What is the substantial cause of an instance of consciousness?
In his commentary to Kamalashila's Stages of Meditation, His Holiness the Dalaï-Lama lists 5 types of causes:


  All those phenomena that  are produced at some times but not others
  depend on causes and conditions, and they are of various types. Causes
  are of different types, such as (1) substantial cause, (2) direct
  cause, (3) indirect cause, (4) cause of equal state, (5) concomitant
  cause, and so forth.


He gives a definition of substantial cause:


  [A substantial cause is] A cause that primarily produces its result as its
  substantial continuity; e.g., production of a sprout from a seed.


In 'Kindness, clarity and insight', HHDL says:


  It is not possible for consciousness to be produced from matter as its
  substantial cause. Once consciousness is produced from a former moment
  of consciousness, a beginning to the continuum of consciousness cannot
  be posited.


In addition, in 'Consciousness at the Crossroards', HHDL gives an interesting line of reasoning establishing that matter (form) is not the substantial cause of awareness:


  Specifically, the awareness must have a substantial cause. Now if the
  substantial cause of awareness is matter, then why is it that some
  things have no consciousness, and other things, such as animals, have
  consciousness?


In Pali and Sanskrit, 'substantial cause' is Upādāna. Regardless of contexts where it refers to specifics, literally it translates fuel. The Tibetan for substantial cause is nyer len.

In 'Buddhist Philosophy: Losang Gönchok’s Short Commentary to Jamyang Shayba’s Root Text on Tenets, by Daniel Cozort and Craig Preston, Jamyang Shayba says:


  Because of being a mind, it is established that [another mind]
  precedes it.


And Losang Gönchok comments:


  The subject, a knower [i.e., a mind] just after conception, is
  preceded by a mind that is its substantial cause because of being a
  knower. The subject, the final mind of death of a common being, is
  able to generate the mind that is its impelled object because of being
  a knower having desire, such as a prior main consciousness.



With respect to the ancillary topic (rebirth) it says:


  Because of movement [to activities immediately after birth] and memory
  [of former lives] also, [the existence of former lives is
  established]. Moreover, since a calf who has just been born seeks the
  milk of the teat and since with respect to animals, etc., the mind
  moves to activities of eating grass and engaging in desire without
  being taught, one can understand that this is done due to the
  conditioning of previous lifetimes. Also, memories of former lifetimes
  exist, and matter cannot act as a substantial cause to produce
  consciousness.