Apomictic Fern Research Articles

Apomictic fern fathers: an experimental approach to the reproductive characteristics of sexual, apomict, and hybrid fern gametophytes.

Apomixis and hybridization are two essential and complementary factors in the evolution of plants, including ferns. Hybridization combines characteristics from different species, while apomixis conserves features within a lineage. When combined, these two processes result in apo-sex hybrids. The conditions leading to the formation of these hybrids are poorly understood in ferns. We cultivated spores from 66 fern samples (43 apomicts, 7 apo-sex hybrids, and 16 sexuals), and measured their development in vitro over 16 weeks. We evaluated germination, lateral meristem formation rates, sexual expression, and production of sporophytes and then compared ontogenetic patterns among the three groups. The three examined groups formed antheridia (male gametangia) but differed in overall gametophyte development. Sexual species created archegonia (female, 86% of viable samples), but no sporophytes. Apomicts rarely created nonfunctional archegonia (8%) but usually produced apogamous sporophytes (75%). Surprisingly, apomictic and sexual species showed similar development speed. The sexually reproducing parents of viable studied hybrids formed about twice as many meristic gametophytes as the apomictic parents (39% vs. 20%, respectively). We present the most thorough comparison of gametangial development of sexual and apomictic ferns, to date. Despite expectations, apomictic reproduction might not lead to earlier sporophyte formation. Apomicts produce functional sperm and thus can contribute this type of gamete to their hybrids. The development patterns found in the parents of hybrids indicate a possible increase of hybridization rates by antheridiogens. The apo-sex hybrids always inherit the apomictic reproductive strategy and are thus capable of self-perpetuation.

An experimental assessment of competitive interactions between sexual and apomictic fern gametophytes using Easy Leaf Area.

PremiseFew studies have explored competition in fern gametophyte populations. One limiting factor is the tedious measurement of gametophyte size as a proxy for biomass in these small plants. Here, an alternative approach of estimating the number of green pixels from photos was employed to measure the competitive interactions among apomictic and sexual Dryopteris gametophytes.MethodsWe cultivated the gametophytes of two apomictic (diploid and triploid) and one sexual (tetraploid) Dryopteris species in monocultures and in two‐species mixtures in the ratios 1 : 1 and 1 : 3. The total gametophyte cover of each population originating from 20 spores was assessed using Easy Leaf Area. Assessments were performed weekly between weeks 4 and 10 of cultivation. Additionally, during week 5, the cover of each species in each mixture was estimated separately.ResultsWe identified a positive correlation between gametophyte size and ploidy level as well as sexual reproduction. The performance of the tested species in mixtures was dependent on the competitor species identity, indicating the importance of competition between gametophytes.DiscussionThe methods outlined can be used for a rapid assessment of fern gametophyte cover in large gametophyte populations. Ploidy level and reproduction type seem to play a major role in the competitive abilities of fern gametophytes, but more research is needed on this topic.

When it only takes one to tango: assessing the impact of apomixis in the fern genus Pteris.

PremiseApomixis (asexual reproduction by seed, spore, or egg) has evolved repeatedly across the tree of life. Studies of animals and angiosperms show that apomictic lineages are often evolutionarily short‐lived and frequently exhibit different distributions than their sexual relatives. However, apomixis is rare in these groups. Less is known about the role of apomixis in the evolution and biogeography of ferns, in which ~10% of species are apomictic. Apomixis is especially common in the fern genus Pteris (34–39% of species); however, because of the limited taxonomic and geographic sampling of previous studies, the true frequency of apomixis and its associations with geography and phylogeny in this lineage remain unclear.MethodsWe used spore analyses of herbarium specimens to determine reproductive mode for 127 previously unsampled Pteris species. Then we leveraged biogeographic and phylogenetic analyses to estimate the global distribution and evolution of apomixis in Pteris.ResultsAmong all Pteris species examined, we found that 21% are exclusively apomictic, 71% are exclusively sexual, and 8% have conflicting reports. Apomixis is unevenly distributed across the range of the genus, with the Paleotropics exhibiting the highest frequency, and has evolved numerous times across the Pteris phylogeny, with predominantly East Asian and South Asian clades containing the most apomictic species.ConclusionsApomixis arises frequently in Pteris, but apomictic species do not appear to diversify. Species that encompass both apomictic and sexual populations have wider ranges than exclusively sexual or apomictic species, which suggests that sexual and apomictic ferns could occupy separate ecological niches.

Awards of JSE Outstanding Papers (2016)

Awards of JSE Outstanding Papers (2016)

Spore abortion index (SAI) as a promising tool of evaluation of spore fitness in ferns: an insight into sexual and apomictic species

Ferns reproduce through small and usually haploid spores. The general paradigm states that whereas species produce good shaped spores, hybrids are sterile and form aborted spores. Apomictic fern species represent an unusual case, and it is believed that they produce an unbalanced spore spectrum. Until now, no comprehensive comparison of sexual and apomictic taxa using extensive spore fitness data has been published. Based on a representative data set of 109 plants from 23 fern taxa, we accomplished the first robust analysis of spore fitness using spore abortion index (SAI), the ratio of aborted to all examined spores. One thousand spores were analyzed for each plant. Focusing mainly on two major European fern taxa (Asplenium, Dryopteris), we compared this trait for different fern reproductive types (sexual/apomicts/hybrids) and ploidy levels (diploid versus polyploid). Our results confirmed the general assumption that shows higher SAI for apomictic taxa (18%) when compared to sexual taxa (3%). Furthermore, hybrids are characterized by having almost all spores aborted (99.8%) with the notable exception of pentaploid Dryopteris × critica (93.1%), the hybrid between sexual and apomictic taxa. We found no significant difference in SAI between sexual taxa of various ploidy levels or between sexual taxa of genera Dryopteris and Asplenium. Additionally, we carried out an optimization of the SAI method, outlying important guidelines for the use of this method in the future.

A current perspective on apomixis in ferns

AbstractThis review provides a synopsis of apogamous reproduction in ferns and highlights important progress made in recent studies of fern apomixis. First, a summary of the apomictic fern life cycle is provided, distinguishing between two pathways to diploid spore production that have been documented in apomictic ferns (premeiotic endomitosis and meiotic first division restitution) and briefly discussing the evolutionary implications of each. Next, recent trends in fern apomixis research are discussed, exposing a shift in focus from the observation and characterization of apomixis in ferns to more integrated studies of the evolutionary and ecological implications of this reproductive mode. Peer‐reviewed contributions from the past decade are then summarized, spanning the identification of new apomictic lineages through to the developmental, phylogenetic, and population genetic insights that have been made in studies of fern apomixis during that time. Gaps in our understanding are also discussed, including the extent and implications of recombinant apomixis in ferns, the possible reversibility of reproductive mode (from apomictic to sexual) in ferns, and the genomic causes and consequences of apomixis in seed free vascular plants. To conclude, future directions for fern apomixis research are proposed in the context of modern technological advances and recent insights from studies of apomixis in other groups.

Genome size expansion and the relationship between nuclear DNA content and spore size in the Asplenium monanthes fern complex (Aspleniaceae)

BackgroundHomosporous ferns are distinctive amongst the land plant lineages for their high chromosome numbers and enigmatic genomes. Genome size measurements are an under exploited tool in homosporous ferns and show great potential to provide an overview of the mechanisms that define genome evolution in these ferns. The aim of this study is to investigate the evolution of genome size and the relationship between genome size and spore size within the apomictic Asplenium monanthes fern complex and related lineages.ResultsComparative analyses to test for a relationship between spore size and genome size show that they are not correlated. The data do however provide evidence for marked genome size variation between species in this group. These results indicate that Asplenium monanthes has undergone a two-fold expansion in genome size.ConclusionsOur findings challenge the widely held assumption that spore size can be used to infer ploidy levels within apomictic fern complexes. We argue that the observed genome size variation is likely to have arisen via increases in both chromosome number due to polyploidy and chromosome size due to amplification of repetitive DNA (e.g. transposable elements, especially retrotransposons). However, to date the latter has not been considered to be an important process of genome evolution within homosporous ferns. We infer that genome evolution, at least in some homosporous fern lineages, is a more dynamic process than existing studies would suggest.

Apomixis in the interspecific triploid hybrid fern Cornopteris christenseniana (Woodsiaceae).

Cornopteris christenseniana is a "sterile" interspecific triploid hybrid of diploid C. crenulatoserrulata and tetraploid C. decurrenti-alata. Morphological and cytological studies show that, of 41 young plants of Cornopteris that have been propagated naturally in the Fern Garden of the Botanical Gardens, University of Tokyo, 30 plants are the sterile C. christenseniana, 10 are fertile C. decurrenti-alata and 1, fertile C. crenulatoserrulata. This proportion supports the view that the young plants of C. christenseniana are derived from spores of reproductively mature plants of the species cultivated. Cytogenetic observations and culture experiments show that C. christenseniana produces normal spores in various proportions in some sporogenetic pathways that are aberrant from the ordinary process in sexual and apomictic ferns. Under culture conditions, normal spores germinate in rough proportion to the frequency of normal spores, and sporophytes are apogamously produced in rough proportion to the frequency of spore germination. As a whole, the rates of spore germination and apogamous sporophyte development vary according to the specific plant. Taken together, these observations suggest that C. christenseniana is an incipient apomict.

Patterns of genetic variation detected by RAPDs suggest a single origin with subsequent mutations and long‐distance dispersal in the apomictic fern Dryopteris remota (Dryopteridaceae)

Debates on speciation processes in pteridophytes have revived. In order to study the evolutionary origin of an apomictic fern species, we investigated the genetic variation in the strictly agamosporous Dryopteris remota. We determined the genotypes of 22 individuals from many different locations within the species' European distribution and of 20 individuals from a Swiss population. A previous study on isozyme variation showed no intraspecific genetic variation in a similar sample set (Schneller and Holderegger, 1994, American Fern Journal 84: 94-98). In contrast to this, four out of 12 random amplified polymorphic DNA (RAPD) primers tested revealed low genetic diversity among individuals of D. remota from different locations. Intrapopulational genetic variation was also very low, but in the single population studied, a unique multiband genotype could be detected. The geographic distribution of genetic variation found in D. remota was best explained by the assumption of a single origin, the accumulation of somatic mutations during spread, and occasional, but effective, events of dispersal over large distances. The present study thus stresses the importance of long-distance dispersal in evolutionary processes and biogeography of ferns.

Spore germination and isozyme patterns in the apomictic fern Cyrtomium falcatum

Progeny from a plant of the apomictic species Cyrtomium falcatum were examined by protein electrophoresis. These progeny were produced from spores originating in sporangia with differing spore numbers. Analysis of zymograms indicates that variant banding patterns were exhibited in 16.1% of the progeny sampled. These anomalous phenotypes suggest that some apomictically reproducing plants can produce genetically variable progeny.

Spore germination and isozyme patterns in the apomictic fern Cyrtomium falcatum

Progeny from a plant of the apomictic species Cyrtomium falcatum were examined by protein electrophoresis. These progeny were produced from spores originating in sporangia with differing spore numbers. Analysis of zymograms indicates that variant banding patterns were exhibited in 16.1% of the progeny sampled. These anomalous phenotypes suggest that some apomictically reproducing plants can produce genetically variable progeny.

APOGAMY AND SOMATIC RESTITUTION IN THE FERN CERATOPTERIS

A mutant stock of the fern Ceratopteris has been derived from an inbreeding study following an interspecific hybridization between two diploid species. The mutant is characterized by gametophytes that produce non‐functional spermatozoids and are incapable of selfing. Sporophytes develop apogamously from the mutant gametophytes and, although they are initially haploid and sterile, portions of the fronds later become doubled somatically and behave like tissues of sexually derived homozygous sporophytes. The mutant segregates from sporophytes in a 1:1 ratio when crosses are made with wild type gametophytes. Certain aspects of the behavior are similar to those seen in some naturally occurring apomictic ferns.

Problems of Apomixis and the Treatment of Agamic Complexes

Problems associated with the systematic treatment of apomictic fern complexes of eastern North America are discussed. Examples are drawn from the literature and include primarily the genera Polypodium and Asplenium, although reference is made also to other genera. The emphasis is on different forms of apomixis rather than a catalogue of all known apomictic ferns of the region. Discussion relates to the treatment of apomictic versus sexual ferns for the Flora North America Project.