The independence number of a square matrix A, denoted by α(A), is the maximum order of its principal zero submatrices. Let S + n be the set of n × n nonnegative symmetric matrices with zero trace, and let J n be the n × n matrix with all entries equal to one. Given any integers n, t with 2 ≤ t ≤ n − 1, we prove that a linear map φ :if and only if there is a permutation matrix P such that φ(X) = P T XP for all X ∈ S + n .