The structure of the irreducible collective spaces of the group
$Sp(12,R)$, which many-particle nuclear states are classified
according to the chain $Sp(12,R) \supset U(6) \supset SO(6) \supset
SU_{pn}(3) \otimes SO(2) \supset SO(3)$ of the proton-neutron
symplectic model (PNSM), is considered in detail. This chain of the
PNSM was recently shown to correspond to a microscopic shell-model version of
the Bohr-Mottelson collective model. The construction of the
relevant shell-model representations of the $Sp(12,R)$ group along
this chain is considered for three nuclei with varying collective
properties and from different mass regions. It is shown that the
$SU_{pn}(3)$ basis states of the $Sp(12,R)$ representations
belonging to $SO(6)$ irreps with seniority
$\upsilon \geq \upsilon_{0}$, with $v_{0}$ denoting the maximal
seniority $SO(6)$ irrep contained in the $Sp(12,R)$ bandhead, are
always Pauli allowed, but organized in a different way into
different $SO(6)$ shells. This is in contrast to the case of filling
the levels of the standard three-dimensional harmonic oscillator and
using the plethysm operation. Although the $SU_{pn}(3)$ multiplets
with $\upsilon < \upsilon_{0}$ are not all Pauli forbidden, it is
safe to discard them. The results obtained in the present work are
important for the practical application of the microscopic version
of the Bohr-Mottelson collective model.