Origin and evolution of the sex chromosomes in birds.

A scheme is proposed to explain the process of sex chromosome differentiation in the class Aves. It is based on the restriction of recombination in an ancestral homomorphic ZW pair and the subsequent degeneration of the non-recombining region of the W chromosome leading to the highly heteromorphic sex chromosomes of carinate birds. The model presented here and Ohno's model of sex chromosome differentiation in Ophidia are the only integral models of sex chromosome evolution based on cytological evidences. The co-existence among living birds of mostly homomorphic (Ratites) and highly heteromorphic ZW pairs (carinates) provides a unique ground to study the mechanisms of sex chromosome differentiation in vertebrates.