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Nearby dwarf galaxies provide a unique laboratory in which to test stellarpopulation models below $Z_\odot/2$. Such tests are particularly important forinterpreting the surprising high-ionization UV line emission detected at $z>6$in recent years. We present HST/COS ultraviolet spectra of ten nearbymetal-poor star-forming galaxies selected to show He II emission in SDSSoptical spectra. The targets span nearly a dex in gas-phase oxygen abundance($7.8<12+\log\mathrm{O/H}<8.5$) and present uniformly large specific starformation rates (sSFR $\sim 10^2$ $\mathrm{Gyr}^{-1}$). The UV spectra confirmthat metal-poor stellar populations can power extreme nebular emission inhigh-ionization UV lines, reaching C III] equivalent widths comparable to thoseseen in systems at $z\sim 6-7$. Our data reveal a marked transition in UVspectral properties with decreasing metallicity, with systems below$12+\log\mathrm{O/H}\lesssim 8.0$ ($Z/Z_\odot \lesssim 1/5$) presenting minimalstellar wind features and prominent nebular emission in He II and C IV. This isconsistent with nearly an order of magnitude increase in ionizing photonproduction beyond the $\mathrm{He^+}$-ionizing edge relative to H-ionizing fluxas metallicity decreases below a fifth solar, well in excess of standardstellar population synthesis predictions. Our results suggest that oftenneglected sources of energetic radiation such as stripped binary products andvery massive O-stars produce a sharper change in the ionizing spectrum withdecreasing metallicity than expected. Consequently, nebular emission in C IVand He II powered by these stars may provide useful metallicity constraints inthe reionization era.

Ultraviolet spectra of extreme nearby star-forming regions --- approaching a local reference sample for JWST