Shrew evolution in hybrid zones: a meiotic undecavalent as an exclusive chromosomal supermultivalent and its weak effect on fertility

Hybrid zones are "natural laboratories" for studying speciation. In the common shrew Sorex araneus, the hybrid zone between extremely divergent in karyotypes the Moscow and Seliger chromosomal races is unique, because complex heterozygotes (interracial hybrids) form the longest meiotic configuration consisting of 11 chromosomes with monobrachial homology (undecavalent or chain-of-eleven - CXI). Different studies suggested that such a multivalent would negatively affect meiotic progression and, in general, would significantly reduce fertility. In this work, using immunocytochemical and electron microscopy methods, we investigated chromosome synapsis, recombination, and meiotic silencing in pachytene spermatocytes carrying undecavalent. Despite some abnormalities detected in spermatocytes, such as single associations of chromosomes, stretched centromeres in the multivalent, shifted recombinational peak towards distal parts of chromosomal arms of superchain, heterozygous shrews were able to form a large number of morphologically normal and active spermatozoa. Possible low stringency of pachytene checkpoints, proper segregation of homologous chromosomes, and the ability of hybrids to form mature germ cells imply rapid evolutionary fixation and circulation of Rb chromosomes within shrew populations, leading to a variety of chromosomal races.