Board Thread:Boruto Discussions/@comment-30415965-20170714114941/@comment-734582-20170714135303

Mr. Grave wrote: ^It was one unbroken chain, honestly. It is, but if Jiraiya had taken it, after the Second Hokage every Hokage taught the next.

Tobirama taught Hiruzen taught Jiraiya taught Minato taught Kakashi taught Naruto (taught Konohameru). In the series Tsunade is a bit of a side-step, but only a bit.