  Context. gamma Doradus (gamma Dor) are late A and F-type stars pulsating with
high order gravity modes (g-modes). The existence of different evolutionary
phases crossing the gamma Dor instability strip raises the question of the
existence of pre-main sequence (PMS) gamma Dor stars. Aims. We intend to study
the differences between the asteroseismic behaviour of PMS and main sequence
(MS) gamma Dor pulsators as it is predicted by the current theory of stellar
evolution and stability. Methods. We explore the adiabatic and non-adiabatic
properties of high order g-modes in a grid of PMS and MS models covering the
mass range 1.2 Msun < Mstar < 2.5 Msun. Results. We derive the theoretical
instability strip (IS) for the PMS gamma Dor pulsators. This IS covers the same
effective temperature range as the MS gamma Dor one. Nevertheless, the
frequency domain of unstable modes in PMS models with a fully radiative core is
larger than in MS models, even if they present the same number of unstable
modes. Moreover, the differences between MS and PMS internal structures are
reflected on the average values of the period spacing as well as on the
dependence of the period spacing on the radial order of the modes, opening the
window to the determination of the evolutionary phase of gamma Dor stars from
their pulsation spectra.
