The frequency of heterologous synapsis increases with aging in Robertsonian heterozygous male mice.

The house mouse is characterised by highly
variable chromosome number due to the presence of
Robertsonian (Rb) chromosomes. During meiosis in
Rb heterozygotes, intricated chromosomal figures are
produced, and many unsynapsed regions are present
during the first prophase, triggering a meiotic silencing
of unsynapsed chromatin (MSUC) in a similar mode
to the sex chromosome inactivation. The presence of unsynapsed chromosome regions is associated with
impaired spermatogenesis. Interestingly, in male mice
carrying multiple Rb trivalents, the frequency of germ
cell death, defective tubules, and altered sperm morphology
decreases during aging. Here, we studied
whether synapsis of trivalent chromosomes and MSUC
are involved in this improvement. By immunocytochemistry,
we analysed the frequency of unsynapsed
chromosomes and of those positive to γH2AX (a
marker of MSUC) labelling in spermatocytes of 3-, 5-
and 7-month-old Rb heterozygotes. With aging, we
observed a decrease of the frequency of unsynapsed
chromosomes, of spermatocytes bearing them and of
trivalents carrying γH2AX-negative unsynapsed
regions. Our quantitative results show that both synapsis
and MSUC processes are better accomplished during
male aging, partially accounting for the improvement of
spermatogenesis.