Tendency towards clonality: deviations of meiosis in parthenogenetic Caucasian rock lizards†

Cytogenetic mechanisms of unisexuality in diploid parthenogenetic species of the genus Darevskia have remained debatable until recently. The mechanism that allows the unisexual form to maintain heterozygosity in a number of generations is important for its long-term existence in nature. In this work, for the first time, for parthenogenetic species of the genus Darevskia, in addition to primary oocytes with the usual ploidy (18 + ZW bivalents in meiotic prophase I), oocytes that underwent premeiotic genome endoduplication and carried a doubled number of bivalents (36 + ZZ + WW) were found. Here, we present a detailed comparative analysis of the preparations of synaptonemal complexes in oocyte nuclei without and with genome endoduplication and the behavior of sex Z and W chromosomes. We show the details of the assembly of bivalents in pachytene nuclei, where either homeologs or doubled identical copies of chromosomes compete for synapsis and form multivalents. For the first time, the WW sex bivalent has been visualized in parthenogenetic reptiles. We show the reverse side of meiotic deviations in obligate parthenogenesis-cases of nonviable embryos with specific abnormalities.