  For any nonorientable closed surface, we determine the minimal dilatation
among pseudo-Anosov mapping classes arising from Penner's construction. We
deduce that the sequence of minimal Penner dilatations has exactly two
accumulation points, in contrast to the case of orientable surfaces where there
is only one accumulation point. One of our key techniques is representing
pseudo-Anosov dilatations as roots of Alexander polynomials of fibred links and
comparing dilatations using the skein relation for Alexander polynomials.
