Greater spire

The zerg greater spire is evolved from the spire (if there is a hive present somewhere in the same command structure) and it produces the genetic code that allows some zerg air breeds to mutate into different forms(such as the Mutalisk evolving in to the Guardian.

StarCraft
The greater spire enables mutalisks to mutate into guardians and devourers. It also provides the same upgrades as the spire and will retain the evolutionary progress so that any aerial units keep all upgrades that were completed at the spire.

StarCraft II
In StarCraft II the greater spire enables production of the brood lord.