Apomictic Lineages Research Articles

A study on the adaptive potential of new sterile cytoplasm of Beta patula and Beta maritima L. from wild beets of the genus Beta L.: cold resistance and traits of early sugar accumulation in introduced alloplasmic lines

Purpose. Assessment of new sugar beet breeding genotypes by a set of economically valuable traits to determine their suitability for the efficiency of the process of managing the production of bioethanol with the study of the activity of photosynthesis according to the content of chlorophylls a and b, early accumulation of carbohydrates, tolerance to low temperatures, germination of apozygous seeds and using new plasma of the substituted lines of sugar beet with introduced sterile cytoplasms from wild species Beta patula and B. maritima L. and apozygous lines A9 with S vulgaris Owen's cytoplasm. Methods. Field methods were used (pollen-free regime for the reproduction of apozygotic seeds in conditions of isolation and seed monogermity of the seed plants), laboratory methods (selection of the best experimental genotypes for cold resistance, analysis of productivity and dynamics of carbohydrate accumulation). Results. Self-fertile pollen-sterile lines A9 Beta vulgaris Sxxzz rr with an apomictic method of seed reproduction of seeds (Yaltushkiv EBS) and commercial hybrids with early carbohydrate accumulation were selected for the recessive colour of the hypocotyl r-r- and gametophytic reduced parthenogenesis for hybridization according to the scheme: A9 Beta vulgaris Sxxzz rr × Beta vulgaris Mm Rr. The influence of cytoplasmic inheritance of B. maritima and B. patula on cold resistance indicators was studied and seed samples of the experimental genotypes No. 17225, 17221, 17222, 17220, 17226, 17223 were selected among substituted lines with seed germination rates at +4°C varying from 14.3 to 40.3%. Experimental genotypes of the substituted lines, grown in the conditions of pots were selected and examined by the indicators of early accumulation of sugars, which had values from 16.2 to 17.6% during four months of vegetation; photosynthetic activity with indicators of the content of chlorophyll varying from 2.06 to 1.32%, and chlorophyll b from 1.02 to 0.65%. Conclusions. Sugar beet hybrids, created with the use of cold-resistant substituted pollen-sterile lines of sugar beet, thanks to the natural evolution of the introduced cytoplasmic genome, ensure the extension of the growing season, high productivity, early accumulation of sugars and enrichment of the crop adaptation potential. The article contains the results of the study of the early carbohydrate accumulation, photosynthetic activity of selected experimental genotypes of the substituted sugar beet lines with high seed quality and 100% monogermity. Root crops of apomictic lines were selected based on R+r hypocotyl colour, generative reduced parthenogenesis, and studied for their specifics of seed set under the pollen-free regime.

Environmental and genealogical effects on DNA methylation in a widespread apomictic dandelion lineage

Environmental and genealogical effects on DNA methylation in a widespread apomictic dandelion lineage

Environmental and genealogical effects on DNA methylation in a widespread apomictic dandelion lineage.

DNA methylation in plant genomes occurs in different sequences and genomic contexts that have very different properties. DNA methylation that occurs in CG (mCG) sequence context shows transgenerational stability and high epimutation rate, and can thus provide genealogical information at short time scales. However, due to meta-stability and because mCG variants may arise due to other factors than epimutation, such as environmental stress exposure, it is not clear how well mCG captures genealogical information at micro-evolutionary time scales. Here, we analysed DNA methylation variation between accessions from a geographically widespread, apomictic common dandelion (Taraxacum officinale) lineage when grown experimentally under different light conditions. Using a reduced-representation bisulphite sequencing approach, we show that the light treatment induced differentially methylated cytosines (DMCs) in all sequence contexts, with a bias towards transposable elements. Accession differences were associated mainly with DMCs in CG context. Hierarchical clustering of samples based on total mCG profiles revealed a perfect clustering of samples by accession identity, irrespective of light conditions. Using microsatellite information as a benchmark of genetic divergence within the clonal lineage, we show that genetic divergence between accessions correlates strongly with overall mCG profiles. However, our results suggest that environmental effects that do occur in CG context may produce a heritable signal that partly dilutes the genealogical signal. Our study shows that methylation information in plants can be used to reconstruct micro-evolutionary genealogy, providing a useful tool in systems that lack genetic variation such as clonal and vegetatively propagated plants.

Environmental and genealogical effects on DNA methylation in a widespread apomictic dandelion lineage

Environmental and genealogical effects on DNA methylation in a widespread apomictic dandelion lineage

Seed size, endosperm and germination variation in sexual and apomictic Boechera.

Asexual reproduction results in offspring that are genetically identical to the mother. Among apomictic plants (reproducing asexually through seeds) many require paternal genetic contribution for proper endosperm development (pseudogamous endosperm). We examined phenotypic diversity in seed traits using a diverse panel of sexual and apomictic accessions from the genus Boechera. While genetic uniformity resulting from asexual reproduction is expected to reduce phenotypic diversity in seeds produced by apomictic individuals, pseudogamous endosperm, variable endosperm ploidy, and the deviations from 2:1 maternal:paternal genome ratio in endosperm can all contribute to increased phenotypic diversity among apomictic offspring. We characterized seed size variation in 64 diploid sexual and apomictic (diploid and triploid) Boechera lineages. In order to find out whether individual seed size was related to endosperm ploidy we performed individual seed measurements (projected area and mass) using the phenoSeeder robot system and flow cytometric seed screen. In order to test whether individual seed size had an effect on resulting fitness we performed a controlled growth experiment and recorded seedling life history traits (germination success, germination timing, and root growth rate). Seeds with triploid embryos were 33% larger than those with diploid embryos, but no average size difference was found between sexual and apomictic groups. We identified a maternal effect whereby chloroplast lineage 2 had 30% larger seeds than lineage 3, despite having broad and mostly overlapping geographic ranges. Apomictic seeds were not more uniform in size than sexual seeds, despite genetic uniformity of the maternal gametophyte in the former. Among specific embryo/endosperm ploidy combinations, seeds with tetraploid (automomous) endosperm were on average smaller, and the proportion of such seeds was highest in apomicts. Larger seeds germinated more quickly than small seeds, and lead to higher rates of root growth in young seedlings. Seed mass is under balancing selection in Boechera, and it is an important predictor of several traits, including germination probability and timing, root growth rates, and developmental abnormalities in apomictic accessions.

Evolution of Transcriptomes in Early-Generation Hybrids of the Apomictic Ranunculus auricomus Complex (Ranunculaceae).

Hybridisation in plants may cause a shift from sexual to asexual seed formation (apomixis). Indeed, natural apomictic plants are usually hybrids, but it is still unclear how hybridisation could trigger the shift to apomixis. The genome evolution of older apomictic lineages is influenced by diverse processes such as polyploidy, mutation accumulation, and allelic sequence divergence. To disentangle the effects of hybridisation from these other factors, we analysed the transcriptomes of flowering buds from artificially produced, diploid F2 hybrids of the Ranunculus auricomus complex. The hybrids exhibited unreduced embryo sac formation (apospory) as one important component of apomixis, whereas their parental species were sexual. We revealed 2915 annotated single-copy genes that were mostly under purifying selection according to dN/dS ratios. However, pairwise comparisons revealed, after rigorous filtering, 79 genes under diversifying selection between hybrids and parents, whereby gene annotation assigned ten of them to reproductive processes. Four genes belong to the meiosis-sporogenesis phase (ASY1, APC1, MSP1, and XRI1) and represent, according to literature records, candidate genes for apospory. We conclude that hybridisation could combine novel (or existing) mutations in key developmental genes in certain hybrid lineages, and establish (together with altered gene expression profiles, as observed in other studies) a heritable regulatory mechanism for aposporous development.

Taxonomic assessment of Sorbus subgenus Aria in the Malé Karpaty Mountains

In this paper we present a taxonomic assessment of Sorbus subgen. Aria in the Malé Karpaty Mts (western Slovakia and north-eastern Austria). Detailed field observations, morphological comparison, herbarium revisions, multivariate morphometric analyses, flow cytometry and molecular (nuclear microsatellite markers) analyses were used to assess the taxonomic diversity within this group. Diploids, triploids and tetraploids were detected. Diploids are represented by a single taxon, S. aria, which shows considerable morphological and genetic variability. Sorbus aria ranges from rare to locally abundant throughout the area studied. Triploids are confined to the central and southern parts of the region and include several single, presumably apomictic individuals and rare apomictic lineages, which are not treated as separate species here due to the very limited number of individuals recorded. Tetraploids show similar distribution patterns to those of triploids: S. danubialis and S. collina occur in the southern part of the mountains, while S. petraea, a new species described here, inhabits the central part of the mountain range. The entire population of S. petraea includes approximately one hundred individuals. It occurs mainly in the surroundings of Vysoká Mt. by the village of Kuchyňa, where it forms several small populations and approximately 30 individuals grow on Čierna skala hill by the village of Plavecký Mikuláš. In addition to these recent occurrences, there are two herbarium specimens from the 1960s documenting the species on Vápená Mt. The low genetic and morphological variability and flow cytometric seed screening indicate an apomictic mode of reproduction in this species and justify its recognition at the species level. Sorbus collina is reported here in Slovakia for the first time. We have not confirmed the occurrence of S. austriaca, S. buekkensis, S. carpatica, S. graeca, S. pannonica, S. thaiszii and S. zolyomii in the Malé Karpaty Mts and consider the previously published records to be erroneous. Lectotypes are designated for S. thaiszii and S. zolyomii. A determination key, detailed distribution maps and photographs of representative herbarium specimens of all taxa of S. subgen. Aria occurring in the Malé Karpaty Mts are provided. A line drawing and a photograph of the newly described species are also presented.

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.

The spread of infectious asexuality through haploid pollen.

The mechanisms of initiation and transmission of apomixis (asexual reproduction through seeds) in natural plant populations are important for understanding the evolution of reproductive variation. Here, we used the phylogenetic diversity of the genus Boechera (Brassicaceae), together with natural diversity in pollen types produced by apomictic lines, to test whether hybridization triggers the transition to asexuality, and whether a 'triploid bridge' is required for the formation of polyploid apomicts. We performed crosses between diploid sexual recipient and diploid apomictic donor lines and tested whether the mating system (interspecific hybridization vs intraspecific outcrossing) or pollen type (haploid vs diploid) influenced the transmission of apomixis from diploid apomictic donors into sexual recipients. We used genetic markers and flow cytometric analyses of embryo and endosperm in seeds to infer the reproductive mode. Within a single generation, initiation of both diploid and polyploid apomixis in sexual Boechera can occur. Diploid apomixis is transmitted through haploid pollen (infectious asexuality) and polyploids can form through multiple pathways. The three functional elements of apomixis occasionally segregate. Variation in pollen ploidy and the segregation of apomixis elements drive reproductive diversity of hybrids and outcrosses and can be utilized for apomixis initiation in crop breeding programs.

Apomictic lines of sugar beet: development and studying

Background. While working with such cross-pollinated crops as sugar beet, the greatest problem is the fixation of valuable genotypes. Using apomixis to produce breeding material helps to accelerate the breeding process and save the desired combination of genes.Materials and methods. The research objects were 110 accessions of sugar beet from the VIR collection. Field experiments and assessments of the resistance to Cercospora leaf spot, monogermity, and non-bolting were performed according to VIR’s guidelines in 2016–2018 at Pushkin and Pavlovsk Laboratories of VIR and Maikop Experiment Station of VIR. The sugar level in roots was measured using an optical refractometer.Results. A comprehensive study of sugar beet accessions resulted in the development of apomictic lines with cytoplasmic male sterility, followed by an evaluation of their economically important characters. An extremely rare occurrence of biotypes with the 0-type sterility (less than 0.5%) was observed in the population. The seeds obtained from apomixis-prone lines demonstrated a significant difference during inbreeding from the seeds of fertile inbred genotypes: no inbreeding depression was observed in apomictic lines. Lines combining sterility and monogermity in their genotype were produced. Testing parent accessions and apomictic forms did not reveal significant differences in the sugar content and root yield, so the resulting forms can be efficiently used in future breeding programs.Conclusion. Using apomixis to develop sugar beet lines helped to fixate the sugar content level, biennial plant development cycle, and Cercospora leaf spot resistance. Thus, apomixis is promising for ensuring maternal inheritance and preserving the desired combination of genes in sugar beet, thereby accelerating the breeding process.

Stomatal size, ploidy and polyembryony in Eriotheca Stellate Trichome Species Complex (Bombacoideae – Malvaceae) in the Cerrados of Brazil

Geographic parthenogenesis, range expansion of apomictic plants after climate changes, has been described for Northern Hemisphere gametophytic apomicts. But similar trends have been observed for sporophytic apomicts of Cerrado, the savannas in Brazil. Eriotheca pubescens is a common Cerrado tree, an agamic complex of either hexaploid/polyembryonic apomicts or tetraploid/monoembryonic sexual individuals. Some populations have been described as a new species, Eriotheca estevesiae, all included in the Eriotheca Stellate Trichome Species Complex (ESTSC). Since breeding systems and ploidy are clearly associated with polyembryony and stomatal size, we used these ancillary features to map the reproductive and ploidy level traits of E. pubescens and E. estevesiae. Leaves and seeds were collected from individuals of 19 populations. Seeds were evaluated for the presence of polyembryony and leaves for stomatal measurements. Eight populations were monoembryonic while another eight were polyembryonic and for other three, the embryonic pattern was not readily verified. E. pubescens polyembryonic and hexaploid populations formed a homogeneous group, but monoembryonic plants were more variable. E. estevesiae populations were monoembryonic with smaller stomata. In contrast, some E. pubescens monoembryonic populations further south presented larger stomata. Despite these outliers, possibly mixed populations, stomatal size and embryonic pattern differed from northern to southern populations. Embryonic pattern and stomatal size indicated that northernmost populations of Eriotheca STSC (E. estevesiae) are diploid and sexual. Southernmost populations, mostly polyembryonic and with large stomata, are hexaploid and apomictic. This is in agreement with geographic parthenogenesis and range expansion of apomictic lineages to southern habitats available after the last glacial maximum.

Gene Function Rather than Reproductive Mode Drives the Evolution of RNA Helicases in Sexual and Apomictic Boechera.

In higher plants, sexual and asexual reproductions through seeds (apomixis) have evolved as alternative strategies. Evolutionary advantages leading to coexistence of both reproductive modes are currently not well understood. It is expected that accumulation of deleterious mutations leads to a rapid elimination of apomictic lineages from populations. In this line, apomixis originated repeatedly, likely from deregulation of the sexual pathway, leading to alterations in the development of reproductive lineages (germlines) in apomicts as compared with sexual plants. This potentially involves mutations in genes controlling reproduction.Increasing evidence suggests that RNA helicases are crucial regulators of germline development. To gain insights into the evolution of 58 members of this diverse gene family in sexual and apomictic plants, we applied target enrichment combined with next-generation sequencing to identify allelic variants from 24 accessions of the genus Boechera, comprising sexual, facultative, and obligate apomicts. Interestingly, allelic variants from apomicts did not show consistently increased mutation frequency. Either sequences were highly conserved in any accession, or allelic variants preferentially harbored mutations in evolutionary less conserved C- and N-terminal domains, or presented high mutation load independent of the reproductive mode. Only for a few genes allelic variants harboring deleterious mutations were only identified in apomicts. To test if high sequence conservation correlates with roles in fundamental cellular or developmental processes, we analyzed Arabidopsis thaliana mutant lines in VASA-LIKE (VASL), and identified pleiotropic defects during ovule and reproductive development. This indicates that also in apomicts mechanisms of selection are in place based on gene function.

Controlling Apomixis: Shared Features and Distinct Characteristics of Gene Regulation.

In higher plants, sexual and asexual reproduction through seeds (apomixis) have evolved as alternative strategies. As apomixis leads to the formation of clonal offspring, its great potential for agricultural applications has long been recognized. However, the genetic basis and the molecular control underlying apomixis and its evolutionary origin are to date not fully understood. Both in sexual and apomictic plants, reproduction is tightly controlled by versatile mechanisms regulating gene expression, translation, and protein abundance and activity. Increasing evidence suggests that interrelated pathways including epigenetic regulation, cell-cycle control, hormonal pathways, and signal transduction processes are relevant for apomixis. Additional molecular mechanisms are being identified that involve the activity of DNA- and RNA-binding proteins, such as RNA helicases which are increasingly recognized as important regulators of reproduction. Together with other factors including non-coding RNAs, their association with ribosomes is likely to be relevant for the formation and specification of the apomictic reproductive lineage. Subsequent seed formation appears to involve an interplay of transcriptional activation and repression of developmental programs by epigenetic regulatory mechanisms. In this review, insights into the genetic basis and molecular control of apomixis are presented, also taking into account potential relations to environmental stress, and considering aspects of evolution.

Morphology mirrors ploidy and reproductive modes in Pilosella officinarum

Pilosella officinarum is represented predominantly by tetraploid (2n = 36), pentaploid (2n = 45) and hexaploid (2n = 54) cytotypes reproducing, to various degrees, both sexually and apomictically. Its current intraspecific taxonomical treatment is based mainly on selected apomictic lineages, the large number of which makes the treatment confusing and not generally applicable. We therefore tested the breeding modes of a representative set of plants from central Europe, cultivated under experimental conditions. Each ploidy level was associated with a different reproductive pattern (4x – sexual, 5x – prevalently apomictic and 6x – sexual and apomictic). Whereas sexual tetraploids occurred in the western part of the study area (the Czech Massif), apomictic pentaploids and hexaploids were scattered in its eastern part (Western Carpathians and Pannonia). Moreover, the hexaploid cytotype formed a distinct exclusively sexual group restricted to steep river canyons of the Czech Massif. Morphometric analyses were performed to determine the set of characters which distinguish major lineages characterized by different ploidy and reproductive modes. Their results confirm the existence of morphological differences between plants of different ploidy levels and, in the case of the hexaploid cytotype, different modes of breeding. Knowledge of ploidy and reproductive modes is therefore essential for elucidating the reticulate infraspecific structure of Pilosella officinarum.

An unexpected new diploid Hieracium from Europe: Integrative taxonomic approach with a phylogeny of diploid Hieracium taxa

AbstractHieracium s.str. (Asteraceae) is one of the largest angiosperm genera notorious for its taxonomic complexity caused by widespread interspecific hybridization. This process is tightly coupled with polyploidization and apomixis – asexual reproduction by seeds, which has ensured the persistence of otherwise sterile interspecific hybrids. As a result, apomictic polyploid taxa dominate the taxonomic diversity of the genus whilst sexual diploid species are extremely rare and mostly confined to southern Europe. As diploid taxa are assumed to be the parents of apomictic lineages, the discovery of any new diploid species is important for understanding evolutionary processes and diversity patterns in the genus. Here, we describe a new diploid species, Hieracium vranceae, a narrow endemic to the Munţii Vrancei (Vrancea Mountains, Eastern Carpathians, Romania). This taxon with a distinctive morphology and a strong affinity to relic rocky habitats was first collected in 2013. This means that, even in Europe, some regions, like the Eastern Carpathians, are botanically underexplored and might still preserve an undiscovered diversity of plants. Phylogenetic analyses with multiple molecular markers (low‐copy nuclear genes gsh1 and sqs, nuclear ribosomal ETS, four cpDNA loci) applied to all diploid Hieracium species support specific rank for H. vranceae. In contrast, molecular data suggest conspecifity of two pairs of species, H. lucidum/H. cophanense from Sicily and H. naegelianum/H. renatae from the Balkans, which are moreover only weakly differentiated morphologically. Molecular (cpDNA) and cytogenetic (GISH, FISH) analyses furthermore revealed that H. vranceae has been involved as a maternal parent in the origin of an allotriploid apomictic species, H. telekianum, to which H. vranceae contributed one haplome. The other putative parents of H. telekianum are the diploid narrow endemic H. pojoritense and probably H. sparsum s.l. Our data thus stress the importance of interspecific hybridization for an evolutionary shift from sexuality to apomixis in Hieracium. Our findings furthermore highlight the significance of that part of the Carpathians as one of the most important evolutionary/refugial centres of Hieracium and suggest dynamic species ranges at a regional scale allowing physical contact of taxa whose distributional ranges are nowadays completely allopatric.

Development of EST-SSR markers in Cenchrus ciliaris and their applicability in studying the genetic diversity and cross-species transferability

Most of the grasses of the genus Cenchrus (20-25 species) and Pennisetum (80-140 species) are distributed throughout the tropical and subtropical regions of the world and reproduce both by sexual and apomictic modes. However, the relationships among the Cenchrus-Pennisetum species are not very clear yet. Molecular markers like expressed sequence tag-simple sequence repeats (EST-SSRs) have been reported to be a better choice for resolving the phylogenetic relationships and to estimate the genetic diversity. The present study describes the identification of EST-SSR markers based on the transcriptome data of Cenchrus ciliaris inflorescence and illustrates the genetic diversity and phylogenetic relationships among these species. Of the 378 primer pairs used across 33 accessions of 21 Cenchrus, Pennisetum, and related grass (Bothriochloa, Dichanthium and Panicum) species, 116 EST-SSR markers were found to be polymorphic with an average polymorphism information content (PIC) of 0.49. Fifty-one EST-SSR loci and 520 alleles showed that where the PIC value is >0.5 there the GAG repeat motif was highly polymorphic. Two EST-SSR markers, CcSSR_80 and CcSSR_102, are polymorphic among the Cenchrus species, while they are absent in Pennisetum and the allied species. Five SSR markers (CcSSR_75, CcSSR_85, CcSSR_87, CcSSR_88 and CcSSR_114) showed 100% cross-transferability among the 21 Cenchrus-Pennisetum species. Species-specific alleles could also be detected for seven species of Cenchrus, Pennisetum and Panicum across 10 SSR markers. Assay of polymorphism across these agamic complexes showed that the three SSR markers (CcSSR_26, CcSSR_97 and CcSSR_109) were associated with Cenchrus-Pennisetum complex, and one (CcSSR_47) with Bothriochloa-Dichanthium complex. Markers with high discriminating power, namely CcSSR_4, CcSSR_38, CcSSR_48, CcSSR_66, CcSSR_67 and CcSSR_70, can be used to estimate the allelic sequence divergence across the sexual and apomictic lineages. Genetic diversity analysis using neighbour-joining (NJ) and principal co-ordinate analysis (PCoA) based approaches showed six and five clusters for the 33 accessions, respectively, having congruence in the pattern of clustering. These accessions were grouped according to their mode of reproduction. Cenchrus and Pennisetum species were grouped separately within the same clade, implying monophyletic group within a 'bristle clade'. Thus, this study showed high discrimination power of microsatellite (EST-SSR) markers to resolve the phylogenetic relationships.

The Rise of Apomixis in Natural Plant Populations.

Apomixis, the asexual reproduction via seed, has many potential applications for plant breeding by maintaining desirable genotypes over generations. Since most major crops do not express natural apomixis, it is useful to understand the origin and maintenance of apomixis in natural plant systems. Here, we review the state of knowledge on origin, establishment and maintenance of natural apomixis. Many studies suggest that hybridization, either on diploid or polyploid cytotypes, is a major trigger for the formation of unreduced female gametophytes, which represents the first step toward apomixis, and must be combined to parthenogenesis, the development of an unfertilized egg cell. Nevertheless, fertilization of endosperm is still needed for most apomictic plants. Coupling of these three steps appears to be a major constraint for shifts to natural apomixis. Adventitious embryony is another developmental pathway toward apomixis. Establishment of a newly arisen apomictic lineage is often fostered by side-effects of polyploidy. Polyploidy creates an immediate reproductive barrier against the diploid parental and progenitor populations; it can cause a breakdown of genetic self-incompatibility (SI) systems which is needed to establish self-fertility of pseudogamous apomictic lineages; and finally, polyploidy could indirectly help to establish an apomictic cytotype in a novel ecological niche by increasing adaptive potentials of the plants. This step may be followed by a phase of diversification and range expansion, mostly described as geographical parthenogenesis. The utilization of apomixis in crops must consider the potential risks of pollen transfer and introgression into sexual crop fields, which might be overcome by using pollen-sterile or cleistogamous variants. Another risk is the escape into natural vegetation and potential invasiveness of apomictic plants which needs careful management and consideration of ecological conditions.

The Boechera Genus as a Resource for Apomixis Research.

The genera Boechera (A. Löve et D. Löve) and Arabidopsis, the latter containing the model plant Arabidopsis thaliana, belong to the same clade within the Brassicaceae family. Boechera is the only among the more than 370 genera in the Brassicaceae where apomixis is well documented. Apomixis refers to the asexual reproduction through seed, and a better understanding of the underlying mechanisms has great potential for applications in agriculture. The Boechera genus currently includes 110 species (of which 38 are reported to be triploid and thus apomictic), which are distributed mostly in the North America. The apomictic lineages of Boechera occur at both the diploid and triploid level and show signs of a hybridogenic origin, resulting in a modification of their chromosome structure, as reflected by alloploidy, aneuploidy, substitutions of homeologous chromosomes, and the presence of aberrant chromosomes. In this review, we discuss the advantages of the Boechera genus to study apomixis, consider its modes of reproduction as well as the inheritance and possible mechanisms controlling apomixis. We also consider population genetic aspects and a possible role of hybridization at the origin of apomixis in Boechera. The molecular tools available to study Boechera, such as transformation techniques, laser capture microdissection, analysis of transcriptomes etc. are also discussed. We survey available genome assemblies of Boechera spp. and point out the challenges to assemble the highly heterozygous genomes of apomictic species. Due to these challenges, we argue for the application of an alternative reference-free method for the comparative analysis of such genomes, provide an overview of genomic sequencing data in the genus Boechera suitable for such analysis, and provide examples of its application.

Apomixis and Hybridization Drives Reticulate Evolution and Phyletic Differentiation in Sorbus L.: Implications for Conservation.

Hybridization and polyploidy are major forces in the evolution of plant diversity and the study of these processes is of particular interest to understand how novel taxa are formed and how they maintain genetic integrity. Sorbus is an example of a genus where active diversification and speciation are ongoing and, as such, represents an ideal model to investigate the roles of hybridization, polyploidy and apomixis in a reticulate evolutionary process. To elucidate breeding systems and evolutionary origins of a complex of closely related Sorbus taxa, we assessed genotypic diversity and population structure within and among taxa, combining data from nuclear DNA microsatellite markers and flow cytometry. Clonal analysis and low genotypic diversity within the polyploid taxa suggest apomixis is obligate. However, genetic variation has led to groups of ‘clone-mates’ within apomictic taxa that strongly suggest mutation is responsible for the genotypic diversity of these apomictic lineages. In addition, microsatellite profiles and site demographics suggest hybridization events among apomictic polyploid Sorbus may have contributed to the extant diversity of recognized taxa in this region. This research demonstrates that both macro- and micro-evolutionary processes are active within this reticulate Sorbus complex. Conservation measures should be aimed at maintaining this process and should therefore be prioritized for those areas of Sorbus species richness where the potential for interspecific gene flow is greatest.

Ecological differentiation facilitates fine-scale coexistence of sexual and asexual Boechera.

Ecological differentiation (ED) between sexual and asexual organisms may permit the maintenance of reproductive polymorphism. Several studies of sexual/asexual ED in plants have shown that the geographic ranges of asexuals extend beyond those of sexuals, often in areas of higher latitude or elevation. But very little is known about ED at fine scales, wherein coexistence of sexuals and asexuals may be permitted by differential niche occupation. We used 149 populations of sexual and apomictic lineages in the genus Boechera (rock cress) collected across a portion of this mustard's vast range. We characterized reproductive mode, ploidy, and species identity or hybrid parentage of each individual, and then used a multipronged statistical approach to (1) identify ED between sexuals and asexuals; (2) investigate the impacts of two confounding factors, polyploidy and hybridization, on ED; and (3) determine the environmental variables underlying ED. We found that sexuals and asexuals are significantly ecologically differentiated across the landscape, despite fine-scale interdigitation of these two reproductive forms. Asexual reproduction was strongly associated with greater disturbance, reduced slope, and greater environmental variability. Although ploidy had little effect on the patterns observed, hybridization has a unique impact on the relationships between asexual reproduction and specific environmental variables. Ecological differentiation along the axes of disturbance, slope, and climatic variability, as well as the effects of heterozygosity, may contribute to the maintenance of sexuality and asexuality across the landscape, ultimately impacting the establishment and spread of asexual lineages.