I went to Sihler: New Comparative Grammar of Greek and Latin
Indo-European fem sg: –e[math]H_2[/math]. Indo-European neuter o-stem plural: –e-[math]H_2[/math]. They are the same; as Sihler notes (p. 263) “identical in form with the nom.sg of -e[math]H_2[/math] stems (=first declension) and probably the point of departure for the creation of that stem.”—
(p. 266) “a connection particularly suggestive because of the otherwise puzzling lack of an overt case marker *-s on the nom.sg of the -e[math]H_2[/math]-stems.” (i.e. why is the feminine –ā and not –ās?)
“A historical connection between o-stem neuter plurals and the feminine –e[math]H_2[/math]-stems was made easier when it was discovered that in Hittite, as in Greek, the “plural” of the neuter was in a very real sense singular, as it construes with 3sg verbs. Before that discovery, there was room for debate over whether Greek syntax of the πάντα ῥεῖ ‘all things flow’ type was an innovation. But now it is clear that it can only be an ancient trait. The reinterpretation of a neuter plural as some kind of derivative (collective) singular is thinkable if *kʷekʷle[math]H_2[/math] (to *kʷekʷlom ‘wheel’) was not so much ‘wheels’ as something like ‘wheelage’, or perhaps indifferently one or the other. But the evolution of a whole new stem type and concord class (‘feminine’) from a single form is not easy to trace in detail.”