Non-random distribution of the pericentromeric heterochromatin in meiotic prophase nuclei of mammalian spermatocytes

The central or peripheral distribution of condensed chromatin (CC) was studied in pachytene spermatocyte nuclei in Mus domesticus, 2n = 40; Pudu puda, 2n = 70; Ctenomys opimus, 2n = 26 and Octodon degus, 2n = 58. Species were chosen according to the morphological characteristics of their chromosomal complements and in particular, the terminal or medial chromosomal localisation of the pericentromeric constitutive heterochromatin. Counts were made by defining the areas corresponding to peripheral and central location in each nuclear section from a series. The null hypothesis (i.e. random distribution of CC) was rejected. In the nuclear sections of Mus domesticus and Pudu puda, 69% and 74% of CC, respectively, was found in the peripheral nuclear space, while in those of Octodon degus and Ctenomys opimus, 69% and 65% of CC, respectively, was found in the central nuclear space. We estimate that if the CC measured in spermatocyte nuclei corresponds mainly to pericentromeric constitutive heterochromatin, the distribution found is consistent with that expected in accordance with the nuclear architecture model for meiocytes (Fernández-Donoso, 1982; Fernández-Donoso & Berrios, 1985). This model proposes a peripheral nuclear localisation for pericentromeric heterochromatin of telocentric bivalents and a relatively central nuclear localisation for pericentromeric heterochromatin of metacentric bivalents. We also discuss some of the biological consequences that could arise from the conservation of such distributions.