Natural selection operates on the mating strategies of hermaphrodites through their functional gender, i.e. their relative success as male versus female parents. Because functional gender will tend to be strongly influenced by sex allocation, it is often estimated in plants by counting seeds and pollen grains. However, a plant's functional gender must also depend on the fate of the seeds and pollen grains it produces. We provide clear evidence of a paternal effect on the functional gender of a plant that is independent of the resources invested in pollen. In the Mediterranean tree Fraxinus ornus, males coexist with hermaphrodites that disperse viable pollen and that sire seeds; the population would thus appear to be functionally androdioecious. However, we found that seedlings sired by hermaphrodites grew significantly less well than those sired by males, suggesting that hermaphrodites may be functionally less male than they seem. The observed 1 : 1 sex ratios in F. ornus, which have hitherto been difficult to explain in the light of the seed-siring ability of hermaphrodites, support our interpretation that this species is cryptically dioecious. Our results underscore the importance of considering progeny quality when estimating gender, and caution against inferring androdioecy on the basis of a siring ability of hermaphrodites alone.
Background and Aims
Plants exhibit a variety of reproductive systems where unisexual (females or males) morphs coexist with hermaphrodites. The maintenance of dimorphic and polymorphic reproductive systems may be problematic. For example, to coexist with hermaphrodites the females of gynodioecious species have to compensate for the lack of male function. In our study species, Geranium sylvaticum, a perennial gynodioecious herb, the relative seed fitness advantage of females varies significantly between years within populations as well as among populations. Differences in reproductive investment between females and hermaphrodites may lead to differences in future survival, growth and reproductive success, i.e. to differential costs of reproduction. Since females of this species produce more seeds, higher costs of reproduction in females than in hermaphrodites were expected. Due to the higher costs of reproduction, the yearly variation in reproductive output of females might be more pronounced than that of hermaphrodites.
Using supplemental hand-pollination of females and hermaphrodites of G. sylvaticum we examined if increased reproductive output leads to differential costs of reproduction in terms of survival, probability of flowering, and seed production in the following year.
Experimentally increased reproductive output had differential effects on the reproduction of females and hermaphrodites. In hermaphrodites, the probability of flowering decreased significantly in the following year, whereas in females the costs were expressed in terms of decreased future seed production.
When combining the probability of flowering and seed production per plant to estimate the multiplicative change in fitness, female plants showed a 56 % and hermaphrodites showed a 39 % decrease in fitness due to experimentally increased reproduction. Therefore, in total, female plants seem to be more sensitive to the cost of reproduction in terms of seed fitness than hermaphrodites.
Gynodioecy; pollen limitation; hand-pollination; Geranium sylvaticum; cost of reproduction; maintenance of gynodioecy; seed production
Background and aims
Sexually dimorphic populations are often located in drier habitats than cosexual populations. Gender plasticity (GP), whereby hermaphrodites alter female and male functions depending on resources, and sex-differential plasticity (SDP) between hermaphrodites and unisexuals are predicted to affect sexual system stability. Here, GP and SDP are evaluated in cosexual and gynodioecious Wurmbea biglandulosa and sub-dioecious and dioecious W. dioica.
GP was evaluated under two resource conditions, compared among sexual systems and assessed as to whether (1) males produced perfect flowers and (2) hermaphrodites altered investment in perfect (female function) and total (male function) flowers. SDP was assessed within sexual systems as differences between sex functions of hermaphrodites vs. unisexuals. Males and hermaphrodites were compared to assess whether size thresholds for female function differed among sexual systems. Plasticity costs were evaluated using correlations between female function and male traits in hermaphrodites, and in W. dioica by comparing hermaphrodite and male regressions between plant size and pollen production.
In dioecious W. dioica no males exhibited GP, whereas 100 % did in gynodioecious and cosexual W. biglandulosa. In sub-dioecious W. dioica, resources affected GP (high, 66 %; low, 42 %). Hermaphrodites in all sexual systems reduced perfect but not total flowers under low resources. Unisexuals were unaffected, demonstrating SDP for female function only. Thresholds for female function were greater in sub-dioecious W. dioica than in W. biglandulosa. Plasticity costs were detected only in sub-dioecious W. dioica.
SDP for female function could assist female establishment in cosexual populations and maintain females in gynodioecious and sub-dioecious populations. Although the absence of male SDP should stabilize sub-dioecy, plasticity costs would render sub-dioecy unstable, favouring canalized males over hermaphrodites. This study highlights the importance of interactions between environmental conditions and hermaphrodite sex expression for the stability of dimorphic sexual systems.
Dioecy; gender plasticity; gynodioecy; plant sexual systems; sex-allocation plasticity; sex-differential plasticity; sub-dioecy; Wurmbea biglandulosa; Wurmbea dioica
Background and Aims
Dimorphism among floral traits can evolve through variation in selection intensity between female and male performance, especially when sex functions are separated between flowers on a plant (monoecy), or between individuals (dioecy). In animal-pollinated species, male floral traits are predicted to be larger because competition for pollinators should favour larger displays. Floral dimorphism may be greater in dioecious than monoecious populations because of trade-offs between female and male function and opportunities for selfing in hermaphrodites.
These predictions were tested by surveying flower size, total flowers per inflorescence and daily display size in the insect-pollinated Sagittaria latifolia (Alismataceae). This species is useful for comparative analysis because populations are mostly either monoecious or dioecious. We examined floral dimorphism in 13 monoecious and 16 dioecious populations in eastern North America.
Male flowers were significantly larger than female flowers in monoecious and dioecious populations, but there was no evidence for greater flower size dimorphism in dioecious populations despite their larger flower sizes overall. Although inflorescences in both dioecious and monoecious populations produced more male flowers, daily floral displays were significantly larger for female than male function due to more synchronous female flower opening. Daily floral display dimorphism was significantly greater in dioecious populations, due to greater female daily floral displays. There was a positive relationship between mean flower size and total flowers per inflorescence for both sexes in dioecious populations, but no relationship for either sex function in monoecious populations. Flower size dimorphism was positively correlated with the frequencies of females in dioecious populations.
The increased size and number of male flowers and protracted male floral displays in S. latifolia are probably shaped by sexual selection for more effective pollen dispersal.
Sexual dimorphism; flower size; daily floral display; sexual selection; sex ratios; monoecy; dioecy; Sagittaria latifolia
Variation in the timing of reproductive functions in dioecious organisms may result in adaptive changes in the direction of sexual dimorphism during the breeding season. For plants in which both pollen and seeds are wind-dispersed, it may be advantageous for male plants to be taller when pollen is dispersed and female plants to be taller when seeds are dispersed. We examined the dynamics of height dimorphism in Rumex hastatulus, an annual, wind-pollinated, dioecious plant from the southern USA. A field survey of seven populations indicated that females were significantly taller than males at seed maturity. However, a glasshouse experiment revealed a more complex pattern of height growth during the life cycle. No dimorphism was evident prior to reproduction for six of seven populations, but at flowering, males were significantly taller than females in all populations. This pattern was reversed at reproductive maturity, consistent with field observations. Males flowered later than females and the degree of height dimorphism was greater in populations with a later onset of male flowering. We discuss the potential adaptive significance of temporal changes in height dimorphism for pollen and seed dispersal, and how this may be optimized for the contrasting reproductive functions of the sexes.
dispersal; phenology; plant height; sexual dimorphism; wind pollination
• Background and Aims In dioecious species male and female plants experience different selective pressures and often incur different reproductive costs. An increase in reproductive investment habitually results in a reduction of the resources available to other demands, such as vegetative growth. Tree-ring growth is an integrative measure that tracks vegetative investment through the plant's entire life span. This allows the study of gender-specific vegetative allocation strategies in dioecious tree species thoughout their life stages.
• Methods Standard dendrochronological procedures were used to measure tree-ring width. Analyses of time-series were made by means of General Mixed Models with correction of autocorrelated values by the use of an autoregressive covariance structure of order one. Bootstrapped correlation functions were used to study the relationship between climate and tree-ring width.
• Key Results Male and female trees invest a similar amount of resources to ring growth during the early life stages of Juniperus thurifera. However, after reaching sexual maturity, tree-ring growth is reduced for both sexes. Furthermore, females experience a significantly stronger reduction in growth than males, which indicates a lower vegetative allocation in females. In addition, growth was positively correlated with precipitation from the current winter and spring in male trees but only to current spring precipitation in females.
• Conclusions Once sexual maturity is achieved, tree rings grow proportionally more in males than in females. Differences in tree-ring growth between the genders could be a strategy to respond to different reproductive demands. Therefore, and responding to the questions of when, how and how much asked in the title, it is shown that male trees invest more resources to growth than female trees only after reaching sexual maturity, and they use these resources in a different temporal way.
Dendrochronology; sexual maturity; resource allocation; trade-offs; Juniperus
Many co-sexual plants segregate female and male function among flowers on an inflorescence through dichogamy or the production of unisexual flowers. Sexual segregation may reduce self-pollination among flowers within inflorescences (geitonogamy), thereby increasing the pollen available for export to other plants. To assess these complementary roles we manipulated the simultaneously hermaphroditic (adichogamous) flowers of Eichhornia paniculata to produce ten-flowered inflorescences with either female above male flowers (female/male inflorescences) or male/female inflorescences, which competed for mating opportunities with five-flowered adichogamous inflorescences. Because of the upward movement of bumble-bees, selfing increased upward in adichogamous inflorescences (overall female selfing rate s+/-s.e.=0.320+/-0.026). Female flowers of male/female inflorescences selfed less than flowers in corresponding positions in adichogamous inflorescences so s fell to 0.135+/-0.027. In contrast, all-female flowers of female/male inflorescences selfed similarly to upper flowers on adichogamous inflorescences, elevating s (0.437+/-0.043). During 1997, male/female inflorescences sired more outcrossed seeds than female/male or adichogamous inflorescences, whereas during 1994 flowers on male/female inflorescences received fewer visits than those of adichogamous inflorescences, reducing their outcross siring success. Hence, sexual segregation limits geitonogamy and enhances outcross siring success when it does not affect pollinator behaviour, illustrating the importance of both female and male function in inflorescence design.
The flowers of most angiosperm species are hermaphroditic. Spatial separation of male and female organs within a flower (hercogamy) is a common character traditionally interpreted as an adaptation to reduce intrafloral self-fertilization, one potential cost of hermaphroditism. Another possible cost that may lead to selection for hercogamy is physical interference between male and female floral functions. Here, I present evidence demonstrating the role of a floral character in reducing female interference with male function. The bi-lobed stigma of the bush monkeyflower closes after receiving pollen, causing increased spatial separation of the anthers and stigma ('movement' hercogamy). Experimental manipulations show that flowers with closed stigmas export more than twice as much pollen to other flowers as those in which the stigma is prevented from closing. However, stigma closure only minimally reduces the potential for intrafloral self-pollination. This study provides the first experimental evidence that selection to reduce intrafloral male female interference can be a strong selective force and can drive the evolution of floral characters usually interpreted as mechanisms to reduce self-fertilization.
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.
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.
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.
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.
Life history trade-offs; competition; wind pollination; separate sexes; sex allocation; sexual size dimorphism; reproductive allocation; resource availability; Mercurialis annua
Background and Aims
Monoecious plants have the capacity to allocate resources separately to male and female functions more easily than hermaphrodites. This can be advantageous against environmental stresses such as leaf herbivory. However, studies showing effects of herbivory on male and female functions and on the interaction with the plant's pollinators are limited, particularly in tropical plants. Here, the effects of experimental defoliation were examined in the monoecious shrub Croton suberosus (Euphorbiaceae), a wasp-pollinated species from a Mexican tropical dry forest.
Three defoliation treatments were applied: 0 % (control), 25 % (low) or 75 % (high) of plant leaf area removed. Vegetative (production of new leaves) and reproductive (pistillate and staminate flower production, pollen viability, nectar production, fruit set, and seed set) performance variables, and the abundance and activity of floral visitors were examined.
Defoliated plants overcompensated for tissue loss by producing more new leaves than control plants. Production of staminate flowers gradually decreased with increasing defoliation and the floral sex ratio (staminate : pistillate flowers) was drastically reduced in high-defoliation plants. In contrast, female reproductive performance (pistillate flower production, fruit set and seed set) and pollinator visitation and abundance were not impacted by defoliation.
The asymmetrical effects of defoliation on male and female traits of C. suberosus may be due to the temporal and spatial flexibility in the allocation of resources deployed by monoecious plants. We posit that this helps to maintain the plant's pollination success in the face of leaf herbivory stress.
Euphorbiaceae; floral sex ratio; foliar herbivory; leaf production; nectar production; Neotropical dry forest; plant–insect interactions; pollen production; pollination success; pollinator activity
Background and Aims
Why are sterile anthers and carpels retained in some flowering plants, given their likely costs? To address this question, a cryptically dioecious species, Petasites tricholobus, in which male and female plants each have two floret types that appear pistillate and hermaphroditic, was studied. The aim was to understand the function of sterile hermaphroditic florets in females. In addition, the first examination of functions of sterile female structures in male plants was conducted in the hermaphroditic florets on males of this species. These female structures are exceptionally large in this species despite being sterile.
Differences in floret morphology between the sex morphs were documented and the possible functions of sterile sex organs investigated using manipulative experiments. Tests were carried out to find out if sterile female structures in male florets attract pollinators and if they aid in pollen dispersal, also to find out if the presence and quantity of sterile hermaphroditic florets in females increase pollinator attraction and reproductive success. To investigate what floret types provide nectar, all types of florets were examined under a scanning electron microscope to search for nectaries.
The sterile female structures in male florets did not increase pollinator visits but were essential to secondary pollen presentation, which significantly enhanced pollen dispersal. Sterile pistillate florets on male plants did not contribute to floral display and disappeared in nearly half of the male plants. The sterile hermaphroditic florets on female plants attracted pollinators by producing nectar and enhanced seed production.
The presence of female structures in male florets and hermaphroditic florets on female plants is adaptive despite being sterile, and may be evolutionarily stable. However, the pistillate florets on male plants appear non-adaptive and are presumably in decline. Differential fates of the sterile sex organs in the species are determined by both the historical constraints and the ecological functions.
Cryptic dioecy; sterile sex organ; secondary pollen presentation; pollinator attraction; breeding system evolution; ecological function; Petasites tricholobus
In many gynodioecious species the nuclear inheritance of male fertility is complex and involves multiple (restorer) genes. In addition to restoring plants from the female (male sterile) to the hermaphrodite (male fertile) state, these genes are also thought to play a role in the determination of the quantity of pollen produced by hermaphrodites. The more restorer alleles a hermaphroditic plant possesses, the higher the pollen production. To test this hypothesis I combined the results of crossing studies of the genetics of male sterility with phenotypic data on investment in stamens and ovules among the progeny of plants involved in these studies. The sex ratio (i.e. the frequency of hermaphrodites among the progeny), being a measure of the number of restorer alleles of the maternal plant, was positively related to the investment in pollen (male function), but negatively related to the investment in ovules (female function), in both field and greenhouse experiments. Consequently, a negative correlation between male and female function was observed (trade-off) and it is suggested that antagonistic pleiotropic effects of restorer genes might be the cause. Phenotypic gender, a measure combining investment in both pollen and ovules, was highly repeatable between field and greenhouse, indicating genetic determination of a more male- or female-biased allocation pattern among the studied plants.
Background and aims
Dioecious plants often show sex-specific differences in growth and biomass allocation. These differences have been explained as a consequence of the different reproductive functions performed by the sexes. Empirical evidence strongly supports a greater reproductive investment in females. Sex differences in allocation may determine the performance of each sex in different habitats and therefore might explain the spatial segregation of the sexes described in many dimorphic plants. Here, an investigation was made of the sexual dimorphism in seasonal patterns of biomass allocation in the subdioecious perennial herb Honckenya peploides, a species that grows in embryo dunes (i.e. the youngest coastal dune formation) and displays spatial segregation of the sexes at the studied site. The water content in the soil of the male- and female-plant habitats at different times throughout the season was also examined.
The seasonal patterns of soil-water availability and biomass allocation were compared in two consecutive years in male and female H. peploides plants by collecting soil and plant samples in natural populations. Vertical profiles of below-ground biomass and water content were studied by sampling soil in male- and female-plant habitats at different soil depths.
The sexes of H. peploides differed in their seasonal patterns of biomass allocation to reproduction. Males invested twice as much in reproduction than females early in the season, but sexual differences became reversed as the season progressed. No differences were found in above-ground biomass between the sexes, but the allocation of biomass to below-ground structures varied differently in depth for males and females, with females usually having greater below-ground biomass than males. In addition, male and female plants of H. peploides had different water-content profiles in the soil where they were growing and, when differences existed (usually in the upper layers of the soil), the water content of the soil was higher for the female plants had than for the male plants.
Sex-differential timing of investment in reproduction and differential availability and use of resources from the soil (particularly water) are factors that probably offset the costs of reproduction in the above-ground growth in males and females of H. peploides. The results suggest that the patterns of spatial segregation of the sexes observed in H. peploides may contribute to maximize each sex's growth and reproduction.
Dioecy; biomass allocation; below-ground structures; reproductive effort; spatial segregation; water availability
Background and Aims
The cost of reproduction in dioecious plants is often female-biased. However, several studies have reported no difference in costs of reproduction between the sexes. In this study, the relative reproductive allocation and costs at the shoot and whole-plant levels were examined in woody dioecious Rhus javanica and R. trichocarpa, in order to examine differences between types of phenophase (i.e. physiological stage of development).
Male and female Rhus javanica and R. trichocarpa were sampled and the reproductive and vegetative allocation of the shoot were estimated by harvesting reproductive current-year shoots during flowering and fruiting. Measurements were made of the number of reproductive and total current-year shoots per whole plant, and of the basal area increment (BAI). The numbers of reproductive and total current-year shoots per 1-year-old shoot were counted in order to examine the costs in the following year at the shoot level.
A female-biased annual reproductive allocation was found; however, the ratio of reproductive current-year shoots per tree and the BAI did not differ between sexes in Rhus javanica and R. trichocarpa. The percentage of 1-year-old shoots with at least one reproductive current-year shoot was significantly male-biased in R. trichocarpa, but not in R. javanica, indicating that there was a relative cost at the shoot level only in R. trichocarpa. The female-biased leaf mass per shoot, an indicator of compensation for costs, was only found in R. javanica.
Relative reproductive costs at the shoot level were detected in Rhus trichocarpa, which has simultaneous leafing and flowering, but not in R. javanica, which has leafing followed by flowering. However, the costs for the whole-plant level were diminished in both species. The results suggest that the phenophase type may produce the different costs for R. javanica and R. trichocarpa through the development of a compensation mechanism.
Modularity; phenology; reproductive allocation; reproductive cost; Rhus javanica; Rhus trichocarpa
Females tend to be smaller than males in woody dioecious plant species, but they tend to be larger in herbs. The smaller size of females in woody species has been attributed to higher reproductive costs, yet no satisfactory explanation has been provided for their larger size in herbs. Because herbs have higher nitrogen concentrations in their tissues than woody plants, and because pollen is particularly rich in nitrogen, we predicted that male growth would be more compromised by reproduction than female growth. To test this hypothesis, we conducted three experiments on the annual dioecious herb Mercurialis annua. First, we compared the timing of reproduction between males and females and found that males started flowering earlier than females; early flowering is expected to compromise growth more than later flowering. Second, we compared plants allowed to flower with those prevented from flowering by experimental debudding and found that males incurred a higher reproductive cost than females in terms of both biomass and, particularly, nitrogen. Third, we grew plants under varying levels of nitrogen availability and found that although sexual size dimorphism was unaffected by nitrogen, females, but not males, decreased their relative allocation to both roots and reproduction under high nitrogen availability. We propose that males deal with the high cost of pollen production in terms of nitrogen by allocating biomass to nitrogen-harvesting roots, whereas females pay for carbon-rich seeds and fruits by investing in photosynthetic organs. Sexual dimorphism would thus seem to be the outcome of allocation to above- versus below-ground sinks that supply resources (carbon versus nitrogen) limiting the female and male reproduction differentially.
life history; Mercurialis annua; trade-off; nitrogen allocation; carbon allocation
Background and Aims
Heterodichogamy (a dimorphic breeding system comprising protandrous and protogynous individuals) is a potential starting point in the evolution of dioecy from hermaphroditism. In the genus Acer, previous work suggests that dioecy evolved from heterodichogamy through an initial spread of unisexual males. Here, the question is asked as to whether the different morphs in Acer opalus, a species in which males co-exist with heterodichogamous hermaphrodites, differ in various components of male in fitness.
Several components of male fertility were analysed. Pollination rates in the male phase were recorded across one flowering period. Pollen viability was compared among morphs through hand pollinations both with pollen from a single sexual morph and also simulating a situation of pollen competition; in the latter experiment, paternity was assessed with microsatellite markers. It was also determined whether effects of genetic relatedness between pollen donors and recipients could influence the siring success. Finally, paternal effects occurring beyond the fertilization process were tested for by measuring the height reached by seedlings with different sires over three consecutive growing seasons.
The males and protandrous morphs had higher pollination rates than the protogynous morph, and the seedlings they sired grew taller. No differences in male fertility were found between males and protandrous individuals. Departures from random mating due to effects of genetic relatedness among sires and pollen recipients were also ruled out.
Males and protandrous individuals are probably better sires than protogynous individuals, as shown by the higher pollination rates and the differential growth of the seedlings sired by these morphs. In contrast, the fertility of males was not higher than the male fertility of the protandrous morph. While the appearance of males in sexually specialized heterodichogamous populations is possible, even in the absence of a fitness advantage, it is not clear that males can be maintained at an evolutionary equilibrium with two classes of heterodichogamous hermaphrodites.
Acer opalus; heterodichogamy; male fertility; microsatellites; paternal effects; pollen competition; pollination rates; genetic relatedness
Sexual selection theory for separate-sexed animals predicts that the sexes differ in the benefit they can obtain from multiple mating. Conventional sex roles assume that the relationship between the number of mates and the fitness of an individual is steeper in males compared with females. Under these conditions, males are expected to be more eager to mate, whereas females are expected to be choosier. Here we hypothesize that the sex allocation, i.e. the reproductive investment devoted to the male versus female function, can be an important predictor of the mating strategy in simultaneous hermaphrodites. We argue that within-species variation in sex allocation can cause differences in the proportional fitness gain derived through each sex function. Individuals should therefore adjust their mating strategy in a way that is more beneficial to the sex function that is relatively more pronounced. To test this, we experimentally manipulated the sex allocation in a simultaneously hermaphroditic flatworm and investigated whether this affects the mating behaviour. The results demonstrate that individuals with a more male-biased sex allocation (i.e. relatively large testes and small ovaries) are more eager to mate compared with individuals with a more female-biased sex allocation (i.e. relatively small testes and large ovaries). We argue that this pattern is comparable to conventional gender roles in separate-sexed organisms.
gender role; Macrostomum lignano; mating behaviour; ovary size; phenotypic plasticity; testis size
Males of plants with separate sexes are often more prone to attack by herbivores than females. A common explanation for this pattern is that individuals with a greater male function suffer more from herbivory because they grow more quickly, drawing more heavily on resources for growth that might otherwise be allocated to defence. Here, we test this ‘faster-sex’ hypothesis in a species in which males in fact grow more slowly than hermaphrodites, the wind-pollinated annual herb Mercurialis annua. We expected greater herbivory in the faster-growing hermaphrodites. In contrast, we found that males, the slower sex, were significantly more heavily eaten by snails than hermaphrodites. Our results thus reject the faster-sex hypothesis and point to the importance of a trade-off between defence and reproduction rather than growth.
Investment in current reproduction should come at the expense of traits promoting future reproduction, such as immunity and longevity. To date, comparative studies of pace-of-life traits have provided some support for this, with slower paced species having greater immune function. Another means of investment in current reproduction is through secondary sexual characters (SSC). Investment in SSC's is considered costly, both in terms of immunity and longevity, with greater costs being borne by species with more elaborate traits. Yet within species, females prefer more ornate males and those males are typically immunologically superior. Because of this, predictions about the relationship between immunity and SSC's across species are not clear. If traits are costly, brighter species should have reduced immune function, but the opposite is true if SSC's arise from selection for more immunocompetent individuals. My approach was to investigate immune investment in relation to SSC's, pace-of-life and longevity while considering potentially confounding ecological factors. To do so I assessed leukocyte counts from in a novel group, the Psittaciformes. Investment in SSC's best explained investment in immunity: species with brighter plumage had higher leukocyte counts and those with a greater degree of sexual dichromatism had fewer. Ecological variables and pace-of-life models tended to be poor predictors of immune investment. However, shorter incubation periods were associated with lower leukocyte counts supporting the notion that species with a fast pace-of-life invest less in immunity. These results suggest that investment in reproduction in terms of fast pace-of-life and sexual dichromatism results in reduced immunity; however, investment in plumage colour per se does not impose a cost on immunity across species.
We studied several flowering traits, namely, male-female cone phenology, male-female cone production per tree, mating system, sex ratio, air-borne pollen grains and pollen migration, over four successive years in two different natural populations of P. roxburghii from Garhwal Himalaya, India. Assessment of each trait mentioned except pollen dispersion was done by selecting five representative trees randomly in each population. The pollen migration was studied on naturally isolated source trees. The pollen trapping was done in all directions up to 2.5 km. The average reproductive period in P. roxburghii was 36 days with 3–5 days protandry. There were significant year and population effects for male and female cone output and pollen grains production per tree. In mass production year (1999), an average production of pollen cone per tree was estimated as 42.44 ± 8.32 × 103 at lower altitude and 28.1 ± 0.89 × 103 at higher altitude. The controlled pollination results in high level of outcrossing with 90% seed setting. We conclude that the high male-female ratio and tremendous pollen production capacity in P. roxburghii indicate high male competition among trees within populations. The isolation strip of 600 m is considered minimal for the management of seed orchard.
The size-advantage model predicts that hermaphroditic organisms adjust sex allocation depending on their resource status. We investigated the relationship between size and sex allocation in the co-sexual perennial herbs Trillium erectum and Trillium grandiflorum at two sites in southern Ontario, Canada by measuring pollen and ovule production and biomass allocation at flowering and fruiting. In both species, there was a strong relationship between size and gender; larger plants allocated proportionately more biomass to female reproduction and produced fewer pollen grains relative to ovules than smaller plants. Variation in gender was better explained by size than age, although age and size were correlated. While the relationship between size and gender was similar between species, T. erectum allocated proportionately more to female reproduction than T. grandiflorum, independent of size. In the absence of pollen limitation, there was no evidence of secondary adjustment of gender at fruiting. The results are discussed in the context of models predicting size-dependent gender modification in animal-pollinated plants. Evidence about the pollination and seed dispersal biology of Trillium spp. suggests that the relative effects of local mate and resource competition may be important in driving size-dependent sex allocation in these species.
In the free-living rhabditid nematode Caenorhabditis elegans, sperm size is a determinant of sperm competitiveness. Larger sperm crawl faster and physically displace smaller sperm to take fertilization priority, but not without a cost: larger sperm are produced at a slower rate. Here, we investigate the evolution of sperm size in the family Rhabditidae by comparing sperm among 19 species, seven of which are hermaphroditic (self-fertile hermaphrodites and males), the rest being gonochoristic (females and males). We found that sperm size differed significantly with reproductive mode: males of gonochoristic species had significantly larger sperm than did males of the hermaphroditic species. Because males compose 50% of the populations of gonochoristic species but are rare in hermaphroditic species, the risk of male-male sperm competition is greater in gonochoristic species. Larger sperm have thus evolved in species with a greater risk of sperm competition. Our results support recent studies contending that sperm size may increase in response to sperm competition.
Trioecy is an uncommon sexual system in which males, females, and hermaphrodites co-occur as three clearly different gender classes. The evolutionary stability of trioecy is unclear, but would depend on factors such as hermaphroditic sex allocation and rates of outcrossing vs. selfing. Here, trioecious populations of Mercurialis annua are described for the first time. We examined the frequencies of females, males and hermaphrodites across ten natural populations and evaluated the association between the frequency of females and plant densities. Previous studies have shown that selfing rates in this species are density-dependent and are reduced in the presence of males, which produce substantially more pollen than hermaphrodites. Accordingly, we examined the evolutionary stability of trioecy using an experiment in which we (a) indirectly manipulated selfing rates by altering plant densities and the frequency of males in a fully factorial manner across 20 experimental plots and (b) examined the effect of these manipulations on the frequency of the three sex phenotypes in the next generation of plants. In the parental generation, we measured the seed and pollen allocations of hermaphrodites and compared them with allocations by unisexual plants. In natural populations, females occurred at higher frequencies in denser patches, a finding consistent with our expectations. Under our experimental conditions, however, no combination of plant densities and male frequencies was associated with increased frequencies of females. Our results suggest that the factors that regulate female frequencies in trioecious populations of M. annua are independent of those regulating male frequencies (density), and that the stable co-existence of all three sex phenotypes within populations is unlikely.
Male plants of spinach (Spinacea oleracea L.) senesce following flowering. It has been suggested that nutrient drain by male flowers is insufficient to trigger senescence. The partitioning of radiolabelled photosynthate between vegetative and reproductive tissue was compared in male (staminate) versus female (pistillate) plants. After the start of flowering staminate plants senesce 3 weeks earlier than pistillate plants. Soon after the start of flowering, staminate plants allocated several times as much photosynthate to flowering structures as did pistillate plants. The buds of staminate flowers with developing pollen had the greatest draw of photosynthate. When the staminate plants begin to show senescence 68% of fixed C was allocated to the staminate reproductive structures. In the pistillate plants, export to the developing fruits and young flowers remained near 10% until mid-reproductive development, when it increased to 40%, declining to 27% as the plants started to senesce. These differences were also present on a sink-mass corrected basis. Flowers on staminate spinach plants develop faster than pistillate flowers and have a greater draw of photosynthate than do pistillate flowers and fruits, although for a shorter period. Pistillate plants also produce more leaf area within the inflorescence to sustain the developing fruits. The 14C in the staminate flowers declined due to respiration, especially during pollen maturation; no such loss occurred in pistillate reproductive structures. The partitioning to the reproductive structures correlates with the greater production of floral versus vegetative tissue in staminate plants and their more rapid senescence. As at senescence the leaves still had adequate carbohydrate, the resources are clearly phloem-transported compounds other than carbohydrates. The extent of the resource redistribution to reproductive structures and away from the development of new vegetative sinks, starting very early in the reproductive phase, is sufficient to account for the triggering of senescence in the rest of the plant.
Carbohydrates; dioecious; female; flowering; flowers; fruits; male; monocarpic; photosynthate partitioning; pistillate; nitrogen; reproduction; respiration; resource allocation; senescence; sink strength; Spinacea oleracea; spinach; staminate; whole plant
Self-fertile hermaphrodites have evolved independently several times in the genus Caenorhabditis [1, 2]. These XX hermaphrodites make smaller sperm than males [3, 4], which they use to fertilize their own oocytes. Since larger sperm outcompete smaller sperm in nematodes [3–5], it had been assumed that this dimorphism evolved in response to sperm competition. However, we show that it was instead caused by a developmental bias. When we transformed females of the species C. remanei into hermaphrodites , their sperm were significantly smaller than those of males. Because this species never makes hermaphrodites in the wild, this dimorphism cannot be due to selection. Instead, analyses of the related nematode C. elegans suggest that this dimorphism might reflect the development of sperm within the distinct physiological environment of the hermaphrodite gonad. These results reveal a new mechanism for some types of developmental bias — the effects of a novel physical location alter the development of ectopic cells in predictable ways.