Evolutionary Ecology of Parasites
What is evolutionary ecology of parasites?
1. Evolution of characters in parasite individual, population
and community
2. Change of strategy for an organism from free living to
parasitic → biological changes
3. Evolution of parasite ability to use one host or most hosts
4. Evolution of life strategy (life traits history) – effects of
selection given by host, environment and phylogeny
5. Evolution of virulence, evolution of manipulation by
parasites (to manipulate host behaviour)
6. Speciation and diversification of parasites, host-parasite
coevolution
7. Host specificity of parasites – effects of phylogeny,
specialization and adaptation
8. Population ecology of parasites – evolutionary causes and
ecological consequences of aggregation, dynamics of
parasite populations – effect of evolution on population
processes
What is evolutionary ecology of parasites?
9. Structure of parasite communities – random or predicted
(organized communities)
10. Interactions between parasites – character of interactions,
response to interspecific competition, causes and
consequences of competition in ecological and evolutionary
times
11. Parasite diversity – determinants, biogeographical
aspects
12. Immunoecology and evolution of host immunity – tradeoff,
immunity vs. parasitism, host immune genes and
parasite-mediated selection, role of parasites in evolution
of host sexual selection
What is evolutionary ecology of parasites?
Evolutionarly ecological approach
► Ecological approach – associations in short time
who harms whom?
► Evolutionary approach – long-termed associations
host-parasite coevolution
evolution of immune components
Evolutionary ecology – study of selective pressures from the
environment and evolutionary responses to them
Natural selection affects the characters of the individual, the
characters of populations and communities
Hierarchical levels of study – individual, population and
community
Parasitism from an ecological point of view
► Reciprocal interaction, one gains an advantage, the other is
damaged
► Some form of symbiosis (close bilateral interaction), benefit
for one species, loss for another species
► Widespread biological phenomenon, high diversity and a
large number of niches
Evolutionary success of the parasitism
strategy
► Strategy to find a host
► Penetration and attachment strategies
► Adaptation to an unfavorable host environment
► Ability of the parasite to feed on the host
► Ability to defend against the immune system
► Ability to reproduce in the host and ability to disperse
Parasite
► It lives for the whole life or part of its life on the body or
inside the body of another organism (= host), it feeds at
the expense of the host
► benefit to the parasite, costs to the host
► consumer versus prey
► biotrophic organism
Typical parasite
► One host, very little or no pathological manifestations
► Host survives
► Following Anderson and May this category is separated :
pathogens – intensity independent models (microparasites)
typical parasites – intensity dependent models
(macroparasites)
!!! Trophically transmissible typical parasite or pathogen host
death necessary
Parasitoid
► One host
► The parasite always causes the death of its host
► Parasitic insect larvae Diptera (Tachinidae) and
Hymenoptera (Chalcidoidea, Braconidae), physiological
adaptations (endosymbiotic viruses)
► The female lays eggs in the host, hatched larvae are
parasitic
Parasitic castrator
► It uses the host's energy for its reproduction
► It kills the host from an evolutionary point of view - it
blocks reproduction and reduces fitness
► Partial castrator - a transition between a typical parasite
and a parasitic castrator
Micropredator and vector
► Micropredator
► Multiple hosts (more specimens of a given species)
► micropredator does not kill the prey
► Vector transmits diseases
► Flies - mechanical transmission
► Anopheles transmits malaria (Plasmodium)
► Phlebotomus and Lutzomia transmit Leichmania
► Lice transmits typhus
► Fleas cause encephalitidis and other diseases
► Tse-tse flies (Glosina) – sleeping sickness (Trypanosoma)
Ecological definition of parasites
► Number of hosts attacked during the life of the parasite
► Influence of the parasite on the biological fitness of the
host
► Link between infection intensity and host mortality
► Benefit of host death for parasites
Effect on
biological
fitness
Number of casualties per 1 attacker
One victim More victims
death
disadvantageous
death
advantageous
Less than
100%
Typical parasite Trophically
transmissible
parasite
micropredator
100%:
victim has
zero fitness
Parcial castrator Trophically
transmissible
castrator
Social
predator
Ecological definition of parasite / prey effects
depending on the number of parasites
Effect on
biological
fitness
Number of casualties per 1 attacker
One victim More
victims
death
disadvantageous
death
advantageous
Less than
100%
Monogenea
Ichthyophthirius
metacercariae of
digeneas
mosquito
100% victim
has zero
fitness
Cysticerkoids of
Hymenolepis
diminuta
Isopoda
Plerocerkoids of
Schistocephalus
wolf
Ecological definition of parasite / prey effects
depending on the number of parasites
Ecological definition of parasite / prey - effects
independing on the number of parasites
Effect on
biological
fitness
Number of casualties per 1 attacker
One victim More
victims
death
disadvantageous
death
advantageous
Less than
100%
patogen trophically
transmissible
patogen
herbivor
100%: victim
has zero
fitness
Parasitic
castrator
parasitoid predator
Effect on
biological
fitness
Number of casualties per 1 attacker
One victim More victims
death
disadvantageous
death
advantageous
Less than
100%
Giardia,
influenza virus
Toxoplasma deer
100% victim
has zero
fitness
Sacculina lumek sparrow
Ecological definition of parasite / prey - effects
independing on the number of parasites
Parasite size versus host
Frequency of strategy
Classification of parasites in ecology
► Microparasite versus macroparasite (Andreson & May,
1979)
► Microparasites - short generation time, high rate of
reproduction, reproduction in the host, no infectious
stages, acute illness - death or recovering, induction of
immunity against reinfection
► Macroparasites - longer generation time, reproduction
outside the host, development and growth in the host,
produce infectious stages, host immune response
(relatively short) depends on the number of parasites,
insignificant mortality, frequent reinfection
► Microparasite versus macroparasite
► The size of the parasite is not a determining criterion
► Microparasite – bacteria, fungi, protozoa
► Macroparasite – helminths, arthropods
► Aphids, digenean larvae in snails – microparasite
► Ichthyophthirius, Eimeria tenela, necrotrophic fungi -
macroparasite
Classification of parasites in ecology
► Zooparasite vs. phytoparasite
► Ectoparasite vs. endoparasite
► Endoparasites according to the location:
- Intestinal Entamoeba histolytica, Trematoda, Cestoda
- In blood plasma (Trypanosoma), in blood
cells(Plasmodium)
- Intracellular tissue (Toxoplasma gondii, Leishmania)
Epicellular (Giardia intestinalis)
Intercellular (Myxosporidia)
- Cavital (Entamoeba gingivalis, Trichomonas vaginalis)
► Ectopic localization: Paragonimus westermani, Fasciola
hepatica in brain
Classification of parasites in ecology
► According to the link to the host
obligatory (Monogenea, Digenea, Cestoda)
optional (facultative) (Nematoda Micronema)
hyperparasite – Udonella on parasitic Crustacea
► According to the time period of the parasitism
permanent (Plasmodium, Entamoeba)
temporal (Argulus, Ixodes)
periodical parasitism
- stage (glochidia of Mollusca,
larvae of Diptera)
- generation (Rabdias bufonis)
Classification of parasites in ecology
► By type of life cycle
monoxenus (stenoxenous) – life cycle with one host
(Monogenea)
heteroxenous (euryxenous) – life cycle with more hosts
(Digenea, Cestoda)
► By type of feeding
monophagous (stenophagous) – feeding on/in one host
(Eudiplozoon nipponicum) (strict-specific, species-specific)
polyphagous (euryphagous) – feeding on/in more hosts
(Trichinella spiralis, Posthodiplostomum cuticola)
Classification of parasites in ecology
Classification of parasites in ecology
► Host specificity = range of used host species (definitive
hosts or intermediate hosts)
► Narrow host specificity (monophagous, specialists, hostspecific)
– Taenia solium, Schistosoma haematobium – DH
humans
► Wide host specificity (polyphagous, generalists) –
Trichinella spiralis – DH warm-blooded vertebrates
► Parasites with complex life cycle – different host specificity
at the levels of intermediate host and definitive host
Classification of hosts
► Definitive host - a host on/in which the parasite matures
sexually and produces eggs or larvae
► Intermediate host – necessary for the development of larval
stages of the parasite
- IH = invasive stage → definitive host
- one or more intermediate hosts (Digenea, Cestoda)
- humans as an intermediate host (Echinococcus, Taenia
solium)
► Paranthenic (transport) host
no development of parasite
parasite survives and retains ability to invade
not necessary to complete the developmental cycle of
the parasite
important source of infection for IH or DH
e.g. mollusks for larvae of nematodes with development in
short-lived crustaceans
► Reservoir host
= source of parasite infection for the ecosystem
- the parasite survives even in conditions unsuitable for a
common host
E.g. Rats and carnivores for Trichinella, Schistosoma
japonicum - eggs into the environment, a source of
infection for humans - epidemiological significance
Classification of hosts
Parasitism less known – parasitic plants
► 1% angiosperm plants (3000 species)
► Presence of chlorophyll – hemiparasites, absence of
chlorophyll – holoparasites
► Depending on the connection point to the host – root and
stem
► Connection to the host vascular system = haustorium
► i.e. cosmopolitanly expanded Viscum (mistletoe)
The course of infection
1) formation of apresorium (organ used to mechanically disrupt cell walls)
2) growth of haustorium (thin fibers), which can exert considerable
pressure on the cell wall
3) penetration through the cell wall to the membrane
4) branching without breaking the membrane
Parasitism less known – parasitic plants
Loranthus europaeus
host: Quercus
Lathraea squamaria
host: woody plants (roots)
Orobanche (more species)
host: legumes, deafblind,
sunflower, hemp
► birds
► Intraspecific parasitism (ducks, songbirds)
► Interspecific (cuckoos…)
► Cuculus canorus – about 100 species of songbirds as hosts,
host with smaller eggs, of similar colour, strong shell,
shorter incubation time
Parasitism less known – nest parasitism
Cowbirds (Icteridae)
► Nest parasitism in Hymenoptera
► Social parasitism in ants
- temporary social parasitism - the new queen kills the host
queen
- slavery - the use of an ant worker of wild species
- permanent parasitism without slavery - the parasitic
species uses the organization to host nests, produces a
sexual caste
Parasitism less known – nest and social
parasitism in insects
Polyergus breviceps
Host: Formica argentea
Evolution of parasitism
► Independent origin in different groups of organisms –
parasitism has multiple origin
► Reversibility of the transition to parasitism (not all)
► It is created on the basis of unique pre-adaptations and
historical events
► The transition to parasitism must be beneficial - increase
fitness
Origin of parasitism
► at least 50 times in Metazoa
► 1. Free living organisms → a part of the group creates
parasitic lineage (e.g. Heteroptera – the two lines switched
to a parasitic way of life, Isopoda, Amphipoda, Copepoda,
Nematoda – 4 times switched to a parasitic way of life)
► 2. Diversification and evolution of parasitic group – splitting
the group and changing the strategy (e.g. Digenea +
Monogenea + Cestoda = monophyletic group,
Acanthocephala)
Evolution of parasitism – increase of fitness
The complexity of the life cycle increases the fitness parasite
e.g. two species of parasites living in sympatry
Evolution of parasitism
► Two evolutionary scenarios in Diplomonadida
Evolution of parasitism
► Regression evolution → lower structural complexity =
simplification
► Morphology - loss of some organs
reduction of some organs (saculinization) - locomotor,
nervous and sensory systems
however, exceptionally complex nervous and sensory organs:
Aspidogastrea Lobatostoma manteri, Monogenea
Polystomoides malayi
larvae 8000 and adult 20.000-40.000 of sensory receptors
► Reduction of genome size (unused parts)
however, big genome in some parasites
► Reducing body size in parasites
dependence on the size of the host's body = Harrison´s
rule
12 m didymozoid trematodean species in Mola mola
40 m Tetragonoporus calyptocephalus in cetaceans
Schistocephalus solidus up to 2x heavier than a fish host
► Optimal virulence
evolution towards optimal virulence (depends on the mode
of transmission, host availability…) - maximizes fecundity
during the life of the parasite
Evolution of parasitism
► High fecundity
► some parasites have behavior that does not require high
fecundity (modify host behavior)
Evolution of parasite fecundity
Number of eggs during
an individual's
lifetime
Multiplication of
larval stages
Turbellaria (free living) 10 1
Monogenea (ectoparasites) 1000 1
Digenea (endoparasites) 10 milions ≥1000
Cestoda (endoparasites) 10 milions 1-1000
► Free living way of life → parasitic way of life – simple life
cycle and sexual reproduction → complex life cycle –
sexual and asexual reproduction, changes of parasite
phenotypes, hermaphroditism, parthenogenesis
Evolution of life cycle
Evolution of life cycle in parasites: increasing
complexity of development
► 2 ways of changes
upward incorporation
downward incorporation
Evolution of life cycle in parasites: increasing
complexity of development
► Survival and growth in the new host generates selection to
shift sexual maturity and reproduction
upward incorporation
of new host
M – sexual maturity
Evolution of parasitism: shortening the
development
► Advantageous under certain conditions
► E.g. Sangunicolidae, Spirorchidae and Schistosomatidae
(Digenea)
Figure: Trematoda of the genus Alloglossidium
Evolution of parasitism: shortening the
development
► Three-host life cycle is reduced to two-host or single-host
life cycle (several ways)
Figure: Trematoda, A – adult, MC – metacercaria, S – sporocyst, R - rediae