Evolutionary patterns in the Dilatata group (Paspalum, Poaceae)

Paspalum dilatatum Poir. and its related species are warm-season grasses native to the grasslands of temperate South America. The group comprises several sexual tetraploid forms and apomictic tetraploids, pentaploids, hexaploids, and heptaploids. Interest in several of these biotypes as forage grasses has led to the accumulation of abundant cytogenetic information, evolutionary hypotheses, and thorough field studies which make the group a very promising model for analysis of evolutionary processes in apomictic complexes. Microsatellite markers were used here to analyze the relationships among the apomictic biotypes and evolutionary pathways. Most apomictic biotypes were shown to be monoclonal and sexual recombination is probably very rare. Suggested mechanisms for the formation of apomicts involve either unreduced female gametes or euploid pollen grains from the pentaploid biotype. Even-ploid apomictics, including those cytologically capable of facultative apomixis, are monoclonal and seem to play a very minor role in the evolution of the complex. The relationships hypothesized among the apomicts are congruent with a single origin of apomixis in the group which in turn would be coded by a non-recombining genome.     

The complex causality of geographical parthenogenesis

Asexual organisms usually have larger and more northern distributions than their sexual relatives. This phenomenon, called geographical parthenogenesis, has been controversially attributed to predispositions in certain taxa; advantages of polyploidy and/or hybrid origin; better colonizing abilities because of uniparental reproduction; introgression of apomixis into sexuals; niche differentiation of clones; or biotic interactions. This review on apomictic plants demonstrates that each of these factors alone has not been able to explain the observed distributions. Establishment of the complex regulatory system of apomixis requires taxonomic and geographical predispositions; hybridization and/or polyploidization do create diversity, but they do not necessarily result in large distributions; colonizing abilities depend on clonal diversity and are outweighed by sexuals by self-compatibility and higher potentials for speciation; niche differentiation, ploidy levels and selfing keep sympatric sexuals and apomicts separated; and the impact of biotic interactions on distributions is uncertain. In conclusion, the distributional success of apomicts has a complex causality and depends on certain circumstances and combinations of factors. The rare establishment of apomixis may help to explain the predominance of sexuality on the large scale.     

Ecological and evolutionary opportunities of apomixis: insights from Taraxacum and Chondrilla

The ecological and evolutionary opportunities of apomixis in the short and the long term are considered, based on two closely related apomictic genera: Taraxacum (dandelion) and Chondrilla (skeleton weed). In both genera apomicts have a wider geographical distribution than sexuals, illustrating the short-term ecological success of apomixis. Allozymes and DNA markers indicate that apomictic populations are highly polyclonal. In Taraxacum, clonal diversity can be generated by rare hybridization between sexuals and apomicts, the latter acting as pollen donors. Less extensive clonal diversity is generated by mutations within clonal lineages. Clonal diversity may be maintained by frequency-dependent selection, caused by biological interactions (e.g. competitors and pathogens). Some clones are geographically widespread and probably represent phenotypically plastic 'general-purpose genotypes'. The long-term evolutionary success of apomictic clones may be limited by lack of adaptive potential and the accumulation of deleterious mutations. Although apomictic clones may be considered as 'evolutionary dead ends', the genes controlling apomixis can escape from degeneration and extinction via pollen in crosses between sexuals and apomicts. In this way, apomixis genes are transferred to a new genetic background, potentially adaptive and cleansed from linked deleterious mutations. Consequently, apomixis genes can be much older than the clones they are currently contained in. The close phylogenetic relationship between Taraxacum and Chondrilla and the similarity of their apomixis mechanisms suggest that apomixis in these two genera could be of common ancestry.     

Differential effects of polyploidy and diploidy on fitness of apomictic Boechera

The co-occurrence of apomixis (asexual reproduction) and polyploidy in plants has been the subject of debate in regard to the origin and evolution of asexuality. In recent years, polyploidy has been postulated as a maintenance and stabilization factor rather than as a source of apomixis origin. It is assumed polyploidy facilitates the compensation for mutation accumulation, and hence, the rare occurrence of diploid apomixis indirectly supports this finding. Nevertheless, diploid apomicts exist and are successful, especially in the genus Boechera. While comparing phenotypic traits, fitness-related traits and apomixis penetrance between both diploid and triploid apomicts in the genus Boechera, it was expected to find trait variance that can be attributed to ploidy. Surprisingly, little trait variation could be assigned to ploidy, but rather trait variations were mainly genotype-specific. Additionally, it is shown that paternal contribution is very important for trait success, even though all offspring are genetically identical to the mother plant. This harbors implications for the introduction of apomixis into crop plants, considering the effects of paternal contribution during asexual reproduction. Nevertheless, polyploidy is an efficient way to buffer deleterious mutations, but the flexibility of diploid apomicts of the genus Boechera for rare sexual events contributes to their success in nature.     

A model for the evolution and control of generative apomixis

A model is presented for the evolution and control of generative apomixis—a collective term for apomixis in animals and diplosporous apomixis in flowering plants. Its development takes into account data obtained from studies of apomictic-like processes in sexual organisms and in non-apomictic parthenogens, as well as data obtained from studies of generative apomicts. This approach provides insights into the evolution and control of generative apomixis that cannot be obtained from studies of generative apomicts alone. It is argued that the control of the avoidance of meiotic reduction during egg production in generative apomicts resides at a single locus, the identity of which can vary between lineages. This variation accounts for the observed variation between taxa in the pattern of avoidance of meiotic reduction. The affected locus contains a wild-type allele that codes for meiotic reduction and excess copies of a mutant allele that codes for its avoidance. The dominance relationship between these is determined by their ratio and by the environment. Environmental differences between female generative cells and somatic cells are such that the phenotypic expression of the mutant allele is favoured in the former, while that of the wild-type allele is favoured in the latter. This is important, for the locus is also involved in the control of mitosis which would be disrupted by the expression of the mutant allele in somatic cells. The requirement to maintain a viable pattern of growth and development explains why the wild-type allele is retained by generative apomicts, and this in turn explains why the ability to produce meiotically reduced eggs is retained by facultative forms and why it appears to be suppressed in, rather than absent from, obligate forms. The requirement for excess copies of the mutant allele in generative cells explains why generative apomicts are typically polyploid, as this condition provides a simple and effective means of generating the correct balance of mutant and wild-type alleles. Environmental effects can also lead to the dominance relationship between wild-type and mutant alleles varying between generative cells. In plants, this can lead to the apomixis gene being expressed, and thus to meiotic reduction being avoided, in only some ovules. Meiotically reduced, as well as meiotically unreduced, eggs are produced when this occurs. If compatible and viable pollen is available the meiotically reduced eggs may be fertilized, resulting in these organisms reproducing as facultative apomicts. It is argued that the control and evolution of parthenogenesis in generative apomicts varies between taxa. In some, the parthenogenetic initiation of embryos may result from the acquisition of a parthenogenesis gene or genes; but there is no reason to believe that this is either a general or a common requirement. Indeed, in some it may be an ancestral trait, these apomicts differing from their sexual ancestors in the ability to mature, rather than in the ability to initiate, embryos from unfertilized eggs; or it may result from physiological or developmental changes induced, for example, by polyploidization, hybridization, or the avoidance of meiotic reduction. In some plants it may be induced by pollination (without fertilization) or by the activity of a developing endosperm. Although it is argued that most generatively apomictic lineages may have acquired this form of reproduction relatively easily, by the acquisition of a mutation at a single locus, it is argued that newly initiated lineages may often be reproductively inefficient. These will begin to accumulate mutations that improve the efficiency of apomictic reproduction. Thus several loci may be involved in the control of generative apomixis in established lineages, even though only a single locus was involved in its initiation in these lineages. Care must be taken to distinguish between these initiator and modifier genes when considering the evolution of generative apomixis. Finally, it is argued that although generatively apomictic lineages have easily acquired this form of reproduction, its evolution in some taxa may be so difficult, requiring the acquisition of mutations simultaneously at two or more loci, that these may never acquire it. Thus, evidence obtained from taxa that have successfully made the transition from sexual reproduction to generative apomixis that its evolution was straightforward should not be used as evidence that its evolution will always be relatively easily achieved. Its uneven taxonomic distribution indicates that it is much more easily evolved by some taxonomic groups than by others.     

The evolution of self-fertility in apomictic plants

Self-fertilization and apomixis have often been seen as alternative evolutionary strategies of flowering plants that are advantageous for colonization scenarios and in bottleneck situations. Both traits have multiple origins, but different genetic control mechanisms; possible connections between the two phenomena have long been overlooked. Most apomictic plants, however, need a fertilization of polar nuclei for normal seed development (pseudogamy). If self-pollen is used for this purpose, self-compatibility is a requirement for successful pollen tube growth. Apomictic lineages usually evolve from sexual self-incompatible outcrossing plants, but pseudogamous apomicts frequently show a breakdown of self-incompatibility. Two possible pathways may explain the evolution of SC: (1) Polyploidy not only may trigger gametophytic apomixis, but also may result in a partial breakdown of SI systems. (2) Alternatively, frequent pseudo self-compatibility (PSC) via aborted pollen may induce selfing of pseudogamous apomicts (mentor effects). Self-fertile pseudogamous genotypes will be selected for within mixed sexual–apomictic populations because of avoidance of interploidal crosses; in founder situations, SC provides reproductive assurance independent from pollinators and mating partners. SI pseudogamous genotypes will be selected against in mixed populations because of minority cytotype problems and high pollen discounting; in founder populations, SI reactions among clone mates will reduce seed set. Selection for SC genotypes will eliminate SI unless the apomict maintains a high genotypic diversity and thus a diversity of S-alleles within a population, or shifts to pollen-independent autonomous apomixis. The implications of a breakdown of SI in apomictic plants for evolutionary questions and for agricultural sciences are being discussed.     

The role of hybridization, polyploidization and glaciation in the origin and evolution of the apomictic Ranunculus cassubicus complex

The Ranunculus cassubicus complex, comprising diploids and polyploids, is a good model for studying the role of hybridization and polyploidy in the origin of apomixis. Results from amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) analyses performed on 448 individuals were combined with evidence from morphology, isozymes, karyology and distribution. Our results indicated a unique hybrid origin for the apomictic hexaploid R. carpaticola from north-western Slovakia, involving two sexual parents: autotetraploid R. cassubicifolius from the northern pre-Alps, and diploid R. carpaticola from central Slovakia. The hybrids were intermediate to the parents, but unique alleles have resulted from genomic reorganisation in the allopolyploids, which might also have triggered apomixis. Their distribution patterns and estimated ages suggest that hybridization may be correlated with the last glacial period. Hybridization seems to be the major origination for apomicts in the R. cassubicus complex. Polyploidy creates novel sexual genotypes and acts as a springboard for the production of new hybrids, but it only results in a combination with hybridization in apomixis. In turn, asexuality has permitted the perpetuation and establishment of ecologically divergent hybrid genotypes.     

Sexual reproduction is the default mode in apomictic Hieracium subgenus Pilosella, in which two dominant loci function to enable apomixis

Asexual seed formation, or apomixis, in the Hieracium subgenus Pilosella is controlled by two dominant independent genetic loci, LOSS OF APOMEIOSIS (LOA) and LOSS OF PARTHENOGENESIS (LOP). We examined apomixis mutants that had lost function in one or both loci to establish their developmental roles during seed formation. In apomicts, sexual reproduction is initiated first. Somatic aposporous initial (AI) cells differentiate near meiotic cells, and the sexual pathway is terminated as AI cells undergo mitotic embryo sac formation. Seed initiation is fertilization-independent. Using a partially penetrant cytotoxic reporter to inhibit meioisis, we showed that developmental events leading to the completion of meiotic tetrad formation are required for AI cell formation. Sexual initiation may therefore stimulate activity of the LOA locus, which was found to be required for AI cell formation and subsequent suppression of the sexual pathway. AI cells undergo nuclear division to form embryo sacs, in which LOP functions gametophytically to stimulate fertilization-independent embryo and endosperm formation. Loss of function in either locus results in partial reversion to sexual reproduction, and loss of function in both loci results in total reversion to sexual reproduction. Therefore, in these apomicts, sexual reproduction is the default reproductive mode upon which apomixis is superimposed. These loci are unlikely to encode genes essential for sexual reproduction, but may function to recruit the sexual machinery at specific time points to enable apomixis.     

Asynchronous expression of duplicate genes in angiosperms may cause apomixis, bispory, tetraspory, and polyembryony

Apomicts that produce unreduced parthenogenetic eggs are generally polyploid and occur in at least 33 of 460 families of angiosperms. Embryo sacs of these apomicts form precociously from ameiotic megaspore mother cells (diplospory) or adjacent somatic cells (apospory). Polysporic species (bisporic and tetrasporic) are sexual and occur in at least 88 families. Their embryo sacs also form precociously, but only non-critical portions of meiosis are affected. It is hypothesized that (i) the partial to complete replacement of meiosis by embryo sac formation in apomictic and polysporic species results from asynchronously-expressed duplicate genes that control female development, (ii) duplicate genes result from polyploidy or paleopolyploidy (diploidized polyploidy with chromatin from multiple genomes), (iii) apomixis results from competition between nearly complete sets of asynchronously-expressed duplicate genes, and (iv) polyspory and polyembryony result from competition between incomplete sets of asynchronously-expressed duplicate genes. Phylogenetic and genomic studies were conducted to evaluate this hypothesis. Apomictic, polysporic, and polyembryonic species tended to occur together in cosmopolitan families in which temporal variation in female development is expected, apomicts were generally polyploid with few chromosomes per genome (X = 9.6pL0.4 SE), and polysporic and polyembryonic species were paleopolyploid with many chromosomes per genome (x= 15.7pL0.6 and 13.2pL0.4, respectively). These findings support the proposed duplicate-gene asynchrony hypothesis and further suggest asexual reproduction in apomicts preserves primary genomes, sexual reproduction in polysporic and polyembryonic polyploids accelerates paleopolyploidization, and pa-leopolyploidization may sometimes eliminate gene duplications required for apomixis while retaining duplications required for polyspory or polyembryony. Hence, apomixis, with its long-term reproductive stability, may occasionally serve as an evolutionary springboard in the evolution of normal and developmentally-novel paleopolyploid sexual species and genera.     

Methods for induction and utilization of variability in the improvement of an apomictic grass,Dichanthium annulatum complex

Summary Many problems and difficulties are encountered in making genetic improvements in plants where both apomixis and polyploidy occur together. From biosystematic studies on an agamic species complex, Dichanthium annulatum, information is presented on: (A) Mechanisms which create variability in apomicts — (i) genome building and reduction, (ii) hybridization between ecotypes of facultative apomicts, (iii) fertilization of unreduced gametes, (iv) introgressive hybridization, (v) preferential pairing and genotypic control of bivalent formation and (vi) induced mutation; (B) Embryo-sac variations, vis-a-vis sexual/apomictic sacs — (i) production of sexual embryo-sac in apomicts, (ii) balance between apomixis and sexual process, (iii) effect of environment and experimental manipulation of the type of embryo-sac; and (C) Heterosis and fixation of apomixis.The utilization and exploitation of these mechanisms and phenomena for accelerating the genetic improvement of apomictic plants is discussed.Mating systems impose certain restrictions on the breeding methodology to be used in the genetic improvement of crop plants. Allogamous species have built-in mechanisms for self-improvement and, for them, the breeding techniques are well worked out. Little information is, however, available on the procedures to be followed for the genetic improvement of apomicts. Recently gathered information on the causal mechanisms of apomixis and its mode of inheritance, the genetic systems which regulate the balance between apomixis and sexuality, the physical and chemical agents for artificial induction of sexuality in apomicts, and the processes which promote variability and adaptive polymorphism in apomicts show a way for the creation, exploitation and fixation of superior genotypes. Such information, based on biosystematic studies on an agamic species complex, Dichanthium annulatum, at the Oklahoma State University, Stillwater, Oklahoma, U.S.A., is presented here.Breeding procedures commonly followed for the genetic improvement of apomicts are outlined below:1. Collection of varieties, strains or ecotypes from diverse sources; 2. Evaluation of the germ plasm for the presence of desirable characters; 3. Building up of selection indices and estimation of genetic parameters; 4. Determination of mode of reproduction and isolation of sexual types or clones; 5. Hybridization using the sexual types; 6. Progeny testing, comparisons, multiplication and release of superior types.Thus, the success of the breeding programme would depend on the range of variability already present in the germ plasm collections, the relative proportion of sexual/apomictic seed produced and the exploitation of variability from the crossbred progenies. Since large collections of plants with different genotypes are not often available, one would like to look for the mechanisms which can create variability in the apomicts. Such mechanisms are as follows.     

INDUCTION OF APOMIXIS BY OUTCROSSING BETWEEN GENETICALLY DIVERGENT ENTITIES OF CALOGLOSSA LEPRIEURII (CERAMIALES,RHODOPHYTA) AND EVIDENCE OF HYBRID APOMICTS IN NATURE1

Our previous study revealed that apomixis, recycling of tetrasporophytes, can be generated through outcrossing between genetically divergent entities of Caloglossa monosticha M. Kamiya, though such apomicts have never been found in nature. In the case of C. leprieurii (Mont.) G. Martens, the most widespread species in this genus, many apomictic strains have been isolated worldwide, but it is unknown whether these apomicts evolved through an outcrossing process similar to that in C. monosticha. In this study, heterogeneity of the apomicts and their sexual relatives as well as their evolutionary relationships was examined using the nuclear‐encoded actin gene and plastid‐encoded RUBISCO spacer region. Thirteen out of 18 apomictic strains were heterogeneous and contained divergent actin alleles, whereas only two out of 23 sexual strains were heterogeneous. The five homogeneous apomicts were genetically identical, or quite similar, to the sexual strains isolated from adjacent sites. Furthermore, three of the five homogeneous apomicts frequently produced tetraspores that grew into gametophytes, while all the heterogeneous apomicts never generated gametophytes. Apomictic strains from Florida were allotriploid, and each of the three actin sequences was closely related to those of sexual strains from Florida, Peru, and Mexico/Guatemala. In crossing tests, obligate apomixis was generated through the outcrossing between the male from Madagascar and the female from the northwestern Atlantic. These results suggest that outcrossing between genetically divergent sexual entities is one factor that induces apomixis in C. leprieurii.     

On the origin and evolution of apomixis in Boechera

The genetic mechanisms causing seed development by gametophytic apomixis in plants are predominantly unknown. As apomixis is consistently associated with hybridity and polyploidy, these confounding factors may either (a) be the underlying mechanism for the expression of apomixis, or (b) obscure the genetic factors which cause apomixis. To distinguish between these hypotheses, we analyzed the population genetic patterns of diploid and triploid apomictic lineages and their sexual progenitors in the genus Boechera (Brassicaceae). We find that while triploid apomixis is associated with hybridization, the majority of diploid apomictic lineages are likely the product of intra-specific crosses. We then show that these diploid apomicts are more likely to sire triploid apomictic lineages than conspecific sexuals. Combined with flow cytometric seed screen phenotyping for male and female components of apomixis, our analyses demonstrate that hybridization is an indirect correlate of apomixis in Boechera.     

DOES HYBRIDIZATION DRIVE THE TRANSITION TO ASEXUALITY IN DIPLOID BOECHERA?

Gametophytic apomixis is a common form of asexual reproduction in plants. Virtually all gametophytic apomicts are polyploids, and some view polyploidy as a prerequisite for the transition to apomixis. However, any causal link between apomixis and polyploidy is complicated by the fact that most apomictic polyploids are allopolyploids, leading some to speculate that hybridization, rather than polyploidy, enables apomixis. Diploid apomixis presents a rare opportunity to isolate the role of hybridization, and a number of diploid apomicts have been documented in the genus Boechera (Brassicaceae). Here, we present the results of a microsatellite study of 1393 morphologically and geographically diverse diploid individuals, evaluating the hypothesis that diploid Boechera apomicts are hybrids. This genus‐wide dataset was made possible by the applicability of a core set of microsatellite loci in 69 of the 70 diploid Boechera species and by our ability to successfully genotype herbarium specimens of widely varying ages. With few exceptions, diploid apomicts exhibited markedly high levels of heterozygosity resulting from the combination of disparate genomes. This strongly suggests that most apomictic diploid Boechera lineages are of hybrid origin, and that the genomic consequences of hybridization allow for the transition to gametophytic apomixis in this genus.     

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.     

Difference in reproductive mode rather than ploidy explains niche differentiation in sympatric sexual and apomictic populations of Potentilla puberula

Apomicts tend to have larger geographical distributional ranges and to occur in ecologically more extreme environments than their sexual progenitors. However, the expression of apomixis is typically linked to polyploidy. Thus, it is a priori not clear whether intrinsic effects related to the change in the reproductive mode or rather in the ploidy drive ecological differentiation. We used sympatric sexual and apomictic populations of Potentilla puberula to test for ecological differentiation. To distinguish the effects of reproductive mode and ploidy on the ecology of cytotypes, we compared the niches (a) of sexuals (tetraploids) and autopolyploid apomicts (penta‐, hepta‐, and octoploids) and (b) of the three apomictic cytotypes. We based comparisons on a ploidy screen of 238 populations along a latitudinal transect through the Eastern European Alps and associated bioclimatic, and soil and topographic data. Sexual tetraploids preferred primary habitats at drier, steeper, more south‐oriented slopes, while apomicts mostly occurred in human‐made habitats with higher water availability. Contrariwise, we found no or only marginal ecological differentiation among the apomictic higher ploids. Based on the pronounced ecological differences found between sexuals and apomicts, in addition to the lack of niche differentiation among cytotypes of the same reproductive mode, we conclude that reproductive mode rather than ploidy is the main driver of the observed differences. Moreover, we compared our system with others from the literature, to stress the importance of identifying alternative confounding effects (such as hybrid origin). Finally, we underline the relevance of studying ecological parthenogenesis in sympatry, to minimize the effects of differential migration abilities.     

Apomixis technology and the paradox of sex

Most plant species produce genetically variable seeds by the fusion of meiotically reduced egg cells and pollen grains. However, a small proportion of seed plants produces clonal, asexual seeds by the process of apomixis. The fixation of heterosis by apomixis is of great interest for plant breeding. The prospect of changing sexual crop species into apomictic crop species by genetic engineering--apomixis technology--has recently caused a boom in apomixis research. According to evolutionary biological theories, a dominant apomixis gene will rapidly become fixed in an outcrossing sexual population. Therefore, in theory, apomixis transgenes could have unconditional advantages that could result in the uncontrollable spread of the transgenes. By contrast, 'classic' transgenes might only have conditional advantages. Paradoxically, sexual reproduction and not apomixis is common in nature. However, this is no guarantee that apomixis transgenes will be ecologically safe because there could be essential differences between natural and transgenic apomicts.     

The role of tetraploids in the sexual-asexual cycle in dandelions (Taraxacum)

Apomictic plants often produce pollen that can function in crosses with related sexuals. Moreover, facultative apomicts can produce some sexual offspring. In dandelions, Taraxacum, a sexual-asexual cycle between diploid sexuals and triploid apomicts, has been described, based on experimental crosses and population genetic studies. Little is known about the actual hybridization processes in nature. We therefore studied the sexual-asexual cycle in a mixed dandelion population in the Netherlands. In this population, the frequencies of sexual diploids and triploids were 0.31 and 0.68, respectively. In addition, less than 1% tetraploids were detected. Diploids were strict sexuals, triploids were obligate apomicts, but tetraploids were most often only partly apomictic, lacking certain elements of apomixis. Tetraploid seed fertility in the field was significantly lower than that of apomictic triploids. Field-pollinated sexual diploids produced on average less than 2% polyploid offspring, implying that the effect of hybridization in the 2x-3x cycle in Taraxacum will be low. Until now, 2x-3x crosses were assumed to be the main pathway of new formation of triploid apomicts in the sexual-asexual cycle in Taraxacum. However, tetraploid pollen donors produced 28 times more triploid offspring in experimental crosses with diploid sexuals than triploid pollen donors. Rare tetraploids may therefore act as an important bridge in the formation of new triploid apomicts.     

Sexual and apomictic development in Hieracium

 Most members of the genus Hieracium are apomictic and set seed without fertilization, but sexual forms also exist. A cytological study was conducted on an apomictic accession of H. aurantiacum (A3.4) and also H. piloselloides (D3) to precisely define the cellular basis for apomixis. The apomictic events were compared with the sexual events in a self-incompatible isolate of H. pilosella (P4). All plants were maintained as vegetatively propagated lines each derived from a single plant. Sexual P4 exhibited characteristic events of polygonum-type embryo sac formation, showed no latent apomitic tendencies, and depended upon fertilization to set seed. In contrast, D3 and A3.4 were autonomous aposporous apomicts, forming both embryo and endosperm spontaneously inside an unreduced embryo sac. The two apomicts exhibited distinct mechanisms, but variation was also observed within each apomictic line. Seeds from apomicts often contained more than one embryo. A degree of developmental instability was also observed amongst germinated seedlings and included variation in meristem and cotyledon number, altered phyllotaxis, callus formation, and seedling fusion. In most cases abnormal seedlings developed into normal plants. Such phenomena were not observed following germination of hybrid seeds derived from crosses between sexual P4 and the apomictic plants. The three plants can now be used in inheritance studies and also to investigate the molecular mechanisms controlling apomixis. Received: 11 February 1998 / Revision accepted: 23 July 1998     

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.