Heterosynapsis and axial equalization of the sex chromosomes of the northern bobwhite quail.

The pairing behavior of the Z and W chromosomes in the female northern bobwhite quail (Colinus virginianus) was analyzed by electron microscopy of silver-stained synaptonemal complexes (SCs). After autosomal pairing was completed, synapsis of the sex chromosomes initiated at the short-arm end of the W chromosome and one end of the Z chromosome. Synapsis then progressed unidirectionally, producing a sex bivalent in which the entire length of the W axis was paired with an equivalent length of the Z axis. Progressive contraction and asymmetrical twisting of the Z axis ultimately resulted in a fully paired configuration with aligned axial ends. Further contraction of the Z axis reduced the extent of asymmetrical twisting such that only the nonaligned centromeric regions distinguished the SC of the ZW bivalent from SCs of similar-sized autosomes in late-pachytene nuclei. Quantitative analyses indicated that the length of the Z axis shortened significantly during the adjustment process, whereas no significant difference occurred in the length of the W axis. The nonalignment of the centromeric regions during transitional stages of ZW synapsis indicates that direct heterosynapsis of nonhomologous segments, followed by axial equalization of the length inequality, is responsible for the length adjustment during synapsis in the sex chromosomes of the bobwhite quail.