Natural male hybrid common shrews with a very long chromosomal multivalent at meiosis appear not to be completely sterile

Among 36 known chromosomal hybrid zones of the common shrew Sorex araneus, the Moscow-Seliger hybrid zone is of special interest because inter-racial complex heterozygotes (F1 hybrids) produce the longest meiotic configuration, consisting of 11 chromosomes with monobrachial homology (undecavalent or chain-of-eleven: CXI). Different studies suggest that such a multivalent may negatively affect meiotic progression and in general should significantly reduce fertility of hybrids. In this work, by immunocytochemical and electron microscopy methods, we investigated for the first time chromosome synapsis, recombination and meiotic silencing in pachytene spermatocytes of natural inter-racial heterozygous shrew males carrying CXI configurations. Despite some abnormalities detected in spermatocytes, such as associations of chromosomes, stretched centromeres, and the absence of recombination nodules in some arms of the multivalent, a large number of morphologically normal spermatozoa were observed. Possible low stringency of pachytene checkpoints may mean that even very long meiotic configurations do not cause complete sterility of such complex inter-racial heterozygotes. Natural hybrids between extremely divergent by karyotypes chromosomal races produce a complex meiotic configuration chain-of-eleven or undecavalent. It is expected that they may suffer from reduced fertility, but our results showed that such a multivalent probably does not negatively affect progression of meiosis, since morphologically normal spermatozoa were revealed.image The Moscow-Seliger F1 hybrids of the common shrew produce a longest meiotic configuration, consisting of 11 chromosomes with monobrachial homology (undecavalent or chain-of-eleven: CXI).Chains-of-eleven have been identified in hybrid spermatocytes at the pachytene and diakinesis stages using immunocytochemical, electron and light microscopy methods.In CXI, arms of chromosomes were synapsed and recombined, although recombination nodules may lack in some arms.Unsynapsed regions of the undecavalents undergo meiotic inactivation.A large number of morphologically normal spermatozoa were observed.It is most likely that common-shrew F1 male hybrids were not sterile.