Fishing in African Great Lakes for answers

Fishing in African Great Lakes for answers
Professor Byrappa Venkatesh - from the Agency for Science, Technology and Research's Institute of Molecular and Cell Biology - and other scientists around the world are studying the cichlid fish to gain insight into how species, including humans, may evolve over time, as well as how genes influence disease.

Luohan, angelfish, tilapia.

Singaporeans know these fish by name and by sight. One is prized for good luck, another is a home aquarium favourite, while the third tends to end up on the dinner table.

Few, though, would think that these three very different-looking fish are, in fact, close relatives.

They are all cichlids, an incredibly varied family of freshwater fish that originate from the Great Lakes of East Africa. Of the roughly 32,000 fish species worldwide, about 6 per cent, or 2,000, are cichlids of all shapes, colours and sizes.

Their mind-boggling diversity finds its source deep in the gigantic lakes of Victoria, Malawi and Tanganyika, freshwater bodies 40 to 100 times the size of Singapore, sprawled across eight African nations.

In Lake Victoria, for instance, the 500 cichlid species there evolved from just a handful of species within less than 100,000 years.

In evolutionary terms, this is light speed. Species usually evolve over tens of millions of years as they gradually adapt to their environments.

This has set scientists on the trail of the cichlid as a good model for studying how species, including humans, may evolve over time.

Studying the fish may also shed light on how genes influence disease, says Professor Byrappa Venkatesh, research director at the Institute of Molecular and Cell Biology under the Agency for Science, Technology and Research.

"This goes back to the basic question in evolution: why there are new species formed, and how they are formed," he says.

The genome scientist joined 76 other researchers from 10 countries to sequence the genome of five cichlid species from the three lakes, and reconstructed their history going back 10 million years.

Their work appeared in leading science journal Nature last month.

It is part of the Genome 10K project that aims to decode the genetic make-up of 10,000 animal species, to let researchers reconstruct their evolutionary history and chart how they evolved over time.

It turns out that there used to be only a few dull-coloured species of cichlids in the Nile River in northeast Africa, where the fish originate.

As millions of years passed, they naturally accumulated variations in their genetic make-up, but had no need to outwardly evolve because they were perfectly adapted to the river they lived in.

This all changed, however, when they began to colonise the three mega-lakes connected to the Nile, beginning with Lake Tanganyika 10 million to 20 million years ago. They populated Lake Malawi five million years ago, and spread out to Lake Victoria 15,000 to 100,000 years ago.

The deep lakes provided a buffet spread of different food sources, and various habitats of sandy beds, rocky shores and plankton-filled surfaces.

In one fell swoop, the evolution genie was let out of the bottle. The cichlids had genome variations at the ready to adapt quickly.

The few drab species became hundreds over time as they adapted. Cichlids became increasingly different in size, form, colour and feeding habits.

Their plain colours gave way to a multitude of vivid shades and striped patterns - blue, yellow, orange, red, purple - as they competed to breed.

Formerly herbivores, some grew powerful teeth to crush the shells of molluscs for food.

In others, their mouths changed so they could feed on the scales of other fish. Yet others evolved to feed on algae or fish eggs.

"The trigger was that the lakes were so huge. They offered different ecosystems," says Prof Venkatesh.

Where the African Great Lakes caused cichlids to evolve in different ways, modern societies may do the same to humans, he adds.

By studying cichlids, he says, it may be possible to one day understand better how the human genome adapts to selective pressures.

For instance, team member Russell Fernald, a professor of biology at Stanford University, and his group are studying fish with specific mating behaviours, cross-breeding them, seeing which behaviours are conserved and then analysing the genomes of the fish for clues as to which genes control the behavioural change.

"Genetics is so highly conserved across species that by doing this type of analysis in animals where we can control variables very closely, we can begin to understand how genes control behaviour in other vertebrates," Prof Fernald says in a statement.

Genes can also cause disease. Once the scientists identify the genetic variations in cichlids that cause diseases, they may learn how genetic human diseases emerge, says Prof Venkatesh.

"Humans also accumulate genome variations. A combination of these can lead to disease, just as they do in cichlids," he explains.

Under the Genome 10K project, scientists will also sequence species more closely related to humans, such as primates.

"This comparison with other closely related vertebrates is a powerful strategy to understand why we are so unique from other animals," Prof Venkatesh adds.

davidee@sph.com.sg


This article was first published on Oct 26, 2014.
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