Asymmetry and resolution of the synaptonemal complex in the XY pair ofChinchilla laniger

Abstract

The meiotic behavior of the XY pair of the chinchilla (C. laniger), has been studied with C-banding and with the microspreading procedure for synaptonemal complex (SC) karyotyping. The large X chromosome of the chinchilla has a paracentromeric and two subterminal (stl and st2) C+ bands, while the minute Y chromosome has a C+ long arm. At metaphase-I the X chromosome is associated end-to-end with the short arm of the Y chromosome. The appearance of the X axis in microspread spermatocytes is delayed up to early pachytene. The X axis, as well as the Y axis appear as thick, separated threads. The thick X and Y axes are then co-aligned without forming a SC. The Y axis becomes thinner and then an asymmetrical SC is formed between the axes. This SC becomes symmetrical by a later thinning of the corresponding part of the X axis. During mid-pachytene an additional SC is formed at the other end of the XY pair in many cells. During late pachytene the SCs become resolved and finally the four termini are separated from each other although they tend to be located in the same region of the nucleus. These observations suggest that the SCs in the XY pair of the chinchilla are formed between non-homologous regions. The presence of non-homologous synapsis suggests that the XY pair ofC. laniger is joined by an achiasmatic mechanism, different from the chiasmatic joining usually found in eutherian mammals. The presence of subterminal C+ bands in the X chromosome may be a hindrance for the formation of a SC during early pachytene. It is concluded that the XY pair of the chinchilla shows an intermediate behavior when compared to that of the sand rat on one side, and that of most mammals having a euchromatic X chromosome, on the other side.