"Selfish" DNA and differential parental investment: some implications for sex chromosomes

Muller's classical theory of why the Y chromosome is small than X is falsified when applied to Bryophytes and is only partially successful when applied to female heterogametic species. Here an alternative based on the "selfish DNA" hypothesis and on the consequences of differential parental investment by sex is offered for the small size, few expressed genes, and occasional absence of Y. It is argued that Y has these properties because with anisogamy or differential egg/sperm investment in the zygote, Y is parasitic on X. The theory is applied to Bryophytes and to female as well as to male heterogametic species.