Chimeric embryos—potential mechanism of avian offspring sex manipulation

Environmental and social effects on offspring sex ratio bias in birds are among the most studied topics in evolutionary biology. Although it is established that offspring sex is determined at the stage of the first meiotic division, there is no direct evidence for the common belief that the bias happens via nonrandom sex chromosome segregation. In this paper, we suggest an alternative mechanism based on the failure of polar body emission, followed by fertilization of multiple haploid nuclei through polyspermy and subsequent competition of Z and W bearing cells within chimeric embryos. Although the occurrence of failure of polar body extrusion is obvious from the observations of chimeric birds, the idea that it could be the first step of offspring sex ratio bias has been entirely overlooked. We review the evidence in support of that idea, demonstrate that it is consistent with the observations of biased offspring sex ratio reported so far, and suggest a way to verify it.