Extreme axial equalization and wide distribution of recombination nodules in the primitive ZW pair of Rhea americana (Aves, Ratitae).

Pachytene oocytes from the ratite bird Rhea americana were used for synaptonemal complex analysis with a surface spreading technique and phosphotungstic acid staining. The ZW bivalent is slightly smaller than the fourth autosomal bivalent and clearly shows unequal W and Z axes only in 27% of the bivalents. Most of the ZW pairs are completely adjusted and thus the W and Z axes are almost equal in length. A sample of 134 recombination nodules (RNs) from 63 ZW pairs showed a striking departure of number and location of these nodules compared with those of carinate birds. The average number of RNs in the ZW pair of R. americana is 2.13, and the average SC length per RN is 4.2 microm. The locations of the RNs along most of the long arms of the Z and W are not random, and the distances between pairs of RNs show interference. Thus, the pattern of RNs in this mostly euchromatic ZW pair is identical to that of autosomes. From the present and previous data, it is concluded that the ZW pair of R. americana is in a primitive stage of chromosomal differentiation, in which recombination is restricted only in the small short arm and in the pericentromeric region.