Molecular evidence for polyploid origins in Saxifraga (Saxifragaceae): the narrow arctic endemic S. svalbardensis and its widespread allies

The recently described polyploid Saxifraga svalbardensis is endemic to the arctic archipelago of Svalbard. We investigated relationships among four closely related species of Saxifraga in Svalbard and tested three previously proposed hypotheses for the origin of S. svalbardensis: (1) differentiation from the morphologically and chromosomally variable polyploid S. cernua; (2) hybridization between the diploid S. hyperborea and S. cernua; and (3) hybridization between the tetraploid S. rivularis and S. cernua. Fifteen populations were analyzed using random amplified polymorphic DNAs (RAPDs) and nucleotide sequences of the chloroplast gene matK and the internal transcribed spacers (ITS) of nuclear ribosomal DNA (rDNA). RAPD and matK data suggest that S. svalbardensis has originated from a hybrid with S. rivularis as the maternal parent and S. cernua as the paternal parent, possibly a single time, whereas ITS data could not be used to discriminate among the hypotheses. The data also suggest that the diploid S. hyperborea is a progenitor of the tetraploid S. rivularis. The four populations examined of S. svalbardensis were virtually identical for RAPD and ITS markers, whereas S. cernua showed high levels of variation, suggesting that the latter polyploid either has formed recurrently or has undergone considerable differentiation since its origin.     

Same parental species, but different taxa: molecular evidence for hybrid origins of the rare endemics Saxifraga opdalensis and S. svalbardensis (Saxifragaceae)

Saxifraga opdolensis was described from Oppdal in southern Norway and hypothesized to have originated as the hybrid S. cernua × rivularis or to have been derived from a S . cernua-like progenitor. We tested these alternative hypotheses using uni- and biparentally inherited molecular markers observed in S. opdalensis and its putative parental species at the type locality: PCR-RFLPs (restriction fragment length polymorphisms in amplified fragments of chloroplast DNA; cpDNA), sequences of the cpDNA intron trnL and the spacer tmL-tmF , and RAPDs (random amplified polymorphic DNAs). The data provided unambiguous support for the hybrid hypothesis. The cpDNA analyses distinguished two well-differentiated chloroplast genomes, one in S. opdalensis and S. rivularis , and another in S. cernua . The majority of the RAPD markers showed distinct additivity in S. opdalensis relative to its postulated parental species. Thus, S. opdalensis has probably originated from a hybrid with S. rivularis as the maternal parent and S. cernua as the paternal parent. We also included S. svalbaardnsis in the present study because previous molecular analyses of Svalbard material have shown that this species had probably also originated as a hybrid between S. cernua and S. rivularis . The chloroplast genome of S. svalbaardnsis was identical to that of S. opdalensis , but the two species differed in many RAF'D markers. Although these two endemics probably have been derived from the same hybrid combination, they are morphologically and genetically distinct and should be referred to separate species. Differences between such independently originated hybrid taxa may result from intraspecific variation in their parental taxa. Saxufraga cernua comprises, for example, several highly divergent evolutionary lineages.     

On the occurrence of sexual reproduction in Saxifraga cernua and S. foliolosa (Saxifragaceae)

In all contemporary floras, the two circumpolar arctic species Saxifraga cernua and S. foliolosa are claimed to reproduce only by vegetative means (bulbils) and to have abandoned sexual reproduction. Scattered records in the literature indicate that this is erroneous, and that seed is set irregularly and at a low rate in both species. An inventory of herbarium material supports this view. Field studies at Latnjajaure, N Swedish Lapland, demonstrate that both species have an androdioecious mating system, a phenomenon which is extremely rare in natural plant populations. The two gender classes within each species are maintained by self-incompatibility in hermaphorodites and a higher rate of vegetative propagation in female-sterile plants. Hermaphrodites are rare, especially in S. cernua , and most populations consist exclusively of female-sterile plants. The sparsity of fruiting material in herbaria is regarded to be, in part, a result of low frequency of collecting, due to the in-conspicuousness of the plants in this stage. Scanning electron micrographs of seeds of the two species are provided.
Flora Nordica Notes no. 5.     

Chromosome numbers of flowering plants from northern Norway and Svalbard

Chromosome numbers of 25 taxa of flowering plants from northern Norway and Svalbard are reported. Four species and one subspecies, Carex halophila F. Nyl. (2n = ca. 78), C. stenolepis Less. (2n = ca. 80), Salicorniapojarkovae N. Sem. (2n = 36), Alnus incano (L.) Moench ssp. kolaensis (Orlova) Love (2n = 28), and Saxifraga svalbardensis D. O. Øvstedal (2n = ca. 64), have not been studied karyologically before. Chromosome numbers of four additional species are reported for the first time from Norway; those of another six species for the first time from northern Norway. The hybrid Salix herbacea L. (2x) x 5. polaris Wahlenb. (6x) is proved tetraploid and may be fertile. A possible hybrid between Saxifraga hyperborea R. Br. and S. rivularis L. was revealed.     

Genetic enrichment of the arctic clonal plant Saxifraga cernua at its southern periphery via the alpine sexual Saxifraga sibirica

Isolation of populations at the margins of a species range may lead to decreasing genetic diversity via genetic drift and inbreeding. Hybridization between peripheral populations of two species can, however, counteract genetic impoverishment. The mainly clonal, polyploid plant Saxifraga cernua has a wide arctic distribution but also extends southwards into alpine sites. In the Ural Mountains, its peripheral distribution overlaps with that of its sexually reproducing, diploid relative Saxifraga sibirica, and fertile polyploids of more or less intermediate appearance are found in this overlap zone. We used amplified fragment length polymorphism (AFLP) analysis to address the potential impact of interspecific gene flow on genetic diversity in the peripheral populations. A total of 149 plants from 17 populations along a 1650 km south-north gradient were analysed for 253 markers. The results suggest that three Middle Ural populations containing fertile and morphologically more or less intermediate plants have been affected by hybridization. All of these plants formed a strongly supported (100%) group with S. cernua in a neighbour-joining tree, but their AFLP phenotypes assigned either to S. cernua or to artificial (simulated) F(1) hybrids between S. cernua and S. sibirica in multilocus assignment tests. The three populations were highly diverse with virtually every plant representing a distinct AFLP phenotype, providing additional evidence for formation of later-generation hybrids and/or backcrossing to S. cernua. In contrast, other peripheral populations of S. cernua were typically monoclonal, suggesting that hybridization with S. sibirica can increase genetic diversity in S. cernua at its southern periphery.     

Male sterility in triploid dandelions: asexual females vs. asexual hermaphrodites

Male reproductive output, pollen in plants and sperm in animals has been shown to constitute a substantial cost for many organisms. In parthenogenetic hermaphrodites, selection is therefore expected to reduce the allocation of resources to male reproductive output. However, sustained production of pollen or sperm has been observed in numerous asexual hermaphrodites. We studied the widespread production of pollen by triploid asexual dandelions, Taraxacum sect. Ruderalia, comparing rare male sterile individuals with pollen producing asexuals. We found that individuals can show plasticity in the production of pollen, but that it is nevertheless possible to distinguish between (facultatively) male sterile asexuals and male fertile asexuals. Based on evidence from genetic markers and crosses, we conclude that the male sterility in asexual dandelions is caused by nuclear genes, in contrast to the cytoplasmically inherited male sterility previously found in sexual dandelions. Male sterile lineages did not produce more seeds per flower head, heavier seeds or seeds that were more viable. However, male sterile plants did produce more seed heads and hence more seeds than pollen producing ones, indicating that they were able to reallocate resources toward seed production. Considering the difference in seed production, it remains puzzling that not more asexual dandelions are male sterile.     

Reproductive strategy, clonal structure and genetic diversity in populations of the aquatic macrophyte Sparganium emersum in river systems

Many aquatic and riparian plant species are characterized by the ability to reproduce both sexually and asexually. Yet, little is known about how spatial variation in sexual and asexual reproduction affects the genotypic diversity within populations of aquatic and riparian plants. We used six polymorphic microsatellites to examine the genetic diversity within and differentiation among 17 populations (606 individuals) of Sparganium emersum, in two Dutch-German rivers. Our study revealed a striking difference between rivers in the mode of reproduction (sexual vs. asexual) within S. emersum populations. The mode of reproduction was strongly related to locally reigning hydrodynamic conditions. Sexually reproducing populations exhibited a greater number of multilocus genotypes compared to asexual populations. The regional population structure suggested higher levels of gene flow among sexually reproducing populations compared to clonal populations. Gene flow was mainly mediated via hydrochoric dispersal of generative propagules (seeds), impeding genetic differentiation among populations even over river distances up to 50 km. Although evidence for hydrochoric dispersal of vegetative propagules (clonal plant fragments) was found, this mechanism appeared to be relatively less important. Bayesian-based assignment procedures revealed a number of immigrants, originating from outside our study area, suggesting intercatchment plant dispersal, possibly the result of waterfowl-mediated seed dispersal. This study demonstrates how variation in local environmental conditions in river systems, resulting in shifting balances of sexual vs. asexual reproduction within populations, will affect the genotypic diversity within populations. This study furthermore cautions against generalizations about dispersal of riparian plant species in river systems.     

Clonality and recombination in the arctic plant Saxifraga cernua

The circumarctic clonal plant Saxifraga cernua reproduces efficiently via bulbils, largely depends on insects for pollination and appears to set seed very rarely. However, high levels of genotypic variation observed at small spatial scales in the arctic archipelago of Svalbard have been taken as evidence of occasional sexual reproduction. Here we assess the relative contributions of mutation and recombination to random amplified polymorphic DNA variation in four populations in East Greenland and re-analyse the Svalbard data. Greater variation due to recombination was predicted in Greenland than in Svalbard, because the higher summer temperatures and longer growing season likely increase the chances for sexual reproduction. Although we observed higher levels of genotypic diversity in Greenland than in Svalbard, matrix incompatibility and linkage disequilibrium measures provided no evidence of more sexuality, suggesting differences in glacial/postglacial history. The genetic structure and spatial distribution of clones suggest that clonal migration may increase variability in local populations, which is consistent with frequent large-scale migration in this species inferred from a circumarctic analysis of chloroplast DNA haplotypes. We conclude that a combination of somatic mutations and sexual reproduction has contributed to the observed patterns of genotypic diversity in the Greenland and Svalbard populations of S. cernua , and that sexual reproduction is important in maintaining genotypic diversity, despite the rarity of observations of seed setting.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 152 , 209–217.     

Population genetic consequences of extreme variation in sexual and clonal reproduction in an aquatic plant

Most plants combine sexual reproduction with asexual clonal reproduction in varying degrees, yet the genetic consequences of reproductive variation remain poorly understood. The aquatic plant Butomus umbellatus exhibits striking reproductive variation related to ploidy. Diploids produce abundant viable seed whereas triploids are sexually sterile. Diploids also produce hundreds of tiny clonal bulbils, whereas triploids exhibit only limited clonal multiplication through rhizome fragmentation. We investigated whether this marked difference in reproductive strategy influences the diversity of genotypes within populations and their movement between populations by performing two large-scale population surveys (n = 58 populations) and assaying genotypic variation using random amplified polymorphic DNA (RAPDs). Contrary to expectations, sexually fertile populations did not exhibit higher genotypic diversity than sterile populations. For each cytotype, we detected one very common and widespread genotype. This would only occur with a very low probability (< 10-7) under regular sexual recombination. Compatibility analysis also indicated that the pattern of genotypic variation largely conformed to that expected with predominant clonal reproduction. The potential for recombination in diploids is not realized, possibly because seeds are outcompeted by bulbils for safe sites during establishment. We also failed to find evidence for more extensive movement of fertile than sterile genotypes. Aside from the few widespread genotypes, most were restricted to single populations. Genotypes in fertile populations were very strongly differentiated from those in sterile populations, suggesting that new triploids have not arisen during the colonization of North America. The colonization of North America involves two distinct forms of B. umbellatus that, despite striking reproductive differences, exhibit largely clonal population genetic structures.     

PATHOGEN IMPACT ON SEXUAL VS. ASEXUAL REPRODUCTIVE SUCCESS IN ARISAEMA TRIPHYLLUM

Populations of Arisaema triphyllum commonly harbor a systemic fungal pathogen, Uromyces ari-triphylli, which reduces leaf area and leaf longevity. Infected plants produced 42% fewer asexual progeny (cormlets). Furthermore, all asexual progeny from infected parents were themselves infected with Uromyces. Mean seed production of infected plants was only 21% of that among healthy plants. A hand pollination experiment demonstrated that 1) infected male plants can produce viable pollen, 2) the low seed production of naturally pollinated infected females was not due to inadequate pollination, and 3) seed production among healthy female plants was strongly pollinator limited. Seeds from infected and healthy parents had similar viability, and all seeds germinating from both types of parents were uninfected. The contrast in disease transmission to sexual vs. asexual progeny suggests that pathogen attack may be one selective factor favoring sexual reproduction in A. triphyllum populations.     

Novel distribution pattern between coexisting sexual and obligate asexual variants of the true estuarine macroalga Ulva prolifera

Where sexual and asexual forms coexist within a species, the asexuals are often found to be prevalent in marginal habitats. This asexual distribution pattern has received evolutionary attention linked to the paradox of sex. In many marine species, there is a distributional trend of asexual modes being more common in lower salinity waters regarded as the ecogeographic marginal, being explained by negative effects of low salinities on sexual reproductive success. However, the distribution pattern of estuarine species recently adapted to low salinity waters has remained unknown. The brackish macroalga Ulva prolifera being a major benthic component of estuarine ecosystems includes a sexual variant and obligate asexual variants by means of motile spores. We examined the sexual–asexual distribution pattern of this alga along a salinity gradient in river estuaries. Surprisingly, opposite to the distributional trend of marine organisms, the results clearly showed the persistent predominance of sexuals in the lower salinity reaches than the asexuals. In addition, a frequent alternating of dioecious gametophytes and sporophytes in the sexual population was observed, suggesting the sexual reproductive process would be robustly performed in the low salinity waters. Considering U. prolifera had evolved from the ancestral marine species to become a true estuarine species of which the core habitat is the low salinity reaches, in a broad sense its sexual–asexual distribution pattern would be involved in asexual marginal occupations of the species range previously reported in other organisms. Based on the frozen niche variation model, we can give a concise explanation for the evolutionary process of this pattern; multiple asexuals with frozen genotypic variation had arisen from sexual ancestors undergoing low salinity adaptation and share the estuarine habitat with the sexuals at present.     

Variation within some Nordic Saxifraga species (Saxifragaceae)

By means of chemometric and morphometric analyses, the North and South Norwegian populations of Saxifraga paniculata are shown to be different from each other. No difference could be found between one North Norwegian and Icelandic populations of S. paniculata ; the name var. neogaea should not be used for the Icelandic plants. The enigmatic S . × blyttii from Western Norway is most probably an aberrant form of S. paniculata. S. hyperborea and S. rivularis were compared by means of morphometric analyses and a study of pollen size. Presence/ absence of stolons and pollen size are apparently sufficient as differential characters. No Icelandic material could be determined as S. hyperborea . By means of morphometric analyses, S × opdalensis was compared to its putative parents, S. cernua and S. rivularis , and shown to be well separated from them.     

Recovery and succession in a multi-species tropical seagrass meadow following experimental disturbance: the role of sexual and asexual reproduction

Recolonisation and succession in a multi-species tropical seagrass meadow was examined by creating gaps (50×50 cm) in the meadow and manipulating the supply of sexual and asexual propagules. Measurements of leaf shoot density and estimates of above-ground biomass were conducted monthly to measure recovery of gaps between September 1995 and November 1997. Measurements of the seeds stored in the sediment (seed bank) and horizontal rhizome growth of colonising species were also conducted to determine their role in the recovery process.Asexual colonisation through horizontal rhizome growth from the surrounding meadow was the main mechanism for colonisation of gaps created in the meadow. The seed bank played no role in recolonisation of cleared plots. Total shoot density and above-ground biomass (all species pooled) of cleared plots recovered asexually to the level of the undisturbed controls in 10 and 7 months, respectively. There was some sexual recruitment into cleared plots where asexual colonisation was prevented but seagrass abundance (shoot density and biomass) did not reach the level of unmanipulated controls. Seagrass species did not appear to form seed banks despite some species being capable of producing long-lived seeds.The species composition of cleared plots remained different to the undisturbed controls throughout the 26-month experiment. Syringodium isoetifolium was a rapid asexual coloniser of disturbed plots and remained at higher abundances than in the control treatments for the duration of the study. S. isoetifolium had the fastest horizontal rhizome growth of species asexually colonising cleared plots (6.9 mm day⁻¹). Halophila ovalis was the most successful sexual coloniser but was displaced by asexually colonising species. H. ovalis was the only species observed to produce fruits during the study.Small disturbances in the meadow led to long-term (>2 years) changes in community composition. This study demonstrated that succession in tropical seagrass communities was not a deterministic process. Variations in recovery observed for different tropical seagrass communities highlighted the importance of understanding life history characteristics of species within individual communities to effectively predict their response to disturbance. A reproductive strategy involving clonal growth and production of long-lived, locally dispersed seeds is suggested which may provide an evolutionary advantage to plants growing in tropical environments subject to temporally unpredictable major disturbances such as cyclones.     

Sexual reproduction of cordgrass hybrids (Spartina foliosa x alterniflora) invading tidal marshes in San Francisco Bay

We genetically analysed cordgrass plants and seedlings throughout the San Francisco, California, USA, estuary and found that hybrids between exotic Spartina alterniflora and native Spartina foliosa are the principal cordgrass invaders and colonizers. We hypothesized that this was due to higher seed set and siring ability by hybrids relative to the native species; too few alien parents remained in San Francisco Bay for our comparative studies. Hybrid seed comprised 91% to 98% of that set in the marsh study plants over the 2 years of the study. Total viable pollen production by hybrid plants was 400 times that of the native plants. Seed and pollen production were highly skewed towards a few hybrid genotypes. In addition to seed produced by hybrid plants, hybrid seed was produced by S. foliosa due to hybrid backcrossing. While the greatest advantage for hybrids was in pollen and seed production, hybrid seeds germinated, and seedlings survived and grew as well or better than the native species. As native S. foliosa becomes increasingly rare, hybrid seed floating on the tides will predominate, overwhelming recruitment sites and resulting in further colonization by hybrids. In an evolutionary context, hybrids with exceptional pollen and seed production will be initially favoured by natural selection, leading to the evolution of even more fertile hybrid genotypes.     

矮牡丹传粉生物学的初步研究

对山西省南部矮牡丹3个居群连续两年的野外观察和实验研究表明,共有5种蜂和4种甲虫参 与矮牡丹的传粉。电镜观察和人工控制昆虫传粉试验证明,蜂类,特别是地蜂类是矮牡丹的主要传粉 者,而甲虫类只是一种不稳定的传粉者。矮牡丹花无蜜液,但可散发气味,主要以花粉吸引昆虫。矮牡 丹不存在无融合生殖,也没有自动自花结实现象,但同株异花能产生少量种子,具微弱的自交性。在花 少的居群中自然结实率与人工异交结实率近相等。矮牡丹的结实率低,平均只有近1／4的胚珠发育成种子。     

Sources of variation in pollinator contribution within a guild: the effects of plant and pollinator factors

Among plants visited by many pollinator species, the relative contribution of each pollinator to plant reproduction is determined by variation in both pollinator and plant traits. Here we evaluate how pollinator movement among plants, apparent pollen carryover, ovule number, resource limitation of seed set, and pollen output affect variation in contribution of individual pollinator species to seed set in Lithophragma parviflorum (Saxifragaceae), a species visited by a broad spectrum of visitors, including beeflies, bees and a moth species. A previous study demonstrated differences among visitor species in their single-visit pollination efficacy but did not evaluate how differences in visitation patterns and pollen carryover affect pollinator efficacy. Incorporation of differential visitation patterns and pollen carryover effects —commonly cited as potentially important in evaluating pollinator guilds — had minor effects (0–0.6% change) on the estimates of relative contribution based on visit frequency and single-visit efficacy alone. Beeflies visited significantly more flowers per inflorescence than the bees and the moth. Seed set remained virtually constant during the first three visited flowers for beeflies and larger bees, indicating that apparent pollen carryover did not reduce per-visit efficacy of these taxa. In contrast, Greya moth visits showed a decrease in seed set by 55.4% and the smaller bees by 45.4% from first to second flower. The larger carryover effects in smaller bees and Greya were diminished in importance by their small overall contribution to seed set. Three variable plant traits may affect seed set: ovule number, resource limitation on seed maturation, and pollen output. Ovule number per flower declined strongly with later position within inflorescences. Numbers were much higher in first-year greenhouse-grown plants than in field populations, and differences increased during 3 years of study. Mean pollen count by position varied 7-fold among flowers; it paralleled ovule number variation, resulting in a relatively stable pollen:ovule ratio. Resource limitation of seed set increased strongly with later flowering, with seed set in hand-pollinated flowers ranging from 66% in early flowers to 0% in the last two flowers of all plants. Variation in ovule number and resource limitation of seed maturation jointly had a strong effect on the number of seeds per flower. Visitation to early flowers had the potential to cause more seed set than visitation to later flowers. Overall, the most important sources of variation to seed production contribution were differences among pollinators in abundance and absolute efficacy (ovules fertilized on a single visit) and potentially differential phenology among visitor species. These effects are likely to vary among populations and years.     

Breeding system and pollen limitation in the masting tree Sorbus aucuparia L. (Rosaceae) in the NW Iberian Peninsula

Mast seeding or masting is the supra-annual periodic production of a large number of seeds by long-lived plants. It has been suggested that this may be a strategy to increase pollination efficiency. Sorbus aucuparia is a masting tree typically showing rather low fruit set, though with some variation among years and populations, together with marked among-year variation in flower and fruit production. Here we report a study of the reproductive biology and insect-visitor spectrum of S. aucuparia in the NW Iberian Peninsula. Results obtained over a 4-year period indicate marked self-incompatibility, so that fruit set is strongly dependent on pollinator service. Nevertheless, fruit and seed set were not limited by pollen supply in any of the years of study, since fruit and seed set after manual cross-pollination were no higher than after natural pollination. Inflorescences were visited by diverse insect species. There was no significant correlation between fruit set and insect visit frequency. Taken together, these findings indicate that the rather low fruit and seed sets observed in this species, and the spatiotemporal variation in these parameters, must be attributed to other factors, such as abiotic resource availability. We conclude that masting in S. aucuparia is probably not a strategy for increasing pollination efficiency.     

Origins of variance in seed number and mass: interaction of sex expression and herbivory in Lomatium salmoniflorum

Summary As in many plant species, Lomatium salmoniflorum (Umbelliferae) individuals produce many flowers, only a subset of which produce mature seeds that escape seed parasitism and enter the seed bank. The interrelationships between the timing and number of flowers produced, sex expression, seed set, and seed parasitism were studied for their direct and indirect effects on the numbers and masses of viable seeds produced by individual plants. In a sample population of 369 plants that produced 161 386 flowers, 76% of the plants produced some hermaphroditic flowers. The percentage of hermaphroditic flowers increased significantly with the total number of flowers produced by a plant. Seed set was 65–90% in plants producing >600 flowers, but was highly variable in plants producing fewer flowers. Hand-pollinated plants showed the same pattern of seed set, suggesting that variable seed set in small plants may result from insufficient resources for seed development. The majority of schizocarps was produced by only 12% of the plants. Parasites killed 24.5% of the seeds prior to dispersal. Another 14.5% of the seeds lacked endosperm. Hence, the initial 161 386 flowers, which included 25874 hermaphroditic flowers each capable of producing two seeds, produced 42 468 seeds of which an estimated 25906 entered the seed bank as undamaged seeds with fully developed endosperm. Path analysis indicated that the number of hermaphroditic flowers on a plant and the percentage of seeds attacked by seed parasites had the greatest direct effects on the number of viable seeds entering the seed bank. The date at which a plant began flowering and the percentage of flowers setting seed had smaller or only indirect effects on viable seed production. Mean seed mass for plants was not significantly related to any of the factors that affected seed number, but little of the variance in seed mass occurred among plants. Masses of intact seeds in the population ranged 9-fold in both 1987 and 1988. Thirty-five percent of the variance was among seeds within umbels, 46% was among umbels within plants, and only 19% was among plants. The large variation among umbels within plants resulted from a seasonal pattern in which seeds from umbels produced late in the spring had lower mean seed masses than seeds from umbels produced early in the spring. Overall, the results indicate that both direct and indirect interactions between number of flowers, the date of initiation of flowering, seed set, and seed parasitism affect the number of viable seeds entering the seed bank. These interactions strongly bias viable seed output to a small minority of plants that produce many seeds with a wide range of masses over the growing season.     

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.     

Elevation-related variation in the population characteristics of distylous Primula nivalis affects female fitness and inbreeding depression

The population characteristics of distylous species are highly sensitive to stochastic natural selection pressure.Therefore,populations growing under different environmental conditions may vary in floral morph ratios,potentially affecting female fitness and leading to inbreeding depression.However,the variation in offspring quality among populations as a result of inbreeding depression is poorly understood in distylous species.This study investigates variations in plant density,seed mass,seed viabilityfemale fitness,and post-dispersal inbreeding depression in both sexual morphs(long-styled and shortstyled plants)of the distylous Primula nivalis that were subjected to different pollination treatments along an elevational gradient from 1657 to 2704 m a.s.l.Population characteristics(morph plant density and ratio)and fruit set were significantly affected by sexual morph and elevation.Plant density and fruitset frequencies were lower for short-styled than for long-styled plants at 2704 m a.s.l.The seeds from the cross-pollinated flowers of both morphs were higher in quality than those of self-pollinated flowers.The female fitness of seeds from cross-pollinated flowers of both morphs was higher than that of seeds from open-pollinated and self-pollinated flowers.The female fitness of seeds from long-styled flowers was higher than that of seeds from short-styled flowers at all elevations.Inbreeding depression increased with elevation among plants with short-styled flowers but not among those with long-styled flowers.Variation in the elevation-dependent mating system might influence female fitness and affect inbreeding depression in both floral morphs.In conclusion,the low quality of seeds from short-styled flowers at high elevations might decrease short-styled flower frequency,affecting population characteristics.