Germ cell restriction and regular transmission of an accessory chromosome that mimics a sex body in the zebra finch, Taeniopygia guttata

Mitotic and meiotic analysis with light and electron microscopy was performed in male and female zebra finches (Taeniopygia guttata). Somatic cells from bone marrow have a 2n = 80 and show the usual sex chromosome mechanism of birds ZZ (male)/ZW (female). In the germ lines of both sexes, a single accessory chromosome was regularly present in all the cells examined from all the individual birds. In synaptonemal complex (SC) spreads of pachytene oocytes and spermatocytes, this accessory chromosome forms a single axis, but it behaves differentially in male and female meiosis. While this accessory chromosome is euchromatic in oocytes, it is strongly heterochromatic in spermatocytes. In pachytene spermatocytes, the accessory chromosome adopts a morphology strikingly similar to that of the XY body (`sex vesicle') of mammalian spermatocytes. This accessory chromosome is eliminated during male meiosis and forms a cytoplasmic dense body in young spermatids that shows strong fluorescence with DAPI. The presence of this germ line-restricted chromosome does not affect the behaviour of the ZW pair in oocytes, as the sex chromosomes pair regularly and show a localized recombination nodule. It is suggested that this accessory chromosome has transcriptional activity during oogenesis, and thus it is regularly transmitted through preferential segregation during female meiosis.