Multi-Host

Photo – historicengland.org.uk

England 1862, Wednesday early afternoon - the postman has delivered a parcel to an estate just outside London.  The box has been taken up to the study on the second floor and placed on a large table.  The head of the household has returned from his midday walk and is making his way up the stairs.  His full whitish-grey beard and poor posture make him seem older than his age of fifty-three.  His movements are slow and he requires the use of a walking stick.  He pauses in the hall for a pinch of snuff before entering his study and taking a seat at the table.  He notices the parcel on the table and sees that it’s from James Bateman a horticulturist. 

He had requested specimens from Bateman to use in his work on the insect pollination of orchids.  Using his penknife he carefully removes the twine and paper and opens the box.  This time Bateman has included something truly unique - Angraecum sesquipedale.  He unconsciously places one hand on top of his bald head as he examines the orchid and discovers the nectar sits at the base of a narrow green spur that descends more than foot below the flower (sesquipedale is Latin for “foot-and-a-half”).  The notes written by Charles Darwin that afternoon ask, “Good Heavens, what insect can suck it?”

There was no known insect at the time capable of reaching the nectar.  Darwin theorized in his book Fertilization of Orchids that a moth with a proboscis long enough to reach the nectar did exist.  In probing for the nectar the moth’s head would contact the pollen from one orchid and then pollinate the next orchid in the same manner.  It seemed somewhat of a stretch to expect a moth to carry around an elongated sucking tube three times as long as its body, but Darwin had found that the existence of every other orchid relied on a symbiotic relationship with another creature.  The fact that no one had ever seen such a moth as would be required for Angraecum sesquipedale was of no concern to Darwin - there simply had to be one.

In 1903, forty years after Darwin’s Fertilization of Orchids was published, a moth with the precise type of equipment as predicted by Darwin was discovered in Madagascar.  Field observations revealed that the African Hawkmoth searches out and identifies the Angraecum sesquipedale by scent, having found one then backs up a foot or so, deploys its proboscis and then flies forward inserting its proboscis down the orchid’s spur.  Photo below from biologicalexceptions.blogspot.

     

The tale of Darwin and the orchid enjoyed a good run.  It was a feel-good yarn demonstrating that despite the long odds the old guy really knew what he was talking about.  The story line began showing up everywhere you looked.  It was handled respectfully in Susan Orleans’ novel The Orchid Thief and even better in the film Adaptation which followed.  It even made its way into tattoo parlors where you can get both the moth and the orchid inked onto you caught in flagrante. 

Ever since the dark ages faded away there’s never been a moment’s rest for any of us.  Especially with science, just when you get comfortable something else always seems to come along.  Move aside Angraecum sesquipedale, it’s time to make room for the new kid in town – the Ribeiroia parasite.   

As with the orchid and the moth, parasites and their hosts also form a symbiotic relationship.  However, any relationship with a parasite is one-sided.  Both orchid and moth benefit with the moth feasting on the nectar and the orchid continuing on as a species.  A host can only look forward to having nutrients drained from it by a parasite.

There’s a good parasite story to scare the bejesus out of just about everybody.  There’s the cautionary tale regarding some places in the undeveloped world where skinny dipping is riskier than sky diving.  My personal favorite is how to get rid of a tape worm.  The host or patient is first required to fast for several days and then made to lie flat on their back with a plate of food resting on their chest.  Tapeworms have but a single mission and that is to find something to eat and devour it.  After days without anything to eat the tapeworm becomes desperate and is drawn up and out of the patient’s mouth by the smell of the food where the attending staff latches on to the worm and yanks it out. 

Photo from Getty stock images

What sets apart the Ribeiroia parasite from others is that it employs three separate hosts through a life cycle that has no beginning or end.  It just circles endlessly through three carriers.  Let’s begin to describe the cycle with the snails that populate wetlands and feed on algae.  The Ribeiroia parasite in the form of a flatworm resides inside these snails and once it has castrated them enlists the snails’ help in pumping out thousands of free-swimming Ribeiroia larvae that go looking for their next host.  The larvae are drawn to tadpoles where they invade the tissue that is associated with the growth of legs.  When the tadpoles become frogs their legs either never develop or do so in such a deformed way that the frogs become easy prey for birds.  Once ingested by a bird the parasite reproduces.  When the bird’s droppings fall into the wetlands they carry parasite eggs which make their way back inside the snails and mature into flatworms. 

Wiki site of the practical exercise of the IV Southern-Summer School on Mathematical Biology

It seems as thought their existence serves no purpose other than as a microscopic cog on the evolutionary wheel.  I suppose if one were able to view the human race from another galaxy, our lives could seem just as unremarkable.  Once born, we live with the earth as our host; then pass away with all of it terribly mundane from a cosmic perspective.