The ZW pairs of two paleognath birds from two orders show transitional stages of sex chromosome differentiation.

Pachytene oocytes from the two presumably most primitive orders (Paleognathae) among living birds were used to study the pairing behaviour and location of recombination nodules (RNs) in the sex pair. In the ratite Pterocnemia pennata (Rheiformes), the 42 analyzed ZW pairs show an average of 2.2 RNs distributed along 80% of the synaptonemal complex (SC) that covers the long arm of the acrocentric Z and W chromosomes in this homomorphic sex pair. In the tinamid Rynchotus rufescens (Tinamiformes), the 60 analyzed ZW pairs show an average of 1.35 RNs distributed along 66% of the SC covering most of the long arms of this visibly heteromorphic ZW pair. RNs are non-randomly distributed and show interference in both species, but in the tinamou they are restricted to a significantly smaller stretch. The discovery of an intermediate degree in the restriction of RN location, between the extremes of free recombination along most of the W in ratites and strict localization of a single RN in Neognath birds, suggests its relationship with the mechanism of sex chromosome differentiation among Aves.