Ex-Plaid Cymru leader Lord Elis-Thomas has said he has no intention of standing down and calling a by-election despite leaving the party and taking a job in the Labour-led Welsh Government.
Re-elected as Plaid AM for Dwyfor Meirionnydd in May 2016, he left the party five months later after a series of rows with Plaid leader Leanne Wood.
Now Plaid MP Liz Saville Roberts has challenged him to call a by-election.
She represents the same constituency at Westminster.
Both politicians appeared on the S4C panel discussion programme Pawb a'i Farn on Thursday evening, broadcast from Penrhyndeudraeth in the constituency.
Since November 2017 Lord Elis-Thomas has been the Welsh Government's culture minister.
Ms Saville Roberts said: "If I can speak plainly - if Dafydd Elis-Thomas had an ounce of respect for his constituents and for the people who have supported him for decades, he would have stepped down."
Lord Elis-Thomas, who was appointed Welsh Government culture minister last November, said: "I am not a turncoat. I remain a cultured nationalist.
"The argument I had was with the leadership and strategy of the Plaid Cymru group in the assembly, and the refusal to co-operate to create a stable government for Wales."
He added: "Everything I promised to the constituents of Dwyfor Meirionnydd is being enacted.
"I have had the honour, since November, to be a Minister in the Welsh Government. It is the greatest honour of my life, and I will continue to represent this constituency as long as my health allows."