Sex expression, sex change and fruiting habit in an Acer rufinerve population

Interspecific variation in mortality and growth and changes in their relationship with size class in an old-growth temperate forest

Understanding trade-offs between demographic parameters is crucial when investigating community assembly rules in high-diversity forests. To this end, we estimated mortality and growth parameters, and correlations among them, across entire size classes for 17 tree species (Betula, Carpinus, Fagus, Quercus, Castanea, Acer, Cerasus, Swida, Kalopanax, and Styrax) using a dataset over 18 years obtained from an old-growth forest in Japan.Size classes were represented by 12 categories determined by age, height, and diameter at breast height (DBH) from new seedlings to stems of DBH >85 cm. We derived the annual mortality and growth for each species and class using estimates of transition probabilities between classes. Trade-offs or synergies in growth and survival among species per size class were analyzed with and without the inclusion of phylogenetic relationships.Annual mortality showed U-shaped patterns across size classes for species that could potentially reach a DBH ≥55 cm: 0.2-0.98 for seedlings, 0.002-0.01 at DBH 35-45 cm, and ca. 0.01 at DBH ≥55 cm. Other species demonstrated monotonically decreasing mortality toward specific maximum size classes. When phylogenetic information was included in analyses, the correlations between survival and growth changed across size classes were significant for some classes: As an overall tendency, synergy was observed in growth and survival for seedling to sapling classes, trade-offs for juvenile to DBH 15-25 cm classes, and synergy again for larger classes. When phylogenetic information was not included, a significant trade-off was observed only at DBH 5-15 cm. Synthesis. Trade-offs at intermediate classes imply differentiation in demographic characteristics related to life history strategies. However, evolutionarily obtained demographic characteristics are not substantial drivers of niche differentiation in the study area. The polylemma of mortality, growth, and other parameters such as the onset of reproduction may also be important factors driving species-specific demographic traits.

Time for a change: patterns of sex expression, health and mortality in a sex-changing tree

BACKGROUND AND AIMS

The ability of individuals to change sex during their lifetime is known as environmental sex determination (ESD). This represents a unique life history trait, allowing plants to allocate resources differentially to male and female functions across lifetimes, potentially maximizing fitness in response to changing environmental or internal cues. In this study, Acer pensylvanicum, a species with an unconfirmed sex determination system, was investigated to see what patterns in sex expression existed across multiple years, if there were sex-based differences in growth and mortality, and whether this species conformed to theoretical predictions that females are larger and in better condition.

METHODS

Patterns of sex expression were documented over 4 years in a phenotypically subdioecious A. pensylvanicum population located in New Jersey, USA, and data on size, mortality, health and growth were collected. A machine-learning algorithm known as a boosted classification tree was used to develop a model to predict the sex of a tree based on its condition, size and previous sex.

RESULTS

In this study, 54 % of the trees switched sex expression during a 4-year period, with 26 % of those trees switching sex at least twice. Consistently monoecious trees could change relative sex expression by as much as 95 %. Both size and condition were influential in predicting sex, with condition exerting three times more relative influence than size on expressed sex. Contrary to theoretical predictions, the model showed that full female sex expression did not increase with size. Healthy trees were more likely to be male; predicted female sex expression increased with deteriorating health. Growth rate negatively correlated with multiple years of female sex expression. Populations maintained similar male-skewed sex ratios across years and locations and may result from differential mortality: 75 % of dead trees flowered female immediately before death.

CONCLUSIONS

This study shows conclusively that A. pensylvanicum exhibits ESD and that femaleness correlates with decreased health, in contrast to prevailing theory. The mortality findings advance our understanding of puzzling non-equilibrium sex ratios and life history trade-offs resulting from male and female sex expression.

Sex change in the subdioecious shrub Eurya japonica (Pentaphylacaceae)

Sex change affects the sex ratios of plant populations and may play an essential role in the evolutionary shift of sexual systems. Sex change can be a strategy for increasing fitness over the lifetime of a plant, and plant size, environmental factors, and growth rate may affect sex change. We described frequent, repeated sex changes following various patterns in a subdioecious Eurya japonica population over five successive years. Of the individuals, 27.5% changed their sex at least once, and these changes were unidirectional or bidirectional. The sex ratio (females/males/all hermaphrodite types) did not fluctuate over the 5 years. In our study plots, although the current sex ratio among the sexes appears to be stable, the change in sex ratio may be slowly progressing toward increasing females and decreasing males. Sex was more likely to change with higher growth rates and more exposure to light throughout the year. Among individuals that changed sex, those that were less exposed to light in the leafy season and had less diameter growth tended to shift from hermaphrodite to a single sex. Therefore, sex change in E. japonica seemed to be explained by a response to the internal physiological condition of an individual mediated by intrinsic and abiotic environmental factors.

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.

The evolution of dioecy, heterodichogamy, and labile sex expression in Acer

The northern hemisphere tree genus Acer comprises 124 species, most of them monoecious, but 13 dioecious. The monoecious species flower dichogamously, duodichogamously (male, female, male), or in some species heterodichogamously (two morphs that each produce male and female flowers but at reciprocal times). Dioecious species cannot engage in these temporal strategies. Using a phylogeny for 66 species and subspecies obtained from 6600 nucleotides of chloroplast introns, spacers, and a protein-coding gene, we address the hypothesis (Pannell and Verdú, Evolution 60: 660-673. 2006) that dioecy evolved from heterodichogamy. This hypothesis was based on phylogenetic analyses (Gleiser and Verdú, New Phytol. 165: 633-640. 2005) that included 29-39 species of Acer coded for five sexual strategies (duodichogamous monoecy, heterodichogamous androdioecy, heterodichogamous trioecy, dichogamous subdioecy, and dioecy) treated as ordered states or as a single continuous variable. When reviewing the basis for these scorings, we found errors that together with the small taxon sample, cast doubt on the earlier inferences. Based on published studies, we coded 56 species of Acer for four sexual strategies, dioecy, monoecy with dichogamous or duodichogamous flowering, monoecy with heterodichogamous flowering, or labile sex expression, in which individuals reverse their sex allocation depending on environment-phenotype interactions. Using Bayesian character mapping, we infer an average of 15 transformations, a third of them involving changes from monoecy-cum-duodichogamy to dioecy; less frequent were changes from this strategy to heterodichogamy; dioecy rarely reverts to other sexual systems. Contra the earlier inferences, we found no switches between heterodichogamy and dioecy. Unexpectedly, most of the species with labile sex expression are grouped together, suggesting that phenotypic plasticity in Acer may be a heritable sexual strategy. Because of the complex flowering phenologies, however, a concern remains that monoecy in Acer might not always be distinguishable from labile sex expression, which needs to be addressed by long-term monitoring of monoecious trees. The 13 dioecious species occur in phylogenetically disparate clades that date back to the Late Eocene and Oligocene, judging from a fossil-calibrated relaxed molecular clock.

Repeated evolution of dioecy from androdioecy in Acer

The evolution of breeding systems was studied in the genus Acer, with special attention to the origin of androdioecy and dioecy, using a phylogenetic approach. Parsimony and maximum-likelihood techniques were used to infer the ancestral character state and trends in the evolution of breeding systems. Information on breeding systems was obtained from the literature, and phylogenetic relationships were taken from three published phylogenies. Although a general trend from duodichogamy to dioecy through heterodichogamy has been proposed for the genus Acer, our results show that a general trend is not detected when phylogenetic relationships are taken into account. Dioecy appeared as a derived state that evolved at least three times and never reversed towards other states. Three different paths to dioecy have been followed in the genus Acer: from heterodichogamous androdioecy; from heterodichogamous trioecy; and from dichogamous subdioecy. Therefore, although the best documented cases of evolution of androdioecy indicate that this breeding system evolves from dioecy, in the genus Acer the opposite situation occurs (androdioecy leading to dioecy). Here we discuss the role of inbreeding avoidance and sexual specialization as selective forces driving the evolution of dioecy in the genus Acer.

The costs of reproduction in plants

This review reports on the processes associated with costs of reproduction, including some theoretical considerations, definitions and methodological aspects, followed by a list of the situations where costs are difficult to find. Despite some exceptions, case studies, examined by trade-offs between reproduction and other life-history traits, generally support the predictions of the cost of reproduction hypothesis. The cost of reproduction as an evolutionary determinant of sexual dimorphism in life history traits in dioecious species was specifically tested, considering that the higher cost of reproduction in females has driven the life history traits related to sexual dimorphism. Females of woody dioecious species were consistently smaller than males supporting the costs of reproduction hypothesis. By contrast, females of herbaceous perennials were generally the larger sex, which did not fit the expectations of the hypothesis. Finally, the mechanisms that enable the compensation of the reproductive costs are detailed, including the plastic responses of photosynthesis and growth, the effects of the timing of investment, plant architecture and plant physiological integration. Contents Summary 321 I. Introduction 321 II. Theory on costs of reproduction 322 III. Methodological aspects 324 IV. Empirical evidence 328 V. Plant size and costs of reproduction 330 VI. Costs of reproduction in sexually dimorphic plants 331 VII. Compensation of the costs 333 VIII. Concluding comments and future perspectives 336 Acknowledgements 337 References 337.