SPONGES are primitive creatures with a body plan unlike that of any other living organism. They are also our most distant animal cousins. Now that their genetic make-up has finally been sequenced, it could explain one of the greatest mysteries of evolution: how single-celled organisms in the primordial oceans evolved into complex multicellular animals with the spectacular diversity of body plans we see today.
The sponge genome confirms that sponges share much the same genetic tool kit for multicellularity as the rest of the animal kingdom. This means that all the key genetic prerequisites for modern animals made up of trillions of cells were in place well before sponges split from other animals 600 million years ago. What’s more, the sponge genome reveals the very ancient origin of genes involved in cancer, which is caused by a derailing of cell replication and is the signature disease of multicellularity.
We had already gained some insights from studies comparing the genomes of single-celled protozoans with those of other primitive animals. But there was a crucial gap, occupied by the sponges. “Sponges really were the last missing piece of this puzzle,” says , an evolutionary biologist at the Whitehead Institute in Cambridge, Massachusetts.
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Srivastava and her team have bridged that gap by sequencing the genome of Amphimedon queenslandica, a dark grey sponge native to Australia’s Great Barrier Reef (Nature, ). “People have been waiting for this for a long time,” says , a molecular biologist at the University of California, Irvine.
Among the sponge’s 20,000 to 30,000 genes, the team found 4670 gene families – groups of related genes – that are common to all animals. Of these, 1286, or just over a quarter, could hold the genetic keys to multicellular life: they are absent in animals’ closest single-celled relatives, the .
A closer look at the genome reveals the necessary genes for what Srivastava calls the “hallmarks of animal multicellularity”: the ability of cells to stick to their neighbours and send chemical signals to them, to divide and grow in a coordinated fashion, to develop into specialised cell types, and to distinguish themselves from cells belonging to other organisms.
Since sponges are the most ancient form of animal life around today, we might assume that their genomes represent the bare essentials for multicellular life. Srivastava found that the gene families implicated in multicellular life were indeed mostly smaller in Amphimedon queenslandica than in higher animals. For example, the gene families for transcription factors, which regulate the activity of other genes, have up to 34 times as many member genes in higher animals as in Amphimedon.
To get an idea of the order in which evolution put together the genetic tool kit for multicellular life, Srivastava’s team compared the sponge’s genome with those of single-celled protozoans and sea anemones, as well as higher animals. This revealed that the most ancient group of genes was one involved in regulating cell division. These genes occur in single-celled bacteria, too – even single-celled organisms need to control when they divide.
To this basic set of genes, sponges and other multicellular animals add a small suite of master-control genes which may allow the greater coordination needed when several cells are dividing together. In contrast, most of the genes involved in programmed cell death – which eliminates cells that are in the wrong place or are misbehaving – are novelties that first appear in sponges, or even later.
Many of the genes in sponges turn out to be implicated in cancer. This link was expected because cancers are essentially errors of multicellularity, where some cells lose the ability to be controlled by their neighbours.
Knowing exactly which cancer genes were present at the dawn of animal life may suggest new avenues for research, Srivastava says. For example, the sponge lacks two genes linked to cancer that are thought to be key to regulating cell division. Figuring out how sponges get by without them may shed light on their role in human cancers.
“Knowing how sponges get by without two key genes regulating cell division could shed light on cancer”
