THE human genome was unveiled last year to much fanfare, self-congratulation and just a little disappointment. For despite our apparent sophistication, we find we have fewer genes than plants such as the humble rice plant.
Now a London-based biologist thinks he has the answer to this apparent paradox. The number of genes we have may be a concession to our complex immune system, says immunologist Andrew George of Imperial College. If we had any more, our bodies would be crippled by autoimmune diseases because our immune cells wouldn鈥檛 be able to learn to recognise and ignore the huge numbers of proteins the genes would churn out.
Rather than the 100,000 plus genes we thought we had, the 鈥渂ook of life鈥� revealed that we make do with a mere 30,000 to 45,000. That鈥檚 startlingly few, considering that a simple sea urchin has only a few thousand fewer, and squid are though to have the same number (see Graphic). And rice plants may have up to 55,000.
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George thinks our immune system has placed an upper limit on the number of genes we can cope with. At the sharp end of the immune system are T cells, which circulate around the body and check the identity of other cells by examining the short sections of proteins presented on their surfaces. These protein fragments are taken from a random selection of those within the cell, so a cell infected with a virus presents viral proteins as well as its own. The T cell recognises the viral fragment and recruits other immune cells to destroy the infected cell.
Crucially though, T cells must ignore all the 鈥渇riendly鈥� proteins coded for by our genes or else they will attack healthy cells. To prevent this, newly formed T cells that get it wrong are weeded out in the thymus gland in the neck.
This only works as long as there aren鈥檛 too many friendly proteins to ignore. Assuming that each gene codes for a distinct protein, George estimates that the immune system of an organism containing 30,000 genes would have to destroy a quarter of all new T cells just to eliminate the rogues that could turn on the body.
But with 100,000 genes, almost two-thirds of all new T cells would have to be eliminated, he says. Maintaining an immune system that won鈥檛 turn on the body simply becomes too wasteful, time consuming and expensive. What鈥檚 more, it raises the chances of a foreign protein looking like a normal one and slipping through the net. 鈥淚t鈥檚 a really exciting idea,鈥� says Laurence Hurst, an expert in genome evolution at Bath University. 鈥淚t implies that simply adding new genes comes at a cost.鈥�
However, George takes his theory a step further. He says that our relatively small genome may hinder our ability to adapt to environmental change. Since it can鈥檛 add new genes, our genome has to respond by coming up with other ways of performing new functions, using the same protein in several different roles, for example. But that has its limits, says George. 鈥淭here must come a stage when you鈥檝e got less room to manoeuvre.鈥� In short, our immune system may be frustrating our ability to evolve (TRENDS in Immunology, vol 23, p 351). The same would apply to birds, reptiles and other mammals, which have similar immune systems.
Dmitri Petrov, an evolutionary geneticist at Stanford University, is cautious. 鈥淭he idea should be kept on the table, but I don鈥檛 think there鈥檚 any evidence that it鈥檚 true yet.鈥� He questions the general assumption that humans must have more genes than other life forms.