Asexual Mode Research Articles

Characterization of active transposable elements and their new insertions in tuber propagated greater yam (Dioscorea alata)

BackgroundGreater yam is a key staple crop grown in tropical and subtropical regions, while its asexual propagation mode had led to non-flowering mutations. How transposable elements contribute to its genetic variations is rarely analyzed. We used transcriptome and whole genome sequencing data to identify active transposable elements (TEs) and genetic variation caused by these active TEs. Our aim was to shed light on which TEs would lead to its intraspecies variation.ResultsAnnotation of de novo assembly transcripts indicated that 0.8 − 0.9% of transcripts were TE related, with LTR retrotransposons (LTR-RTs) accounted for 65% TE transcripts. A large portion of these transcripts were non-autonomous TEs, which had incomplete functional domains. The majority of mapped transcripts were distributed in genic deficient regions, with 9% of TEs overlapping with genic regions. Moreover, over 90% TE transcripts exhibited low expression levels and insufficient reads coverage to support full-length structure assembly. Subfamily analysis of Copia and Gypsy, the two LTR-RTs revealed that a small number of subfamilies contained a significantly larger number of members, which play a key role in generating TE transcript. Based on resequencing data, 15,002 L-RT insertion loci were detected for active LTR-RT members. The insertion loci of LTR-RTs were highly divergent among greater yam accessions.ConclusionsThis study showed the ongoing transcription and transpositions of TEs in greater yam, despite low transcription levels and incomplete proteins insufficient for autonomous transposition. While our research did not directly link these TEs to specific yam traits such as tuber yield and propagation mode, it lays a crucial foundation for further research on how these TE insertion polymorphisms (TIPs) might be related to variations in greater yam traits and its tuber propagation mode. Future research may explore the potential roles of TEs in trait variations, such as tuber yield and stress resistance, in greater yam.

Exploring the Arsenal of Novel Antifungal Drug Targets for Combating Fungal Infections.

Fungal infections contribute to over 1.5 million fatalities each year, with cutaneous mycoses standing as prominent global infections. The spectrum of these mycoses varies widely, encompassing enduring afflictions like ringworm, localized infections such as tinea capitis, recurrent instances like vaginal candidiasis, and potentially fatal systemic infections impacting multiple organ systems. The escalating recognition of the health and socioeconomic ramifications associated with fungal pathogens underscores their importance in contemporary discourse. On a global scale, projections indicate that over 300 million individuals experience significant fungal infections annually, resulting in a mortality rate exceeding 1.5 million deaths per year. Alarmingly, resistance to commonly used antifungal drugs was on the rise, with some reports suggesting that over 10% of Candida bloodstream isolates worldwide were resistant to fluconazole, a commonly prescribed antifungal medication. Therefore, there is an immediate need to increase the accessibility of new antifungal medications while minimizing their costs and adverse effects. Fungi, as heterotrophic organisms, acquire nutrients through absorption. Their filamentous structure, composed of hyphae, facilitates efficient nutrient uptake by secreting enzymes that break down complex organic matter into simpler compounds. These organisms exhibit remarkable adaptability in responding to environmental cues, adjusting growth rates, and altering morphological features. Fungi regulate their metabolism intricately, undergoing various metabolic pathways for energy production and utilizing diverse substrates for respiration. Additionally, they exhibit distinctive reproductive strategies, employing both sexual and asexual modes of reproduction, contributing to their genetic diversity and resilience in diverse ecosystems. We now have more information than ever on the origins of infection as well as the physiology of fungi cells, giving us the chance to use it to produce new generations of antifungals. This review includes various novel antifungal drug targets showing their possible effects via different mechanisms aiming at vital functions like GPI synthesis, cell wall synthesis, hyphal growth, and other essential pathways responsible for fungal growth.

Effects of mode of reproduction on genetic polymorphism and divergence in wild yams (Dioscoreaceae: Dioscorea)

Evolutionary transitions from sexual to asexual reproduction should have significant influences on genetic divergence and polymorphism at the genome level. Plant lineages with diverse reproductive systems provide opportunities to investigate this question using comparative approaches and studies of molecular evolution. We investigated evidence for differences among the transcriptomes of 19 Dioscorea species (wild yams) with diverse reproductive systems. These included sexual species, those that propagate primarily by bulbils, and those with mixed sexual and asexual reproductive modes. We examined how transitions between these reproductive systems affected between-species divergence and within-species polymorphism. Primarily asexual species exhibited a reduced efficacy of natural selection and accumulation of deleterious mutations for both divergence and polymorphism. In contrast, species with mixed reproductive strategies involving both seed and clonal reproduction showed no evidence of an increased fixation of harmful mutations at the divergence level, while an accumulation of genetic load present in polymorphism was evident. Our study indicates that the genetic consequences of evolutionary transitions from sexual to predominantly clonal reproduction is likely to depend on both the duration and extent of asexuality occurring in populations.

The microbial cataclysm that forever changed life’s history

After a full century of search, evidence of Darwin’s “missing” pre-Cambrian fossil record was finally discovered in the mid-1950s. Unsurprisingly, given the then widely held belief that Precambrian life was not only unknown but unknowable, the initial report engendered widespread skepticism. Nevertheless, new finds soon followed, discoveries that set the science on its present course by establishing the role of O2-producing phototrophic cyanobacteria as prime components of the Precambrian biota. Remarkably, however, many of the newly found ancient specimens were all but indistinguishable from cyanobacteria living today, a similarity that although bolstering their interpretation as authentic fossils raised questions about their apparent lack of evolutionary change. As the Precambrian fossil record has become increasingly better documented over recent decades, the seeming identity of modern and fossil cyanobacterial taxa has been repeatedly confirmed by workers worldwide. To accommodate these extraordinarily long-lived taxa, a new category of evolutionary rate-distribution, hypobradytely, has been added to G.G. Simpson’s classic listing and a second group of hypobradytelic Precambrian organisms, the microbial components of sulfuretum biocoenoses, has been discovered. Both of these groups originated early in earth history, the origin of O2-producing cyanobacteria ∼2700 Ma ago producing an enormous upheaval of the world’s ecosystem -- the microbial cataclysm -- as they supplanted long-established oxygen-intolerant anoxygenic photosynthetic bacteria to become the predominant primary producers, the fundament of the global ecosystem. At least three principal characteristics of these early-evolved hypobradytelic prokaryotes, both cyanobacteria and sulfuretum bacteria, have contributed to their striking differences in tempo and mode of evolution from those of the later-evolved Phanerozoic biota: (1) Their lack of outgroup biotic competition; (2) their within-group competition-minimizing ecologic preferences; and (3) their phenotype variability-limiting asexual mode of reproduction. Clearly, both the tempo and mode of evolution evolved as the cyanobacterially induced microbial cataclysm forever changed life’s history.

Does ecology shape geographical parthenogenesis? Evidence from the facultatively parthenogenetic stick insect Megacrania batesii.

Closely related sexual and parthenogenetic species often show distinct distribution patterns, known as geographical parthenogenesis. Similar patterns, characterized by the existence of separate sexual and parthenogenetic populations across their natural range, can also be found in facultative parthenogens - species in which every female is capable of both sexual and parthenogenetic reproduction. The underlying mechanisms driving this phenomenon in nature remain unclear. Features of the habitat, such as differences in host-plant phenotypes or niche breadth, could favour sexual or asexual reproductive modes and thus help to explain geographical parthenogenesis in natural insect populations. Megacrania batesii is a facultatively parthenogenetic stick insect that displays geographical parthenogenesis in the wild. We aimed to explore whether sexual and parthenogenetic populations of M. batesii displayed niche differentiation or variations in niche breadth that could explain the separation of the two population types. To do this, we sampled host plants from across the range of M. batesii and quantified phenotypic traits that might affect palatability or accessibility for M. batesii, including leaf thickness, toughness, spike size and density, plant height, and chemical composition. We also quantified host-plant density, which could affect M. batesii dispersal. We found little evidence of phenotypic differences between host plants supporting sexual versus asexual M. batesii populations, and no difference in host-plant density or niche breadth between the two population types. Our results suggest that habitat parameters do not play a substantial role in shaping patterns of geographical parthenogenesis in wild populations of M. batesii. Instead, population sex ratio variation could result from interactions between the sexes or dispersal dynamics.

Development of Apomictic 56-Chromosomal Maize-Tripsacum Hybrids: A Potential Breakthrough in Heterosis Fixation.

Maize (Zea mays L.) is one of the most demanded grain crops in the world. Currently, production has exceeded one billion tons and is increasing by 3-5% annually. Such growth is due to the genetic potential of the crop and the use of heterosis F1 hybrids in production. However, the need to produce first-generation seed annually poses significant challenges and is an economically costly technology. A solution to this problem may be the transfer of the asexual (apomictic) mode of reproduction to maize from its wild relative, eastern gamagrass (Tripsacum dactyloides L.). In this work, we report the production of 56-chromosome apomictic hybrids of maize (Zea mays L.) with eastern gamagrass (T. dactyloides L.) with restored anther fertility. The mode of reproduction of the plant was confirmed by counting chromosomes and sequencing the nuclear gene (Pox3) and chloroplast tRNA-Leu (trnL) gene. These apomictic hybrids had karyotypes of 2n = 56 = [(10Zm(573MB) + 36Td) + 10Zm(611CB)] and 2n = 56 = [(10Zm(611CB) + 36Td) + 10Zm(611CB)]. The resulting hybrids can be widely used as a fodder crop.

Genome dosage alteration caused by chromosome pyramiding and shuffling effects on karyotypic heterogeneity, reproductive diversity, and phenotypic variation in Zea-Tripsacum allopolyploids.

We developed an array of Zea-Tripsacum tri-hybrid allopolyploids with multiple ploidies. We unveiled that changes in genome dosage due to the chromosomes pyramiding and shuffling of three species effects karyotypic heterogeneity, reproductive diversity, and phenotypic variation in Zea-Tripsacum allopolyploids. Polyploidy, or whole genome duplication, has played a major role in evolution and speciation. The genomic consequences of polyploidy have been extensively studied in many plants; however, the extent of chromosomal variation, genome dosage, phenotypic diversity, and heterosis in allopolyploids derived from multiple species remains largely unknown. To address this question, we synthesized an allohexaploid involving Zea mays, Tripsacum dactyloides, and Z. perennis by chromosomal pyramiding. Subsequently, an allooctoploid and an allopentaploid were obtained by hybridization of the allohexaploid with Z. perennis. Moreover, we constructed three populations with different ploidy by chromosomal shuffling (allopentaploid × Z. perennis, allohexaploid × Z. perennis, and allooctoploid × Z. perennis). We have observed 3 types of sexual reproductive modes and 2 types of asexual reproduction modes in the tri-species hybrids, including 2n gamete fusion (2n + n), haploid gamete fusion (n + n), polyspermy fertilization (n + n + n) or 2n gamete fusion (n + 2n), haploid gametophyte apomixis, and asexual reproduction. The tri-hybrids library presents extremely rich karyotype heterogeneity. Chromosomal compensation appears to exist between maize and Z. perennis. A rise in the ploidy of the trihybrids was linked to a higher frequency of chromosomal translocation. Variation in the degree of phenotypic diversity observed in different segregating populations suggested that genome dosage effects phenotypic manifestation. These findings not only broaden our understanding of the mechanisms of polyploid formation and reproductive diversity but also provide a novel insight into genome pyramiding and shuffling driven genome dosage effects and phenotypic diversity.

The species problem in Artemia Leach, 1819 (Crustacea: Anostraca), a genus with sexual species and obligate parthenogenetic lineages

Abstract Parthenogenesis is an asexual reproduction mode characterized by the development of a female oocyte without fertilization. From an evolutionary perspective, parthenogenesis seems less successful than the predominant sexual mode, though there are groups in which both reproductive types exist, an example of which is the genus Artemia Leach, 1819. This salt-tolerant crustacean inhabiting hypersaline environments contains regionally endemic sexual species and obligate parthenogenetic groups with different ploidy levels, collectively referred to as Artemia parthenogenetica. Here, we discuss the difficulties of using a common species concept in Artemia Leach, 1819. While sexual species are widespread and fit the Biological Species Concept (BSC), which emphasizes reproductive isolation to maintain species genetic integrity or cohesiveness, it does not apply to uniparental organisms originating from sexual species with major meiotic changes. We show that different ploidy levels of parthenogenetic Artemia groups with uniform nuclear gene pools are maternally independent genetic entities (or cohesive), collectively and wrongly referred to as Artemia parthenogenetica. Thus, we conclude that ‘Artemia parthenogenetica’ is an invalid nominal specific name. Additionally, parthenogenetic Artemia groups cannot be considered a form of Artemia species (A. urmiana and/or A. sinica). In conclusion, we recommend using the term ‘parthenogenetic lineage(s)’ instead of ‘parthenogenetic population(s)’ to describe asexual Artemia group(s), because in modern population genetics and systematics, the term ‘population’ refers to interbreeding individuals with sexual reproduction. Furthermore, it clarifies that parthenogenetic lineages of Artemia are native to Mediterranean biodiversity.

Is apomixis occurring in walnut (Juglans regia L.)? New data from progeny molecular tests and cytological investigations shed light on its reproductive system.

Persian walnut (Juglans regia) is an economically important nut fruit species cultivated worldwide for its nutritious kernel and timber quality wood. Walnut trees are mostly hetero-dichogamous and, depending on the genotype, some cultivars are protogynous, while others are protandrous. Although selfing is possible when male and female blooms overlap, the dichogamy of the species promotes outcrossing. In addition to sexual reproduction, some reports indicate that elements of apomixis may occur in commercial orchards of walnut varieties and in the last two decades, nut production by apomixis has been reported in walnut. However, there are no reliable studies on the occurrence of apomictic reproduction based on cytoembryological observations and/or molecular marker-progeny tests. This study addresses the combined use of molecular and cytological analyses to gain new insights into the population genetics and reproduction systems of J. regia. We systematically analyzed the reproductive origin of individual progeny plants from 8 different cultivated walnut genotypes using microsatellite genotyping and carried out cytohistological investigations of 5 cultivated walnut genotypes arising seed sets from isolated flowers, to shed light on the mode of reproduction. These cytometric and genotyping analyses did not support any asexual mode of reproduction or asexual propagation by seed and all individuals studied were identified as zygotic plants produced by crossing. Likewise, the cytological findings did not confirm completely the first component of apomixis, namely apomeiosis. On the other hand, according to histological evidence, adventitious embryony seems to take place at low frequency. Overall, our findings suggest that the occurrence of gametophytic apomixis is unlikely in J. regia, but sporophytic apomixis cannot be completely ruled out.

Morph-specific fitness throughout the life cycle of the grain aphid, nonhost-alternating, holocyclic Sitobion avenae (Hemiptera: Aphididae).

Aphids exhibit seasonally alternating asexual and sexual reproductive modes. Different morphs are produced throughout the life cycle. To evaluate morph-specific fitness during reproductive switching, holocyclic Sitobion avenae were induced continuously under short light conditions, and development and reproduction were compared in each morph. Seven morphs, including apterous and alate virginoparae, apterous and alate sexuparae, oviparae, males, and fundatrices, were produced during the life cycle. The greatest proportions of sexuparae, oviparae, males, and virginoparae were in the G1, G2, G3, and G4 generations, respectively. Regardless of asexual or sexual morphs, alate morphs exhibited a marked delay in age at maturity compared with that of apterous morphs. Among the alate morphs, males had the longest age at maturity, followed by sexuparae and virginoparae. Among the apterous morphs, sexuparae were older at maturity than the fundatrices, virginoparae, and oviparae. The nymphs of each morph had equal survival potentials. For the same wing morphs, apterous sexuparae and oviparae exhibited substantial delays in the pre-reproductive period and considerable reductions in fecundity, compared with those of apterous virginoparae and fundatrices, whereas alate sexuparae and alate virginoparae had similar fecundity. The seven morphs exhibited Deevey I survivorship throughout the life cycle. These results suggest that sexual production, particularly in males, has short-term development and reproduction costs. The coexistence of sexual and asexual morphs in sexuparae offspring may be regarded as an adaptive strategy for limiting the risk of low fitness in winter.

Gene flow and species boundaries of the genus Salmonella.

The genus Salmonella comprises two species, Salmonella bongori and Salmonella enterica, which are infectious to a wide variety of animal hosts. The diversity within S. enterica has been further partitioned into 6-10 subspecies based on such features as host range, geography, and most recently, genetic relatedness and phylogenetic affiliation. Although Salmonella pathogenicity is attributable to large numbers of acquired virulence factors, the extent of homologous exchange in the species at large is apparently constrained such that the species and subspecies form distinct clusters of strains. To explore the extent of gene flow within and among subspecies, and to ultimately define true biological species, we evaluated patterns of recombination in over 1,000 genomes currently assigned to the genus. Those Salmonella subspecies containing sufficient numbers of sequenced genomes to allow meaningful analysis-i.e., subsp. enterica and diarizonae-were found to be reproductively isolated from one another and from all other subspecies. Based on the configuration of genomic sequence divergence among subspecies, it is expected that each of the other Salmonella subspecies will also represent a biological species. Our findings argue against the application of prescribed nucleotide-identity thresholds to delineate bacterial species and contend that the Biological Species Concept should not be disregarded for bacteria, even those, like Salmonella, that demonstrate complex patterns of species and subspecies divergence. IMPORTANCE The Biological Species Concept (BSC), which defines species boundaries based on the capacity for gene exchange, is widely used to classify sexually reproducing eukaryotes but is generally thought to be inapplicable to bacteria due to their completely asexual mode of reproduction. We show that the genus Salmonella, whose thousands of described serovars were formerly considered to be strictly clonal, undergoes sufficient levels of homologous recombination to be assigned to species according to the BSC. Aside from the two recognized species, Salmonella enterica and Salmonella bongori, several (and likely all) of the subspecies within S. enterica are reproductively isolated from one another and should each be considered a separate biological species. These findings demonstrate that species barriers in bacteria can form despite high levels of nucleotide identity and that commonly applied thresholds of genomic sequence identity are not reliable indicators of bacterial species status.

Warming increases survival and asexual fitness in a facultatively sexual freshwater cnidarian with winter diapause.

Temperature is a key abiotic factor controlling population dynamics. In facultatively sexual animals inhabiting the temperate zone, temperature can regulate the switch between asexual and sexual modes of reproduction, initiates growth or dormancy, and acts together with photoperiod to mediate seasonal physiological transitions. Increasing temperature due to recent global warming is likely to disrupt population dynamics of facultatively sexual animals because of the strong temperature dependence of multiple fitness components. However, the fitness consequences of warming in these animals are still poorly understood. This is unfortunate since facultatively sexual animals-through their ability for asexual reproduction resulting in quick population growth and sexual reproduction enabling long-term persistence-are key components of freshwater ecosystems. Here, I studied the fitness effects of warming in Hydra oligactis, a freshwater cnidarian that reproduces asexually throughout most of the year but switches to sexual reproduction under decreasing temperatures. I exposed hydra polyps to a simulated short summer heatwave or long-term elevated winter temperature. Since sexual development in this species is dependent on low temperature, I predicted reduced sexual investment (gonad production) and elevated asexual fitness (budding) in polyps exposed to higher temperatures. The results show a complex effect of warming on sexual fitness: While gonad number decreased in response to warming, both male and female polyps exposed to high winter temperature were capable of multiple rounds of gamete production. Asexual reproduction and survival rate, on the contrary, clearly increased in response to higher temperature, especially in males. These results predict increased population growth of H. oligactis in temperate freshwater habitats, which will likely affect the population dynamics of its main prey (freshwater zooplankton), and through that, the whole aquatic ecosystem.

Asexual reproduction strategies in the moon jellyfish Aurelia (Cnidaria: Scyphozoa)

The genusAureliais one of the major contributors to jellyfish blooms in coastal waters, possibly due to its adaptive reproduction strategies. DifferentAurelialineages have adapted their reproduction modes to varying environmental conditions in their respective habitats. To understand the successful adaptation strategies, three strains ofAurelia coeruleaand two strains ofAurelia solidapolyps from different geographical areas were exposed to a range of temperatures and two food regimes, and the effects on reproduction rates were assessed. Asexual reproduction was significantly affected by the changes in these factors. The highest reproduction rate under sufficient food conditions was observed in the United States strain and the lowest was observed in the Israel strain, regardless of temperature, indicating the differences in the blooming potential. Six asexual reproduction modes were observed, of which lateral budding, lateral budding by means of stolons, and reproduction from parts of stolons were the main modes used by allAureliastrains, exceptAurelia solidaISR, for which reproduction by stolons was the main mode. The capability to switch reproductive strategies in response to environmental cues depending on the lineage predetermines the highly frequent blooming events ofAurelia.

The venom composition and parthenogenesis mechanism of the parasitoid wasp Microctonus hyperodae, a declining biocontrol agent

A biocontrol system in New Zealand using the endoparasitoid Microctonus hyperodae is failing, despite once being one of the most successful examples of classical biocontrol worldwide. Though it is of significant economic importance as a control agent, little is known about the genetics of M. hyperodae. In this study, RNA-seq was used to characterise two key traits of M. hyperodae in this system, the venom, critical for the initial success of biocontrol, and the asexual reproduction mode, which influenced biocontrol decline. Expanded characterisation of M. hyperodae venom revealed candidates involved in manipulating the host environment to source nutrition for the parasitoid egg, preventing a host immune response against the egg, as well as two components that may stimulate the host's innate immune system. Notably lacking from the venom-specific expression list was calreticulin, as it also had high expression in the ovaries. In-situ hybridisation revealed this ovarian expression was localised to the follicle cells, which may result in the deposition of calreticulin into the egg exochorion. Investigating the asexual reproduction of M. hyperodae revealed core meiosis-specific genes had conserved expression patterns with the highest expression in the ovaries, suggesting M. hyperodae parthenogenesis involves meiosis and that the potential for sexual reproduction may have been retained. Upregulation of genes involved in endoreduplication provides a potential mechanism for the restoration of diploidy in eggs after meiosis.

The Adaptive Capacity of Alien and Rare Species in China

Alien and rare plants tend to adapt to contrasting biotic and abiotic conditions. However, adaptability assessments of alien and rare plants using biological and habitat characteristics in stressful and disturbed environments are limited. We collected and demonstrated the biological and habitat characteristics and assessed the adaptive capacity of alien and rare plants in China using the analytic hierarchy process. Biological characteristics, such as dispersal strategy, sexual and asexual reproduction modes, life form, and habitat characteristics (e.g., habitat type and distribution spatial extent), are important indicators of the adaptability of alien and rare plants to stressful and disturbed environments. Alien plants have a higher adaptive capacity to disturbed environments than rare plants, while rare plants have a higher adaptive capacity to stressful environments than alien plants. Stressful and disturbed environments constrain the adaptive capacity of alien and rare plants, respectively. However, the constraint on alien plants from stressful environments is weaker than that on rare plants from disturbed environments. Understanding the adaptive capacity of alien and rare plants will help researchers and policymakers develop strategies for preventing the invasion of alien plants as well as protecting rare and endangered plants.

Annual rhythmicity in the switching of reproductive mode in planarians

Species of planarians include both asexually reproducing individuals (reproduce through fission and regeneration) and sexually reproducing individuals (hermaphrodites that mate to produce cocoons). While some individuals can switch between the asexual and sexual modes of reproduction. In this study, we examined the reproductive modes and ploidy of Dugesia japonica and Dugesia ryukyuensis from three spring wells in Okinawa (Japan) during two consecutive years. D. japonica are mostly asexual and triploid. In contrast, only 40 % of D. ryukyuensis are asexual and triploid; the remaining are sexual, and diploid or triploid. The sexually reproductive season of D. ryukyuensis is winter. In July, the reproductive organs disappear, and the individuals start asexual reproduction through fission and regeneration. In January of the following year, the individuals develop ovaries and necessary reproductive organs and start sexual reproduction. When these species were lab-reared for a longer period, the reproductive cycles in three strains were repeated for three years. These results confirm that D. ryukyuensis population in Okinawa switches between reproductive modes on an annual cycle, even when kept under constant temperature and no light/dark cycle.

Bryophytes of conservation concern decline and traits change in retention patches during two decades following forest harvest

Setting aside tree retention patches at final harvest is a common forest conservation measure, but the long-term responses of species of conservation concern to different types of forest retention remain unknown. This is the first long-term study of the dynamics of bryophytes of conservation concern and associated traits in small forest retention patches (0.01–0.53 ha) over almost two decades and in relation to environmental drivers. We applied joint species distribution models to compare how bryophytes changed in terms of occurrence, abundance, species richness and traits between year 2000, 2006 and 2018, and in relation to five different retention patch types, patch area, retained living tree volume per hectare, structural heterogeneity, and surrounding habitat openness. Bryophytes responded mostly negatively to the long-term environmental change, although with considerable variation depending on retention patch type. Only negative trends were detected for individual species. Still, no regional extinctions (loss of species from all local patches) occurred and there was a tendency for recovery of a few species. Bryophytes with predominantly asexual reproductive modes and small colony size (<100 cm2) increased in occurrence probability over time, indicating that these traits were beneficial for the long-term persistence of species in small and exposed forest patches. We recommend forest managers to create large (preferably up to 0.5 ha) and variable retention patches, both in terms of patch type (i.e. buffer zones and free-standing groups of different dominant tree species and ground moisture) and structural heterogeneity (i.e. living tree volumes, large and small trees, and deadwood).

Complete and rapid regeneration of fragments from the upside-down jellyfish Cassiopea

The upside-down jellyfish Cassiopea increasingly occurs in many (sub-) tropical coastal habitats such as mangrove forests, seagrass meadows, and coral reefs. Its mixotrophic lifestyle and ecophysiological plasticity as well as a high regenerative capacity may be reasons for its success. While the regeneration of umbrella tissue and body structures (i.e. rhopalia and oral arms) was already demonstrated, it remains unclear whether a fully functioning medusa can regenerate from only umbrella tissue. In this study, we thus investigated the regeneration of umbrella fragments over time. We conducted a laboratory experiment for which we used 18 Cassiopea medusae of three different size classes that were cut into two pieces each, one fragment with oral arms and one without. Over a total observation period of 5 weeks, we regularly monitored survival, pulsation behavior, growth and the regeneration pattern of fragments. Findings revealed that 100% of the fragments with oral arms and 88% of the fragments without oral arms survived. Pulsation behavior occurred in all fragments and lasted until the end of the experiment in 94% of all fragments. The umbrella area of fragments without oral arms showed a significantly higher decrease in the first two weeks compared to fragments with oral arms. A complete regeneration of umbrella tissue was observed in all fragments, with and without oral arms alike, and 50% of all fragments even regenerated rhopalia or oral arms as body structures after 33 days. These results suggest an outstanding regenerative capacity of Cassiopea jellyfish after fragmentation. This may contribute to (i) explain the currently observed success of upside-down jellyfish and (ii) extend our knowledge about its regeneration process, which might even act as an asexual reproduction mode in Cassiopea.

Patterns of gene expression in ovaries of sexual vs. asexual lineages of a freshwater snail

Why sexual reproduction is so common when asexual reproduction should be much more efficient and less costly remains an open question in evolutionary biology. Comparisons between otherwise similar sexual and asexual taxa allow us to characterize the genetic architecture underlying asexuality, which can, in turn, illuminate how this reproductive mode transition occurred and the mechanisms by which it is maintained or disrupted. Here, we used transcriptome sequencing to compare patterns of ovarian gene expression between actively reproducing obligately sexual and obligately asexual females from multiple lineages of Potamopyrgus antipodarum, a freshwater New Zealand snail characterized by frequent separate transitions to asexuality and coexistence of otherwise similar sexual and asexual lineages. We also used these sequence data to evaluate whether population history accounts for variation in patterns of gene expression. We found that source population was a major source of gene expression variation, and likely more influential than reproductive mode. This outcome for these common garden-raised snails is strikingly similar to earlier results from field-collected snails. While we did not identify a likely set of candidate genes from expression profiles that could plausibly explain how transitions to asexuality occurred, we identified around 1,000 genes with evidence of differential expression between sexual and asexual reproductive modes, and 21 genes that appear to exhibit consistent expression differences between sexuals and asexuals across genetic backgrounds. This second smaller set of genes provides a good starting point for further exploration regarding a potential role in the transition to asexual reproduction. These results mark the first effort to characterize the causes of asexuality in P. antipodarum, demonstrate the apparently high heritability of gene expression patterns in this species, and hint that for P. antipodarum, transitions to asexuality might not necessarily be strongly associated with broad changes in gene expression.

Occasional sexual reproduction significantly affects the population structure of the widespread, predominantly asexually reproducing marine worm Lineus sanguineus (Nemertea: Pilidiophora)

Asexual reproduction by fissiparity has only been described from very few of the approximately 1300 nemertean species that otherwise mainly reproduce sexually. The best studied fissiparous species, Lineus sanguineus (Rathke 1799), is a cosmopolitan heteronemertean species inhabiting intertidal habitats of temperate coasts. Although sexual reproduction has never been described, molecular data suggest that sexual reproduction substantially shapes the genetic structure of the investigated populations. In an attempt to clarify the extent of sexual reproduction, three gene fragments (COI, 16S, ITS) were sequenced for 108 specimens sampled in 8 European localities. The results of a phylogenetic analysis and haplotype network showed no clear distinction between different populations, thus indicating the presence of sexual reproduction. Furthermore, we provide circumstantial evidence for the presence of a comparably long-lived planktonic larval stage as present in the closest relatives of L. sanguineus. To further understand the impact of abiotic factors on sexual reproduction and fissiparity, the effect of different temperature and illumination regimes on reproductive behavior and fragmentation was studied in specimens from a population from Bergen, Norway that share the same haplotype. Experimental setups represented summer (long light period and elevated temperature) and winter (short light and decreased temperature) conditions. Under winter conditions, a higher number of animals remained sexually mature and at least one specimen shed eggs on one occasion. Thus, although short light and/or low temperatures are most likely the influential factors on sexual maturity, the factors that influence fissiparity are less clear. The results of this study further solidify the cosmopolitan status of L. sanguineus and clarify the population structuring of this species. In addition, the study provides first data on the dynamics of sexual and asexual reproduction modes on which future investigations will have to expand, especially regarding genetic and physiological aspects.