Centromere localization at meiosis and the position of chiasmata in the male and female mouse

Techniques for obtaining differential Giemsa staining of the paracentromeric (p.c.) regions of male and female mouse meiotic chromosomes (centromeric heterochromatin) were explored and standard procedures developed for the different meiotic cells in the two sexes. The best result followed the use of heat at controlled pH in Sorensen's phosphate buffer or in Standard Saline Citrate (SSC) solutions. With these techniques, morphological features of the p.c. regions and their variation were studied in normal animals (CFLP strain) and in a strain (AKR) homozygous for a centric fusion [T(11; ?)-1 Ald] between chromosomes No. 6 and No. 15 (Miller et al., 1971). The Y chromosome was often found to show distinct p. c. staining at first and apparently at second meiotic metaphase, and the X and Y chromosomes were found to associate as bivalents by their long arms. Autosomal p.c. regions showed variation in size which might indicate differences between non-homologous chromosomes but a tendency to similarity between homologues. Differences were found between males and females in respect to proportions and variation of bivalents with single and double chiasmata. The relative positions of chiasmata were different in the two sexes. The presence of the centric fusion in the males did not seem to affect the pairing behaviour of the remaining autosomes or of those taking part in the centric fusion. The possibility is discussed that the p.c. regions, to which also other functions would seem to appertain, may be important for chromosome recognition and pairing, possibly on a quantitative basis.