Among vertebrates, obligate parthenogenesis occurs exclusively in squamate reptiles. Premeiotic endoreplication in a small subset of developing oocytes has been documented as the mechanism of production of unreduced eggs in minutely explored obligate parthenogenetic lineages, namely in teiids and geckos. The situation in the lacertid genus Darevskia has been discussed for decades. Certain observations suggested that the ploidy level is restored during egg formation through a fusion of egg and polar body nuclei in Darevskia unisexualis and D. armeniaca. In this study, we re-evaluated the fusion hypothesis by studying diplotene chromosomes in adult females of sexual species D. raddei nairensis and obligate parthenogens D. armeniaca, D. dahli and D. unisexualis. We revealed 19 bivalents in the sexual species and 38 bivalents in the diploid obligate parthenogens, which uncovers premeiotic endoreplication as the mechanism of the production of non-reduced eggs in parthenogenetic females. The earlier contradicting reports can likely be attributed to the difficulty in identifying mispairing of chromosomes in pachytene, and the fact that in parthenogenetic reptiles relying on premeiotic endoreplication only a small subset of developing oocytes undergo genome doubling and overcome the pachytene checkpoint. This study highlights co-option of premeiotic endoreplication for escape from sexual reproduction in all independent hybrid origins of obligate parthenogenesis in vertebrates studied to date.

Department of Ecology Faculty of Science Charles University Prague Viničná 7 128 44 Czech Republic

Laboratory of Fish Genetics Institute of Animal Physiology and Genetics Czech Academy of Sciences Rumburská 89 Liběchov 277 21 Czech Republic

Laboratory of Non Mendelian Evolution Institute of Animal Physiology and Genetics Czech Academy of Sciences Rumburská 89 Liběchov 277 21 Czech Republic

Research Institute of Biology Yerevan State University Yerevan 0025 Armenia

Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences Leninsky Prospect 33 Moscow Russia

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Premeiotic endoreplication is the mechanism of obligate parthenogenesis in rock lizards of the genus Darevskia