Themes of Parasitology: What's Mine is Yours & Yours Is Mine.

Exchange of Genetic Material

The Give and Take

When a parasitic relationship takes place, the parasite essentially becomes a part of the host. Though we typically think of parasites as organisms that steal nutrients from the host and give nothing in return, they sometimes share something valuable with their host. Though not common, parasites can exchange genetic information with their host, integrating their genes and forming a recombinant genome. This can be thought of as a form of horizontal gene transfer, where genes are exchanged between lineages (non-sexually) rather than within a lineage (sexually).1

How Does This Happen?

TRANSPOSONS

Rafflesia cantleyi (red flower) attached to it's host

Tetrastigma rafflesiae. [9]

Within a DNA sequence there are transposable elements, which move segments of nucleic information elsewhere.These "jumping genes" can be a type of transposons called class 2 transposons. Type 2 transposons use a “cut and paste” function so that they can “cut” a transposon out from their genome and then “paste” themselves into a target area, which could be either in a different segment of their own genome or a different genome altogether.2 An example of this can be seen between the parasitic plant Rafflesia cantleyi and it's host Tetrastigma rafflesiae. Researches have found that R. cantleyi has been retrieving genetic information from it's host to either gain nutrients more efficiently or to bypass the host's immune response to it.3 By taking advantage of transposons and integrating the two different genomes, parasitic R. cantleyi is able to improve its fitness and gives itself an edge over it's host.

RETROTRANSPOSONS

Transposable elements can also act as class 1 retrotransposons and use RNA as an intermediate between the two different genomes.4 An important parasite-host relationship that exploits the use of retrotransposons is the the HIV-human relationship. Human immunodeficiency virus is a retrovirus that uses RNA, reverse transcriptase, and integrase to fuse it’s own genetic information with the DNA of human cells. Once recombination occurs, the host's behavior is changed since it's own immune system begins to attack the infected cells, which are actually the body's immune cells. As the body attacks this hybrid of "host-parasite" cell, it also creates more immune cells, which HIV is patiently waiting to infect. This interaction allows HIV, the parasite, to continually infect other cells and create more copies of itself. This, in turn, increases HIV’s “fitness” due to an increase in reproductive success.

BACTERIOPHAGES

Parasites do not only exchange genetic information with their host, but also other symbiotic parasites that live in the same host. An example of this can be seen between different types of bacteria who take advantage of bacteriophages to integrate unique genetic information with one another.5 Bacteriophages help exchange DNA from bacteria such as, Staphylococcus aureus, to other cells to either continually reproduce the genetic information (lytic cycle) or to propagate its genetic information within its host species (lysogenic cycle).6 This recombination in the new parasitic cells can ultimately lead to resistance and increased efficiency in reproduction for the parasite.

The lytic and lysogenic cycle of bacteriophages which transfers genetic information from cell to cell. [10]

Why does it happen?

In many parasite-host relationship, an adaptive strategy made by either organism is done to improve fitness or to become a more efficient organism. When a parasite either gains or gives genetic information with it’s host, it is contributing in this exchange so that it has a more favorable environment and conditions to reproduce. The gene a parasite gives to its host could be one of antibiotic resistance, so that if the host tries to treat itself, it will prove ineffective. Likewise, a parasite could gain genetic information that prevents the host immune system from attacking it.7 Exchange of genetic information becomes an evolutionary tactic for the parasite to survive and to produce future generations. The parasite takes advantage of horizontal gene transfer in order to improve vertical gene transfer, and ultimately gives the parasite a more favorable opportunity to thrust its genes into the next generation.

SOURCES

Themes of Parasitology

Saturday, January 25, 2014

What's Mine is Yours & Yours Is Mine.

Exchange of Genetic Material

The Give and Take

How Does This Happen?

Why does it happen?

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