Schistocephalus Solidus
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''Schistocephalus solidus'' is a
tapeworm Eucestoda, commonly referred to as tapeworms, is the larger of the two subclasses of flatworms in the class Cestoda (the other subclass being Cestodaria). Larvae have six posterior hooks on the scolex (head), in contrast to the ten-hooked Ce ...
of fish, fish-eating birds and rodents. This
hermaphroditic A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...
belongs to the Eucestoda subclass, of class Cestoda. This species has been used to demonstrate that cross-fertilization produces a higher infective success rate than
self-fertilization Autogamy or self-fertilization refers to the Cell fusion, fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a Reproduction, reproductive mechanism employed by many flowering pl ...
.


Life cycle

It parasitizes fish and fish-eating water birds. The fish-eating water bird is the
definitive host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include ...
, and reproduction occurs in the bird's intestine. Eggs of the tapeworm are passed with the bird's feces and hatch in the water, where the first larval stage, the coracidium, is produced. The coracidium is then ingested by the first
intermediate host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include ...
, a cyclopoid copepod (e.g. '' Macrocyclops albidus''). The second larval stage then subsequently develops in the tissue of this host. Within one to two weeks, the infected copepod is ingested by the second intermediate host, the
three-spined stickleback The three-spined stickleback (''Gasterosteus aculeatus'') is a fish native to most inland and coastal waters north of 30°N. It has long been a subject of scientific study for many reasons. It shows great morphological variation throughout its ra ...
, ''Gasterosteus aculeatus''. The third larval stage, the plerocercoid, grows in the abdomen of the fish. When the fish is eaten by a bird, the larvae mature and adults start to produce eggs within two days. Reproduction takes place within one to two weeks, after which the parasite dies.


Ecology

Prevalence In epidemiology, prevalence is the proportion of a particular population found to be affected by a medical condition (typically a disease or a risk factor such as smoking or seatbelt use) at a specific time. It is derived by comparing the number o ...
— the proportion of host population infected — in naturally infected populations of the first intermediate hosts is likely low. Conversely, in populations where ''Schistocephalus solidus'' infects the second intermediate host (three-spined stickleback) it can reach high prevalence, up to 93% in both European and North American populations The growth of ''S. solidus'' in the second intermediate host is largely dependent upon the environmental temperatures. At an increase of temperature from 15 °C to 20 °C the growth of ''S. solidus'' can grow four times as fast. At the same time, the growth rate of the stickleback is significantly reduced.


Reproduction

Reproduction of ''S. solidus'' in the definitive bird host in which it resides for a maximum of two weeks. Because adult worms are
hermaphroditic A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...
eggs can be fertilised in three different ways; (1)
self-fertilization Autogamy or self-fertilization refers to the Cell fusion, fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a Reproduction, reproductive mechanism employed by many flowering pl ...
(2) breeding with a sibling (3) breeding with an unrelated individual. In most species
outbreeding Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing distantly related genetic material into a breeding line, thereby increasing genetic diversity. Outcrossing in animals Out ...
(mating with an unrelated individual) would be preferred, but advantages and disadvantages of each of these breeding strategies have been argued. In short, self-fertilization is advantageous when no mating partners are around, but might lead to
inbreeding depression Inbreeding depression is the reduced biological fitness caused by loss of genetic diversity as a consequence of inbreeding, the breeding of individuals closely related genetically. This loss of genetic diversity results from small population siz ...
—the reduced fitness of offspring because of the unmasking of deleterious recessive alleles due to the breeding of closely related individuals. Similarly, breeding with a sibling, also known as incestuous mating, also shares some of the same disadvantages as self-fertilization does—inbreeding depression and lack of genetic variation. But incestuous mating is advantageous because it helps maintain gene complexes within the family which may be important for
local adaptation Local may refer to: Geography and transportation * Local (train), a train serving local traffic demand * Local, Missouri, a community in the United States Arts, entertainment, and media * ''Local'' (comics), a limited series comic book by Bria ...
. Breeding with unrelated individuals might seem to be most advantageous choice of mating because it increases genetic variation and avoids inbreeding depression, but it could be more time-consuming as partners might not always be available. In ''Schistocephalus solidus'' inbreeding is indeed disadvantageous, as mating between siblings generally produce a 3.5 times reduction in hatching success of the eggs produced from these matings compared to mating with unrelated individuals. Outcrossing also increases the chances of infecting the second intermediate host. However, there is also a preference to pair with larger mates, and to avoid very small mates. The later means that self-fertilisation can also occur when potential partners are available. Under some circumstances, there could exist a significant advantage for incestuous mating, despite inbreeding depression. In species where there is low parental investment and sexual encounters are rare and sequential, incestuous breeding is indirectly beneficial. If the prospective mates are related there is an increase mutual interest in finding a resolution with respect to playing the unpreferred sexual role. With less time allotted to conflicting over sexual roles and dominating one another, procreation is more cost-effective. Under these conditions, the greater effectiveness of inbreeding prevails over the detriment of incestuous mating and evolutionarily select for a preference for related mates.


Infectivity

Corracidia are more infective to male copepods than to female copepods. This has been suggested to be due to the negative impacts sex hormones such as
testosterone Testosterone is the primary male sex hormone and androgen in Male, males. In humans, testosterone plays a key role in the development of Male reproductive system, male reproductive tissues such as testicles and prostate, as well as promoting se ...
can have on the immune system.


Viruses

''Schistocephalus solidus'' itself a parasite, can also get infected by parasites (known as
hyperparasite A hyperparasite, also known as a metaparasite, is a parasite whose host, often an insect, is also a parasite, often specifically a parasitoid. Hyperparasites are found mainly among the wasp-waisted Apocrita within the Hymenoptera, and in two ot ...
s), including viruses. These viruses are likely to affect the evolution of the virulence and broader interactions of ''S. solidus'' with its hosts.


Host manipulation

The ''Schistocephalus solidus'' parasite is capable of host manipulation in both intermediate hosts, the copepod and the three-spined stickleback.


First intermediate host

In the copepod host, it is able to suppress activity while uninfective to the stickleback host. This reduces the likelihood of the copepod host being consumed and consequently unsuccessful transmission of the parasite. Once the parasite becomes infective, after approximately two weeks, activity increases and, as a consequence, the risk of consumption by three-spined sticklebacks increases. However, when multiple, non-simultaneous infections by ''S. solidus'' occur, host manipulation is orchestrated by the first infecting parasite. This increases the risk of premature consumption of the subsequent infections by the fish host. Consistent differences in manipulation are seen between parasite genotypes and populations. Differences in host genotypes are maintained after infections, but less pronounced.


Second intermediate host

In the fish host, host manipulation induces more risk taking behaviour like positive
geotaxis Gravitaxis (or ''geotaxis'') is a form of taxis characterized by the directional movement of an organism in response to gravity. There are a few different causes for gravitaxis. Many microorganisms have receptors like statocysts that allow them to ...
and negative
thigmotaxis Thigmotaxis (from Greek ''thigma'', "touch" meaning contact with an object, and ''taxis'', "arrangement, order", meaning reaction by movement) is a behavioral response to tactile stimuli, typically referring to an organism's movement in response t ...
. This change in behaviour is unlikely to be caused solely by the mechanical presence of the parasite.
Phenotype modification Phenotype modification is the process of experimentally altering an organism's phenotype to investigate the impact of phenotype on the fitness. Phenotype modification has been used to assess the impact of parasite Parasitism is a Symbiosis ...
, through injecting silicon "parasites", with densities and sizes similar to infective plerocercoids (~150 mg) did not alter behaviour. Physiologically, ''S. solidus'' is a parasite that inhibits egg production in female three-spined sticklebacks in European populations, but not in Alaskan populations where only egg mass is reduced. The egg mass of fish was correlated to the
parasite index Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ento ...
, which indicates that the reduction in egg mass is a non-adaptive side effect of parasite infection.


Model species

''Schistocephalus solidus'' is effectively used a
model species A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mode ...
for studying the evolutionary dynamics of host-parasite interactions. More recently, it was proposed a model to study host-parasite-microbe interactions


''in vitro'' breeding

The option to breed ''S. solidus'' in the laboratory makes them a useful model for studying host-parasite interactions. For 'culturing' of the worm progenetic plerocercoids are dissected from the stickleback host. The worm can then be incubated in a dialysis tube embedded in
culture medium A growth medium or culture medium is a solid, liquid, or semi-solid designed to support the growth of a population of microorganisms or cells via the process of cell proliferation or small plants like the moss '' Physcomitrella patens''. Diffe ...
and kept at 40 °C. These worms are then ideally incubated in pairs of similar size to maximise
outcrossing Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing distantly related genetic material into a breeding line, thereby increasing genetic diversity. Outcrossing in animals Out ...
and egg hatching. Optionally, large ''S. solidus'' worms can also be cut into smaller pieces and incubated separately.


References

{{Taxonbar, from=Q7431517 Cestoda Parasitic helminths of fish Parasites of birds Parasites of rodents