  Crosslinks and molecular motors play an important role in the organization of
cytoskeletal filament networks. Here we incorporate the effect of crosslinks
into our model of polar motor-filament organization [Phys. Rev. E {\bf 71},
050901 (2005)], through suppressing the relative sliding of filaments in the
course of motor-mediated alignment. We show that this modification leads to a
nontrivial macroscopic behavior, namely the oriented state exhibits a
transverse instability in contrast to the isotropic instability that occurs
without crosslinks. This transverse instability leads to the formation of dense
extended bundles of oriented filaments, similar to recently observed structures
in actomyosin. This model also can be applied to situations with two oppositely
directed motor species or motors with different processing speeds.
