Apomixis,hybridization, and taxonomic complexity in eastern north AmericanAmelanchier (Rosaceae)

Apomixis and hybridization together contribute to taxonomic complexity inAmelanchier. Hybridization combines genetically divergent genomes and spawns new forms that apomixis perpetuates. Apomixis is aposporous, facultative, and pseudogamous in the genus, and apomicts are generally polyploid, pollen fertile, and pollinated by generalists. That gene flow actually occurs is empirically evident. As apomixis is genetically dominant over sexuality, hybrids involving at least one apomictic parent are apomictic. Clonal reproduction may thus perpetuate F₁ individuals and generate agamospecies. Alternatively hybrids may interbreed and backcross to create hybrid swarms or cross with species other than the parents. In eastern North America, the abundance of published names and general taxonomic confusion in the genus doubtless result at least in part from this interplay of apomixis and hybridization. The roles of apomixis and hybridization in diversification withinAmelanchier are examined in light of new data about breeding system of an apomictic, hybrid microspecies, informally namedA. “erecta” and its formation of a hybrid swarm with anotherAmelanchier apomict,A. laevis.     

Sexual Hieracium pilosella plants are better inter-specific,while apomictic plants are better intra-specific competitors

Apomixis, asexual reproduction through seeds, occurs in over 40 plant families. This widespread phenomenon can lead to the fixation of successful genotypes, resulting in a fitness advantage. On the other hand, apomicts are expected to lose their fitness advantage if the environment changes because of their limited evolutionary potential, which is due to low genetic variability and the potential accumulation of deleterious somatic mutations. Nonetheless, some apomicts have been extremely successful, for example certain apomictic accessions of Hieracium pilosella L. from New Zealand, where the plant is invasive. Here, we investigate whether the success of these apomictic accessions could be due to a fitness advantage by comparing the vegetative competitiveness of apomictic H. pilosella from New Zealand with sexual accessions of H. pilosella from Europe. Sexual and apomictic plants were grown either (A) alone (no competition), (B) in competition with the other type (intra-specific competition), (C) in competition with the grass Bromus erectus (inter-specific competition), and (D) in competition with the other type and the grass B. erectus (intra- and inter-specific competition). To distinguish effects of apomixis and the region of origin, different H. pilosella lineages were compared. Furthermore, experiments were carried out to investigate effects of the ploidy level. We show that sexual plants are better inter-specific competitors than apomicts in terms of vegetative reproduction (number of stolons) and vegetative spread (stolon length), while apomicts do better than sexuals in intra-specific competition. The magnitude of the effect was in some cases dependent on the ploidy levels of the plants. Furthermore, apomicts always produced more stolons than sexuals, suggesting potential displacement of sexuals by apomicts where they co-occur.     

Breeding systems,hybridization and continuing evolution in Avon Gorge Sorbus

Background and Aims

Interspecific hybridization and polyploidy are key processes in plant evolution and are responsible for ongoing genetic diversification in the genus Sorbus (Rosaceae). The Avon Gorge, Bristol, UK, is a world ‘hotspot’ for Sorbus diversity and home to diploid sexual species and polyploid apomictic species. This research investigated how mating system variation, hybridization and polyploidy interact to generate this biological diversity.

Methods

Mating systems of diploid, triploid and tetraploid Sorbus taxa were analysed using pollen tube growth and seed set assays from controlled pollinations, and parent–offspring genotyping of progeny from open and manual pollinations.

Key Results

Diploid Sorbus are outcrossing and self-incompatible (SI). Triploid taxa are pseudogamous apomicts and genetically invariable, but because they also display self-incompatibility, apomictic seed set requires pollen from other Sorbus taxa – a phenomenon which offers direct opportunities for hybridization. In contrast tetraploid taxa are pseudogamous but self-compatible, so do not have the same obligate requirement for intertaxon pollination.

Conclusions

The mating inter-relationships among Avon Gorge Sorbus taxa are complex and are the driving force for hybridization and ongoing genetic diversification. In particular, the presence of self-incompatibility in triploid pseudogamous apomicts imposes a requirement for interspecific cross-pollination, thereby facilitating continuing diversification and evolution through rare sexual hybridization events. This is the first report of naturally occurring pseudogamous apomictic SI plant populations, and we suggest that interspecific pollination, in combination with a relaxed endosperm balance requirement, is the most likely route to the persistence of these populations. We propose that Avon Gorge Sorbus represents a model system for studying the establishment and persistence of SI apomicts in natural populations.     

Harnessing apomictic reproduction in grasses: what we have learned from Paspalum

Background

Apomixis is an alternative route of plant reproduction that produces individuals genetically identical to the mother plant through seeds. Apomixis is desirable in agriculture, because it guarantees the perpetuation of superior genotypes (i.e. heterotic hybrid seeds) by self-seeding without loss of hybrid vigour. The Paspalum genus, an archetypal model system for mining apomixis gene(s), is composed of about 370 species that have extremely diverse reproductive systems, including self-incompatibility, self-fertility, full sexual reproduction, and facultative or obligate apomixis. Barriers to interspecific hybridization are relaxed in this genus, allowing the production of new hybrids from many different parental combinations. Paspalum is also tolerant to various parental genome contributions to the endosperm, allowing analyses of how sexually reproducing crop species might escape from dosage effects in the endosperm.

Scope

In this article, the available literature characterizing apomixis in Paspalum spp. and its use in breeding is critically reviewed. In particular, a comparison is made across species of the structure and function of the genomic region controlling apomixis in order to identify a common core region shared by all apomictic Paspalum species and where apomixis genes are likely to be localized. Candidate genes are discussed, either as possible genetic determinants (including homologs to signal transduction and RNA methylation genes) or as downstream factors (such as cell-to-cell signalling and auxin response genes) depending, respectively, on their co-segregation with apomixis or less. Strategies to validate the role of candidate genes in apomictic process are also discussed, with special emphasis on plant transformation in natural apomictic species.     

Transmission of the apomictic mode of reproduction in Pennisetum: co-inheritance of the trait and molecular markers

Summary Apomixis, asexual reproduction through seed, is an obligate mode of reproduction in several species from the genus Pennisetum. Transfer of apomixis to sexual, cultivated pearl millet (P. glaucum) from a wild species P. squamulatum has resulted in an obligate apomictic backcross line with a low, but unknown number, of chromosomes from the wild species. Molecular markers (restriction fragment length polymorphisms and random amplified polymorphic DNAs) have been identified that unequivocally demonstrate the presence of P. squamulatum DNA in BC₃. Three of the informative RFLP clones have been sequenced and converted to sequence-tagged sites that can be amplified by the polymerase chain reaction. Molecular genetic analysis of more advanced back-cross individuals, using the two types of polymerase chain reaction-based molecular markers, has demonstrated co-inheritance of apomictic reproduction and two of the molecular markers. The remaining five molecular markers generally co-segregate with each other but are not linked with the mode of reproduction. These results suggest that genes for apomixis apparently can be transmitted by a single chromosome. Chromosome-specific markers will provide a starting point for the mapping of this genetically intractable reproductive trait.     

Apomixis and reticulate evolution in the Asplenium monanthes fern complex

Background and Aims

Asexual reproduction is a prominent evolutionary process within land plant lineages and especially in ferns. Up to 10 % of the approx. 10 000 fern species are assumed to be obligate asexuals. In the Asplenium monanthes species complex, previous studies identified two triploid, apomictic species. The purpose of this study was to elucidate the phylogenetic relationships in the A. monanthes complex and to investigate the occurrence and evolution of apomixis within this group.

Methods

DNA sequences of three plastid markers and one nuclear single copy gene were used for phylogenetic analyses. Reproductive modes were assessed by examining gametophytic and sporophyte development, while polyploidy was inferred from spore measurements.

Key Results

Asplenium monanthes and A. resiliens are confirmed to be apomictic. Asplenium palmeri, A. hallbergii and specimens that are morphologically similar to A. heterochroum are also found to be apomictic. Apomixis is confined to two main clades of taxa related to A. monanthes and A. resiliens, respectively, and is associated with reticulate evolution. Two apomictic A. monanthes lineages, and two putative diploid sexual progenitor species are identified in the A. monanthes clade.

Conclusions

Multiple origins of apomixis are inferred, in both alloploid and autoploid forms, within the A. resiliens and A. monanthes clades.     

Cytotype stability, facultative apomixis and geographical parthenogenesis in Ranunculus kuepferi (Ranunculaceae)

Background and Aims

Asexual organisms are more widespread in previously glaciated areas than their sexual relatives (‘geographical parthenogenesis’). In plants, this pattern is probably dependent on reproductive isolation and stability of cytotypes within their respective distribution areas. Both partial apomixis and introgressive hybridization potentially destabilize the spatial separation of sexual and apomictic populations. The wide distribution of apomicts may be further enhanced by uniparental reproduction which is advantageous for colonization. These factors are studied in the alpine species Ranunculus kuepferi.

Methods

Geographical distribution, diversity and mode of reproduction of cytotypes were assessed using flow cytometry and flow cytometric seed screening on samples from 59 natural populations of Ranunculus kuepferi. Seed set of cytotypes was compared in the wild.

Key Results

Diploid sexuals are confined to the south-western parts of the Alps, while tetraploid apomicts dominate in previously glaciated and in geographically isolated areas despite a significantly lower fertility. Other cytotypes (3x, 5x and 6x) occur mainly in the sympatric zone, but without establishing populations. The tetraploids are predominantly apomictic, but also show a partial apomixis via an uncoupling of apomeiosis and parthenogenesis in the seed material. Both pseudogamy and autonomous endosperm formation are observed which may enhance uniparental reproduction.

Conclusions

Diploids occupy a glacial relic area and resist introgression of apomixis, probably because of a significantly higher seed set. Among the polyploids, only apomictic tetraploids form stable populations; the other cytotypes arising from partial apomixis fail to establish, probably because of minority cytotype disadvantages. Tetraploid apomicts colonize previously devastated and also distant areas via long-distance dispersal, confirming Baker''s law of an advantage of uniparental reproduction. It is concluded that stability of cytotypes and of modes of reproduction are important factors for establishing a pattern of geographical parthenogenesis.     

Molecular understandings on ‘the never thirsty’ and apomictic <Emphasis Type="Italic">Cenchrus</Emphasis> grass

The genus Cenchrus comprises around 25 species of ‘bristle clade’ grasses. Cenchrus ciliaris (buffel grass) is a hardy, perennial range grass that survives in poor sandy soils and limiting soil moisture conditions and, due to the very same reasons, this grass is one of the most prevalent fodder grasses of the arid and semi-arid regions. Most of the germplasms of Cenchrus produce seeds asexually through the process of apomeiosis. Therefore, the lack of sufficient sexual lines has hindered the crop improvement efforts in Cenchrus being confined to simple selection methods. Many attempts have been initiated in buffel grass to investigate the various molecular aspects such as genomic signatures of different species and genotypes, molecular basis of abiotic stress tolerance and reproductive performance. Even though it is an important fodder crop, molecular investigations in Cenchrus lack focus and the molecular information available on this grass is scanty. Cenchrus is a very good gene source for abiotic stress tolerance and apomixis studies. Biotechnological interventions in Cenchrus can help in crop improvement in Cenchrus as well as other crops through transgenic technology or marker assisted selection. To date no consolidated review on biotechnological interventions in Cenchrus grass has been published. Therefore we provide a thorough and in depth review on molecular research in Cenchrus focusing on molecular signatures of evolution, tolerance to abiotic stress and apomictic reproductive mechanism.     

Apomixis is not developmentally conserved in related, genetically characterized Hieracium plants of varying ploidy

Apomixis is facultative in characterized members of the genus Hieracium. The three components that comprise the apomictic mechanism include apospory followed by autonomous embryo and endosperm formation. The time of aposporous embryo sac initiation and mode of embryo sac formation are different in Hieracium piloselloides (D3) and Hieracium aurantiacum (A3.4). Genetic studies have shown that a single dominant locus encodes all three components of apomixis in both species (Bicknell et al. 2000). We histologically examined a range of related, genetically characterized apomictic Hieracium plants derived from D3 and A3.4 to assess conservation of the apomictic mechanism in different genetic backgrounds. The plants varied in ploidy, and also in the amount of DNA introduced from sexual Hieracium pilosella (P4). An apomictic hybrid from a cross between the two apomicts was also examined. The developmental processes observed in the parental apomicts were not conserved in the examined plants and alterations occurred in the components of apomixis. One plant also exhibited adventitious embryony. The results show that other genetic factors can modify apomixis with respect to time of initiation, spatial location, and mode of developmental progression. Both the apomictic locus and the modifiers are essential for efficient penetrance of the trait in Hieracium. Some of the findings in Hieracium correspond with observations in Ranunculus and this is discussed in terms of models for apomictic development and the control of apomixis in crops. Received: 21 June 1999 / Revision accepted: 17 November 1999     

Enriching Ploidy Level Diversity: the Role of Apomictic and Sexual Biotypes of Hieracium subgen. Pilosella (Asteraceae) that Coexist in Polyploid Populations

The capacity to generate variation in ploidy and reproductive mode was compared in facultatively apomictic versus sexual maternal plants that coexist in two model populations. The population structure was studied in polyploid hybrid swarms comprised of Hieracium pilosella (usually sexual, less commonly apomictic), H. bauhini (apomictic), and their hybrids (sexual, apomictic, or sterile). Relationships among established biotypes were proposed on the basis of their DNA ploidy level/chromosome number, reproductive mode and morphology. Isozyme phenotypes and chloroplast DNA haplotypes were assayed in the population that was richer in hybrids. The reproductive origin of seed progeny was identified in both sexual and apomictic mothers, using alternative methods: the karyological, morphological and reproductive characters of the cultivated progeny were compared with those of respective mothers, or flow cytometric seed screening was used. In both populations, the progeny of sexual mothers mainly retained a rather narrow range of ploidy level/chromosome number, while the progeny of facultatively apomictic mothers was more variable. The high-polyploid hybrids, which had arisen from the fertilization of unreduced egg cells of apomicts, mainly produced aberrant non-maternal progeny (either sexually and/or via haploid parthenogenesis). Apparently, such versatile reproduction resulted in genomic instability of the recently formed high-polyploid hybrids. While the progeny produced by both true apomictic and sexual mothers mostly maintained the maternal reproductive mode, the progeny of those ‘versatile’ mothers was mainly sexual. Herein, we argue that polyploid facultative apomicts can considerably increase population diversity.     

Evolution of apomixis loci in Pilosella and Hieracium (Asteraceae) inferred from the conservation of apomixis-linked markers in natural and experimental populations

The Hieracium and Pilosella (Lactuceae, Asteraceae) genera of closely related hawkweeds contain species with two different modes of gametophytic apomixis (asexual seed formation). Both genera contain polyploid species, and in wild populations, sexual and apomictic species co-exist. Apomixis is known to co-exist with sexuality in apomictic Pilosella individuals, however, apomictic Hieracium have been regarded as obligate apomicts. Here, a developmental analysis of apomixis within 16 Hieracium species revealed meiosis and megaspore tetrad formation in 1 to 7% of ovules, for the first time indicating residual sexuality in this genus. Molecular markers linked to the two independent, dominant loci LOSS OF APOMEIOSIS (LOA) and LOSS OF PARTHENOGENESIS (LOP) controlling apomixis in Pilosella piloselloides subsp. praealta were screened across 20 phenotyped Hieracium individuals from natural populations, and 65 phenotyped Pilosella individuals from natural and experimental cross populations, to examine their conservation, inheritance and association with reproductive modes. All of the tested LOA and LOP-linked markers were absent in the 20 Hieracium samples irrespective of their reproductive mode. Within Pilosella, LOA and LOP-linked markers were essentially absent within the sexual plants, although they were not conserved in all apomictic individuals. Both loci appeared to be inherited independently, and evidence for additional genetic factors influencing quantitative expression of LOA and LOP was obtained. Collectively, these data suggest independent evolution of apomixis in Hieracium and Pilosella and are discussed with respect to current knowledge of the evolution of apomixis.     

Introgression of apomixis into sexual species is inhibited by mentor effects and ploidy barriers in the Ranunculus auricomus complex

Background and Aims

Apomictic plants maintain functional pollen, and via pollination the genetic factors controlling apomixis can be potentially transferred to congeneric sexual populations. In contrast, the sexual individuals do not fertilize apomictic plants which produce seeds without fertilization of the egg cells. This unidirectional introgressive hybridization is expected finally to replace sexuality by apomixis and is thought to be a causal factor for the wide geographical distribution of apomictic complexes. Nevertheless, this process may be inhibited by induced selfing (mentor effects) of otherwise self-incompatible sexual individuals. Here whether mentor effects or actual cross-fertilization takes place between diploid sexual and polyploid apomictic cytotypes in the Ranunculus auricomus complex was tested via experimental crosses.

Methods

Diploid sexual mother plants were pollinated with tetra- and hexaploid apomictic pollen donators by hand, and the amount of well-developed seed compared with aborted seed was evaluated. The reproductive pathways were assessed in the well-developed seed via flow cytometric seed screen (FCSS).

Key Results

The majority of seed was aborted; the well-developed seeds have resulted from both mentor effects and cross-fertilization at very low frequencies (1·3 and 1·6 % of achenes, respectively). Pollination by 4x apomictic pollen plants results more frequently in cross-fertilization, whereas pollen from 6x plants more frequently induced mentor effects.

Conclusions

It is concluded that introgression of apomixis into sexual populations is limited by ploidy barriers in the R. auricomus complex, and to a minor extent by mentor effects. In mixed populations, sexuality cannot be replaced by apomixis because the higher fertility of sexual populations still compensates the low frequencies of potential introgression of apomixis.Key words: Apomixis, Ranunculus auricomus, evolution, geographical parthenogenesis, crossing experiments, flow cytometry     

Synergids and filiform apparatus in the sexual and apomictic dandelions from section Palustria (Taraxacum, Asteraceae)

An evolutionary trend to reduce “unnecessary costs” associated with the sexual reproduction of their amphimictic ancestors, which may result in greater reproductive success, has been observed among the obligatory apomicts. However, in the case of the female gametophyte, knowledge about this trend in apomicts is not sufficient because most of the ultrastructural studies of the female gametophyte have dealt with amphimictic angiosperms. In this paper, we tested the hypothesis that, in contrast to amphimictic plants, synergids in apomictic embryo sacs do not form a filiform apparatus. We compared the synergid structure in two dandelions from sect. Palustria: the amphimictic diploid Taraxacum tenuifolium and the apomictic tetraploid, male-sterile Taraxacum brandenburgicum. Synergids in both species possessed a filiform apparatus. In T. brandenburgicum, both synergids persisted for a long time without any degeneration, in spite of the presence of an embryo and endosperm. We propose that the persistent synergids in apomicts may play a role in the transport of nutrients to the embryo.     

A microsatellite linkage map of Drosophila mojavensis

Background

Drosophila mojavensishas been a model system for genetic studies of ecological adaptation and speciation. However, despite its use for over half a century, no linkage map has been produced for this species or its close relatives.

Results

We have developed and mapped 90 microsatellites in D. mojavensis, and we present a detailed recombinational linkage map of 34 of these microsatellites. A slight excess of repetitive sequence was observed on the X-chromosome relative to the autosomes, and the linkage groups have a greater recombinational length than the homologous D. melanogaster chromosome arms. We also confirmed the conservation of Muller's elements in 23 sequences between D. melanogaster and D. mojavensis.

Conclusions

The microsatellite primer sequences and localizations are presented here and made available to the public. This map will facilitate future quantitative trait locus mapping studies of phenotypes involved in adaptation or reproductive isolation using this species.     

Polymorphisms in bovine immune genes and their associations with somatic cell count and milk production in dairy cattle

Background

Chromosome rearrangements are an important part of the speciation process in many taxa. The study of chromosome evolution in bivalves is hampered by the absence of clear chromosomal banding patterns and the similarity in both chromosome size and morphology. For this reason, obtaining good chromosome markers is essential for reliable karyotypic comparisons. To begin this task, the chromosomes of the mussels Brachidontes puniceus and B. rodriguezi were studied by means of fluorochrome staining and fluorescent in situ hybridization (FISH).

Results

Brachidontes puniceus and B. rodriguezi both have 2n = 32 chromosomes but differing karyotype composition. Vertebrate-type telomeric sequences appear at both ends of every single chromosome. B. puniceus presents a single terminal major rRNA gene cluster on a chromosome pair while B. rodriguezi shows two. Both mussels present two 5S rDNA and two core histone gene clusters intercalary located on the long arms of two chromosome pairs. Double and triple-FISH experiments demonstrated that one of the 5S rDNA and one of the major rDNA clusters appear on the same chromosome pair in B. rodriguezi but not in B. puniceus. On the other hand, the second 5S rDNA cluster is located in one of the chromosome pairs also bearing one of the core histone gene clusters in the two mussel species.

Conclusion

Knowledge of the chromosomal distribution of these sequences in the two species of Brachidontes is a first step in the understanding of the role of chromosome changes on bivalve evolution.     

The Detection,Rate and Manifestation of Residual Sexuality in Apomictic Populations of Pilosella (Asteraceae,Lactuceae)

The effect of maternal, facultatively apomictic plants on population diversity was evaluated in seven hybridizing polyploid Pilosella populations, where apomictic (P. bauhini or P. aurantiaca) and sexual (P. officinarum) biotypes coexist. The ploidy level, reproductive system, morphology, clonal structure and chloroplast DNA haplotypes were used to characterize these plants and their hybrids. The reproductive origins of the progeny were assessed through either a flow cytometric seed screen and/or a comparison between the ploidy level of progeny embryos/seedlings and the maternal ploidy level. The cultivated progeny derived from residual sexuality in maternal apomicts were also identified based on their morphology and reproductive behaviour. The progeny different from their maternal parents (0.6?92.3 % of progeny embryos and 0?100 % of progeny seedlings) originated either sexually or via haploid parthenogenesis. Comparing the facultatively apomictic and sexual mothers, the progeny arrays generated in the field showed that apomictic mothers produce progeny that is more variable in ploidy level. This effect was demonstrated at both the embryonic and seedling stages of progeny development. Residual sexuality in apomicts was also effective in experimental crosses, generating progeny similar to spontaneous hybrids in the field. The 2n + n hybrids produced from an apomictic and a sexual parent displayed similar reproductive behaviour, producing polyhaploid, sexual and apomictic progeny in variable ratios. Repeated hybridizations between parental species and/or multi-step crosses can result in hybrid swarms rich in cytotypes and morphotypes. The variation recorded in these populations suggests prevailing introgressive hybridization towards the sexual species P. officinarum.     

Molecular markers shared by diverse apomictic Pennisetum species

Two molecular markers, a RAPD (randomly amplified polymorphic DNA) and a RFLP/STS (restriction fragment length polymorphism/sequence-tagged site), previously were found associated with apomictic reproductive behavior in a backcross population produced to transfer apomixis from Pennisetum squamulatum to pearl millet. The occurrence of these molecular markers in a range of 29 accessions of Pennisetum comprising 11 apomictic and 8 sexual species was investigated. Both markers were specific for apomictic species in Pennisetum. The RFLP/STS marker, UGT 197, was found to be associated with all taxa that displayed apomictic reproductive behavior except those in section Brevivalvula. Neither UGT197 nor the cloned RAPD fragment OPC-04₆₀₀ hybridized with any sexually reproducing representatives of the genus. The cloned C04₆₀₀ was associated with 3 of the 11 apomictic species, P. ciliare, P. massaicum, and P. squamulatum. UGT197 was more consistently associated with apomictic reproductive behavior than OPC04₆₀₀ or cloned C04₆₀₀, thus it could be inferred that UGT197 is more closely linked to the gene(s) for apomixis than the cloned C04₆₀₀. The successful use of these probes to survey other Pennisetum species indicates that apomixis is a trait that can be followed across species by using molecular means. This technique of surveying species within a genus will be useful in determining the relative importance of newly isolated markers and may facilitate the identification of the apomixis gene(s).