Science Snaps: why aren’t flies as big as hippos?

When it comes to ‘before and after’ photos, these microscopic snaps of two very different fly eyes show a big difference on a small scale.

This latest Science Snaps selection comes from Dr Nic Tapon and his team at our London Research Institute. They are using these striking images to investigate what happens when the signals telling our cells whether to grow or not become disrupted.

Why aren’t flies as big as hippos? It seems an odd thing to ask at first, but at its heart is a question about the fundamentals of cell biology and cancer development.

Most animals start off life as a single cell, which divides and multiplies to give a fully grown adult.

So what tells a fly’s cells to stop multiplying once the fly is full size?

And how is the message different in a hippo, allowing it to carry on growing until it reaches its vast mass?

These ponderings may seem abstract, but they underpin one of the key research areas in cancer biology: what makes cancer cells grow out of control?

Size matters

Dr Nic Tapon and his team study a collection of key genes and proteins that control the growth and death of cells.

By using images to uncover their roles in fruit flies – or to give them their full name Drosophila melanogaster – Dr Tapon is searching for the genetic mistakes that can lead to cancer.

“For the past 15 years my key interest has been growth control – trying to understand how an organism senses its size and keeps itself at the right size,” says Dr Tapon.

Like something out of the 1986 sci-fi horror classic The Fly – in which Jeff Goldblum begins to metamorphose into a giant fly after a teleportation experiment goes horribly wrong – the images here show the dramatic changes that can take place when mistakes happen.

Why Hippo?

As Dr Tapon points out, there’s a reason why the fruit fly isn’t the same size as a hippo.

“The fly always knows what size it’s supposed to be and tries to keep to that size by having control mechanisms that stop cells from growing out of control,” he explains.

But cancer is a disease that ignores these stop signals.

Instead, the cells divide uncontrollably, which results in a tumour.

Dr Tapon’s research focuses on a key relay of signals – known as the Hippo pathway – that tell a cell when to multiply and can become faulty in cancer.

But why Hippo?

For starters, ‘hippo’ is the name of one of the genes Dr Tapon studies. And fly genes are often named after what happens to the fly when faults in that gene emerge. So ‘hippo’ refers to the fact that, when the flies carry a faulty form of this particular gene, they become bigger. Not as big as a hippo, of course – that would be quite difficult to manage in the lab – but bigger nevertheless.

The eyes have it

The ‘before and after’ image above was produced using a Scanning Electron Microscope and shows the heads of two fruit flies. On the left you can see a healthy fly, with its large compound eye. On the right you can see the head is larger and the eye is grossly misshapen.

A healthy developing fly eye

This is the result of a genetic mistake in the Hippo pathway – the cells are no longer aware of when to stop multiplying.

The fly’s eye is an ideal organ for our researchers to look at because it is made up of hundreds of small compound eyes, called ommatidia, arranged in a regular hexagonal lattice. If the Hippo pathway is not working you get too many cells, and enlarged ommatidia.

The third image shows what a healthy developing eye should look like, with the purple dots highlighting each individual ommatidia. By simply comparing the eyes our researchers can spot a fly with faults in the Hippo pathway that could be linked to cancer.

The fruits of research

They may have faces only a mother could love, but the humble fruit fly has a lot to offer research.

They’re small, have simple needs and the genes that control their growth share many similarities to those found in humans.

They also reproduce very quickly, allowing genetic traits to be followed through a number of generations in a far shorter time than with other animals.

Studies in fruit flies have already yielded breakthroughs in our understanding of the genetic changes linked to cancer, and many other diseases.

We’ll be keeping an eye out for the next.

Greg Jones is a press officer at Cancer Research UK