An irreducible representation $(\pi,V_\pi)$ of a compact group $G$ is called spherical with respect to the pair $(G,K)$, $K$ is closed subgroup of $G$, if $V_\pi$ has a non-zero vector invariant by $K$, or equivalently, the trivial representation $1_K$ of $K$ appears in the decomposition of $\pi|_K$.

The pair $(G,K)$, $G=SO(2n)$, $K=SO(2n-1)$, has spherical representations $\pi_k$ with highest weight $k\varepsilon_1$ for $k\geq0$ (under the standard choice of root system and positivity). The same happens with the pair $G=SO(2n+1)$ and $K=SO(2n)$.

The pair $G=Sp(n)$ and $K=Sp(n-1)\times Sp(1)$ has spherical representation $\pi_k$ with highest weight $k(\varepsilon_1+\varepsilon_2)$ for $k\geq0$.

> Is there $K\subset G=Sp(n)$ (closed) such that the spherical representations have highest weight $k\varepsilon_1$ for $k\geq0$?.