Mating patterns and pollen dispersal in a heterodichogamous tree, Juglans mandshurica (Juglandaceae)

Distinct ancient structural polymorphisms control heterodichogamy in walnuts and hickories

The maintenance of stable mating type polymorphisms is a classic example of balancing selection, underlying the nearly ubiquitous 50/50 sex ratio in species with separate sexes. One lesser known but intriguing example of a balanced mating polymorphism in angiosperms is heterodichogamy - polymorphism for opposing directions of dichogamy (temporal separation of male and female function in hermaphrodites) within a flowering season. This mating system is common throughout Juglandaceae, the family that includes globally important and iconic nut and timber crops - walnuts (Juglans), as well as pecan and other hickories (Carya). In both genera, heterodichogamy is controlled by a single dominant allele. We fine-map the locus in each genus, and find two ancient (>50 Mya) structural variants involving different genes that both segregate as genus-wide trans-species polymorphisms. The Juglans locus maps to a ca. 20 kb structural variant adjacent to a probable trehalose phosphate phosphatase (TPPD-1), homologs of which regulate floral development in model systems. TPPD-1 is differentially expressed between morphs in developing male flowers, with increased allele-specific expression of the dominant haplotype copy. Across species, the dominant haplotype contains a tandem array of duplicated sequence motifs, part of which is an inverted copy of the TPPD-1 3' UTR. These repeats generate various distinct small RNAs matching sequences within the 3' UTR and further downstream. In contrast to the single-gene Juglans locus, the Carya heterodichogamy locus maps to a ca. 200-450 kb cluster of tightly linked polymorphisms across 20 genes, some of which have known roles in flowering and are differentially expressed between morphs in developing flowers. The dominant haplotype in pecan, which is nearly always heterozygous and appears to rarely recombine, shows markedly reduced genetic diversity and is over twice as long as its recessive counterpart due to accumulation of various types of transposable elements. We did not detect either genetic system in other heterodichogamous genera within Juglandaceae, suggesting that additional genetic systems for heterodichogamy may yet remain undiscovered.

The fertilization process in Lithocarpus dealbatus (Fagaceae) and its implication on the sexual reproduction evolution of Fagales

MAIN CONCLUSION

The pistillate flowers of Lithocarpus dealbatus show two pollen tube (PT) arresting sites (the style-joining and micropyle) within the pistil during the postpollination-prezygotic stage. The PT, arrested at the pre-ovule stage, enhanced PT competition allowing the most compatible PTs to enter the ovary to ensure the highest fertilization success. During the shift from animal pollination to wind pollination, plants require a series of changes in reproductive traits. The mode of pollination is striking labile in Fagaceae. Lithocarpus is insect pollinated and is closely related to wind-pollinated Quercus. Little is known about the sexual reproduction of Lithocarpus. This study aimed to reveal the sexual reproduction of Lithocarpus dealbatus and to explore the evolutionary pattern of the key sexual reproduction traits to better understand their possible role in labile pollination. We found that after pollination, L. dealbatus PTs grew slowly in the style reaching style-joining in mid-January of the second year; then PT growth was arrested at style-joining for four months. Only two to three PTs resumed growth in mid-May to reach the micropyle, where PT growth ceased for one month before one PT resumed growth and passed through the micropyle to the embryo sac. Fagaceae showed a generalized mating system. Vast pollen production, small-sized pollen grains, long stigmatic receptive time, and reduced perianth were compatible with beetle pollination syndrome, representing the plesiomorphic status in Fagaceae. A large stigmatic surface and dry pollen grains linked to wind pollination might be independently derived several times in fagaceous lineages. Beetle pollination syndrome can cope with the uncertainty of pollinators to ensure conspecific pollen capture, which represents pre-adaptation status and has a selective advantage when conditions change, favouring wind pollination. The arrest of the PT at style-joining is a unique mechanism in later derived fagaceous lineages to enhance PT competition and promote outcrossing.

Whole-genome Duplication Reshaped Adaptive Evolution in A Relict Plant Species, Cyclocarya paliurus

Cyclocarya paliurus is a relict plant species that survived the last glacial period and shows a population expansion recently. Its leaves have been traditionally used to treat obesity and diabetes with the well-known active ingredient cyclocaric acid B. Here, we presented three C. paliurus genomes from two diploids with different flower morphs and one haplotype-resolved tetraploid assembly. Comparative genomic analysis revealed two rounds of recent whole-genome duplication events and identified 691 genes with dosage effects that likely contribute to adaptive evolution through enhanced photosynthesis and increased accumulation of triterpenoids. Resequencing analysis of 45 C. paliurus individuals uncovered two bottlenecks, consistent with the known events of environmental changes, and many selectively swept genes involved in critical biological functions, including plant defense and secondary metabolite biosynthesis. We also proposed the biosynthesis pathway of cyclocaric acid B based on multi-omics data and identified key genes, in particular gibberellin-related genes, associated with the heterodichogamy in C. paliurus species. Our study sheds light on evolutionary history of C. paliurus and provides genomic resources to study the medicinal herbs.

Multiplexed ISSR Genotyping by Sequencing (MIG-Seq)

Multiplexed inter-simple sequence repeat (ISSR) genotyping by sequencing (MIG-seq) is a simple, rapid, and inexpensive method for detecting single-nucleotide polymorphisms (SNPs) using next-generation sequencing (NGS). The advantages of MIG-seq include easy application to various species without prior genetic information. In addition, this method opens the door to genome-wide nucleotide sequence analyses of low-quality and trace-level deoxyribonucleic acid (DNA) samples, which have previously been difficult to analyze. Another advantage is that the procedure is simple, time-saving, and inexpensive. Recently, MIG-seq has been applied to wild and cultivated plants and has produced novel results. Using invisible DNA information, questions related to gene flow through pollination and seed dispersal, the genetic structure and diversity of populations, clonality, and the hybridization of wild and cultivated plants are being rapidly answered. In this chapter, I present the results of plant research based on MIG-seq and describe the procedure for this method as a user of MIG-seq.

Transcriptome Analysis Reveals the Role of GA3 in Regulating the Asynchronism of Floral Bud Differentiation and Development in Heterodichogamous Cyclocarya paliurus (Batal.) Iljinskaja

Cyclocarya paliurus is an important medical plant owing to the diverse bioactive compounds in its leaves. However, the heterodichogamy with female and male functions segregation within protandry (PA) or protogyny (PG) may greatly affect seed quality and its plantations for medicinal use. To speculate on the factor playing the dominant role in regulating heterodichogamy in C. paliurus, based on phenotypic observations, our study performed a multi comparison transcriptome analysis on female and male buds (PG and PA types) using RNA-seq. For the female and male bud comparisons, a total of 6753 differentially expressed genes (DEGs) were detected. In addition, functional analysis revealed that these DEGs were significantly enriched in floral development, hormone, and GA-related pathways. As the dominant hormones responsible for floral differentiation and development, gibberellins (GAs) in floral buds from PG and PA types were quantified using HPLC-MS. Among the tested GAs, GA₃ positively regulated the physiological differentiation (S0) and germination (S2) of floral buds. The dynamic changes of GA₃ content and floral morphological features were consistent with the expression levels of GA-related genes. Divergences of GA₃ contents at S0 triggered the asynchronism of physiological differentiation between male and female buds of intramorphs (PA-M vs. PA-F and PG-F vs. PG-M). A significant difference in GA₃ content enlarged this asynchronism at S2. Thus, we speculate that GA₃ plays the dominant role in the formation of heterodichogamy in C. paliurus. Meanwhile, the expression patterns of GA-related DEGs, including CPS, KO, GA20ox, GA2_OX, GID1, and DELLA genes, which play central roles in regulating flower development, coincided with heterodichogamous characteristics. These results support our speculations well, which should be further confirmed.

Distance Dependent Contribution of Ants to Pollination but Not Defense in a Dioecious, Ambophilous Gymnosperm

Dioecious plants are obligate outcrossers with separate male and female individuals, which can result in decreased seed set with increasing distance between the sexes. Wind pollination is a common correlate of dioecy, yet combined wind and insect pollination (ambophily) could be advantageous in compensating for decreased pollen flow to isolated females. Dioecious, ambophilous gymnosperms Ephedra (Gnetales) secrete pollination drops (PDs) in female cones that capture airborne pollen and attract ants that feed on them. Plant sugary secretions commonly reward ants in exchange for indirect plant defense against herbivores, and more rarely for pollination. We conducted field experiments to investigate whether ants are pollinators and/or plant defenders of South American Ephedra triandra, and whether their contribution to seed set and seed cone protection varies with distance between female and male plants. We quantified pollen flow in the wind and assessed the effectiveness of ants as pollinators by investigating their relative contribution to seed set, and their visitation rate in female plants at increasing distance from the nearest male. Ants accounted for most insect visits to female cones of E. triandra, where they consumed PDs, and pollen load was larger on bigger ants without reduction in pollen viability. While wind pollination was the main contributor to seed set overall, the relative contribution of ants was distance dependent. Ant contribution to seed set was not significant at shorter distances, yet at the farthest distance from the nearest male (23 m), where 20 times less pollen reached females, ants enhanced seed set by 30% compared to plants depending solely on wind pollination. We found no evidence that ants contribute to plant defense by preventing seed cone damage. Our results suggest that, despite their short-range movements, ants can offset pollen limitation in isolated females of wind-pollinated plants with separate sexes. We propose that ants enhance plant reproductive success via targeted delivery of airborne pollen, through frequent contact with ovule tips while consuming PDs. Our study constitutes the first experimental quantification of distance-dependent contribution of ants to pollination and provides a working hypothesis for ambophily in other dioecious plants lacking pollinator reward in male plants.

Flower Development of Heterodichogamous Juglans mandshurica (Juglandaceae)

Juglans mandshurica is a monoecious heterodichogamous species with protogynous and protandrous mating strategies that occur at a 1:1 ratio and are randomly distributed in the population. The inconsistent male and female flowering periods of the same mating type result in an imbalance of the ratio of male and female flowers, contributing to the low yield of this species. However, little more is known about its floral development. Following three consecutive years of observations, histological analysis, and scanning electron microscopy, we found that the morphological and anatomical development of the male and female flowers were synchronous. The male floral morphological development of J. mandshurica was divided into seven phases, while that of the female flower was nine. Four stages were shared between the male and female flower's anatomical development. Our findings indicate that there was minimal overlap between sexual functions within the same mating type, guaranteeing synchronization, mutual non-interference, outcrossing, and avoidance of self-fertilization. These results provide a theoretical basis for the improvement of fruit yield and quality through the reasonable allocation of protogynous and protandrous individuals in a population, and for artificial pollination control. Further, these findings lay a foundation for further research on the genetic mechanisms and environmental effects on flower development of heterodichogamous J. mandshurica.

Intraspecific Seasonal Variation of Flowering Synchronization in a Heterodichogamous Tree

Heterodichogamous reproduction in plants involves two flowering morphs, reciprocal in their timing of male and female sexual functions. The degree of synchrony in floral sex phase, within and between individuals of each morph, determines the flowers’ potential fertilization partners. Complete within-morph synchrony enables across-morph mating alone, whereas unsynchronized floral sex phases may allow fertilization within a plant individual (geitonogamy) or within a morph. We documented the disruption of flowering synchrony in the heterodichogamous Ziziphus spina-christi towards the end of its seven-month flowering season. This desert tree has self-incompatible, protandrous, short-lived (2-day) flowers that open before dawn (‘Early’ morph) or around noon (‘Late’ morph). We counted flowers in the male and female phase on flowering branches that were sampled monthly during the 2016–2018 flowering seasons. In 2018, we also tagged flowers and followed their sex-phase distributions over two days at the start, middle, and end of the season. The switch to the female phase was delayed at the end-season (November-December), and 74% of the flowers did not develop beyond their male phase. Differences in male-phase duration resulted in asynchrony among flowers within each tree and among trees of both flowering morphs. Consequently, fertilization between trees of the same morph becomes potentially possible during the end-season. In controlled hand-pollination assays, some within-morph fertilizations set fruit. The end-season breakdown of synchronous flowering generates variability within morphs and populations. We suggest that this variability may potentially enable new mating combinations in a population and enhance its genetic diversity.

Pollen dispersal patterns differ among sites for a wind-pollinated species and an insect-pollinated species

PREMISE

Pollen dispersal, the main component of overall plant gene flow, generally decreases with increasing distance from the pollen source, but the pattern of this relationship may differ among sites. Although site-based differences in pollen dispersal may lead to over- or underestimation of gene flow, no studies have investigated pollen dispersal patterns among differing urban site types, despite the incongruent range of habitats in urban areas.

METHODS

We used paternity assignment to assess pollen dispersal patterns in a wind-pollinated species (waterhemp; Amaranthus tuberculatus) and in an insect-pollinated species (tomato; Solanum lycopersicum) in experimental arrays at four disparate sites (two roof-level sites, two ground-level sites) in the New York (New York, USA) metropolitan area.

RESULTS

The number of seeds or fruits, a proxy for the number of flowers pollinated, decreased with increasing distance from the pollen donors at all sites for both species. However, the mean number of Amaranthus tuberculatusseeds produced at a given distance differed two-fold among sites, while the slope of the relationship between Solanum lycopersicumfruit production and distance differed by a factor of four among sites.

CONCLUSIONS

Pollen dispersal patterns may differ substantially among sites, both in the amount of pollen dispersed at a given distance and in the proportional decrease in pollen dispersal with increasing distance, and these effects may act independently. Accordingly, the capacity of plant species to adapt to climate change and other selection pressures may be different from predictions based on pollen dispersal patterns at a single location.

Unisexual flower initiation in the monoecious Quercus suber L.: a molecular approach

Several plant species display a temporal separation of the male and female flower organ development to enhance outbreeding; however, little is known regarding the genetic mechanisms controlling this temporal separation. Quercus suber is a monoecious oak tree with accentuated protandry: in late winter, unisexual male flowers emerge adjacent to the swollen buds, whereas unisexual female flowers emerge in the axils of newly formed leaves formed during spring (4-8 weeks after male flowering). Here, a phylogenetic profiling has led to the identification of cork oak homologs of key floral regulatory genes. The role of these cork oak homologs during flower development was identified with functional studies in Arabidopsis thaliana. The expression profile throughout the year of flower regulators (inducers and repressors), in leaves and buds, suggests that the development of male and female flowers may be preceded by separated induction events. Female flowers are most likely induced during the vegetative flush occurring in spring, whereas male flowers may be induced in early summer. Male flowers stay enclosed within the pre-dormant buds, but complete their development before the vegetative flush of the following year, displaying a long period of anthesis that spans the dormant period. Our results portray a genetic mechanism that may explain similar reproductive habits in other monoecious tree species.

An analysis of mating biases in trees

Assortative mating is a deviation from random mating based on phenotypic similarity. As it is much better studied in animals than in plants, we investigate for trees whether kinship of realized mating pairs deviates from what is expected from the set of potential mates and use this information to infer mating biases that may result from kin recognition and/or assortative mating. Our analysis covers 20 species of trees for which microsatellite data is available for adult populations (potential mates) as well as seed arrays. We test whether mean relatedness of observed mating pairs deviates from null expectations that only take pollen dispersal distances into account (estimated from the same data set). This allows the identification of elevated as well as reduced kinship among realized mating pairs, indicative of positive and negative assortative mating, respectively. The test is also able to distinguish elevated biparental inbreeding that occurs solely as a result of related pairs growing closer to each other from further assortativeness. Assortative mating in trees appears potentially common but not ubiquitous: nine data sets show mating bias with elevated inbreeding, nine do not deviate significantly from the null expectation, and two show mating bias with reduced inbreeding. While our data sets lack direct information on phenology, our investigation of the phenological literature for each species identifies flowering phenology as a potential driver of positive assortative mating (leading to elevated inbreeding) in trees. Since active kin recognition provides an alternative hypothesis for these patterns, we encourage further investigations on the processes and traits that influence mating patterns in trees.

Morphological Characterization of Flower Buds Development and Related Gene Expression Profiling at Bud Break Stage in Heterodichogamous Cyclocarya paliurus (Batal.) lljinskaja

Cyclocarya paliurus (Batal.) Iljinskaja, a unique species growing in southern China, is a multi-function tree species with medicinal, healthcare, material, and ornamental values. So far, sexual reproduction is the main method for extensive cultivation of C. paliurus plantations, but this is limited by low seed plumpness resulted from the character of heterodichogamy. Phenological observations have revealed the asynchronism of flower development in this species. However, its molecular mechanism remains largely unknown. To reveal molecular mechanism of heterodichogamy in C. paliurus, transcriptome of female (F) and male (M) buds from two mating types (protandry, PA; protogyny, PG) at bud break stage were sequenced using Illumina Hiseq 4000 platform. The expression patterns of both 32 genes related to flowering and 58 differentially expressed transcription factors (DETFs) selected from 6 families were divided four groups (PG-F, PG-M, PA-F, and PA-M) into two categories: first flowers (PG-F and PA-M) and later flowers (PA-F and PG-M). The results indicated that genes related to plant hormones (IAA, ABA, and GA) synthesis and response, glucose metabolism, and transcription factors (especially in MIKC family) played significant roles in regulating asynchronism of male and female flowers in the same mating type. The expression of DETFs showed two patterns. One contained DETFs up-regulated in first flowers in comparison to later flowers, and the other was the reverse. Nine genes related to flowering were selected for qRT-PCR to confirm the accuracy of RNA-seq, and generally, the RPKM values of these genes were consistent with the result of qRT-PCR. The results of this work could improve our understanding in asynchronism of floral development within one mating type in C. paliurus at transcriptional level, as well as lay a foundation for further study in heterodichogamous plants.

Heterodichogamy, Pollen Viability, and Seed Set in a Population of Polyploidy Cyclocarya Paliurus (Batal) Iljinskaja (Juglandaceae)

Research Highlights: Cyclocarya paliurus, native to the subtropical region of China, is a monoecious species with a heterodichogamous mating system. Its flowering phenology and low seed success characteristics differ from other typical heterodichogamous Juglandaceae species. This could be caused by the existence of polyploidy in the population. Background and Objectives: C. paliurus has been attracting more attention as a result of its medicinal value. To meet the needs for leaf harvest, cultivation expansion is required, but this is limited by a shortage of seeds. This study aims to profile the flowering phenology and the efficacy of pollen dispersal as well as elucidate on the mechanism of low seed success in the population. Materials and Methods: The flowering phenology pattern of C. paliurus was observed in a juvenile plantation containing 835 individuals of 53 families from 8 provenances at the individual (protandry, PA and protogyny, PG) and population levels for 5 consecutive years (2014–2018). Slides with a culture medium of 10% sucrose and 0.01% boric acid were used to estimate pollen density and viability in the population, and seeds were collected from 20 randomly selected PA and PG individuals to assess seed success during 2017–2018. Results: Four flowering phenotypes and strongly skewed ratios of PA/PG and male/female occurred in the juvenile population. Sexual type and ratio changed significantly with the growth of the population over the years, showing an increasing monoecious group (11.1% to 57.2%) and a decreasing unisexual group (33.6% to 16.3%), as well as a tendency for the sexual ratio to move towards equilibrium (5.42:1 to 1.39:1 for PG:PA). Two flowering phases and bimodality in gender were displayed, as in other heterodichogamous species. However, the high overlap of inter-phases and within individuals was quite different from many previous reports. Owing to the low pollen viability of C. paliurus (~30%), low seed success was monitored in the plantation, as well as in the investigated natural populations. Conclusions: Female-bias (PG and F) and a skewed ratio of mating types corresponded to nutrient accumulation in the juvenile population. Heterodichogamy in C. paliurus was verified, but was shown to be different from other documented species in Juglandaceae. The latest finding of major tetraploidy in a natural population could explain the characteristics of the flowering phenology and seed set of C. paliurus and also give rise to more questions to be answered.

Genetic Mapping of the Incompatibility Locus in Olive and Development of a Linked Sequence-Tagged Site Marker

The genetic control of self-incompatibility (SI) has been recently disclosed in olive. Inter-varietal crossing confirmed the presence of only two incompatibility groups (G1 and G2), suggesting a simple Mendelian inheritance of the trait. A double digest restriction associated DNA (ddRAD) sequencing of a biparental population segregating for incompatibility groups has been performed and high-density linkage maps were constructed in order to map the SI locus and identify gene candidates and linked markers. The progeny consisted of a full-sib family of 229 individuals derived from the cross 'Leccino' (G1) × 'Dolce Agogia' (G2) varieties, segregating 1:1 (G1:G2), in accordance with a diallelic self-incompatibility (DSI) model. A total of 16,743 single nucleotide polymorphisms was identified, 7,006 in the female parent 'Leccino' and 9,737 in the male parent 'Dolce Agogia.' Each parental map consisted of 23 linkage groups and showed an unusual large size (5,680 cM in 'Leccino' and 3,538 cM in 'Dolce Agogia'). Recombination was decreased across all linkage groups in pollen mother cells of 'Dolce Agogia,' the parent with higher heterozygosity, compared to megaspore mother cells of 'Leccino,' in a context of a species that showed exceptionally high recombination rates. A subset of 109 adult plants was assigned to either incompatibility group by a stigma test and the diallelic self-incompatibility (DSI) locus was mapped to an interval of 5.4 cM on linkage group 18. This region spanned a size of approximately 300 Kb in the olive genome assembly. We developed a sequence-tagged site marker in the DSI locus and identified five haplotypes in 57 cultivars with known incompatibility group assignment. A combination of two single-nucleotide polymorphisms (SNPs) was sufficient to predict G1 or G2 phenotypes in olive cultivars, enabling early marker-assisted selection of compatible genotypes and allowing for a rapid screening of inter-compatibility among cultivars in order to guarantee effective fertilization and increase olive production. The construction of high-density linkage maps has led to the development of the first functional marker in olive and provided positional candidate genes in the SI locus.

Pollen flow and paternity in an isolated and non-isolated black walnut (Juglans nigra L.) timber seed orchard

Artificial pollination of black walnut (Juglans nigra L.) is not practical and timber breeders have historically utilized only open-pollinated half-sib families. An alternate approach called “breeding without breeding,” consists of genotyping open-pollinated progeny using DNA markers to identify paternal parents and then constructing full-sib families. In 2014, we used 12 SSR markers to genotype 884 open-pollinated half-sib progeny harvested from two clonal orchards containing 206 trees, comprised of 52 elite timber selections. Seed was harvested in 2011 from each of two ramets of 23 clones, one upwind and one downwind, based on prevailing wind direction from the west—southwest. One orchard was isolated from wild black walnut and composed of forward selections while the other orchard was adjacent to a natural forest containing mature black walnut composed of backward selections. Isolation significantly increased within-orchard pollination (85%) of the progeny from the isolated orchard compared to 42% from the non-isolated orchard. Neither prevailing wind direction nor seed tree position in the orchard affected paternity patterns or wild pollen contamination. Genetic diversity indices revealed that progeny from both orchards were in Hardy–Weinberg equilibrium with very little inbreeding and no selfing. A significant level of inbreeding was present among the forward selected parents, but not the first generation (backward selected) parents. Some orchard clones failed to sire any progeny while other clones pollinated upwards of 20% of progeny.

Precocious genotypes and homozygous tendency generated by self-pollination in walnut

BACKGROUND

Observations of precocious (early bearing) genotypes of walnut (Juglans regia L.) under natural conditions encouraged us to study the origin and genetic control of these fascinating traits.

RESULTS

In this study, the self-fertility, progeny performance, and simple sequence repeat (SSR) locus variation of iron walnut (Juglans sigillata Dode), an ecotype of J. regia, were investigated. The average self-pollinated fruit set rate of J. sigillata cv. 'Dapao' (DP) was 7.0% annually from 1979 to 1982. The average germination rate of self-pollinated seeds was 45.2% during the 4-year period. Most progeny had inbreeding depression. Nine representative self-pollinated progeny (SP₁-SP₉), with special or typical traits of DP, were selected. SP₁-SP₄ were precocious because they initiated flowers as early as 2 years after germination, compared to the 7-10-yr period that is typical of DP. SP₉ had not flowered since 1980. Twelve SSR markers were used to analyze the SP and DP. The genome of SP had a tendency toward high levels of homozygosity. The high levels of homozygosity reported in 18 additional precocious walnut genotypes complemented the results of this study.

CONCLUSIONS

These results provide evidence of precocious phenotypes and genomes with high levels of homozygosity that might be generated from self-pollinating walnut. This suggests that self-pollination might facilitate the generation of unique homozygous parents for subsequent use in walnut-breeding programs. The results also indicate that more attention should be focused on adequate management of precocious walnut to avoid early depression in the production of nuts.

Skewed male reproductive success and pollen transfer in a small fragmented population of the heterodichogamous tree Machilus thunbergii

Heterodichogamy is defined as the presence of two flower morphs that exhibit the male and female functions at different times among individuals within a population. Heterodichogamy is regarded as an adaptation to promote outcrossing through enhanced inter-morph mating, together with a 1:1 morph ratio. However, in highly fragmented populations, the morph ratio may be more likely to be biased by stochastic events. In such a situation, individuals of a minority morph within a population are expected to have higher reproductive success than those of a majority morph, which may suffer from pollen shortages of the minority morph. In this paper, we evaluated mating patterns and male reproductive success in a highly fragmented population of Machilus thunbergii, a putative heterodichogamous evergreen laurel tree. Results of paternity analysis indicated that the selfing rate was not clearly different between the two morphs. In contrast, the proportion of intra-morph mating was higher in the majority-morph (MM) mother trees than in the minority-morph (MF) mother trees. Bayesian estimated male reproductive success indicated that male reproductive success was higher in minority-morph (MF) than in majority-morph (MM) mother trees. These findings indicate that (1) the majority morph mothers, suffering a shortage of the opposite morph pollen, could partly compensate for the reduced reproductive success by intra-morph mating rather than by selfing, and (2) negative-frequency dependent selection may be involved in the maintenance of the two morphs.

Mating patterns and pollen dispersal in a Japanese larch (Larix kaempferi) clonal seed orchard: a case study

Pollination dynamics highly determines the genetic quality of seed orchard crops. However, there is less research about the effect of mating patterns on seed productivity of orchard crops. So far, clonal seed orchards have been producing genetically improved seedlings used for most Japanese larch (Larix kaempferi (Lamb.) Carr.) plantations in China. In the present study, a total of 17 highly variable simple sequence repeat (SSR) markers were used for genotyping a progeny trial population consisting of 647 open-pollinated progenies germinated from seeds which were collected from 63 maternal clones with 140 potential paternal clones in a Japanese larch clonal seed orchard in China. Paternity analysis was used in the present case study in order to evaluate the level of paternal gametic contribution, estimate pollen contamination and selfing rates, and investigate pollination patterns, pollen dispersal patterns and the impact of mating patterns on seed productivity of orchard crops. We observed 93.7% of the success rate of the parental assignment, unequal paternal gametic contribution (0-12.4%) with 6.3% of the progenies derived from pollen contamination or unsampled pollen donors, and absence of evidence for selfing. We also found that pollination rate highly depended on the distance between pollen donors and maternal parents, the majority of the identified crossing (65.7%) occurred between clones within a 150-m radius, and large variations in growth performance existed among the paternal half-siblings. Progeny growth performance (diameter at breast (DBH) and height (HGT)) was measured at Age-20 in order to investigate the impact of mating patterns on timber production of orchard crops. As either the paternal or maternal, two clones (i. e., clones Z38 and Z62) were identified to have produced progenies with higher average stem volume breeding values than that of all of the progenies. Specifically, the genetic gains for volume were 3.53% for the two clones as paternal parents, and 8.26% as the maternal parents at Age-20. Thus, both elite clones were ideal candidates for the construction of next-generation clonal seed orchards due to their synchronous reproductive phenology with greater crossing rate and higher genetic gain. These results improved the pedigree information to provide solid evidence of mating patterns for future design and effective management of seed orchards and for the development of viable long-term breeding strategies for other coniferous species.

Phenological match drives pollen-mediated gene flow in a temporally dimorphic tree

Variation in flowering phenology is common in natural populations, and is expected to be, together with inter-mate distance, an important driver of effective pollen dispersal. In populations composed of plants with temporally separated sexual phases (i.e. dichogamous or heterodichogamous populations), pollen-mediated gene flow is assumed to reflect phenological overlap between complementary sexual phases. In this study, we conducted paternity analyses to test this hypothesis in the temporally dimorphic tree Acer opalus. We performed spatially explicit analyses based on categorical and fractional paternity assignment, and included tree size, pair-wise genetic relatedness and morph type as additional predictors. Because differences between morphs in flowering phenology may also influence pollination distances, we modelled separate pollen dispersal kernels for the two morphs. Extended phenological overlap between male and female phases (mainly associated with inter-morph crosses) resulted in higher siring success after accounting for the effects of genetic relatedness, morph type and tree size, while reduced phenological overlap (mainly associated with intra-morph crosses) resulted in longer pollination distances achieved. Siring success also increased in larger trees. Mating patterns could not be predicted by phenology alone. However, as heterogeneity in flowering phenology was the single morph-specific predictor of siring success, it is expected to be key in maintaining the temporal dimorphism in A. opalus, by promoting not only a prevalent pattern of inter-morph mating, but also long-distance pollination resulting from intra-morph mating events.

Population Differentiation in Common Walnut (Juglans regia L.) across Major Parts of Its Native Range-Insights from Molecular and Morphometric Data

Juglans regia is an economically highly important species for fruit and wood production in the warm temperate and subtropical zones of the Northern Hemisphere. Besides the natural influence of climatic and geomorphological barriers, its genetic structure has been strongly modified by humans and the population history is still unclear. For this reason, we investigated mainly natural walnut populations across the Eurasian continent on a molecular (44 populations, 581 trees) and morphometric level (23 populations, 1391 ripe nuts). Population genetic diversity and differentiation were examined by using 7 microsatellite loci. Morphometric characteristics of the nuts (mainly roundness index and nut density) were used to estimate trait variation and population differentiation. Highest allelic richness Rs12 = 7.05 was observed in a Pakistani and the lowest value Rs12 = 3.04 in a Kyrgyz population. The genetic differentiation among populations was high (FST = 0.217; RST = 0.530) indicating a strong phylogeographic pattern. While variation of the roundness index within single populations was high, this trait neither differentiated geographical regions nor was it associated to genetic clusters. Approximated QST based on this trait equalled FST, while approximated QST based on nut density considerably exceeded FST, indicating selection. Nut density was moderately correlated with altitude, latitude, and longitude, and differentiated populations according to their origin. Pakistani and Indian populations showed highest nut densities. These South Asian populations contain putatively ancestral nut forms, which probably have been lost in other populations as a consequence of human selection.

Characterization of 19 microsatellite loci in the clonal monkshood Aconitum kusnezoffii (Ranunculaceae)

PREMISE OF THE STUDY

Microsatellite loci were isolated and characterized from Aconitum kusnezoffii (Ranunculaceae) to estimate male and female reproductive success and evaluate the effects of clonal growth on sexual reproduction.

METHODS AND RESULTS

A genomic enrichment approach was used to develop microsatellite markers. In three investigated A. kusnezoffii populations, a total of 19 microsatellite loci were successfully amplified, and 13 of these loci were polymorphic. Most of the primer pairs designed for the identified loci also amplified corresponding microsatellite loci in A. barbatum var. puberulum and A. alboviolaceum.

CONCLUSIONS

The identified microsatellite loci will be useful for quantifying male and female fitness in A. kusnezoffii and evaluating the effects of clonal growth on sexual reproduction.

Inflorescence dimorphism, heterodichogamy and thrips pollination in Platycarya strobilacea (Juglandaceae)

BACKGROUND AND AIMS

Unlike other taxa in Juglandaceae or in closely related families, which are anemophilous, Platycarya strobilacea has been suggested to be entomophilous. In Juglandaceae, Juglans and Carya show heterodichogamy, a reproductive strategy in which two morphs coexist in a population and undergo synchronous reciprocal sex changes. However, there has been no study focusing on heterodichogamy in the other six or seven genera, including Platycarya.

METHODS

Inflorescence architecture, sexual expression and pollination biology were examined in a P. strobilacea population in Japan. Flowering phenology was monitored daily for 24 trees in 2008 and 27 in 2009. Flower visitors and inhabitants were recorded or collected from different sexes and stages.

KEY RESULTS

The population of P. strobilacea showed heterodichogamous phenology with protogynous and duodichogamous-protandrous morphs. This dimorphism in dichogamy was associated with distinct inflorescence morphologies. Thrips pollination was suggested by the frequent presence of thrips with attached pollen grains, the scarcity of other insect visitors, the synchronicity of thrips number in male spikes with the maturation of female flowers, and morphological characters shared with previously reported thrips-pollinated plants. Male spikes went through two consecutive stages: bright yellow and strong-scented M1 stage, and brownish and little-scented M2 stage. The latter contained more thrips, synchronized better with the receptive stage of female flowers of the reciprocal morph and is probably the main period of pollen export.

CONCLUSIONS

Platycarya strobilacea is heterodichogamous and thrips-pollinated, both of which are relatively rare conditions in angiosperms. In male spikes of P. strobilacea, there is probably a temporal decoupling of pollinator attraction and pollen export.

Disassortative mating and the maintenance of sexual polymorphism in painted maple

Since Darwin's pioneering research on plant reproductive biology (e.g. Darwin 1877), understanding the mechanisms maintaining the diverse sexual strategies of plants has remained an important challenge for evolutionary biologists. In some species, populations are sexually polymorphic and contain two or more mating morphs (sex phenotypes). Differences in morphology or phenology among the morphs influence patterns of non-random mating. In these populations, negative frequency-dependent selection arising from disassortative (intermorph) mating is usually required for the evolutionary maintenance of sexual polymorphism, but few studies have demonstrated the required patterns of non-random mating. In the current issue of Molecular Ecology, Shang et al. (2012) make an important contribution to our understanding of how disassortative mating influences sex phenotype ratios in Acer pictum subsp. mono (painted maple), a heterodichogamous, deciduous tree of eastern China. They monitored sex expression in 97 adults and used paternity analysis of open-pollinated seed to examine disassortative mating among three sex phenotypes. Using a deterministic 'pollen transfer' model, Shang et al. present convincing evidence that differences in the degree of disassortative mating in progeny arrays of the sex phenotypes can explain their uneven frequencies in the adult population. This study provides a useful example of how the deployment of genetic markers, demographic monitoring and modelling can be integrated to investigate the maintenance of sexual diversity in plants.

Genetic and environmental control of temporal and size-dependent sex allocation in a wind-pollinated plant

Sex allocation in hermaphrodites can be affected by spatial and temporal variation in resources, especially in plants where size-dependent gender modification is commonplace. The evolution of sex allocation will depend on the relative importance of genetic and environmental factors governing patterns of investment in female and male function. In wind-pollinated plants, theoretical models predict a positive relation between size and male investment because of the fitness advantages associated with more effective pollen dispersal. Theory also predicts that the timing and allocation to each sex function should depend on available resources. We grew maternal half-sibling families of annual, wind-pollinated, Ambrosia artemisiifolia in sun and shade treatments to investigate these predictions. There was significant genetic variation for female and male flower production in both sun and shade treatments. Size-dependent sex allocation occurred in the direction predicted by theory, with male flower production increasing more rapidly in larger plants. The timing of sex function also varied, with significant genetic variation for dichogamy within environments and plasticity of this trait between environments. Protandry was expressed more commonly in the sun and protogyny in the shade. The occurrence of dynamic sex allocation with changing size and experimental treatment indicates the potential for adaptive responses under different ecological conditions.

Nuclear and chloroplast DNA phylogeography reveal two refuge areas with asymmetrical gene flow in a temperate walnut tree from East Asia

• Recently, there has been a debate about whether the temperate forests of East Asia merged or fragmented during glacial periods in the Pleistocene. Here, we tested these two opposing views through phylogeographical studies of the temperate-deciduous walnut tree, Juglans mandshurica (Juglandaceae) in northern and northeastern China, as well as Japan and Korea. • We assessed the genetic structure of 33 natural populations using 10 nuclear microsatellite loci and seven chloroplast DNA (cpDNA) fragments. • The cpDNA data showed the complete fixation of two different haplotype lineages in northeastern vs northern populations. This pronounced phylogeographic break was also indicated by nuclear microsatellite data, but there were disparities regarding individual populations. Among those populations fixed for haplotype A (the northeastern group), three were clustered in the northern group and four showed evidence of mixed ancestry based on microsatellite data. • Our results support the hypothesis that two independent refugia were maintained across the range of J. mandshurica in the north of China during the last glacial maximum, contrary to the inference that all temperate forests migrated to the south (25-30°N). The discordance between the patterns revealed by cpDNA and microsatellite data indicate that asymmetrical gene flow has occurred between the two refugia.

Disassortative mating, sexual specialization, and the evolution of gender dimorphism in heterodichogamous Acer opalus

In sexually polymorphic species, the morphs are maintained by frequency-dependent selection through disassortative mating. In heterodichogamous populations in which disassortative mating occurs between the protandrous and protogynous morphs, a decrease in female fitness in one morph is hypothesized to drive sexual specialization in the other morph, resulting in dimorphic populations. We test these ideas in a population of the heterodichogamous species, Acer opalus. We assessed both prospective gender of individuals in terms of their allocations and actual parentage using microsatellites; we found that most matings in A. opalus occur disassortatively. We demonstrate that the protogynous morph is maintained by frequency-dependent selection, but that maintenance of males versus protandrous individuals depends on their relative siring success, which changes yearly. Seeds produced later in the reproductive season were smaller than those produced earlier; this should compromise reproduction through ovules in protandrous individuals, rendering them male biased in gender. Time-dependent gender and paternity analyses indicate that the sexual morphs are specialized in their earlier sexual functions, mediated by the seasonal decrease in seed size. Our results confirm that mating patterns are context-dependent and change seasonally, suggesting that sexual specialization can be driven by seasonal effects on fitness gained through one of the two sexual functions.