Frank Jiggins & John Maclennan – Genetics & Geology

The demand for places at lunch is not only driven by an appetite for the excellent food, but also by a recognition of the potential for discovery and enrichment, which cannot be found when eating sandwiches while hunched over a computer keyboard!

A recent example of a successful collaboration that has grown out of a discussion at High Table involves fruit fly genetics and the geology of the Hawaiian Islands.

It might have been during the jam roly-poly, just as the custard was leaving its boat. Or perhaps half-way through a mouthful of bread-and-butter pudding, smeared in chocolate sauce. Dr Jiggins was eyeing up the hand of bananas in the fruit bowl, wondering which flies they might have fed if they had never made it out of the plantation. Dr Maclennan was daydreaming of basalt and accretionary lapilli. Amongst the silverware of High Table, collaboration was about to strike. Frank Jiggins posed the opening question: ‘Do you know anything about the age of Hawaiian islands?’

Frank Jiggins, an evolutionary biologist working on fruit flies and mosquitoes, had been thinking about this problem, on and off, for a couple of years. Evolution is an historical process, and, as is the case in history, knowing the dates when events in the past occurred is essential to understanding why they happened. Unfortunately this is far from straightforward, as these events often happened many millions of years ago. Occasionally you get lucky: seven or eight fruit fly fossils have been discovered, where the unfortunate fly was trapped in tree resin and preserved in amber, and these can be linked to the geological age of the rocks in which they were found. When there are no fossils, we can still reconstruct how species are related by comparing the DNA sequence of their genomes. Once we have produced an evolutionary tree of species in this way, we can date when species split from a common ancestor if we know how fast their genome evolved. The difficulty we face in using this molecular clock is knowing how fast it is ticking: if we see 20 changes in the DNA sequence of a gene, does this equate to one, ten or 100 million years? The solution to this problem lies in Hawaii.

The Hawaiian archipelago sits above a hot spot in the earth’s mantle where molten magma breaks through the crust to form volcanic islands. As the Pacific plate has slid across this hot spot, a string of islands of decreasing age has formed, ranging from the youngest of Hawaii in the south to the oldest island, Kaua’i, in the north. Every time a new island emerged, it was colonised by fruit flies from the neighbouring island, and these eventually diverged to become new species. This provides the solution to our conundrum, as new species develop at the same time as new islands, so if we can count the genetic differences between fly species and put dates on when the islands formed, we can calibrate the rate at which our molecular clock is running.

From John Maclennan’s volcanological point of view, the growth of each Hawaiian island appears to follow a common life cycle. From the inception of a volcano as bulbous little eruptions three miles or more beneath the sea, a giant pile of frozen lava forms and eruptions breach the ocean surface, eventually creating a new island for fruit flies to colonise. Eruptions continue long after the volcano’s first emergence, repaving the forested slopes, burying old lava flows deep into the structure of the island, and building an enormous shield-shaped mountain.

Mauna Loa, one of these shield volcanoes, has a summit that watches over the Big Island of Hawaii from a height of almost 14 000 feet. This magnificent volcanism is followed by the death of the volcano, with a few sorry basaltic coughs and splutters building pimply cones around the shield summit. Then erosion takes over, with rainfall carving deep canyons through the apron of the volcano and cutting back towards the chilled innards of the mountain.

The crucial part of this cycle for fly genetics is that the lava flows that first formed each island, the first hosts for the colonising flies, are now encased deep within the earth, buried by the weight of hundreds of younger eruptions. These rocks have only been sampled by drilling to depths of 3000 feet, but previous genetic studies used ages of volcanic rocks currently found on the surface of each island to date its inception. Clearly, given what geology tells us about the construction of the volcanoes, this method underestimates the age of the birth of each island by as much as half a million years, equivalent to about half the total duration of volcanism at any island. This misunderstanding of the geology meant that geneticists thought the molecular clock was running faster than it was in reality.

As Frank and John talked on, they realised it would be straightforward to bring proper geological understanding to bear on the problem of fruit fly speciation in Hawaii and to produce a more accurate measure of the running of life’s molecular clock than had been presented in previous studies. This work has now been written up along with colleagues in Edinburgh, and is shortly to be published in the journal Molecular Biology and Evolution. Scientific progress fed by sticky toffee pudding.