There was an error published in the ePress version of Development135, 3881-3889on 23rd October 2008.
The first name of Caiying Guo was spelt incorrectly. The correct spelling appears above.
On p. 3886, left column, second paragraph, lines 3 and 6: Figs S2 and S3 should be S4 and S5,and Fig. S5 should be Fig. S4.
The final online and print versions are correct.
We apologise to the authors and readers for this mistake.