Host specificity of parasites
Definition of host specificity
► The extent to which a particular parasitic taxon is restricted
to a number of host species
► A real property of parasites - it occurs repeatedly in
different populations of the parasite
► It is the result of evolutionary events and given ecological
conditions
► Specialist versus generalist concept
Specialist (specific species) – on/in one host species
Generalist – on/in two or more host species
Basic host specificity
► Host range – measure of host specificity, number of
infected host species by a given parasite species
► High host range = low host specificity
► Each of two species parasitize the same number of host
species. Do they exhibit the same host specificity?
► Basic host specificity – number of host species infected by
a given parasite species
► Ecological availability of host species, phylogenetic
relatedness of host species, geographical distribution of
host species
Effect of sampling on host specificity
► High host specificity - a possible sampling artifact
► Host specificity of parasites affected by rare and frequently
sampled hosts
► Sample size correction
►Ex. (a) ectoparasites, and (b) endoparasites in freshwater fish of Canada
Structural specificity
► Ecological importance of the host for the parasite population
► Differences in abundance, intensity of infection and
prevalence of parasite species
A B
C D
► Host specificity indices
► the intensity and frequency of use of the different host
species by the parasite
Rohde (1980)
S = ∑(xi/nihi)/∑(xi/ni)
where xi is the number of individuals of the parasite on ith
host species, ni is the number of individuals studied of the
ith host species, xi/ni is the abundance of the parasite on
the host species i, and hi is the position of the host species
i (the host species with higher parasite abundance has the
position 1).
Structural specificity
Structural specificity
► Ex. Comparison of host specificity of two digenean species
parasitizing marine fishes
► Rohde and Rohde (2008) - index removing sensitivity for
the number of infected hosts
► Species diversity indices - number of species plus species
abundance (e.g. Shannon index) or Levin´s niche width
► Ideal indices of structural specificity - use and availability of
the host (abundance of the parasite in combination with
abundance of the host in the environment)
Structural specificity
Phylogenetic specificity
► host relatedness included in host specificity
A B
C D
► Host specificity indices from a phylogenetic perspective
► Desdevises et al. (2002) and others
Index based on semi-quantitative classification
IHS = 1 - strict specialist (species specific, highly host
specific)
IHS = 2 - intermediate specialist - congeneric host species
IHS = 3 - intermediate generalist - phylogenetically related
hosts
IHS = 4 - generalist - phylogenetically unrelated hosts
Phylogenetic specificity
► Poulin & Mouillot (2003)
STD = 2 ∑∑i<j ωij /s (s -1)
s is the number of host species used by a given species of
parasite, the double sum includes the set {i = 1,...s; j =
1,...s, where i < j} and ωij is the taxonomic difference
between species i and j , or the number of taxonomic steps
required to node representing a common ancestor of both
species
Phylogenetic specificity
Index of phylogenetic specificity
Minimum IS = 1 – host-specific parasite species
Hostspecies
Comparison of host specificity of 3 species of parasites
Host specificity
► Expressed using basic, structural and phylogenetic
components
Hypothetical relationship between structural and phylogenetic components of
host specificity. Abundance is affected by physiological or other factors.
A
B
C
D
► It combines evolutionary and ecological aspects of HS
► Poulin & Mouillot (2003)
index of HS – phylogenetic distance of host species plus
parasite prevalence in different host species
Prevalence weight factor 1 = 100% in two host species, 0 very
low
STD* = ∑∑i<j ωij (pipj)/ ∑∑i<j (pipj)
ωij is the taxonomic difference between species i and j or
the number of taxonomic steps required to node of a
common ancestor of both species, pi a pj are the
prevalence of the parasite in host species i and j.
Phylostructural specificity
► 3 types of parasites
► Parasite "A" achieves high abundance in two congeneric
host species and low abundance in two other species from
different genera
► Parasite "B" achieves the same abundance in 4 different
(phylogenetically distant) species
► Parasite "C" achieves higher abundance in two host
species from different taxa and lower in the other two also
from different taxa
Which one is more specific?
Phylostructural specificity
Phylostructural specificity index
Host specificity in geographical space
► Host communities with different composition and relative
density in different localities
► Consistency of the parasite to use of the host species in
geographical space
► Ex. Host specificity of parasite A vs. parasite B
parasite A - same host species in 2 localities, same
abundance in 2 localities, phylogenetically related host species
parasite B – different host species in 2 localities, but same
abundance in 2 localities, phylogenetically related host species
which is more specific?
Phylobetaspecificity - a combination of
phylogenetic and geographical specificity
A B
C D
Host specificity for a given site (locality) – α-specificity
in all localities – β-specificity
(analogy to α-diversity and β-diversity)
!!! The problem of defining host specificity
local level - species-specific parasite (specialist)
regional level - non-specific parasite (generalist)
α-specificity and β-specificity
Definition of host specificity
► Local level: absence of a certain suitable host species, low
sampling size
► Global level: erroneous taxonomic determination, problem of
random hosts
► For some generalist parasites:
global level: a wide range of host species
preferred (common) host species - abundance of the
parasites is the highest to maintain the parasite population
additional host species - the abundance of the parasite is low
local level: preferred host species only
Macroevolutionary processes - history of hostparasitic
associations
► Cospeciaton between hosts and host-specific parasites identical
phylogenetic reconstruction of hosts and parasites
(Fahrenholz rule)
► Host specificity does not
have to reflect co-speciation
– highly specific parasitehost
systems with
coevolution form intra-host
speciation a "host switching"
Macroevolutionary processes - history of hostparasitic
associations
Evolution of host specificity
► A specialist evolves from a generalist, and many specialists
arise from other specialists
► Evolution leads to host preference → rapid coevolution
between specialists and the immune system
► !! Specialist ancestral character, generalist derived
Microevolutionary processes
► Physiological and ecological factors of the host and
parasites
► Natural selection favors certain changes in host specificity increasing
or decreasing?
► trade-off between the ability to use a lot of hosts and the
average parasite fitness on/in those hosts
► Trade off documented in metazoan parasites of freshwater
fish in North America
► The opposite relationship is also documented (fleas
parasitizing small mammals)
► no relationship (helminths parasitizing birds)
► Existence of a trade-off linked to the costs associated with
adaptation to multiple host species
► Sometimes small costs, for example, the nematode
Howardula aoronymphium parasitizing several species of
Drosophila
experiment 25 generations - selection for one host species,
parasite still retains the ability to parasitize on different
host species
Microevolutionary processes
► Host specificity is determined by the possibilities for
colonization and the availability of a suitable host
► Ex. Host hybridization - a genetic and ecological bridge
between host species for parasites
Microevolutionary processes
Ex. Prevalence of infection Paradiplozoon homoin as a function of hybridization
between Barbus meridionalis and B. barbus
Microevolutionary processes - adaptation
► Host specificity is associated with adaptation and
specialization
► Evolution of host specificity with evolution of morphological
adaptation (e.g. morphology of parasite attachment
organs)
► Possible biochemical interactions
► !!! Morphological adaptation in generalists – e.g. robust
hooks in Dactylogyrus (Monogenea) - necessity of
adaptation to more hosts?
Host specificity and adaptation in congeneric
monogeneans
► Ex. 51 species of the genus Dactylogyrus from 20 species
of freshwater fish
Host specificity and adaptation in congeneric
monogeneans
Rasborinae*
Alburninae*
Leuciscinae*
Percidae
Cyprininae*
Gobioninae*
equivocal
Anchors
Ventral connective bar
Dorsal connective bar
Host mapping
Microevolutionary processes - adaptation
► Selection for higher specificity at the population level
► Adaptation to the local host population
Ex. Diplostomum phoxini (Digenea) in fish in two Swiss lakes, Microphallus sp.
at the intermediate host of a snail from two lakes in New Zealand
Observed patterns of host specificity
► Species-rich parasite taxa most
parasites tend to be
host-specific
► Distribution of host specificity
within taxa, distribution is skewed
► Ex. Frequency distributions of number
of known host species for Cestoda,
Digenea and Nematoda in birds
► Frequency distribution of two measures of host specificity number
of host species and index of host specificity
Ex. Helmints of freshwater fish of Canada
Observed patterns of host specificity
► Parasitic groups differ in their host specificity
► Parasites with a simple life cycle - more host-specific
► Parasites with a complex life cycle - higher specificity for
intermediate hosts than for the final host
► Host specificity of helminths
► Ectoparasitic Monogenea - highly specific, more than half
on one host species
► Endoparasitic helminths - higher range of DH species but
strict specificity of miracidia to IH (snails)
Observed patterns of host specificity
► Increasing the number of host species due to host
switching processes
► The alternative host species shows some physiological or
ecological similarity
Observed patterns of host specificity
►Ex. Relationship between the number of known host species and the number of
potential host species for ectoparasites and endoparasites of freshwater fish in
Canada
Determinants of host specificity
► Specialization on predictable sources
- specialization on stable sources minimizes the risk of
extinction
- - stable host for parasites = large, long-lived, with high
density, at the top of the food chain → higher fecundity and
survival of parasites
Ex. 44 species of the genus Dactylogyrus (Monogenea) on 19
cyprinid species
1,55
1,6
1,65
1,7
1,75
1,8
1,85
Hostlength(inlog)
G S
Generalist versus specialist
► Ex. Dactylogyrus (Monogenea) specialists living on longer
hosts have longer attachment hooks = optimization of
morphological adaptation
Determinants of host specificity
y = 0,0755x - 0,1338
R2
= 0,017
-0,3
-0,2
-0,1
0
0,1
0,2
0,3
0,4
0,5
1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2 2,1
Host length (in log)
Totallengthofanchor
(inlogcontrolledfor
parasitelength)
y = 0,311x - 0,5484
R2
= 0,3882
-0,15
-0,1
-0,05
0
0,05
0,1
0,15
0,2
1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2 2,1
Host length (in log)
Totallengthofanchor
(inlogcontrolledfor
parasitelength)
Specialists Generalists
Ecological specialization
► Species that use more resources show higher local
abundance and are more widespread in nature than
species that use a narrow range of resources (Brown,
1984)
► Host-parasitic systems: generalists use more hosts →
higher abundance and prevalence (measure of distribution
within hosts) than specialists
Specializing on enemy-free space
y = -0,4251x + 6E-18
R2
= 0,1432
-0,8
-0,7
-0,6
-0,5
-0,4
-0,3
-0,2
-0,1
0
0,1
0,2
0,3
-0,3 -0,2 -0,1 0 0,1 0,2 0,3 0,4 0,5
Host range
(in log controlled for sampling size)
MeanRSP(inlogcontrolled
forsamplingsize)
► Species specialization in the absence of potential
competitors (Jeffries & Lawton, 1984) = specialists occur in
species-poor communities
Not confirmed for parasites