Basic

The Basic is tier 1. Upgrades branch from nothing. It can upgrades from following:

Maple Syrup

Hurm04ka

Shrinker

Trapper

Twin

Flank

Machine Gun

Auto Basic

Sniper

Humor

Meterscope

Propeller

Hivemind

Switcheroo

As the Basic
Basic is d**k a tank it's off-limits to kill sentries