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What is the advantage of asexual reproduction to Fasciola ?           What is the advantage of sexual reproduction to Fasciola?

 

Two forms of sex (sexual and asexual) are part of the survival strategy of the liver fluke.

Some animals nourish and care for their young (are K selected) and others (and the parasites are great examples) do not (are r -selected).

Adult flukes are hermaphrodite (carrying both sexes) and can self or cross fertilise where there are more than one adult in a liver i.e. Sexual reproduction

As a result the fertilised adult Fasciola releases vast numbers of offspring into the environment to provide for survival of the species.

Adult liver flukes each produce as an estimate 20,000 * eggs per day and and as they can survive for years in  host sheep or cattle millions of eggs are released onto pasture

in a life-time each egg producing a miracidium.

And then each miracidium ,should it  locate and penetrate a mud snail Lymnaea truncatula, can massively increases its offspring by asexual reproduction

i.e.high numbers of cercariae in the environment increase ovthe erall chance of survival of the species contacting the next host.

A miracidium transforms into a sporocyst on penetration of a mud snail. Each body sac then generates within it (like a Chinese Box)  another sac like body (a redia).

Sometimes these redia can generate within themselves a second generation of rediae (2ndry rediae or daughter rediae).

And the final act within the snail is for the rediae to generate within their body sacs a new form the cercaria with its whip like tail to help it to exit the snail and swim off carrying

by the way within its body all of the cells ready primed with secretions to form the protective cyst of the metacercaria.

It has been estimated that a single snail infected with just one miracidium can release about 600 cercariae   (J D Smyth Animal Parasitology 1962)
It has been estimated that a single infected snail can carry  up to 344 rediae and release  up to 2,275 cercariae. (Kendall 1949 Journal of Helminthology 25, 77-92)

All of this from just one egg and its miracidium !

With the production of the cercaria multiplication by asexual means comes to an end.

Thereafter one cercaria ............... becomes one metacercaria ...........becomes  one juvenile fluke which grows into an adult fluke              as it  :    

                    encysts on grass..............................is ingested................ and penetrates the gut wall and grows as it migrates

                                                                                                              through  the liver to the bile ducts.

 

So what is the advantage of sexual reproduction if asexual reproduction is so successful in the life cycle? adult

Remember Fasciola is hermaphrodite (having both sexes in the same body) and are capable of self fertilisation.

No-one knows the frequency of sexual interaction between adult flukes but they do have a protrusible penis and an adjacent female opening

Nontheless the potential for sexual reproduction benefits the population in biological warfare and the opens the opportunity for changing tactics with time of the parasite

in response to newly arising and effective host attack.

Flukes that were all the same would be found out by a host immune response. Sexual variation means that the unique and variant parasites that have survived  have adapted

and survived. Equally we need to be aware that a single vaccine antigen would be unlikely to kill all liver and that variation and mutation can mean that parasites that survive

additionally drug treatments given over a period of time will in the same way deliver 'drug resistant' strains into the environment.

 

Dr. Clive Bennett 2013

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* Reference Ecological Animal Parasitology CR Kennedy 1975 Blackwell