On the origin of the luminosity - metallicity relation for late-type   galaxies: Spirals to irregulars transition
  We consider the roles of two widely invoked mechanisms for the metallicity-luminosity correlation among late-type galaxies: higher astration level and decreasing efficiency of heavy-element loss with increasing luminosity. We find that both mechanisms contribute about equally to the range in oxygen abundance between low (logL_B = 8) and high (logL_B = 10.5) luminosity galaxies.   We also consider the transition from spirals to irregulars, finding that both the oxygen abundance deficiency (indicating the degree of mass exchange between a galaxy and its environment) and the gas fraction vary smoothly along the sequence. Thus we find no "irregular versus spiral dichotomy". 
L.S. Pilyugin (Main Astron. Obs. of Nat. Academy of Sc. of Ukraine),   F.Ferrini (University of Pisa, Italy)