( ( ( ( ( The ( primary host ) ) , ) ( where ( ( the flukes ) ( sexually reproduce ) ) ) ) , ) ( ( is ( a vertebrate ) ) . ) )
(ROOT (S (NP (NP (DT The) (JJ primary) (NN host)) (, ,) (SBAR (WHADVP (WRB where)) (S (NP (DT the) (NNS flukes)) (ADVP (RB sexually)) (VP (VB reproduce)))) (, ,)) (VP (VBZ is) (NP (DT a) (NN vertebrate))) (. .)))
The primary host, where the flukes sexually reproduce, is a vertebrate.
(ROOT (S (NP (NNS Vertebrates)) (VP (VB reproduce) (ADVP (RB sexually))) (. .)))