  Massive rotating single stars with an initial metal composition appropriate
for the dwarf galaxy I Zw 18 ([Fe/H]=$-$1.7) are modelled during hydrogen
burning for initial masses of 9-300 M$_{\odot}$ and rotational velocities of
0-900 km s$^{-1}$. Internal mixing processes in these models were calibrated
based on an observed sample of OB-type stars in the Magellanic Clouds. Even
moderately fast rotators, which may be abundant at this metallicity, are found
to undergo efficient mixing induced by rotation resulting in quasi
chemically-homogeneous evolution. These homogeneously-evolving models reach
effective temperatures of up to 90 kK during core hydrogen burning. This,
together with their moderate mass-loss rates, make them Transparent Wind
Ultraviolet INtense stars (TWUIN star), and their expected numbers might
explain the observed HeII ionizing photon flux in I Zw 18 and other
low-metallicity HeII galaxies. Our slowly rotating stars above $\sim$80
M$_{\odot}$ evolve into late B- to M-type supergiants during core hydrogen
burning, with visual magnitudes up to 19$^{\mathrm{m}}$ at the distance of I Zw
18. Both types of stars, TWUIN stars and luminous late-type supergiants, are
only predicted at low metallicity. Massive star evolution at low metallicity is
shown to differ qualitatively from that in metal-rich environments. Our grid
can be used to interpret observations of local star-forming dwarf galaxies and
high-redshift galaxies, as well as the metal-poor components of our Milky Way
and its globular clusters.
