Life hinges on your nose

Danger. Food. The right path. Friends and enemies. Sex...
These are some essential things you can gauge with your “nose”, whether you’re a fruit fly, elephant…or even a human.

Humans, and our primate cousins are usually called animals that live by sight, but we’re just as dependent on our sense of smell, which is the oldest sense from an evolutionary perspective.

When our nose or antenna detects a scent, its various chemical components are picked up by scent receptors, receptor molecules, which are located on the surface of nerve cells. The signals are sent to the olfactory bulbs, which interpret the information and then send it on to the higher brain centres.

• Humans have 388 different receptors, each one specialised for a certain number of scent molecules
• The fruit fly has 62
• The rat has 1,259

Does that mean the rat’s sense of smell is over three times sharper than ours, and twenty times better than the fruit fly’s?

We approached Matthias Laska, professor of Zoology, who has experimented for many years on the ability of animals to use their sense of smell. It’s not that simple, he says. 

“The more studies we do, the clearer it becomes that the connection between the sense of smell and behaviour is the critical factor. Evolution forms organisms so that they are adapted to their environment as best as possible,” he says.

Laska is supported by Mattias Alenius a developmental biologist, who at another part of LiU uses fruit flies to study the sense of smell at the nerve cell level.“The fruit fly is specialized for nutrients in a small number of fruits, and therefore does not need the same broad sense of smell as an omnivore. Everything we call a smell, a scent, or maybe even a stench, are mixtures of various chemicals.

• Fruity scents are mixes of various esters
• Meaty scents are dominated by fatty acids
• Fishy scents come primarily from aldehydes

Their complexity has helped specialists to fine-tune their olfactory systems as needed. The spider monkey, which lives in the rain forests of South America, has a weakness for figs and can sniff out small differences in ripeness that are invisible to the naked eye. The objective is to be able to choose those fruits that contain the maximum amount of sugars.

“The task of the nerve cell systems is to see to it that the individual functions optimally in the environment. The nerve cells are like sprinters; they burn themselves out quickly, especially regarding the nose where they are exposed to many olfactory stimuli which, moreover, could be toxic. In humans, all the nerve cells in the nose are replaced every three months. This allows a microevolution in the nose, where our abilities change over time,” Alenius says (pictured alongside doctoral student Anujaianthi Kuzhandaivel).

Alenius is pursuing the basal mechanism behind the sense of smell: how nerve cells in the brain are divided up into different populations, each connected to its own olfactory receptor, and how they “know” that they must always send their growths, called axons, to the same spot in the brain’s olfactory bulbs. The fruit fly proved to be the perfect model.

• The fruit fly’s central nervous system contains 100,000 nerve cells
• Humans have 100 billion

“The fruit fly’s olfactory system is manageable. It involved 1,200 nerve cells, compared with the six million in a human. We’ve been able to completely map how the signal paths go,” Alenius says.

OK, it’s easy to work with, but is it really possible to draw parallels with humans?

“We’re trying to work on the basic principles, like how nerve cell populations are formed; this is the necessary knowledge to be able to cultivate human nerve cells from stem cells in the future.”

A recent breakthrough for Alenius’ research group relates to the discovery of seven regulators, which are genes that divide the nerve cells into different populations. These remain throughout the life of the fly and are used in various combinations.
The group work with different techniques to see what happens when genes are turned on or off.

• One technique uses the colouring agent GFP (2008 Nobel Prize) causes the flies to express a certain olfactory receptor green when illuminated with fluorescent light.
• Another is RNA interference, where genes are turned off through the cell’s virus defences (2006 Nobel Prize).

“We have a really good toolbox to avail ourselves of. The fly research community is quite open and we ourselves have sent flies to one hundred labs around the world.”

Laska and his colleagues visit their subjects, often at Kolmården Zoo, where they’ve conducted experiments on animals such as Asiatic elephants and sea bears (a species of seal). He’s worked on fruit bats and spider monkeys in Mexico with colleagues there. An easier project was testing the sense of smell in bees. Regardless of whether it deals with elephants or bees, the principle is the same: animals must learn to respond to a certain smell with a certain reaction. The motivation is the reward they get when they perform the correct task.

Captured bees are exposed to a scent associated with a drop of nectar as a reward. They are then released into a room with 48 bottles, each of which contains various floral scents, and one of which is the same as in the training round. 

The bottlenecks are covered with something that prevents the bee from flying in and instead it must hover over the mouth of the bottle and try to navigate its way to the correct bottle using its sense of smell. When it locates the correct bottle the bee receives its reward, a drop of nectar dispensed to land in front of its nose.

“They are allowed three attempts and after that they’re very rarely mistaken,” Laska says.

The experiment indicates a very sensitive sense of smell, despite a modest set of different receptors (160) in their antennae. Once again, it’s a question of a reflection of the behaviour:
At the age of four weeks, a worker bee flies out of the hive to start collecting nectar and pollen. It comes to a field with many different flowering plants. How is it to choose? The trick, chiselled out over millions of years of evolution, is that the first flower chosen at random is the flower that bee will always return to even if many alternatives are present. The scent of that flower is etched into the bee’s memory; even on the second flight, the bee seeks out the same flower. The pattern is repeated constantly over three weeks. For the rest of its life the bee stays in the hive, as a guard or nurse.

 

Åke Hjelm
30-01-2012

 

Source for the number of olfactory receptors: Nei, Niimura and Nozawa: The evolution of animal chemisensory receptor gene repertoires: roles of chance and necessity.

Bua the elephant takes part in an experiment led by Matthias Laska and master’s student Alisa Rizvanovic
Photo: Vibeke Mathiesen
Photos from the fruit fly lab: Göran Billeson
The nose at the top of the page belongs to Hanieh Khoshjabinzadeh

Related Links

• Matthias Laska 
• Zoology at LiU
• Mattias Alenius 
• Developmental Biology at LiU
• Kolmården Zoo

 

Page responsible: anna.nilsen@liu.se
Last updated: Wed Feb 15 14:13:59 CET 2012