Widespread co-occurrence of multiple ploidy levels in fragile ferns (Cystopteris fragilis complex; Cystopteridaceae) probably stems from similar ecology of cytotypes, their efficient dispersal and inter-ploidy hybridization

An adventurous journey toward and away from fern apomixis: Insights from genome size and spore abortion patterns

PREMISE

Apomixis in ferns is relatively common and obligatory. Sterile hybrids may restore fertility via apomixis at a cost of long-term genetic stagnation. In this study, we outlined apomixis as a possible temporary phase leading to sexuality and analyzed factors relating to transitioning to and away from apomixis, such as unreduced and reduced spore formation in apomict and apo-sex hybrid ferns.

METHODS

We analyzed the genome size of 15 fern species or hybrids ("taxa") via flow cytometry. The number of reduced and unreduced gametophytes was established as a proxy for viable spore formation of either type. We also calculated the spore abortion ratio (sign of reduced spores) in several taxa, including the apo-sex hybrid Dryopteris × critica and its 16 apomictically formed offspring.

RESULTS

Four of 15 sampled taxa yielded offspring variable in genome size. Specifically, each variable taxon formed one viable reduced plant among 12-451 sampled gametophytes per taxon. Thus, haploid spore formation in the studied apomicts was very rare but possible. Spore abortion analyses indicated gradually decreasing abortion (haploid spore formation) over time. In Dryopteris × critica, abortion decreased from 93.8% to mean 89.5% in one generation.

CONCLUSIONS

Our results support apomixis as a transitionary phase toward sexuality. Newly formed apomicts hybridize with sexual relatives and continue to form haploid spores early on. Thus, they may get the genomic content necessary for regular meiosis and restore sexuality. If the missing relative goes extinct, the lineage gets locked into apomixis as may be the case with the Dryopteris affinis complex.

Plant-soil microbe feedbacks depend on distance and ploidy in a mixed cytotype population of Larrea tridentata

PREMISE

Theory predicts that mixed ploidy populations should be short-lived due to strong fitness disadvantages for the rare ploidy. However, mixed ploidy populations are common, suggesting that the fitness costs for rare ploidies are counterbalanced by ecological benefits that emerge when rare. We investigated whether differences in ecological interactions with soil microbes help to maintain a tetraploid-hexaploid population of Larrea tridentata (creosote bush) in the Sonoran Desert, California, United States, where prior work documented ploidy-specific root-associated microbes.

METHODS

We used a plant-soil feedback (PSF) experiment to test whether host-specific soil microbes can alter the outcomes of intraploidy vs. interploidy competition. Host-specific soil microbes can build up over time; thus, distance from a host plant can affect the fitness of nearby plants.

RESULTS

Seedlings grown in soils from near plants of a different ploidy produced greater biomass relative to seedlings grown in soils from near plants of the same ploidy. Moreover, seedlings grown in soils from near plants of a different ploidy produced more biomass than those grown in soils that were farther from plants of a different ploidy. These results suggest that the ecological consequences of PSF may facilitate the persistence of mixed ploidy populations.

CONCLUSIONS

This is the first evidence, to our knowledge, that is consistent with plant-soil microbe feedback as a viable mechanism to maintain the coexistence of multiple ploidy levels in a single population.

Interploidy gene flow via a 'pentaploid bridge' and ploidy reduction in Cystopteris fragilis fern complex (Cystopteridaceae: Polypodiales)

KEY MESSAGE

Our results indicate the existence of interploidy gene flow in Cystopteris fragilis, resulting in sexual triploid and diploid gametophytes from pentaploid parents. Similar evolutionary dynamics might operate in other fern complexes and need further investigation. Polyploidization and hybridization are a key evolutionary processes in ferns. Here, we outline an interploidy gene flow pathway operating in the polyploid Cystopteris fragilis complex. The conditions necessary for the existence of this pathway were tested. A total of 365 C. fragilis individuals were collected covering representatives of all three predominant ploidy levels (tetraploid, pentaploid, and hexaploid), cultivated, had their ploidy level estimated by flow cytometry, and their spores collected. The spores, as well as gametophytes and sporophytes established from them, were analysed by flow cytometry. Spore abortion rate was also estimated. In tetraploids, we observed the formation of unreduced (tetraploid) spores (ca 2%). Collected pentaploid individuals indicate ongoing hybridization between ploidy levels. Pentaploids formed up to 52% viable spores, ca 79% of them reduced, i.e. diploid and triploid. Reduced spores formed viable gametophytes, and, in the case of triploids, filial hexaploid sporophytes, showing evidence of sexual reproduction. Some tetraploid sporophytes reproduce apomictically (based on uniform ploidy of their metagenesis up to filial sporophytes). Triploid and diploid gametophytes from pentaploid parents are able to mate among themselves, or with "normal" reduced gametophytes from the sexual tetraploid sporophytes (the dominant ploidy level in the sporophytes in this populations), to produce tetraploid, pentaploid, and hexaploid sporophytes, allowing for geneflow from the pentaploids to both the tetraploid and hexaploid populations. Similar evolutionary dynamics might operate in other fern complexes and need further investigation.

Morphological and environmental differentiation as prezygotic reproductive barriers between parapatric and allopatric Campanula rotundifolia agg. cytotypes

BACKGROUND AND AIMS

Reproductive isolation and local establishment are necessary for plant speciation. Polyploidy, the possession of more than two complete chromosome sets, creates a strong postzygotic reproductive barrier between diploid and tetraploid cytotypes. However, this barrier weakens between polyploids (e.g. tetraploids and hexaploids). Reproductive isolation may be enhanced by cytotype morphological and environmental differentiation. Moreover, morphological adaptations to local conditions contribute to plant establishment. However, the relative contributions of ploidy level and the environment to morphology have generally been neglected. Thus, the extent of morphological variation driven by ploidy level and the environment was modelled for diploid, tetraploid and hexaploid cytotypes of Campanula rotundifolia agg. Cytotype distribution was updated, and morphological and environmental differentiation was tested in the presence and absence of natural contact zones.

METHODS

Cytotype distribution was assessed from 231 localities in Central Europe, including 48 localities with known chromosome counts, using flow cytometry. Differentiation in environmental niche and morphology was tested for cytotype pairs using discriminant analyses. A structural equation model was used to explore the synergies between cytotype, environment and morphology.

KEY RESULTS

Tremendous discrepancies were revealed between the reported and detected cytotype distribution. Neither mixed-ploidy populations nor interploidy hybrids were detected in the contact zones. Diploids had the broadest environmental niche, while hexaploids had the smallest and specialized niche. Hexaploids and spatially isolated cytotype pairs differed morphologically, including allopatric tetraploids. While leaf and shoot morphology were influenced by environmental conditions and polyploidy, flower morphology depended exclusively on the cytotype.

CONCLUSIONS

Reproductive isolation mechanisms vary between cytotypes. While diploids and polyploids are isolated postzygotically, the environmental niche shift is essential between higher polyploids. The impact of polyploidy and the environment on plant morphology implies the adaptive potential of polyploids, while the exclusive relationship between flower morphology and cytotype highlights the role of polyploidy in reproductive isolation.

Autopolyploid establishment depends on life-history strategy and the mating outcomes of clonal architecture

Polyploidy is a significant component in the evolution of many taxa, particularly plant groups. However, new polyploids face substantial fitness disadvantages due to a lack of same-cytotype mates, and the factors promoting or preventing polyploid establishment in natural populations are often unclear. We develop spatially explicit agent-based simulation models to test the hypothesis that a perennial life history and clonal propagation facilitate the early stages of polyploid establishment and persistence. Our models show that polyploids are more likely to establish when they have longer life spans than diploids, especially when self-fertilization rates are high. Polyploids that combine sexual and clonal reproduction can establish across a wide range of life histories, but their success is moderated by clonal strategy. By tracking individuals and mating events, we reveal that clonal architecture has a substantial impact on the spatial structure of the mixed diploid-polyploid population during polyploid establishment: altering patterns of mating within or between cytotypes via geitonogamous self-fertilization, the mechanisms through which polyploid establishment proceeds, and the final composition of the polyploid population. Overall, our findings provide novel insight into the role of clonal structure in modulating the complex relationship between polyploidy, perenniality, and clonality and offer testable predictions for future empirical work.

Application-based guidelines for best practices in plant flow cytometry

Flow cytometry (FCM) is currently the most widely-used method to establish nuclear DNA content in plants. Since simple, 1-3-parameter, flow cytometers, which are sufficient for most plant applications, are commercially available at a reasonable price, the number of laboratories equipped with these instruments, and consequently new FCM users, has greatly increased over the last decade. This paper meets an urgent need for comprehensive recommendations for best practices in FCM for different plant science applications. We discuss advantages and limitations of establishing plant ploidy, genome size, DNA base composition, cell cycle activity, and level of endoreduplication. Applications of such measurements in plant systematics, ecology, molecular biology research, reproduction biology, tissue cultures, plant breeding, and seed sciences are described. Advice is included on how to obtain accurate and reliable results, as well as how to manage troubleshooting that may occur during sample preparation, cytometric measurements, and data handling. Each section is followed by best practice recommendations; tips as to what specific information should be provided in FCM papers are also provided.

Intricate Distribution Patterns of Six Cytotypes of Allium oleraceum at a Continental Scale: Niche Expansion and Innovation Followed by Niche Contraction With Increasing Ploidy Level

The establishment and success of polyploids are thought to often be facilitated by ecological niche differentiation from diploids. Unfortunately, most studies compared diploids and polyploids, ignoring variation in ploidy level in polyploids. To fill this gap, we performed a large-scale study of 11,163 samples from 1,283 populations of the polyploid perennial geophyte Allium oleraceum with reported mixed-ploidy populations, revealed distribution ranges of cytotypes, assessed their niches and explored the pattern of niche change with increasing ploidy level. Altogether, six ploidy levels (3x-8x) were identified. The most common were pentaploids (53.6%) followed by hexaploids (22.7%) and tetraploids (21.6%). Higher cytotype diversity was found at lower latitudes than at higher latitudes (>52° N), where only tetraploids and pentaploids occurred. We detected 17.4% of mixed-ploidy populations, usually as a combination of two, rarely of three, cytotypes. The majority of mixed-ploidy populations were found in zones of sympatry of the participating cytotypes, suggesting they have arisen through migration (secondary contact zone). Using coarse-grained variables (climate, soil), we found evidence of both niche expansion and innovation in tetraploids related to triploids, whereas higher ploidy levels showed almost zero niche expansion, but a trend of increased niche unfilling of tetraploids. Niche unfilling in higher ploidy levels was caused by a contraction of niche envelopes toward lower continentality of the climate and resulted in a gradual decrease of niche breadth and a gradual shift in niche optima. Field-recorded data indicated wide habitat breadth of tetraploids and pentaploids, but also a pattern of increasing synanthropy in higher ploidy levels. Wide niche breadth of tetra- and pentaploids might be related to their multiple origins from different environmental conditions, higher "age", and retained sexuality, which likely preserve their adaptive potential. In contrast, other cytotypes with narrower niches are mostly asexual, probably originating from a limited range of contrasting environments. Persistence of local ploidy mixtures could be enabled by the perenniality of A. oleraceum and its prevalence of vegetative reproduction, facilitating the establishment and decreasing exclusion of minority cytotype due to its reproductive costs. Vegetative reproduction might also significantly accelerate colonization of new areas, including recolonization of previously glaciated areas.

So Closely Related and Yet So Different: Strong Contrasts Between the Evolutionary Histories of Species of the Cardamine pratensis Polyploid Complex in Central Europe

Recurrent polyploid formation and weak reproductive barriers between independent polyploid lineages generate intricate species complexes with high diversity and reticulate evolutionary history. Uncovering the evolutionary processes that formed their present-day cytotypic and genetic structure is a challenging task. We studied the species complex of Cardamine pratensis, composed of diploid endemics in the European Mediterranean and diploid-polyploid lineages more widely distributed across Europe, focusing on the poorly understood variation in Central Europe. To elucidate the evolution of Central European populations we analyzed ploidy level and genome size variation, genetic patterns inferred from microsatellite markers and target enrichment of low-copy nuclear genes (Hyb-Seq), and environmental niche differentiation. We observed almost continuous variation in chromosome numbers and genome size in C. pratensis s.str., which is caused by the co-occurrence of euploid and dysploid cytotypes, along with aneuploids, and is likely accompanied by inter-cytotype mating. We inferred that the polyploid cytotypes of C. pratensis s.str. are both of single and multiple, spatially and temporally recurrent origins. The tetraploid Cardamine majovskyi evolved at least twice in different regions by autopolyploidy from diploid Cardamine matthioli. The extensive genome size and genetic variation of Cardamine rivularis reflects differentiation induced by the geographic isolation of disjunct populations, establishment of triploids of different origins, and hybridization with sympatric C. matthioli. Geographically structured genetic lineages identified in the species under study, which are also ecologically divergent, are interpreted as descendants from different source populations in multiple glacial refugia. The postglacial range expansion was accompanied by substantial genetic admixture between the lineages of C. pratensis s.str., which is reflected by diffuse borders in their contact zones. In conclusion, we identified an interplay of diverse processes that have driven the evolution of the species studied, including allopatric and ecological divergence, hybridization, multiple polyploid origins, and genetic reshuffling caused by Pleistocene climate-induced range dynamics.

Asymmetric hybridization in Central European populations of the Dryopteris carthusiana group

PREMISE

Hybridization is a key process in plant speciation. Despite its importance, there is no detailed study of hybridization rates in fern populations. A proper estimate of hybridization rates is needed to understand factors regulating hybridization.

METHODS

We studied hybridization in the European Dryopteris carthusiana group, represented by one diploid and two tetraploid species and their hybrids. We sampled ~100 individuals per population in 40 mixed populations of the D. carthusiana group across Europe. All plants were identified by measuring genome size (DAPI staining) using flow cytometry. To determine the maternal parentage of hybrids, we sequenced the chloroplast region trnL-trnF of all taxa involved.

RESULTS

We found hybrids in 85% of populations. Triploid D. ×ambroseae occurred in every population that included both parent species and is most abundant when the parent species are equally abundant. By contrast, tetraploid D. ×deweveri was rare (15 individuals total) and triploid D. ×sarvelae was absent. The parentage of hybrid taxa is asymmetric. Despite expectations from previous studies, tetraploid D. dilatata is the predominant male parent of its triploid hybrid.

CONCLUSIONS

This is a thorough investigation of hybridization rates in natural populations of ferns. Hybridization rates differ greatly even among closely related fern taxa. In contrast to angiosperms, our data suggest that hybridization rates are highest in balanced parent populations and support the notion that some ferns possess very weak barriers to hybridization. Our results from sequencing cpDNA challenge established notions about the correlation of ploidy level and mating tendencies.