Dislocation nucleation in homogeneous crystals initially unfolds as a linear
symmetry-breaking elastic instability. In the absence of explicit nucleation
centers, such instability develops simultaneously all over the crystal and due
to the dominance of long range elastic interactions it advances into the
nonlinear stage as a collective phenomenon through pattern formation. In this
paper we use a novel mesoscopic tensorial model (MTM) of crystal plasticity to
study the delicate role of crystallographic symmetry in the development of the
dislocation nucleation patterns in defect free crystals loaded in a hard
device. The model is formulated in 2D and we systematically compare lattices
with square and triangular symmetry. To avoid the prevalence of the
conventional plastic mechanisms, we consider the loading paths represented by
pure shears applied on the boundary of the otherwise unloaded body. These
loading protocols can be qualified as exploiting the 'softest' and the
'hardest' directions and we show that the associated dislocation patterns are
strikingly different