Pseudogamous Apomicts Research Articles

Breeding system and spatial isolation from congeners strongly constrain seed set in an insect-pollinated apomictic tree: Sorbus subcuneata (Rosaceae)

In plants, apomixis results in the production of clonal offspring via seed and can provide reproductive assurance for isolated individuals. However, many apomicts require pollination to develop functional endosperm for successful seed set (pseudogamy) and therefore risk pollination-limitation, particularly in self-incompatible species that require heterospecific pollen. We used microsatellite paternity analysis and hand pollinations to investigate pollen-limitation in Sorbus subcuneata, a threatened endemic tree that co-occurs with its congener, S. admonitor. We confirmed that S. subcuneata is an obligate pseudogamous apomict, but open-pollinated flowers rarely produced seed (flower-to-seed conversion < 1%) even though they rapidly accumulated pollen on their stigmas. Manual heterospecific pollination by S. admonitor resulted in a high flower-to-seed conversion rate (65%), however, we estimate that the ratio of self: heterospecific pollination in open-pollinated flowers was at least 22:1. Despite the efficacy of heterospecific pollination, the contribution of S. admonitor trees to paternity in seed from open-pollinated flowers of S. subcuneata decreased rapidly with the spatial separation between paternal and maternal trees. Conservation efforts aimed at maintaining species with this breeding system must therefore manage the congeners in tandem which will also maintain the potential for rare heterospecific fertilisation that typically cause rapid diversification in these lineages.

Floral sex allocation and reproductive success within inflorescences of Hosta ventricosa, a pseudogamous apomict

Within inflorescences of sexually reproducing hermaphrodites, the production of ovules, fruits and seeds commonly declines from basal (early-opening) to distal (late-opening) flowers, while pollen production remains constant or only changes slightly, with the result that distal (late-opening) flowers become functionally male. However, few empirical studies have specifically examined whether or not changes in allocation to pollen production actually lead to changes in the number of seeds sired, a more direct measure of male fitness. In pseudogamous apomicts, fitness depends on the number of seeds produced; thus, a contrasting pattern of variation in the pollen-to-ovule (P/O) ratio within inflorescences might be expected. We investigated floral sex allocation and reproductive success within racemes of Hosta ventricosa, a pseudogamous apomictic hermaphrodite possessing flowers that open acropetally. We quantified variations in pollen number, ovule number, the P/O ratio and fruit and seed production, from 2007 to 2011, among flowers within racemes of four populations of H. ventricosa in southwest China. Ecological causes for fruit and seed production were evaluated by observing patterns of pollen deposition, flower removal and supplemental pollination. Pollen number, ovule number and the P/O ratio declined from basal-to-distal positions in all sampled populations (years). Fruit and seed production decreased distally in most populations (years). Low fruit and seed set of distal flowers was not due to pollen limitation because pollen deposition never declined distally and the low fruit and seed set of distal flowers remained even after supplementary pollination was provided. The flower-removal experiment indicated that inter-fruit competition for resources among flowers was common. The low P/O ratio of distal flowers in H. ventricosa might be favored because they were unable to obtain fitness by donating pollen and siring seeds on other plants. Our study may help to understand the adaptive significance of sex allocation among flowers within inflorescences of sexually reproducing hermaphrodites.

Breeding systems, hybridization and continuing evolution in Avon Gorge Sorbus.

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. 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. 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. 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.

Seasonal expression of apospory in bahiagrass

Flowering plants can reproduce sexually (outcrossing and/or selfing) and/or asexually. Sexual reproduction implies the successful completion of meiosis and double fertilization for the formation of both the embryo and the endosperm. In contrast, gametophytic apomixis is an asexual mode of reproduction through seeds, that in-volves parthenogenetic embryo development from a cytologically unreduced egg cell (2n). Apospory is the process by which unreduced gametophytes are formed after a series of mitotic divisions of somatic cells (2n) in the ovary. This occurs independently from the sexual meiotic process; and therefore, both sexual and apomic-tic pathways may coexist simultaneously. Apospory is inherited in bahiagrass (Paspalum notatum Flugge) as a single dominant Mendelian factor with distorted segregation (Martinez et al. 2001), and its degree of expression was reported to vary throughout the flowering season in P. cromyorrhizon, a close relative of bahiagrass (Quarin1986). Bahiagrass is a perennial warm-season grass widely used for forage and utility turf in the south-eastern US due to its persistence in sandy, infertile soils. Diploid races reproduce sexually and are highly self-incompatible (Acuna et al. 2007), while polyploids are classified as pseudogamous apomicts (pollination is required) (Quarin 1999). Sexual tetraploid genotypes have been experimentally created (Quesen-berry and Smith 2003; Quesenberry et al. 2010) and suc-cessfully used in crosses (Acuna et al. 2009). Cyto-embryological analysis has been used to determine the mode of reproduction in bahiagrass (Martinez et al. 2001; Acuna et al. 2007). At anthesis, sexual plants pro-duce spikelets having only a single Polygonum type

Meiotic behavior in apomictic Brachiaria ruziziensis × B. brizantha (Poaceae) progenies

Hybrids combining desirable traits from divergent parents are the main objective of some Brachiaria (Syn. Urochloa P. Beauv.) breeding programs. There is great interest in the development of apomictic hybrid cultivars that combine desirable genes such as resistance to spittlebugs, high nutritive value, and tolerance to acid soils. Microsporogenesis of six apomictic progenies resulting from a tetraploid (2n = 4x = 36) cross between B. ruziziensis × B. brizantha was evaluated under light microscopy. Genetic recombination, ensured by multivalent chromosome association and crossing-over at prophase occurred in low frequency among progenies, and in one, recombination was almost nonexistent. The percentage of meiocytes with meiotic abnormalities among progenies ranged from 16.6 % to 85.6 %. Besides an observed irregular chromosome segregation typical of polyploid hybrids in these five progenies, putative meiotic mutations characterized as desynapsis and divergent spindle organization occurred in three progenies. These anomalies caused frequent fractionation of the genome into several microspores of different sizes. In Brachiaria, new cultivars must be apomictic to fix the genotype. However, Brachiaria is a pseudogamous apomict, and viable gametes are necessary to produce viable seeds. Considering meiotic behavior, only two progenies are promising for advancement in the breeding program.

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.

Population genetic structure of diploid sexual and polyploid apomictic hawthorns (Crataegus; Rosaceae) in the Pacific Northwest

Polyploidy and gametophytic apomixis are two important and associated processes in plants. Many hawthorn species are polyploids and can reproduce both sexually and apomictically. However, the population genetic structure of these species is poorly understood. Crataegus douglasii is represented exclusively by self-compatible tetraploid pseudogamous apomicts across North America, whereas Crataegus suksdorfii found in the Pacific Northwest is known to include self-incompatible diploid sexuals as well as polyploid apomicts. We compare population structure and genetic variability in these two closely related taxa using microsatellite and chloroplast sequence markers. Using 13 microsatellite loci located on four linkage groups, 251 alleles were detected in 239 individuals sampled from 15 localities. Within-population multilocus genotypic variation and molecular diversity are greatest in diploid sexuals and lowest in triploid apomicts. Apart from the isolation of eastern North American populations of C. douglasii, there is little evidence of isolation by distance in this taxon. Genetic diversity in western populations of C. douglasii suggests that gene flow is frequent, and that colonization and establishment are often successful. In contrast, local populations of C. suksdorfii are more markedly differentiated. Gene flow appears to be limited primarily by distance in diploids and by apomixis and self-compatibility in polyploids. We infer that apomixis and reproductive barriers between cytotypes are factors that reduce the frequency of gene flow among populations, and may ultimately lead to allopatric speciation in C. suksdorfii. Our findings shed light on evolution in woody plants that show heterogeneous ploidy levels and reproductive systems.

Charecterization of the PHERES1 Orthologs in the Apomict Boechera

ABSTRACTHere we report the structure and expression patterns of orthologs for PHERES1 (PHE1) gene in the apomict Boechera holboellii. The PHE1 cDNAs were obtained by RT-PCR from Boechera anthers and carpels before and after fertilization. PHE1 encodes a type 1 MADS box protein and promotes seed growth. Also, PHE1 shows monoallelic expression and is the only plant gene known to be paternally express and maternally silenced imprinted gene. In Arabidopsis, PHE1 is repressed by the FERTILIZATION INDEPENDENT SEED (FIS) complex, also known as the Polycomb group complex 2. The FIS complex represses endosperm development in Arabidopsis thaliana prior to fertilization and plays an important role in regulating the development of endosperm following fertilisation. However, it is not known how apomicts have adjusted endosperm development to deal with parental genome imbalance due to non-reduction of maternal gametes. Therefore, the triploid apomictic Boechera species, as a close relative of A. thaliana, provide an attractive opportunity to investigate PHE1 function in pseudogamous apomicts. The 310 bp cDNA fragment of Boechera PHE1 transcript showed homology with AtPHE1. Phylogenetic analyses revealed that Boechera PHE1 is closely related to A. thaliana. The expression patterns of PHE1 in natural apomict Boechera during seed development were also revealed that the maternal BhPHE1 allele is also expressed before fertilization in carpels.

Specific features of early embryogenesis in apomictic Poa pratensis L.

We studied the early stages of embryo formation in apomictic Poa partensis L. It was shown that during transition to parthenogenesis, at least at the initial stages of embryogenesis, the algorithm of development of the sexual embryo is preserved. This could be due to the system of genetic control of embryogenesis, common for amphimixis and apomixis. We described asynchrony of developmental processes both within the efflorescence (asynchronous maturation of ovules) and within the ovule and even gametophyte (different timing of induction of apoarchesporic initials and oospores). This feature of pseudogamous apomicts allows them to produce simultaneously both sexual and apomictic progenies.

Facultative apomixis and hybridization in arctic Potentilla section Niveae (Rosaceae) from Svalbard

The degree of sexual seed set and the ability to cross were investigated in three taxa of Potentilla section Niveae (P. chamissonis, P. insularis and P. nivea) from the Svalbard Archipelago. Emasculated and bagged flowers had little or no seed set, while 71% of the emasculated, bagged and pollinated flowers had some. The taxa are interpreted as pseudogamous apomicts. Parental plants and offspring were subjected to isozyme analysis. Most of the offspring were clones of the mother plant, but 1.7% had bands from the pollen donor. The sexual offspring were all produced by mother plants of P. insularis, which is interpreted as a facultative apomict. Surprisingly, crosses between different taxa gave higher seed to ovule ratios and numbers of sexually produced offspring than crosses within taxa. Some of the sexually produced hybrid offspring had the same kind of hairs on the petioles as their mother taxon, showing that hybrids may not be intermediate in hair characters, which are considered important in the delimitation of these taxa. The results of the present study indicate very close relationships among the investigated plants. © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 142, 373−381.

Genetic mechanisms of apomixis.

The introduction of apomixis to crops would allow desirable genotypes to be propagated while preventing undesirable gene flow, but so far there has been little success in transferring this trait from a natural apomict to another species. One explanation is the sensitivity of endosperm to changes in relative maternal and paternal contribution owing to parental imprinting, an epigenetic system of transcriptional regulation by which some genes are expressed from only the maternally or paternally contributed allele. In sexual species, endosperm typically requires a ratio of two maternal genomes to one paternal genome for normal development, but this ratio is often altered in apomicts, suggesting that the imprinting system is altered as well. We present evidence that modification of DNA methylation is one mechanism by which the imprinting system could be altered to allow endosperm development in apomicts. Another feature of natural apomixis is the modification of the normal fertilization programme. Sexual reproduction uses both sperm from each pollen grain, but pseudogamous apomicts, which require a sexual endosperm to support the asexual embryo, often use just one. We present evidence that multiple fertilization of the central cell is possible in Arabidopsis thaliana, suggesting that pseudogamous apomicts may also need to acquire a mechanism for preventing more than one sperm from contributing to the endosperm. We conclude that strategies to transfer apomixis to crop species should take account of endosperm development and particularly its sensitivity to parental imprinting, as well as the mechanism of fertilization.

Autonomous endosperm development in flowering plants: how to overcome the imprinting problem?

In the vast majority of sexually reproducing flowering plants, a ratio of 2 maternally derived genomes to 1 paternally derived genome (2m:1p) is essential for normal endosperm development, and therefore ultimately for seed development. Even in many pseudogamous apomicts, where the embryo develops without a paternal contribution, fertilisation of the endosperm to obtain the correct 2m:1p parental ratio is still necessary. How do autonomous apomicts, where both embryo and endosperm develop autonomously, circumvent this requirement? The background for the 2m:1p requirement is that the parental genomes are epigenetically different; in either genome, a set of genes is silenced in a sex-specific way by genomic imprinting. Removal of the imprints from the maternally derived endosperm genome leads to expression of normally maternally silenced genes, and effectively supplies the missing paternal genome. In Arabidopsis, we propose that a combination of the fie mutation and hypomethylation of the genome creates such a situation in the endosperm genome. As a result, in a fie mutant, hypomethylated ovule complete autonomous endosperm development takes place in the absence of fertilisation.

Evidence of sexuality in European Rubus (Rosaceae) species based on AFLP and allozyme analysis

Reproduction of polyploid Rubus species is described as facultatively apomictic. Pollination is needed for seed set, but most seedlings are produced asexually by pseudogamy. Although sexual processes may occur, clonal diversity can be extremely low. We performed a pollination experiment to investigate the breeding system and used allozyme and AFLP markers to analyze genetic variation among and within seed families in R. armeniacus and R. bifrons. Pollination either with self or outcross pollen was necessary to trigger seed set. Outbreeding marginally increased the number and quality of seeds compared with selfing. The enzyme PGI revealed some genetic variation within seed families. Seven other enzyme systems were monomorphic. The more detailed AFLP analyses with five primer pairs detected the same rate of genetic variation (14-17% of seedlings were genetically distinct) and confirmed the allozyme results for the same individuals. No genetic variation was found between the seed families from within a species collected in widely separated populations, but clear species-specific differences were observed. The results support the view that polyploid Rubus species are pseudogamous apomicts with low genetic diversity among and within seed families. However, sexual reproduction occasionally occurs and contributes to the maintenance of genetic variation within natural populations.

Main role of self-pollination rate on reproductive allocations in pseudogamous apomicts

Most apomicts are hermaphroditic and pseudogamous (pollination is necessary to trigger parthenogenesis). In these plants, fitness depends on the number of progeny obtained by maternal reproduction. We determined the evolutionary stable strategy for male and female sex allocation. We show that the efficiency of pollination determines male and female resource allocations. Predictions are made of these allocations, of pollen/ovule ratio and of seed-set. We show that self-compatibility in apomicts is necessary for the maintenance of an apomictic population, and thus can account for the association between the loss of self-incompatibility and pseudogamous apomixis. In contrast to sexuals, male investment in pseudogamous apomicts increases with the rate of self-pollination.

The breeding systems of six species of Arabis (Brassicaceae)

The ability of organisms to produce genetic variation for any trait, including resistance to pathogens, is partially determined by breeding system. I used enzyme electrophoresis, crossing experiments, and cytology to assess the breeding systems of cooccurring Arabis species that are often infected by rust fungi. The Arabis holboellii surveyed were pseudogamous apomicts with relatively high population‐level ailozyme variation, and variable chromosome numbers (2n, 3n, 4n). Arabisgunnisoniana (3n) and Arabis lignifera (2n) were also pseudogamous but showed no allozyme variation either at the population level, or within progeny arrays. Arabis hirsuta may be an autogamous polyploid or it may be a pseudogamous apomict; more work is needed to clarify the breeding system of this species. Arabis drummondii and Arabis crandallii were sexual, but exhibited little genetic variability due to a predominance of self‐fertilization. The use of several techniques was necessary to evaluate these breeding systems. Insect exclusion indicated when pollen was necessary for seed set, but could not differentiate between sexual reproduction and pseudogamy. Electrophoresis yielded information on the degree of selling (as evidenced by homozygosity) and apomixis (fixed heterozygosity), but could not differentiate between autogamy and apomixis in polyploids without ailozyme variation. Pseudogamy was confirmed when crosses between dissimilar genotypes yielded only the maternal genotype, and cytologically by irregular meiosis.

Pseudogamous embryogenesis and the degree of parthenogenesis inAllium tuberosum

Early stages of egg embryony and antipodal embryony in ovules from both pollinated and unpollinated flowers ofAllium tuberosum, a diplosporous apomict, were observed. Whether flowers were pollinated or not, autonomous egg and antipodal embryonies occurred at nearly equal frequencies and progressed almost synchronously for several days. An electrophoretic analysis of endosperm esterase demonstrated the fertilization of polar nuclei. It was thus confirmed thatA. tuberosum is a pseudogamous apomict. The degree of parthenogenesis, calculated as the percentage of ovules in which egg cells showed autonomous embryonic development, ranged between 62% and 94% among six cultivars and was nearly equal to the degree of diplospory previously reported. The present results show that most unreduced egg cells and egg-like antipodal cells ofA. tuberosum have the ability to start embryogenesis independently of pollination.

Studies IN POTENTILLA L.

SummaryBritish material of Potentilla tabemaemontani Aschers. and P. crantsii (Cr.) Beck ex Fritsch consists of pseudogamous apomicts with chromosome complements at the hexaploid level (2n =c. 42) or higher. Some detailed information is given on the distribution of the various cytodemes, and the morphological variation is described. An ‘apomictic hybrid’ origin is suggested for certain topodemes of intermediate morphology. Comparison is made with Continental European studies on the two species.