and De Lange 1999), reliable estimates of host preference rely on representative estimates of host availability (Norton et al. 1995; Mourao et al. 2016). This is further complicated when host use varies across the geographic range of a parasite (Poulin et al. 2011) which may reflect both spatial variation in host specificity and/or spatial variation in host availability, neither of which are considered in many assessments of host spectrum (Grenfell and Burns 2009; Blick et al. 2012; Mans et al. 2014). Given this inherent complexity coupled with methodological difficulties in obtaining comparable samples, specialisation, to our knowledge, has not been determined quantitatively for parasite-host relationships.
Mistletoes are an ideal system forDraft testing theories in parasite ecology and developing novel approaches to collect empirical data to inform and test them. Mistletoes are hemiparasites, physically and physiologically connected to their host through haustoria, establishing a functional connection to the host xylem (Yan 1993) to acquire water, minerals, nutrients and some photosynthates (Mathiasen et al. 2008). The sessile nature of host and parasite and mistletoes’ macroparasitic nature mean infection is apparent in the environment (Reid 1989; Overton 1994).
These characteristics mean accurate estimates of abundance and density of mistletoes, hosts and potential hosts can be made, especially for vegetation types with more open canopies and lower trees. A primary aim of this study was to quantify host spectrum beyond qualitative descriptions.
Here, we use data on host use and host availability for three mistletoe species with divergent host ranges. We apply an index of specialization developed in plant-pollinator systems to derive quantitative estimates of host specificity. We established the primary or ecologically meaningful hosts for each of the three mistletoe species using resource selection ratios to identify host preference. Until now the host preference of Australian mistletoes has been considered at a highly
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localised scale (Yan 1990; Ward 2005) but host spectrum varies with scale; a parasite could be
either a specialist or generalist at a given location and/or across its geographic distribution
(Krasnov et al. 2005). Therefore, a second aim was to sample in multiple habitats across bioregions
to explore how host spectrum varies with scale. To our knowledge, this is the first time these
indices have been applied to parasites.
Method
Study species
Based on qualitative descriptions of recorded hosts in floras and host lists derived from
herbarium specimens (Barlow 1984; Cunningham et al. 1992; Quirico 1992; Downey 1998; Watson 2019), three mistletoe species withDraft differing host spectrums were selected, all members of the Loranthaceae family. The mistletoe with the narrowest host spectrum was leopardwood
mistletoe Amyema lucasii (Blakely) Danser, considered to be almost exclusively dependent on
leopardwood (Flindersia maculosa) hosts, but across its range has been recorded on Grevillea
striata and Geijera parviflora. Grey mistletoe Amyema quandang (Lindl.) Tiegh. has been
recorded on 53 host species, of which 40 are in the Acacia genus, and was chosen to represent
intermediate host spectrum. The study species with the broadest host spectrum was harlequin
mistletoe Lysiana exocarpi (Behr) Tiegh. It has been recorded infecting 114 species in 45 genera
from 21 families, with the most common hosts in the genera Acacia, Senna, Casuarina,
Eremophila, Alectryon, Exocarpos, Santalum and Amyema (epiparasitic on the latter three parasitic
genera in the Santalales). Greater detail on mistletoe host species and distributions are given in
Watson et al. (2017).
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Sampling mistletoe hosts
Field sampling was conducted on roadsides across New South Wales, Australia in April–
September 2014, in six bioregions (Sydney Basin, Riverina, Darling Riverine Plains, Cobar
Peneplain, Mulga Lands and Broken Hill Complex). A spatially-explicit results-based stopping rule was used to estimate the number of host species infected by each mistletoe species, independent of sampling effort (see Watson et al. 2017 for details). Although standardized by estimated completeness to ensure comparability within and between vegetation types, we recognize that our roadside samples may not be representative of more continuous woodlands far from the influence of roads (Milner 2014; Watson et al. 2017). Sample plots commenced where a target mistletoe species was encountered and continued until 25 individual mistletoe-host pairs were recorded. Sampling occurred in at leastDraft three different habitats within a bioregion before host species richness was predicted with a sample coverage estimator (Chao and Lee 1992), using
EstimateS (v 9.1.0, Colwell 2013). If observed host richness was ≥80 % of predicted host richness sampling in the bioregion was considered complete. If not, sampling continued in the bioregion and predicted host richness was re-calculated after each sampling plot until the 80 % threshold was reached.
Nearest-neighbour sampling
For each target mistletoe species, the 10 woody plants closest to each of the 25 infected hosts in each sample plot were identified to species. These potential hosts reflect those individuals most likely to receive mistletoe seeds within the sample plot (after Rödl and Ward 2002;
Okubamichael et al. 2011;). Nearest neighbours were recorded if they had a diameter at breast height (DAB) >150 mm and were considered a single individual if multiple trunks were touching at the base. Nearest neighbour data for each woody plant species sampled was decomposed into
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two variables: number of individuals infected (used resource) and number uninfected (unused
resource) with the target mistletoe species.
Analysis
Host specificity—specialist/generalist Index (G)
Host spectrum was calculated using Medan et al. (2006) specialisation/generalisation index
(G), which minimises the bias from the increased likelihood that generalist interactions will be
encountered more often than specialist interactions. Also, the index includes a measure of the
evenness of species—a measure of hosts used relative to their proportion in the environment—
giving weight to the more ecologically meaningful interactions. Specialisation/generalisation G index for species i is calculated as the productDraft of resource usage (R) and evenness (E):
(1) 퐺푖 = 푅푖 ∙ 퐸푖
where Gi takes a value from zero to one. A value close to zero indicates a highly specialised parasite,
while a value close to one is highly generalised parasite. The component Ri was calculated by:
(2) 푆푖 푅 = 푖 푁
where Si is the number of host species interacting with parasite species i and N is the total number
of potential host species. Ei, based on the Shannon evenness measure (Taki and Kevan 2007), was
calculated by:
∑ (3) ― 푗푝푗 ∙ 푙푛 푝푗 퐸푖 = 푙푛 푆푖
where pj is the proportion of hosts in the environment. When parasite species i has one partner, a
value of 1 is assigned because Ei cannot be calculated (lnSi = 0).
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Preferred host analysis—Resource selection ratios
Host preference was estimated using resource selection ratios—a proportional comparison
of used resource (hosts) and unused resources (non-hosts). The sampling design fits the
assumptions of Manly et al.’s (2002) Design I: known proportions of available resources with the
appropriate resource selection ratio equation applied (Manly et al. 2002, equation 4.14):
(4) 표푖 휔푖 = 휋푖
where oi is the proportion of used resource in category i (i.e. number of infected individuals of
species i / infected + uninfected individuals of species i) and πi is the proportional availability of resource in category i in relation to all available resources (i.e. infected + uninfected individuals of species i / total number of individual Draftnearest neighbours). Therefore, ωi is the preference score for resource in category i (i.e. host species i).
A value of ωi > 1 suggests a parasite may prefer host species i; however, the significance
of the resource selection ratio must then be tested using the confidence limits for ωi. If the upper
confidence interval is <1 the host species was significantly avoided. If the lower confidence
interval is <1, there is no preference for the host species, i.e., it is used in proportion to its
availability. When the lower confidence interval is > 1, the host species is significantly preferred
(Manly et al. 2002). To allow for multiple comparisons between all resource categories (host
species) a Bonferroni correction was applied by dividing the significance level (α = 0.05) by the
number of host species.
Scale
Since we were evaluating patterns of host use at different scales, data were analysed at
multiple scales. Specialisation/generalisation (G) values were calculated at the sample plot level,
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plot data were pooled into habitats within bioregion, and all data were pooled to generate a G value
over the entire study range (herewith referred to as state-level, so as not to be confused with species
geographic range). As with host spectrum data, the additional effect of scale on host preference
was analysed at two levels: habitat within a bioregion and across the state.
Spatial turnover in host species - β-specificity—Similarity index (Chao-Sørensen)
Having evaluated host use at multiple scales, patterns of host use across different areas of
their geographic range were explored. In free-living organisms, geographical diversity can be
recorded as α-diversity—local species diversity—and β-diversity—the difference in composition
among regions. Similarly, in parasitic organisms, α-specificity reflects host specificity of a parasite
in one location, while β-specificity reflects spatial turnover in host species used across regions
(Poulin et al. 2011). β-specificity becomesDraft a measure of local host specialisation and the extent to
which hosts can be exchanged in space (Krasnov et al. 2011). β-specificity allows for the pairwise
comparison of bioregions to reveal how similar the suite of hosts used in one bioregion are to the
suite of hosts used in the next. Spatial turnover in host species or β-specificity was calculated in
using the Chao-Sørensen similarity index. This index is a probabilistic approach extended from
the Sørensen index (previously used by Krasnov et al. 2011; Poulin et al. 2011), which uses
abundance data and corrects for unseen shared species (Chao et al. 2005, 2006). Abundance data
were entered as repeated measures within bioregion in EstimateS v 9.1.0 (Colwell 2013). A
parasite with high β-specificity will have a score close to one where all host species are shared
between bioregions, while, a generalist parasite will have a score close to zero, indicating
extremely low geographic specificity meaning that a parasite species uses entirely different hosts
from location to location (Fadini 2011; Okubamichael et al. 2011).
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Results
A total of 11,825 plants were recorded as potential hosts in this study, with 2,516 infected with one of the three mistletoe species. A total of 1,339 individual hosts from 31 plant species were encountered as hosts for L. exocarpi. For A. quandang, 952 individual hosts were recorded, from 8 host species and A. lucasii was present on 225 individual hosts from a single species. A similar proportion of potential hosts were infected with L. exocarpi, A. quandang or A. lucasii
(0.19 ± 0.01, 0.27 ± 0.03 and 0.16 ± 0.04, respectively; mean ± SE).
Host specificity—specialist/generalist Index (G)
Mean specialist/generalist index G of L. exocarpi were greater than 0.2 (Fig. 1a), A. quandang were less than 0.5 (Fig. 1b)Draft and G did not exceed 0.11 for A. lucasii (Fig. 1c) at all spatial scales from plot to habitat to bioregion to state-level. From these values the degree of specialisation/generalisation (G) categorised the host spectrums of L. exocarpi and A. lucasii as having low specificity and very high specificity respectively, and A. quandang intermediate with high specificity. With further applications of our approach, these terms can be defined more explicitly with reference to specified index values.
The G for each of the three mistletoes differed with scale (Fig. 1). The ‘generalist’ mistletoe,
L. exocarpi, displayed progressively less specialisation with increasing spatial scale (Fig. 1a).
Nevertheless, even as mean G increased with scale at the habitat and bioregion scales, L. exocarpi showed a range of G values from a highly specialist parasite to a generalist parasite (G = 0.08–
0.85 and 0.12–0.91 respectively). With increasing scale, Amyema quandang initially followed a similar pattern to L. exocarpi, moving from a local specialist to less specialised-parasite at the regional scale. Unlike L. exocarpi, at the state-level scale, A. quandang showed high host specificity, similar to its specificity at the smallest scale (Fig. 1b). Despite showing high specificity
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at most scales, in the Riverine Plain Woodlands habitat of the Riverina bioregion, A. quandang
infected a single host species (Acacia pendula) where all the neighbouring trees were of the same
species resulting in G = 1, the maximum possible value and the highest G (lowest specificity) we
recorded. When this extreme case is omitted, A. quandang showed a narrow spectrum of specificity
at the habitat (G = 0.09–0.25) and bioregion (G = 0.14–0.22) scales. Finally, A. lucasii showed the
opposite pattern of the generalist mistletoe, with its high local-scale specialisation becoming
progressively more specialised at increasing spatial scales (Fig. 1c). This mistletoe displayed the
highest average degree of specificity at all scales (Fig. 1c) within the narrowest of ranges at the
sample plot scale (G = 0.07–0.11).
Host preference
All mistletoes recorded had a significantDraft preference for at least some of their recorded hosts.
Preference was considered at two scales, habitat within bioregion and across the sampled state-
level range. The three species showed differences in host preference with scale: Lysiana exocarpi
significantly preferred 26 % of hosts at the habitat level and 13 % of hosts at the state-level, while
the proportional preference for hosts at different scales did not differ for A. quandang or A. lucasii.
Amyema quandang showed significant preference for 50 % of its hosts and A. lucasii had 100%
preference for its host at the habitat and state-level scales.
Lysiana exocarpi showed significant preference for eight hosts (of 31 species infected) at
the habitat level (Fig. 2, Supplementary Table S1) of which three species occurring in multiple
habitats were significantly preferred (Casuarina pauper (Fig. 2a-c), Pittosporum angustifolium
(Fig. 2a,b) and Allocasuarina luehmannii (Fig 2e)). Acacia stenophylla (Fig. 2b) and Casuarina
cristata (Fig. 2d) were significantly preferred in the one habitat in which they occurred. Alectryon
oleifolius (Fig. 2a–d), Eremophila sturtii (Fig. 2a,b), and Acacia salicina (Fig. 2e) were
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significantly preferred in half of the habitats in which they occurred. In two habitats L. exocarpi
showed no preference for any host species (Fig. 2b,c). At the whole-of-distributional-range scale,
the number of significantly preferred hosts was reduced to four: P. angustifolium, A. oleifolius,
A. salicina and E. sturtii (in order of preference; Fig. 3, Supplementary Table S2). Genera of
available hosts that were never recorded as hosting L. exocarpi include Brachychiton, Callitris,
Chenopodium, Choretrum, Dodonaea, Eucalyptus, Geijera, Grevillea, Lycium, Maireana,
Myoporum, Notelaea, Pimelea, Prostanthera and Schinus (Supplementary Table S2).
At the habitat scale A. quandang significantly preferred four of eight hosts (Fig. 4,
Supplementary Table S3); two in all habitats in which they occurred (Acacia homalophylla (Fig.
4a,d) and A. loderi (Fig. 4c)) and two for most of the habitats in which they occurred (A. aneura
(Fig. 4a,c) and A. pendula (Fig. 4b,d)).Draft The remaining four hosts were infected in proportion to their relative abundance in the habitat (results for Acacia burkittii and Amyema linophylla, regarded as provisional as there were <5 individuals (Fig. 4a)). At the state-level scale of sampling,
A. quandang showed significant preference for the same four hosts (Fig. 5, Supplementary Table
S4). Aside from the single instance of an epiparasitic infection of Amyema linophylla, all recorded hosts were in the genus Acacia (Supplementary Table S4), despite the availability of 26 potential host species from 17 genera and 12 families.
The strict species-specialist A. lucasii showed preference for its sole host F. maculosa in both habitats sampled (North-west Plain Shrublands and Sand Plain Mulga Shrublands) and at the state-level scale (Supplementary Table S5). There were 26 potential host species (15 genera, 12 families) growing beside infected F. maculosa that were never recorded hosting A. lucasii
(Supplementary Table S6).
Spatial turnover in host species - β-specificity—Similarity index (Chao-Sørensen)
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Pairwise analysis of bioregions gives β-specificity values that reveal host turnover across
biogeographic regions by comparing similarity of host-use. As expected, based on its extreme high
specificity (Fig. 1), A. lucasii scored maximum similarity (1) between the Broken Hill Complex
and Cobar Peneplain, resulting from high β-specificity. By contrast, host-use between bioregions
for L. exocarpi ranged from high similarity of host species used for adjacent regions (0.85 for
Broken Hill Complex and Mulga Lands) to no shared host species for the most distant regional
pair (Cobar Peneplain and Sydney Basin) (Table 1). Host species present in the Sydney Basin were
the most distinct suite of species for L. exocarpi. Amyema quandang used similar hosts in the
Darling Riverine Plains and Cobar Peneplain (0.90) and also in the Darling Riverine Plains and
Riverina (0.93) (Table 2). Although the Darling Riverine Plains shared many species with both the Cobar Peneplain and Riverina, these twoDraft bioregions have low similarity to one another (0.17) (Table 2). For this species, Broken Hill Complex supported the most distinct suite of hosts.
Discussion
Given the confounding effects of sampling effort, previous work has been unable to
disentangle observed patterns of host use nor develop robust hypotheses concerning their
mechanistic basis. By using a quantitative approach to evaluate mistletoe infection relative to host
availability, we were able to test descriptions of host specificity and identify host species
significantly preferred by each mistletoe species. In addition to enabling a more thorough
exploration of the multiple determinants of a parasite’s host-spectrum, our application of results-
based stopping rules allows this work to be replicated efficiently at multiple scales, from localised
infections to across the distributional range of the parasite.
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Findings from each of the quantitative tests in this study support the qualitative descriptions
of L. exocarpi as generalist, A. quandang as genus specialist and A. lucasii as a strict specialist
(Watson 2019 and references therein). Lysiana exocarpi showed a low degree of specificity (high
G), with significant preference for only a small proportion of its hosts at the state-level scale and
an ability to switch hosts (low β-specificity) between bioregions. These findings suggest that this
species is an opportunistic parasite that can use a multitude of hosts (Krasnov et al. 2005; Takken
and Verhulst 2013). In stark contrast, Amyema lucasii had the highest degree of specificity (low
G) and β-specificity (spatial host turnover), displaying a strong preference for a single host species at all scales covered in this study. This finding confirms A. lucasii as one of the most specialised
mistletoes, occurring almost exclusively on F. maculosa (Barlow 1984; Cunningham et al. 1992). Along the host spectrum Amyema quandangDraft is intermediate, having a relatively high specificity and showing significant preference for half its Acacia hosts. Tests of β-specificity suggest
A. quandang is indeed a genus specialist; it has the ability to switch to other Acacia hosts when
needed, for example, when dispersed into a new environment where its source Acacia host does
not occur. However, qualitative descriptions were most applicable at the state-level range of the
study as host-specificity varied with scale.
As host use by a parasite is constrained at the smallest scales (localised infection at the
stand scale) by both adaptation to local conditions (Giorgi et al. 2004) and availability of hosts
(Jaenike 1990; Norton and Carpenter 1998) it was expected that all mistletoes would have high
specificity at the local scale. Amyema lucasii always recorded high specificity, however in both
A. quandang and L. exocarpi instances of high and low specificity were recorded at small scales.
Generally, A. quandang displayed low G within bioregions, but in Riverine Plain Woodlands, a
stand of A. pendula was infected by the mistletoe, resulting in the lowest specificity recorded for
any mistletoe (G = 1). This apparent expression as a generalist at the local scale was because there
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was no choice of host and aggregated infections are common due to disperser behaviour (Mourao
et al. 2016; Messias et al. 2014) and host compatibility (Fonturbel et al. 2017; Fonturbel et al.
2019). In the case of L. exocarpi, it is a specialist (G = 0.18) in Riverine Sandhill Woodlands
habitat of the Riverina, whereas in a sample plot in the Sand Plain Mulga Shrublands habitat of
the Broken Hill Complex it showed a low level of specificity (G = 0.57), suggesting it can be a
generalist at even the smallest scale. These variations in host spectrum for L. exocarpi result, in
part, from availability of hosts (Jaenike 1990; Norton and Carpenter 1998) but also highlight that
simple host lists do not inform us on the importance of host species to parasites. If we consider the
importance of hosts species, despite conflicting G values, in both these habitats A. oleifolius was
the only significantly preferred host. Host preference is a far more important measure of host use because preference can inform about anDraft organism’s survival requirements (Manly et al. 2002).
All study mistletoe species showed preference for at least some of their hosts, as recorded
in past studies (Norton et al. 1995; Giorgi et al. 2004; Fadini 2011; Blick et al. 2012). It was
expected that a specialist parasite would show preference for its hosts in a particular habitat and
continue to show this same preference at a larger scale, as seen for A. lucasii and A. quandang,
where both species recorded the same proportion of preferred hosts at the scales of habitat and
state-level. Generalist parasites, however, may show preference for certain hosts at the local scale
but use hosts in proportion to their availability at a larger scale (Krasnov et al. 2005; Takken and
Verhulst 2013). Lysiana exocarpi significantly preferred a higher proportion of its hosts at the
habitat scale (0.26) than over the state-level range (0.13). The four species preferred as hosts across
the state-level range were present in two or more habitats, suggesting that these hosts are
encountered sufficiently frequently to develop preference. Given that mistletoes on occasional host
species are smaller than those on main hosts (Yan 1990) host preference reflects the trade-off
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between benefits of high reproductive rates and costs associated with infecting more host species
(Tripet et al. 2002).
Host quality may also confer advantages that result in preference of certain host species.
Only a single taxon was preferred host to more than one mistletoe study species. Various Acacia
species were hosts of either or both A. quandang and L. exocarpi. Acacias are regarded as high- quality hosts due to nitrogen fixing abilities (Dean et al. 1994; Watson 2008). Casuarinaceae are also nitrogen fixers (Reddell et al. 1986) a that trait may explain L. exocarpi preference for three species in this family. Additionally, the open canopies of Acacias and Casuarinaceae allow high light to mistletoes, a necessary resource for these hemiparasites (Fonturbel et al. 2017; Sultan et al. 2018; Fonturbel et al. 2019).
In measuring nearest neighbours,Draft it was possible to identify those hosts that show high susceptibility and those that show high resistance to mistletoe infection. Host compatibility, which relates to how susceptible a host is to infection, plays a role in host preference. Specificity and preference develop as local adaptions to host availability (Blick et al. 2012). For example, seedling establishment is greater when seeds are from mistletoes infecting the same host species (Ramírez and Ornelas 2012) or growing in the same location (Reid and Stafford Smith 2000). Therefore, mistletoe survival rather than initial seed dispersal can explain aggregations (Fonturbel et al. 2017;
Fonturbel et al. 2019). The aggregation of A. quandang in a stand of pure A. homalophylla and
L. exocarpi using clonally-growing Casuarinaceae species highlights that low genetic diversity of hosts may increase host compatibility leading to host preference. These host-mistletoe interactions confirm that when hosts are abundant and predictable, parasites should show preference (Giorgi et al. 2004).
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Although our data relate to only a subset of the geographic range of the three mistletoes
studied, we observed some noteworthy patterns regarding susceptibility of particular host species.
Of particular interest, there were some host species that are apparently resistant to infection. In this
study Callitris glaucophylla was often recorded as a nearest neighbour but not once as a host. As
a gymnosperm, tracheids are used for water transport rather than the wider xylem vessels of an
angiosperm, perhaps making the vascular system incompatible. Note that this species has been
previously recorded as a host, but most records relate to the genus-specific mistletoe Muellerina
bidwillii. Muellerina is an early branch of the Loranthaceae, endemic to Australia, with the recently
described Muellerina flexialabastra dependent on Araucaria cunninghamii as primary host
suggesting that the association between these mistletoes and conifers may be a very old one (Watson 2019). Within the family Scrophulariaceae,Draft the genus Myoporum was resistant to infection but Eremophila species were not. This is an interesting observation as parasites often use
closely related hosts (Poulin and Mouillot 2005; Poulin et al. 2011) and Myoporum has been
recorded as a host of both A. quandang and L. exocarpi previously (Downey 1998). This finding
requires further work to evaluate the basis of differential susceptibility, highlighting the value of
unbiased measures of host preference to identify patterns and frame hypotheses to stimulate further
work.
Until now, the G index has only been applied to mutualistic associations (Taki and Kevan
2007), the data presented here representing the first application of Medan et al.’s (2006) approach
to quantify host specificity in a parasite. To inspire further work on other mistletoe assemblages
and host-parasite interactions more broadly, we suggest the following nominal thresholds. For
resultant G values less than 0.2, parasites are strict specialists; G values between 0.2 and 0.4 are
specialists, and G values greater than 0.4 are generalists. The use of Manly et al. (2002) preference
scores which includes proportional availability of potential hosts is a robust way to assess the
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18 importance of a host to a parasite and identify species that are resistant to parasite infection. In addition to quantifying patterns of host specialisation and preference, our analyses revealed the importance of spatial scale as a moderating influence, patterns of preference changing as data from multiple locations, habitats and bioregions are incorporated.
Acknowledgements
We thank the following people for assistance with field work: Karen Avery, Adrian Boyd, Melinda
Cook, David Roach and Wendy Scott, and to Maggie Watson for logistical support during field work. We acknowledge the constructive input of two anonymous referees on an earlier version of this manuscript. Draft References
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Table 1. Similarity matrix of β-specificity values of host use for Lysiana exocarpi. Pairwise
comparisons of bioregions based on Chao-Sørensen similarity index.
Broken Hill Mulga Riverina Cobar Sydney Complex Lands Peneplain Basin Broken Hill - Complex Mulga Lands 0.85 - Riverina 0.43 0.56 - Cobar Peneplain 0.31 0.15 0.42 - Sydney Basin 0.20 0.01 0.01 0 -
Draft
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Table 2. Similarity matrix of β-specificity values for Amyema quandang. Pairwise comparisons of bioregions based on Chao-Sørensen similarity index.
Darling Riverine Broken Hill Cobar Peneplain Riverina Plains Complex Cobar Peneplain - Darling Riverine 0.90 - Plains Broken Hill 0.19 0 - Complex Riverina 0.17 0.93 0 -
Draft
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Figure 1. Mean mistletoe specialisation/generalisation scores (G) in response to scale. (a) Lysiana
exocarpi (sample plot, n =25; habitat, n =12; bioregion, n =5; state-level, n =1); (b) Amyema
quandang (sample plot, n =13; habitat, n =6; bioregion, n =4; state-level, n =1). (c) Amyema
lucasii (plot, n =5; bioregion, n =2, habitat was nested in bioregion so only bioregion displayed;
state-level, n =1). Error bars show the standard error.
Figure 2. Preference scores (ω) for Lysiana exocarpi hosts pooled into habitat by bioregion.
Dashed line shows ω = 1, lower confidence interval >1 indicates host is significantly preferred.
Species denoted with * are significantly preferred hosts of L. exocarpi. (a) Broken Hill Complex
(b) Mulga Lands (c) Riverina (d) Cobar Peneplain (e) Sydney Basin. Note y axes have different
scales. Draft Figure 3. Preferred host species of Lysiana exocarpi over the state-level range of the study.
Dashed line shows ω = 1, lower confidence interval >1 indicates host is significantly preferred.
Species denoted with * are significantly preferred hosts of L. exocarpi.
Figure 4. Preference scores (ω) for Amyema quandang hosts pooled into habitat by bioregion.
Dashed line shows ω = 1, lower confidence interval >1 indicates host is significantly preferred.
Species denoted with * are significantly preferred hosts of A. quandang. Bioregions sampled are:
(a) Cobar Peneplain; (b) Riverina; (c) Broken Hill Complex; and (d) Darling Riverine Plains. Note
Y axes show different scales.
Figure 5. Preferred hosts of Amyema quandang over the state-level range of the study. Dashed
line shows ω = 1, lower confidence interval >1 indicates host is significantly preferred. Species
denoted with * are significantly preferred hosts of A. quandang.
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Draft
Figure 1. Mean mistletoe specialisation/generalisation scores (G) in response to scale. (a) Lysiana exocarpi (sample plot, n = 25; habitat, n = 12; bioregion, n = 5; state-level, n = 1); (b) Amyema quandang (sample plot, n = 13; habitat, n = 6; bioregion, n = 4; state-level, n = 1). (c) Amyema lucasii (plot, n = 5; bioregion, n = 2, habitat was nested in bioregion so only bioregion displayed; state-level, n = 1). Error bars show the standard error.
185x159mm (300 x 300 DPI)
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Draft
Figure 2. Preference scores (ω) for Lysiana exocarpi hosts pooled into habitat by bioregion. Dashed line shows ω = 1, lower confidence interval >1 indicates host is significantly preferred. Species denoted with * are significantly preferred hosts of L. exocarpi. (a) Broken Hill Complex (b) Mulga Lands (c) Riverina (d) Cobar Peneplain (e) Sydney Basin. Note y axes have different scales.
177x288mm (300 x 300 DPI)
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Figure 3. Preferred host species of Lysiana exocarpi over the state-level range of the study. Dashed line shows ω = 1, lower confidence interval >1 Draftindicates host is significantly preferred. Species denoted with * are significantly preferred hosts of L. exocarpi.
177x101mm (300 x 300 DPI)
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Draft
Figure 4. Preference scores (ω) for Amyema quandang hosts pooled into habitat by bioregion. Dashed line shows ω = 1, lower confidence interval >1 indicates host is significantly preferred. Species denoted with * are significantly preferred hosts of A. quandang. Bioregions sampled are: (a) Cobar Peneplain; (b) Riverina; (c) Broken Hill Complex; and (d) Darling Riverine Plains. Note Y axes show different scales.
177x204mm (300 x 300 DPI)
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Figure 5. Preferred hosts of Amyema quandang over the state-level range of the study. Dashed line shows ω = 1, lower confidence interval >1 indicatesDraft host is significantly preferred. Species denoted with * are significantly preferred hosts of A. quandang.
177x101mm (300 x 300 DPI)
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