The First Analysis of Synaptonemal Complexes in Jawless Vertebrates: Chromosome Synapsis and Transcription Reactivation at Meiotic Prophase I in the Lamprey Lampetra fluviatilis (Petromyzontiformes, Cyclostomata).

Transcription is known to be substage-specific in meiotic prophase I. If transcription is reactivated in the mid pachytene stage in mammals when synapsis is completed, then this process is observed in the zygotene stage in insects. The process of transcriptional reactivation has been studied in a small number of different taxa of invertebrates and vertebrates. Here, for the first time, we investigate synapsis and transcription in prophase I in the European river lamprey Lampetra fluviatilis (Petromyzontiformes, Cyclostomata), which is representative of jawless vertebrates that diverged from the main branch of vertebrates between 535 and 462 million years ago. We found that not all chromosomes complete synapsis in telomeric regions. Rounded structures were detected in chromatin and in some synaptonemal complexes, but their nature could not be determined conclusively. An analysis of RNA polymerase II distribution led to the conclusion that transcriptional reactivation in lamprey prophase I is not associated with the completion of chromosome synapsis. Monomethylated histone H3K4 is localized in meiotic chromatin throughout prophase I, and this pattern has not been previously detected in animals. Thus, the findings made it possible to identify synaptic and epigenetic patterns specific to this group and to expand knowledge about chromatin epigenetics in prophase I.