A NEW HYPOTHESIS ON THE EVOLUTION OF SEX DETERMINATION IN VERTEBRATES- BIG FEMALES ZW, BIG MALES XY

Why are there two chromosomal sex-determining mechanisms in vertebrate s; ZW/ZZ, meaning female heterogamety, and XX/XY, meaning male heterog amety? We propose an evolutionary explanation. Transition from environ mental sex determination to genetic sex determination can result when an allele that accelerates embryonal or larval growth takes over the r ole of environment in bringing about differential growth. Species with environmental sex determination showing female size advantage, are pr edisposed to evolve the ZW/ZZ mechanism, whereas species with male siz e advantage are predisposed to evolve the XX/XY mechanism.