Biomass Allocation in the Dioecious Tropical Palm Chamaedorea tepejilote and Its Life History Consequences

Abstract The patterns of resource allocation are described for a dioecious tropical palm, Chamaedorea tepejilote. Resource allocation was measured by harvesting fifteen plants of C. tepejilote. The relative allocation of biomass in the stem increased with the size of the plant; that in the leaves decreased and that in the other structures remained roughly constant. Female plants showed a greater total reproductive effort, though male plants produced more inflorescences during the flowering season. Both male and female plants allocated more resources to prop root than to hypogeal roots. The annual productivity of reproductive and vegetative parts of C. tepejilote was estimated using allometric relationships for different plant structures and from demographic data obtained from the field. Annually, female plants allocated significantly more resources to leaves than male plants. Yearly productivity of inflorescences was higher for male plants, while female plants had greater total reproductive productivity (inflorescences and fruits). Correlation analysis showed an increase in reproductive effort with plant size, and an inverse relationship between fecundity and probability of survival, fecundity and residual reproductive value, and reproductive effort and life expectancy; these relationships suggested a cost in reproduction. Additionally, mature plants with different growth rates exhibited differences in fecundity: tall plants (>2.5m height) that grew more than 40 cm in height in four years had lower values of fecundity in comparison to plants of slower growth. These data were discussed in the context of the implications in the life history of a dioecious tropical plant.     

Sexual dimorphism in a dioecious population of the wind-pollinated herb Mercurialis annua: the interactive effects of resource availability and competition

Background and Aims

Male-biased sex allocation commonly occurs in wind-pollinated hermaphroditic plants, and is often positively associated with size, notably in terms of height. Currently, it is not well established whether a corresponding pattern holds for dioecious plants: do males of wind-pollinated species exhibit greater reproductive allocation than females? Here, sexual dimorphism is investigated in terms of life history trade-offs in a dioecious population of the wind-pollinated ruderal herb Mercurialis annua.

Methods

The allocation strategies of males and females grown under different soil nutrient availability and competitive (i.e. no, male or female competitor) regimes were compared.

Key Results

Male reproductive allocation increased disproportionately with biomass, and was greater than that of females when grown in rich soils. Sexual morphs differentially adjusted their reproductive allocation in response to local environmental conditions. In particular, males reduced their reproductive allocation in poor soils, whereas females increased theirs, especially when competing with another female rather than growing alone. Finally, males displayed smaller above-ground vegetative sizes than females, but neither nutrient availability nor competition had a strong independent effect on relative size disparities between the sexes.

Conclusions

Selection appears to favour plasticity in reproductive allocation in dioecious M. annua, thereby maintaining a relatively constant size hierarchy between sexual morphs. In common with other dioecious species, there seems to be little divergence in the niches occupied by males and females of M. annua.     

Floral sex allocation at individual and branch levels in Betula platyphylla var. japonica (Betulaceae), a tall, wind-pollinated monoecious tree species

Floral sex allocation at the individual and first-order branch levels and the relation between these levels were examined in Betula platyphylla var. japonica, a wind-pollinated monoecious tree. Floral sex allocation at the individual level varied with resource availability in a pattern similar to that predicted by the Masaka and Takada model (Journal of Theoretical Biology 240: 114-125). Thus, individual trees with few reproductive resources produced only female or male inflorescences, whereas individuals with many resources rarely had a high male ratio (i.e., number of male inflorescences/total number of inflorescences). Furthermore, the number of male inflorescences tended to reach an upper limit, whereas the number of female inflorescences increased monotonically with increasing reproductive investment. The patterns of floral sex allocation at the first-order branch level were analogous to those at the individual level. Thus, each first-order branch of B. platyphylla var. japonica behaves like an individual, and the floral sex allocation of a given branch does not necessarily represent the individual tree. The effect of the individual-level floral sex ratio on branch-level floral sex allocation indicates that branch behavior is controlled by the individual.     

毛翠雀花花序内的性分配和繁殖成功

两性花植物花序内不同位置的性分配和繁殖成功一般存在差异,通常认为资源竞争、结构效应和交配环境是形成这种差异的主要原因。为了研究雄性和雌性繁殖资源在花序内不同位置间的最优分配问题,该文以青藏高原高寒草甸典型高山植物毛翠雀花为材料,通过比较花序内不同位置的花部特征和种子性状,对其花序内的性分配模式和雌性繁殖成功进行研究,并通过观察传粉者运动特点以及人工去花和补授花粉实验,探讨花序内资源竞争和交配环境对繁殖资源分配的影响。结果表明:(1)不同位置间的雄蕊数、雄蕊鲜重/雌蕊鲜重、花粉数及花粉胚珠比从花序基部到上部显著增加,而雌蕊鲜重和胚珠数逐渐减少,表现出上部花偏雄的性分配;上部花的结籽率显著低于基部花和中部花,不同位置间的发育种子数/果实和发育种子重/果实随着花位置的升高而显著降低,说明基部花具有更佳的雌性繁殖成效。(2)去花处理后,剩余果实的单个种子重/果实显著增加,但发育种子数/果实没有显著增加;而给上部花人工补授异花花粉后,位置间结籽率的差异消失,说明传粉限制而非资源竞争导致了花序内位置依赖的种子生产模式。(3)毛翠雀花雄性先熟的开花特征,以及传粉者苏氏熊蜂从花序基部到上部的定向访花行为,导致了花序内交配环境的变化。综上结果表明,毛翠雀花花序内的性分配和繁殖成功差异是对交配环境适应的结果,对其在高山环境中实现雌雄适合度最优化具有重要意义。     

Sex Allocation Ratio in a Wind-Pollinated Self-Incompatible Monoecious Tree, Alnus firma Sieb. et Zucc. (Betulaceae)

Abstract Biomass sex allocation of Alnus firma , a wind-pollinated self-incompatible monoecious tree species, was directly measured at the inflorescence (prezygotic) stage. Sex allocation ratio at the fruit stage was estimated from the number of female inflorescences, percent fruit production, and the mean dry weight of a female inflorescence and a fruit catkin. Investment in male function far exceeds that in female function at the prezygotic stage. When the costs of fruit catkins and seeds were included in the estimation of female function, then the sex allocation bias was reversed towards female. Intrapopulational variation of the sex allocation ratio with respect to tree height and size was also investigated. A strong negative correlation was observed in both cases, although a positive relationship has been predicted by theorists and some previous empirical data have supported this prediction.     

Laboratory growth rates of six species of freshwater Gymnamoebia

Summary Amaranthus and several other wind-pollinated species of plants are used to test some of the theoretical models of relative reproductive effort towards the male and female sexes. Consistent with these models, in self-compatible, monoecious Amaranthus, Chenopodium, Digitaria, Setaria, and Lepidium, female effort represented over 90% of the total reproductive effort. Also consistent with predictions, Lolium, a self-incompatible wind-pollinated species, was found to have about equal male and female effort. A method is described here that should prove useful in quantifying male and female effort in both wind and insect-pollinated species of plants.     

Size-dependent resource allocation among vegetative propagules and male and female functions in the forest herb Laportea bulbifera

Reproductive resource investment among vegetative propagules and male and female sexual function and their size-dependence were investigated in a perennial forest herb, Laportea bulbifera . A theoretical model based on fitness gain curves predicts that optimal investments in three reproductive modes will increase with plant size if fitness returns in all three modes increase but become saturated with investment. In a field population, large plants of L. bulbifera produced both male and female inflorescences with propagules, while small plants produced only vegetative propagules. Biomass of propagules, male inflorescences, and infructescences with achenes were all positively correlated with plant size. The increase in investment with plant size was larger for propagule production than for sexual reproduction. The relationship between propagule biomass and plant size was constant irrespective of year, while the relationship between the biomass of sexual reproductive organs and plant size differed between two successive years. Annual change of individual sex expression was investigated for 25 transplanted plants. Although each plant changed its sex expression variously among male, female and bisexual from year to year, 23 out of 25 plants produced both male and female inflorescences in at least one year. The number of viable (germinated and survived) offspring from seeds was not significantly different from the number from propagules. The production cost of a propagule was higher than that of a seed. Resource allocation theory does not seem to be applicable to size-dependent resource allocation, especially the allocation between seeds and propagules in this species.     

Size-sex relationship and sexual dimorphism in JapaneseArisaema (Araceae)

The size-sex relationship and sexual differences of sixArisaema species native to Japan were investigated. The size-sex relationship showed almost the same pattern in all species. When the plant was small in size, the sex expression was male, and sex expression changed from male to female as the plant grew larger. Male ratios decreased rapidly around a critical size, but this critical size differed from one species to another. Sexual differences were detected in reproductive structures and behavior, although no difference was detected in vegetative structures. The stoutness, longevity and inner tissue of the scape showed remarkable differences between males and females, and this difference was represented most clearly as the size-weight relationship. Earlier initiation of flowering in males was also observed. No difference was found in resource allocation to reproductive structures between male and female plants at the flowering stage. However, a broad variation in the amount of resource allocation to reproductive structures was found at the fruiting stage in female individuals, which was attributed to differences in the setting rate of mature fruits.     

Influence of inflorescence size on sexual expression and female reproductive success in a monoecious species

Sex allocation theory forecasts that larger plant size may modify the balance in fitness gain in both genders, leading to uneven optimal male and female allocation. This reasoning can be applied to flowers and inflorescences, because the increase in flower or inflorescence size can differentially benefit different gender functions, and thus favour preferential allocation to specific floral structures. We investigated how inflorescence size influenced sexual expression and female reproductive success in the monoecious Tussilago farfara, by measuring patterns of biomass, and N and P allocation. Inflorescences of T.?farfara showed broad variation in sex expression and, according to expectations, allocation to different sexual structures showed an allometric pattern. Unexpectedly, two studied populations had a contrasting pattern of sex allocation with an increase in inflorescence size. In a shaded site, larger inflorescences were female-biased and had disproportionately more allocation to attraction structures; while in an open site, larger inflorescences were male-biased. Female reproductive success was higher in larger, showier inflorescences. Surprisingly, male flowers positively influenced female reproductive success. These allometric patterns were not easily interpretable as a result of pollen limitation when na?vely assuming an unequivocal relationship between structure and function for the inflorescence structures. In this and other Asteraceae, where inflorescences are the pollination unit, both male and female flowers can play a role in pollinator attraction.     

Size-dependent variation of gender in high density stands of the monoecious annual,Ambrosia artemisiifolia (Asteraceae)

Summary We examined the relationship between size variability and the distribution of functional gender in stands of the monoecious, wind-pollinated annual Ambrosia artemisiifolia. Populations of 60 individuals were grown in the greenhouse at a density of 372 m^–2 and at two nutrient levels. Among the surviving plants, after self-thinning, variability in above-ground biomass and gender was higher in the high nutrient treatment. Among individuals there was a significant positive correlation between maleness and both height and biomass. Fecundity was also positively correlated with both measures of size. Based on the pattern of distribution of male and female flowers within the plant, it appears that the increase in maleness in larger plants is due to increased branching and axis elongation. These results demonstrate that competitive interactions, which lead to increased variability in biomass and fecundity, can also generate variability in gender within populations.     

Reproductive consequences of mate quantity versus mate diversity in a wind-pollinated plant

Since most pollen travels limited distances in wind-pollinated plants, both the local quantity and diversity of mates may limit female reproductive success. Yet little evidence exists on their relative contribution, despite the importance of viable seed production to population dynamics.To study how variation in female reproductive success is affected by the quantity versus the diversity of surrounding mates contributing pollen, we integrated pollination experiments, data on natural seed set and seed viability, and AFLP genetic marker data in the wind-pollinated dioecious clonal forest herb Mercurialis perennis.Pollination experiments indicated weak quantitative pollen limitation effects on seed set. Among-population crosses showed reduced seed viability, suggesting outbreeding depression due to genetic divergence. Pollination with pollen from a single source did not negatively affect reproductive success. These findings were consistent with results of the survey of natural female reproductive success. Seed set decreased with the distance to males in a female plants’ local neighborhood, suggesting a shortage of pollen in isolated female plants, and increased with the degree of local genetic diversity. Spatial isolation to other populations and population size did not affect seed set. None of these variables were related to seed viability.We conclude that pollen movement in M. perennis is likely very limited. Both male proximity and the local degree of genetic diversity influenced female reproductive success.     

Sexual and vegetative reproduction in the aboveground part of a dioecious clonal plant, <Emphasis Type="Italic">Dioscorea japonica</Emphasis> (Dioscoreaceae)

In dioecious clonal plants, the reproductive effort required to set seeds will be responsible for the larger investment in sexual reproduction by females. If there will be a trade-off in resource allocation between sexual and clonal reproduction, this differential sexual reproduction will lead to sexual differentiation in the relative amount of clonal reproduction. To test this prediction, we studied differences between the sexes in their phenologies and investments in sexual and vegetative reproduction (clonal reproduction by means of bulbils) with respect to ramet size in a dioecious clonal plant, Dioscorea japonica Thunb. The period of bulbil production overlapped the period during which infructescences developed. Females flowered later, produced heavier inflorescences, and fewer flowers per inflorescence than did males. Regression analysis using the size of the individual plants demonstrated that large females made smaller investments in inflorescences and larger investments in sexual reproduction than did large males. In contrast, females invested fewer resources in vegetative reproduction than did males. However, the total investments in sexual and vegetative reproduction did not differ between the sexes. These results supported our hypothesis on the sexual differentiation in sexual and clonal reproduction.     

三种同域分布喉毛花的繁殖分配

以青藏高原高寒草甸中三种同域分布的喉毛花为研究对象,通过比较三个种的植株性状和繁殖分配,探讨繁殖分配的种间差异及其与植株个体大小的关系。结果表明:(1)三个种的植株高度、顶花大小和单株花数目、繁殖分配均存在种间差异,这可能与其各自的交配系统和具体的生境以及相应的生活史对策有关;(2)在三种喉毛花中,投入到营养器官和繁殖器官的绝对资源量均呈显著正相关,未检测到植株生长和繁殖间的权衡关系;(3)三个种的个体大小与繁殖器官生物量均呈显著正相关,而与繁殖分配均呈显著负相关,这表明个体越大,繁殖投入越高,而繁殖分配越低,与以往研究结果一致,这可能是由于繁殖分配与个体大小之间存在异速关系。     

Female freshwater crayfish adjust egg and clutch size in relation to multiple male traits

Females may invest more in reproduction if they acquire mates of high phenotypic quality, because offspring sired by preferred partners may be fitter than offspring sired by non-preferred ones. In this study, we tested the differential maternal allocation hypothesis in the freshwater crayfish, Austropotamobius italicus, by means of a pairing experiment aimed at evaluating the effects of specific male traits (body size, chelae size and chelae asymmetry) on female primary reproductive effort. Our results showed that females laid larger but fewer eggs for relatively small-sized, large-clawed males, and smaller but more numerous eggs for relatively large-sized, small-clawed males. Chelae asymmetry had no effects on female reproductive investment. While the ultimate consequences of this pattern of female allocation remain unclear, females were nevertheless able to adjust their primary reproductive effort in relation to mate characteristics in a species where inter-male competition and sexual coercion may mask or obscure their sexual preferences. In addition, our results suggest that female allocation may differentially affect male characters, thus promoting a trade-off between the expression of different male traits.     

Flower choice by honey bees (Apis mellifera L.): sex-phase of flowers and preferences among nectar and pollen foragers

Bees foraging for nectar should choose different inflorescences from those foraging for both pollen and nectar, if inflorescences consist of differing proportions of male and female flowers, particularly if the sex phases of the flowers differ in nectar content as well as the occurrence of pollen. This study tested this prediction using worker honey bees (Apis mellifera L.) foraging on inflorescences of Lavandula stoechas. Female flowers contained about twice the volume of nectar of male flowers. As one would predict, bees foraging for nectar only chose inflorescences with disproportionately more female flowers: time spent on the inflorescence was correlated with the number of female flowers, but not with the number of male flowers. Inflorescence size was inversely correlated with the number of female flowers, and could be used as a morphological cue by these bees. Also as predicted, workers foraging for both pollen and nectar chose inflorescences with relatively greater numbers of both male and female flowers: time spent on these inflorescences was correlated with the number of male flowers, but not with the number of females flowers. A morphological cue inversely associated with such inflorescences is the size of the bract display. Choice of flowers within inflorescences was also influenced predictably, but preferences appeared to be based upon corolla size rather than directly on sex phase.     

Patterns of resource allocation in male and female plants of Rumex acetosa and R. acetosella

Summary Patterns of resource allocation in the dioecious Rumex acetosa and R. acetosella were investigated. Males were found to allocate more to reproduction during flower production than females, whereas females invested considerably more in reproduction during seed production. Altogether, females allocated both a higher total amount and a higher proportion of energy to reproduction than did males. By regression analysis, the influence of plant size on reproductive effort was examined separately for males and females. The results indicated that while reproductive effort is sometimes lower for tall plants than for small plants, size-independent effects have a greater influence on reproductive effort than size distribution. An analysis of variance was conducted to investigate the effects of population, season, sex and their interactions on plant size, and an analysis of covariance was used to study differences in resource allocation patterns. Different interaction effects were found to be most important in the two species of Rumex.     

Allocation of reproductive effort in female Arrenurus spp. water mites (Acari: Hydrachnidia; Arrenuridae)

Summary Twenty-seven species of water mites of the genus Arrenurus were compared with respect to allocation of reproductive effort. Clutch volume was positively correlated to female volume, female volume was positively correlated with clutch size and with larval volume, while clutch size and larval volume were negatively correlated. In threespace, corresponding to female volume, clutch size and larval volume, species were arranged along two trajectories representing separate reproductive strategies. The strategy characterized by small larvae, large clutches and large females corresponded to species of the subgenus Arrenurus, known to be larval parasites of adult Odonata. Members of three subgenera, known to be larval parasites of adult Diptera, followed the second strategy of small clutch size and alrge larvae. Of nine species of mites tested for intraspecific relationships, only one significant relationship was found, between female volume and clutch size for A. (Megaluracarus) bartonensis.     

Allometric gender allocation in Ambrosia Artemisiifolia (Asteraceae) has adaptive plasticity

Evidence is reported for size-dependent (allometric) gender allocation in the monoecious, wind-pollinated annual Ambrosia artemissifolia. Consistent with established theory, the pattern of allometry displayed adaptive plasticity, depending on the environmental cause of variation in plant size. Plant size gradients were generated in both field and greenhouse experiments using separate and combined gradients of shading, soil nutrient levels, and neighbor proximity. When plant size constraints involved light limitation from shading (e.g., because of close neighbor proximity), decreasing plant size was generally associated with decreasing maleness and increasing femaleness (based on relative male and female flower production, respectively). This is consistent with the "pollen-dispersal" hypothesis in which the consequences of relatively small plant size (among larger neighbors) imposes less severe limitation for female reproductive success than for male reproductive success (because success as an outcrossing donor of wind-dispersed pollen increases with increasing plant height, especially when neighbors are present). However, when size was constrained by soil nutrient limitation alone (i.e., without shading effects), the results had the converse allometric relationship; i.e., decreasing plant size was generally associated with increasing maleness and decreasing femaleness. This is consistent with the "size-advantage" and "time-limitation" hypotheses in which energetic and time limitations (respectively) associated with relatively small plant size impose a less severe limitation for male reproductive success than for female reproductive success.     

Patterns of resource allocation in the dioecious alpine herb Aciphylla simplicifolia (Apiaceae)

Abstract Patterns of reproductive and vegetative biomass allocation were compared in male and female plants of the alpine herb Aciphylla simplicifolia. Male and female plants had similar vegetative biomass but differed in the pattern of resource allocation. Inflorescences of males and females were similar in weight at the time of flowering, but differed in biomass allocation to some structures within the inflorescences, particularly those associated with ovule production and pollinator attraction (number and size of flowers). At the time of fruit production, female inflorescences were 2.6 times heavier than at flowering with developing fruit six times heavier than flowers. In addition to the increase in biomass allocated to structures associated with the provisioning and dissemination of seed, support structures (main and side stalks) were also heavier. As a result of this additional investment of resources at the time of fruit production, the reproductive effort (RE) of female plants was much higher than that of males: 37% of above ground biomass compared with 21% for males. Differences in RE did not change with plant size; however, allocation to reproduction appeared to be a constant proportion of biomass over nearly all plant sizes sampled. These results show that sex‐specific resource allocation can be a complex of temporal and morphological patterns.     

A comparative test of adaptive hypotheses for sexual size dimorphism in lizards

It is commonly argued that sexual size dimorphism (SSD) in lizards has evolved in response to two primary, nonexclusive processes: (1) sexual selection for large male size, which confers an advantage in intrasexual mate competition (intrasexual selection hypothesis), and (2) natural selection for large female size, which confers a fecundity advantage (fecundity advantage hypothesis). However, outside of several well-studied lizard genera, the empirical support for these hypotheses has not been examined with appropriate phylogenetic control. We conducted a comparative phylogenetic analysis to test these hypotheses using literature data from 497 lizard populations representing 302 species and 18 families. As predicted by the intrasexual selection hypothesis, male aggression and territoriality are correlated with SSD, but evolutionary shifts in these categorical variables each explain less than 2% of the inferred evolutionary change in SSD. We found stronger correlations between SSD and continuous estimates of intrasexual selection such as male to female home range ratio and female home range size. These results are consistent with the criticism that categorical variables may obscure much of the actual variation in intrasexual selection intensity needed to explain patterns in SSD. In accordance with the fecundity advantage hypothesis, SSD is correlated with clutch size, reproductive frequency, and reproductive mode (but not fecundity slope, reduced major axis estimator of fecundity slope, length of reproductive season, or latitude). However, evolutionary shifts in clutch size explain less than 8% of the associated change in SSD, which also varies significantly in the absence of evolutionary shifts in reproductive frequency and mode. A multiple regression model retained territoriality and clutch size as significant predictors of SSD, but only 16% of the variation in SSD is explained using these variables. Intrasexual selection for large male size and fecundity selection for large female size have undoubtedly helped to shape patterns of SSD across lizards, but the comparative data at present provide only weak support for these hypotheses as general explanations for SSD in this group. Future work would benefit from the consideration of alternatives to these traditional evolutionary hypotheses, and the elucidation of proximate mechanisms influencing growth and SSD within populations.