Heard spelling

jienoecium

Spelled gynoecium. two changes at once

Can that vowel be spelled ie?

In plenty of English words it can, which is why jienoecium looks reasonable. In this one it cannot: the letter is y, from Greek upsilon.

The ie spelling of that sound belongs to native words, tie, lie, pie. Greek loans keep their y whatever the mouth does with it. Add a j for the soft g and jienoecium is two borrowed spellings replaced by two English habits.

Say

guy-NEE-see-um, and jy-NEE-see-um is heard just as often

Letters

G-Y-N-O-E-C-I-U-M