Equal frequencies of recombination nodules in both sexes of the pigeon suggest a basic difference with eutherian mammals

The total number of recombination nodules (RNs) in the autosomal synaptonem al complexes (SCs) is statistically equivalent in oocytes and spermatocytes from the domestic pigeon Columba livia. The distribution on RNs along the three longest autosomes is also equivalent in oocytes and spermatocytes. Th e numbers of RNs show a linear relationship when plotted against SC length both in oocytes and spermatocytes. On the other hand, the ZW pair shows a s ingle and strictly localized RN near the synaptic termini, but the ZZ pair shows unrestricted location of RNs (average 3.8). The ZW and ZZ pairs of th e pigeon are euchromatic and do not show specific chromatin packing at pach ytene in either sex. The lack of sex-specific differences in the number and location of RNs in the autosomal bivalents of C. livia and previous data o n the chicken, suggest that the regulation of crossing-over is basically di fferent in birds and mammals.