In seasons six and seven, it appears that the Tyrell line is wiped out as Margaery, Loras, and Mace die in the Sept of Baelor, and Olenna died last weekend at Highgarden. But in the books there are two other Tyrell sons who aren't dead.
The Tyrell line isn't over in the books! Willas Tyrell, the oldest born, is holding fort ruling at Highgarden as of "A Dance with Dragons." Garlan "the Gallant" Tyrell has been seen in King’s Landing with his wife, but as of "A Feast for Crows" he was out taking back the Shield Islands from the Ironborn.
Olenna Tyrell briefly tried to marry Sansa Stark to Willas Tyrell in "A Storm of Swords." The show replaced this with Sansa being (almost) married to Loras Tyrell in season three instead, preventing Willas from making an appearance. In both cases, Sansa was married to Tyrion to prevent the union.