I know that virtually all masculine and feminine nouns in Latin have an e or i in the nominative plural and that the genitive singular is often similar. This is quite widespread in Indo-European languages (at least the western ones I know about). Linguists that study this in different languages used a wide range of terminology (palatalization, umlaut, slenderization).
The main exemption in Latin is words like manus in the forth declension. Is there any evidence that the vowel was ever palatalized/slenderized in the nominative plural or genitive singular? (I am aware that the vowel lengthened but I don't know if this has anything to do with it.)