One of the hardest tasks in Biology is classifying things. Despite what your Biology textbook would have you believe, nature doesn’t put organisms in neat little boxes. Many animals like the platypus straddle the line between what we traditionally consider to be a mammal (being warm blooded and having fur) and reptile (laying eggs). Yet, viruses are rather simple creatures (If they even are creatures, which is a whole different debate).
A standard virus particle does not have much going on, quoting Wikipedia:
(i) the genetic material made from either DNA or RNA, long molecules that carry genetic information;
(ii) a protein coat, called the capsid, which surrounds and protects the genetic material; and in some cases
(iii) an envelope of lipids that surrounds the protein coat.
There isn’t much room for confusion above. But as of all things, nature came along with something that scientists didn’t quite know how to classify. Enter the Polydnavirus.
Many animals, including humans have viral genes in their cells. This is not very surprising because of the way viruses replicate – They rely on cellular machinery of more complex prokaryotic and eukaryotic cells in order to multiply and reproduce and ever so often some genetic sequence for reproduction is lost, leading viruses to instead be integrated into the cells genome. Viruses occasionally make it into germ cells as well, and even the human genome has a lot of viral dna.
So when a set of genes existing inside the genome of parasitic wasps was found, we didn’t think much of it. There was probably some endosymbiotic event in the past where the polydnavirus genome was incorporated into the wasp genome. Along the line the genes for transcription and replication were lost.
The parasitic wasp has a bit of an unusual mode of reproduction. It stings caterpillars to paralyze them, and then injects its eggs into the caterpillar (which is alive, hence why it is known as a parasitic wasp) . The wasp eggs then hatch, and devour the caterpillar alive before going their own way. But when the wasps were studied in detail, their was one troubling thing – they seemed to produce viral particles from their ovaries.
For a long time the scientists thought this was just some unique feature of wasp reproduction that viral type particles were produced during reproduction till we finally figured it out. The wasp had hijacked the virus. The wasps ovaries would produce viral particles, which would then be injected into the caterpillar along with the eggs. Over there the viral dna would destroy the immune system of the caterpillar, so that it couldn’t destroy the eggs or react to the presence of foreign objects (wasp eggs) in its body cavity. This would enable the wasp to successfully reproduce.
Scientists have recently sequenced the genome of the polydnavirus and have concluded that they originated as nudivirus, and instead of the virus taking over the organism (as is common) the organism took over the virus! it is one of the most unique examples of symbiosis- two organisms in a mutually beneficial relationship.
Thank you for reading 🙂
Links:
1.) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4424053/
Sapience 