( They ( ( ( can only ) ( reproduce sexually ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (MD can) (ADVP (RB only)) (VP (VB reproduce) (ADVP (RB sexually)))) (. .)))
They can only reproduce sexually.
(ROOT (S (NP (NNS Vertebrates)) (VP (VB reproduce) (ADVP (RB sexually))) (. .)))