Why do parasites harm their hosts

Evolutionary biologists still have a headache with the question Charcles Darwin published in his 1859 book 'Origin of Species' : The parasite lives on the host, so why do they harm animals? master?

A new study of emperor butterflies along with microscopic parasites living on their bodies of the University of Georgia and Emory University found that the parasite stands in dilemma when it has just taken advantage Rapid reproduction has brought the price to the host. The study was published online in the Proceedings of the National Academy of Sciences which provided the first evidence of the so-called 'compromise hypothesis' in a system of nature.

The main author of the study - Jacobus de Roode, a former postdoctoral researcher at UGA, saves an assistant professor at Emory University - saying: 'The parasite is forced to harm the host to replicate itself. dear and passed on to another generation. But what our research found is that if they cause too much damage to the host, they themselves must suffer. On the other hand, if a parasite with 'high benevolence' for a host is not beneficial, it will not be able to reproduce enough levels to pass on to the next generation. '

Emperor butterfly.(Photo: NationalGeographic)

In a three-year arduous study at Sonia Altizer's laboratory - assistant professor at UGA's Odum ecological school, the researchers infested the imperial moth with the gamete of single-celled animals. Births are common in wildlife populations at different levels. After the butterfly matures, the female will mate and arrange the pitfalls in the environment. The butterfly will spread parasites when laying eggs and gametes or spreading gametes on the leaves of the ear which are the favorite of the caterpillars.

These gametes will be caterpillars ingested. Each butterfly has a earring that serves as a shelter in its area. Regularly daily for a 30-day period, scientists give a butterfly a new stem while carrying the previous tree to the lab for analysis. They count the gametes on eggs and on the earring tree, this is a no-brainer because a gamet is about 1/100 the size of a pollen grain on a butterfly's wing. Only one egg has over 1000 gametes.

The researchers collected data and found a clear evidence for a hypothesis that is largely sympathized with by the scientific community but so far that hypothesis has not been well studied in the natural environment of the relationship between parasites and hosts. They found that the female emperor was infected with a parasite that was too severe that it died before she could mate, or if it lived it did not perform mating. Moderately infected children live long and spawn many eggs, while mild-level infections also live long and have many children, but the number of its offspring is relatively small parasitic animals. .

De Roode said: 'The results of the study are significant because it provides an explanation as to why so many parasitic animals cause disease and death for the host'.

The results also demonstrate that the compromise hypothesis occurs in emperor butterflies in a controlled environment, but scientists want to find out how natural selection affects this toxicity in the environment. wild. They isolated the parasitic flow of emperor butterflies in the east and west of North America with different migratory behaviors. The eastern emperor butterfly emigrated from the spawning area of ​​Canada to the winter break in central Mexico and then returned, completing a distance of about 5,000 km. Western emperor butterflies migrate only a third of the distance to stay at the California coast.

Altizer said: 'We think that if the parasite is' more gentle' in any of the two butterfly populations mentioned above it is the eastern emperor butterfly population. Since these butterflies move over a long distance, the parasite that kills the host along this long journey will not be able to produce offspring. '

The research team infects butterfly populations to the east and west with parasites from their own populations and also from the population. Butterflies in both populations are equally susceptible to infection, but the parasite in the western populations causes its host to die faster. This confirms the group's prediction that the parasite of the eastern population is more gentle.

In addition to clarifying the evolutionary question, which is a headache because why parasites harm a host, this study has important implications for the health of butterflies and humans.

In some cases, researchers predict the extent to which public health interventions may affect the evolution of the virulence of disease based on the compromise hypothesis. To anticipate requires thorough knowledge of the biological characteristics of parasites as well as hosts. But the thing is that interventions to inhibit the parasite's chance of infectious parasites can make natural selection prioritize the lower toxic parasite lines. The results of the study show that when the parasite's chance of infection is limited, for example, to the eastern emperor butterfly populations, natural selection will favor the long-lived parasite flow. more on the host body which is already infected and causes less harm.

The study also questioned the feeding of emperor butterflies and released them for some events such as weddings. The incarceration conditions can enhance the chance of infectious parasites and the advantage of a higher toxic parasite strain that could harm wild populations associated with them.

Altizer points out that migratory butterflies, which have faced a changing climate challenge, have reduced their migration rate along with their environmental hazards in the breeding area in the winter.

Altizer said: 'There is a very easy possibility to realize that eastern migratory butterfly populations will no longer appear in the next 50 to 100 years. Smaller non-migrant populations will still exist, but we expect more butterflies to be infected with parasitic animals and parasites will have higher toxicity. '