Challenges and Costs of Asexuality: Variation in Premeiotic Genome Duplication in Gynogenetic Hybrids from Cobitis taenia Complex

The transition from sexual reproduction to asexuality is often triggered by hybridization. The gametogenesis of many hybrid asexuals involves premeiotic genome endoreplication leading to bypass hybrid sterility and forming clonal gametes. However, it is still not clear when endoreplication occurs, how many gonial cells it affects and whether its rate differs among clonal lineages. Here, we investigated meiotic and premeiotic cells of diploid and triploid hybrids of spined loaches (Cypriniformes: Cobitis) that reproduce by gynogenesis. We found that in naturally and experimentally produced F1 hybrids asexuality is achieved by genome endoreplication, which occurs in gonocytes just before entering meiosis or, rarely, one or a few divisions before meiosis. However, genome endoreplication was observed only in a minor fraction of the hybrid’s gonocytes, while the vast majority of gonocytes were unable to duplicate their genomes and consequently could not proceed beyond pachytene due to defects in bivalent formation. We also noted that the rate of endoreplication was significantly higher among gonocytes of hybrids from natural clones than of experimentally produced F1 hybrids. Thus, asexuality and hybrid sterility are intimately related phenomena and the transition from sexual reproduction to asexuality must overcome significant problems with genome incompatibilities with a possible impact on reproductive potential.

Clonal gametogenesis is triggered by intrinsic stimuli in the hybrids germ cells but is dependent on sex differentiation

Interspecific hybridization may trigger the transition from sexual reproduction to asexuality, but mechanistic reasons for such a change in a hybrids reproduction are poorly understood. Gametogenesis of many asexual hybrids involves a stage of premeiotic endoreduplication (PMER), when gonial cells duplicate chromosomes and subsequent meiotic divisions involve bivalents between identical copies, leading to production of clonal gametes. Here, we investigated the triggers of PMER and whether its induction is linked to intrinsic stimuli within a hybrids gonial cells or whether it is regulated by the surrounding gonadal tissue. We investigated gametogenesis in the Cobitis taenia hybrid complex, which involves sexually reproducing species (Cobitis elongatoides and C. taenia) as well as their hybrids, where females reproduce clonally via PMER while males are sterile. We transplanted spermatogonial stem cells (SSCs) from C. elongatoides and triploid hybrid males into embryos of sexual species and of asexual hybrid females, respectively, and observed their development in an allospecific gonadal environment. Sexual SSCs underwent regular meiosis and produced normally reduced gametes when transplanted into clonal females. On the other hand, the hybrids SSCs lead to sterility when transplanted into sexual males, but maintained their ability to undergo asexual development (PMER) and production of clonal eggs, when transplanted into sexual females. This suggests that asexual gametogenesis is under complex control when somatic gonadal tissue indirectly affects the execution of asexual development by determining the sexual differentiation of stem cells and once such cells develop to female phenotypes, hybrid germ cells trigger the PMER from their intrinsic signals.

Morphohistological studies of the gonads of hybrid polyploid loach cobitis from the Lower Dniester

In the lower Dniester, polyploid females of the hybrid complex Cobitis taenia are a portion spawning species with asynchronous development of germ cells during the entire breeding period. The spawning season in this water basin begins in the second decade of May and ends in July. In the population, in parallel with females, hermaphrodite individuals are also present, in which the ovary zone is functional, and the testis zone is sterile. A decrease in the relative mass of the gonads of subsequent generations was noted, which leads to a consistent decrease in the value of the gonadosomatic index, in females, before the second and third spawning.

Challenges and costs of asexuality: Variation in premeiotic genome duplication in gynogenetic hybrids from Cobitis taenia complex.

The transition from sexual reproduction to asexuality is often triggered by hybridization. The gametogenesis of many hybrid asexuals involves a stage of premeiotic genomic endoreduplication leading to the production of clonal gametes and bypassing genomic incompatibilities that would normally cause hybrid sterility. However, it is still not clear at what gametogenic stage the endoreplication occurs, how many gonial cells it affects, and whether its rate differs among clonal lineages. Here, we investigated meiotic and premeiotic cells of diploid and triploid hybrids of spined loaches (Cypriniformes: Cobitis) that reproduce by gynogenesis. We found that naturally as well as experimentally produced F1 hybrid strains undergo an obligatory genome duplication event to achieve asexuality, occurring in the gonocytes just before entering meiosis or, rarely, one or few divisions before meiosis. Surprisingly, however, the genome endoreplication was observed only in a minor fraction of the hybrid's gonocytes, while the vast majority were unable to duplicate their genomes and consequently could not proceed beyond pachytene due to defects in pairing and bivalent formation. We also noted that the rate of endoreplication was significantly higher among gonocytes of hybrids from successful natural clones than of experimentally produced F1 hybrids, indicating that interclonal selection may favor lineages that maximize the rate of premeiotic endoreduplication. We conclude that asexuality and hybrid sterility are intimately related phenomena and the transition from sexual reproduction to asexuality must overcome significant problems with genome incompatibilities with possible impact on reproductive potential.

SPECIES COMPOSITION OF MACROPHYTES AND ICHTHYOFAUNA OF LAKE MARTYSHIV (KYIV, OSOKORKY) AS IMPORTANT ELEMENT OF THE INTACT FLOODPLAIN COMPLEXES

The article examines the species composition of macrophytes (higher aquatic plants) and ichthyofauna of Lake Martyshiv (waterbody of the Osokorky floodplain, eco-corridor of Kyiv). The features of the ecological distribution of higher aquatic plants within the lake Martyshiv are analyzed. The macrophytes are represented by typical limnophilous(lacustrine) complexes. The presence of various groups of macrophytes in the lake indicates a low level of anthropogenic load and is not so differentfrom the values characteristic of a surface water filesunder standard conditions. 22 species of higher aquatic plants have been registered. Phragmites australis (Cav.) Trin. ex Steud., Typha angustifolia L. and Typha latifolia Lare the most common swamping indicator species. Among the studied plants, it should be noted Nuphar lutea (L.) Smith which is protected on the territory of Kyiv in accordance with the decision of the Kyiv Council of December 23, 2004 No. 880/2290. In the ichthyofauna of Lake Martyshiv, 30 species of fish that belong to 9 familieswere identified. In the structure of ichthyocenosis, phytophils (50%) and zoobenthophages (50%) were the dominant ecological groups in terms ofthe method of oviposition and the type of nutrition, respectively. According to the habitat in the reservoir, bottomfishand thicket-livingcategories prevailed (43.3 and 33.3%, respectively). Four species were found in the ichthyofauna of the lake (Rhodeus amarus (Bloch, 1782), Cobitis taenia (sl) Linnaeus, 1758, Misgurnus fossilis (Linnaeus, 1758), Aspius aspius Linnaeus, 1758), which are included to the listof Resolution of the 6thBerne Conventionthat wasadopted to create the Emerald Network. In addition, Carassius carassius Linnaeus, 1758, listed in the Red Book of Ukraine, as well as the elmLeuciscus idus Linnaeus, 1758, which was included in the new edition of the Red Book of Ukraine, were discovered.The research results illustrate preliminary data on the current distribution of species and biological diversity of aquatic ecosystems of the Osokorkyfloodplain. The presence of a number of ecological groups of macrophytes creates a number of biotopes that are important for the existence and reproduction of fish. Thepreservation of LakeMartyshiv in a lesstransformed state providesgroundsforthe creation of a landscape reserve of local importance.Key words:Lake Martyshiv, macrophytes, ichthyofauna, biodiversity, environmental transformation, habitat. У статті розглянуто видовий склад макрофітів (вищі водяні рослини) та іхтіофауни оз. Мартишів (водойма Осокорківської заплави, екокоридору м. Києва). Проаналізовано особливості екологічного розподілу вищих водяних рослин у межах оз. Мартишів. Макрофіти представлені типовими лімнофільними (озерними) комплексами. Наявність різних груп макрофітів у межах озера вказує на низький рівень антропогенного навантаження та мало відрізняється від значень, характерних для масиву поверхневих вод у референційних умовах.Відзначено 22 види вищих водяних рослин. Найпоширеніші види-індикатори процесів заболочення: Phragmitesaustralis(Cav.)Trin. exSteud., Typha angustifolia L. та Typha latifolia L. Серед досліджуваних рослин варто виокремити глечики жовті Nuphar lutea(L.) Smith,які натериторії Києва охороняються на підставі рішенняКиївської міської ради від 23.12.2004 року No880/2290.У складі іхтіофауни оз. Мартишів виявлено 30 видів риб, які належали до 9 родин. У структурі іхтіоценозу за домінуючими екологічними групами за способомвідкладання ікри та типом живлення були фітофіли (50%) та зообентофаги (50%) відповідно. За середовищем існування у водоймі домінували риби придонних та заростевих груп (43,3 та 33,3% відповідно). В іхтіофауні озера було виявлено 4 види (Rhodeus amarus (Bloch, 1782), Cobitis taenia (s.l.) Linnaeus, 1758, Misgurnus fossilis (Linnaeus, 1758), AspiusaspiusLinnaeus, 1758.), які включені до списку Резолюції 6 Бернської конвенції, прийнятої для створення Смарагдової мережі. Крім того, був виявлений CarassiuscarassiusLinnaeus, 1758., що включений до Червоної книги України, а також в’язь Leuciscus idus Linnaeus, 1758, якого буде внесено до нового видання Червоної книги України.Результати досліджень ілюструють попередні дані щодо сучасного розподілу видів та біологічного різноманіття водних екосистем Осокорківської заплави. Наявність низки екологічних груп макрофітів створює низку оселищ, важливих для існування та розмноження риб. Збереження оз. Мартишів у мало трансформованому стані дає підстави для створення ландшафтного заказника.Ключові слова:озеро Мартишів, макрофіти, іхтіофауна, біорізноманіття, трансформація середовища, середовище існування.

Fish in water bodies of the Emerald Network of Ukraine

The paper provides information on the structure of sites of the Emerald Network of Ukraine as special areas of conservation, focused at ensuring the protection of natural fauna, flora and habitats. The development of this network was initiated for the implementation of a set of resolutions of the Berne Convention (1979), and principles of its formation are basically similar to Natura 2000. As of 2019, a total of 271 Emerald sites were designated in Ukraine. According to studies, the water bodies of the network are inhabited by 25 fish species. The data were extracted from up-to-date ichthyological publications on the distribution of fish species in Ukrainian water bodies, catalogues of collections of various zoological museums, regional faunal cadastres, and own research. Analysis of the fish species composition in the sites has shown that the most common and found in more than 60% of the sites are Cobitis taenia, Rhodeus sericeus amarus, Misgurnus fossilis. This can be explained by characteristics of their biology and tolerance to environmental conditions. Endemic species are extremely rare in the Emerald sites and narrowly localized in several river basins. In particular, they are Zingel zingel, Umbra krameri, Eudontomyzon danfordi, Hucho hucho, and Leuciscus souffia recorded in 5-6 sites of the network. Further research is required that will be focused at the development of an integral network of Emerald sites in Ukraine. For this purpose, ichthyological reserves of general and local significance may be considered as promising areas. The analysis of the species number and distribution in water bodies of Ukraine is quite relative and requires additional special field studies. In addition, a qualitative assessment of the number of fish species in promising areas requires ichthyological research methods to be unified and an effective monitoring system be introduced.

The Legacy of Sexual Ancestors in Phenotypic Variability, Gene Expression, and Homoeolog Regulation of Asexual Hybrids and Polyploids

Hybridization and polyploidization are important evolutionary processes whose impacts range from the alteration of gene expression and phenotypic variation to the triggering of asexual reproduction. We investigated fishes of the Cobitis taenia-elongatoides hybrid complex, which allowed us to disentangle the direct effects of both processes, due to the co-occurrence of parental species with their diploid and triploid hybrids. Employing morphological, ecological, and RNAseq approaches, we investigated the molecular determinants of hybrid and polyploid forms. In contrast with other studies, hybridization and polyploidy induced relatively very little transgressivity. Instead, Cobitis hybrids appeared intermediate with a clear effect of genomic dosing when triploids expressed higher similarity to the parent contributing two genome sets. This dosage effect was symmetric in the germline (oocyte gene expression), interestingly though, we observed an overall bias toward C. taenia in somatic tissues and traits. At the level of individual genes, expression-level dominance vastly prevailed over additivity or transgressivity. Also, trans-regulation of gene expression was less efficient in diploid hybrids than in triploids, where the expression modulation of homoeologs derived from the “haploid” parent was stronger than those derived from the “diploid” parent. Our findings suggest that the apparent intermediacy of hybrid phenotypes results from the combination of individual genes with dominant expression rather than from simple additivity. The efficiency of cross-talk between trans-regulatory elements further appears dosage dependent. Important effects of polyploidization may thus stem from changes in relative concentrations of trans-regulatory elements and their binding sites between hybridizing genomes. Links between gene regulation and asexuality are discussed.

First Note on Fish Parasites in Polissky Nature Reserve, Northern Ukraine

Nineteen species of parasites belonging to seven taxonomical groups were detected in nine species of fish (Gobio gobio (Linnaeus, 1758), Cobitis taenia Linnaeus, 1758, Squalius cephalus (Linnaeus, 1758), Rutilus rutilus (Linnaeus, 1758), Misgurnus fossilis (Linnaeus, 1758), Sabanejewia baltica Witkowski, 1994, Barbatula barbatula (Linnaeus, 1758), Esox lucius Linnaeus, 1758 and Perca fluviatilis Linnaeus, 1758) from two small rivers in Polissky Nature Reserve in the northern part of Ukraine. Th e highest species richness of parasites was registered for G. gobio (7 species). Four species of parasites were found in S. cephalus. Three parasite species were found in E. lucius. Two parasite species were detected in R. rutilus and P. fluviatilis. C. taenia, M. fossilis, and S. baltica harboured each one species of parasites. Gyrodactylus sp. from S. baltica was reported for the first time. All parasites represent new geographical finding for the north of Ukraine.