Two question with InnoDB XA transaction (no replies)

I have some question with InnoDB XA transaction.

1. In the MySQL Manual, it is said that using XA transaction you have to set transaction isolation to serializable. But from my test, it do not have to. So why have to set it to serializable ?

2. If there are some distribute XA transaction runing on the MySQL server, dose the local transaction isolation need to be set as serializable ?