  We prove that the dilatation of any pseudo-Anosov homeomorphism on a
translation surface that belong to a hyperelliptic component is bounded from
below uniformly by sqrt{2}. This is in contrast to Penner's asymptotic. Penner
proved that the logarithm of the least dilatation of any pseudo-Anosov
homeomorphism on a surface of genus g tends to zero at rate 1/g (as g goes to
infinity). We also show that our uniform lower bound sqrt{2} is sharp. More
precisely the least dilatation of a pseudo-Anosov on a genus g>1 translation
surface in a hyperelliptic component belongs to the interval
]sqrt{2},sqrt{2}+2^{1-g}[. The proof uses the Rauzy-Veech induction.
