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.

Assessment of the consumptive safety of mercury in fish from the surface waters of the Vologda region in northwestern Russia.

It is known that the consumption of fish is the main source of mercury intake in human beings. In this study, the concentration of mercury in the muscle tissue of fish from different reservoirs of northwestern Russia was found to be 0.01-1.68 µg/g wet weight. The features of mercury accumulation in the muscle tissue of fish, depending on their type, trophic specialization, body weight, length, and the type of water body, were also revealed. Of the fish studied, 7% had mercury concentrations above the regulatory levels of the Russian Federation. The proportion of examined fish, the consumption of which will lead to an excess of the permissible weekly intake of mercury in the individual, is 44% for preschool children (2-5 years old), 34% for children of primary school age (6-10 years old), and 17% for adults. Special attention is drawn to the fact that the mercury content in fish that does not exceed the sanitary and hygienic standards (normative levels) of the Russian Federation may still be unsafe for the health of the population, especially children.