Homogeneous structures with nonuniversal automorphism groups

We present three examples of countable homogeneous structures (also called Fraïsse limits) whose automorphism groups are not universal, namely, fail to contain isomorphic copies of all automorphism groups of their substructures. Our first example is a particular case of a rather general construction on Fraïsse classes, which we call diversification, leading to automorphism groups containing copies of all finite groups. Our second example is a special case of another general construction on Fraïsse classes, the mixed sums, leading to a Fraïsse class with all finite symmetric groups appearing as automorphism groups and at the same time with a torsion-free automorphism group of its Fraïsse limit. Our last example is a Fraïsse class of finite models with arbitrarily large finite abelian automorphism groups, such that the automorphism group of its Fraïsse limit is again torsion-free.