The influence of deer browsing on the reproductive biology of Canada yew (Taxus canadensis marsh.)

Summary Browsed Canada yew (Taxus canadensis) populations have a higher proportion of males and a lower proportion of monoecious plants than unbrowsed yew populations. The proportion of monoecious plants increases with time following protection from browsing suggesting that deer browsing causes male-biased sex expression in Canada yew. In contrast, results from comparing browsed and unbrowsed populations, exclosure studies, and browse simulation experiments indicate that strobilus ratios and phenotypic gender of browsed yews may be female-biased. In part, these results correspond to the influence of size on sex expression in Canada yew; small yews tend to be male, but if monoecious, have female-biased strobilus ratios. Large yews are monoecious, but have male-biased strobilus ratios. There is, however, no consistent relationship between size and gender in Canada yew, suggesting that in some circumstances, yews shift allocation to female function in response to browsing.     

EFFECT OF DENSITY ON SECONDARY SEX CHARACTERISTICS AND SEX RATIO IN SILENE ALBA (CARYOPHYLLACEAE)

A field survey of plant and flower sex ratio and secondary sex characteristics was made in Silene alba. Female-biased plant sex ratios were found, as seems typical for the species. Sex ratio distribution correlated with a gradient of soil moisture (with the more moist area having a more female-biased ratio) and with changes in the density of Silene (intermediate and higher density areas having greater female bias). The floral sex ratio was significantly female-biased only at the site that was most female-biased in terms of plant sex ratio. Otherwise the population of flowers was significantly male-biased. Male and female plants harvested from the field differed in secondary sexual characteristics. Males had more flowers and invested proportionately more biomass in leaf, but less in root, stem and reproductive tissue than did females. Although both males and females were larger in terms of total dry weight at the moist site, males produced more flowers at the driest (high density) site. Here the female bias in plant sex ratio was intermediate, but the floral sex ratio was significantly male-biased. A glasshouse experiment was performed in which plants were grown at four densities. Density significantly influenced plant survivorship and the probability of flowering, and increased female bias in the pots, but it did not affect patterns of biomass allocation in flowering plants. Patterns of male and female biomass allocation did not differ in the experiment, except in terms of reproductive allocation (greater in females) and allocation to leaf, greater in males, but only at the lowest density. This work urges caution in interpreting differences between males and females in the field as secondary sex characteristics, since we find such properties to be overlapping under experimental conditions. It supports the idea that males and females of a species may sustain different reproductive output under differing conditions.     

Patterns of change in secondary and tertiary sex ratios of the Terebrantian thrips,Frankliniella occidentalis

Adult sex ratios (tertiary sex ratios) of the haplo-diploid Terebrantian thrips species, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) were examined from several native and introduced hosts in an apple (Malus spp.) ecosystem. The ratios were variable and most were female-biased, ranging from 0 to 61% males. Populations in apple bud clusters during early bloom were female-biased (0–10% males). At later bloom states, populations were less female-biased in open blossom clusters (up to 33% males) compared with those in either unopened buds or older petalless blossoms (<11% males). Changes in tertiary sex ratio in all hosts were associated with changes in resource quality, suggesting a possible effect of host quality on sex allocation. In contrast, larval (secondary) sex ratios within apples were slightly male-biased (57%) with little change across apple bud reproductive stages or over time, indicating little to no differential sex allocation. Sex ratios of populations in flight were less female-biased than those found on plants in 62 of 65 comparisons, which suggests that males are more likely to be in flight relative to females. The evidence supports that differential dispersal and distribution of sexes toward various host qualities together with a shorter longevity of adult males determines the pattern of adult sex ratios rather than any significant differential sex allocation.     

Interplant movement by pear psylla (Homoptera: Psyllidae): Effects of sex ratio and reproductive status

Sticky trap catch of pear psylla,Cacopsylla pyricola Foerster, is male biased during the reproductive generations, but not the diapausing generation. In cage studies, we monitored movement by male and female pear psylla between host plants, and tested whether reproductive and diapausing psylla exhibit similar rates of movement. We also experimentally varied sex ratio to determine whether sex ratio affected movement. Male-biased sex ratios prompted increased movement off of the original host by male psylla of the reproductive generations; no such effect was noted for diapausing insects. We interpret these results to indicate that male movements increased under male-biased conditions due to mate-searching activities. There was also evidence in two experiments that severely male-biased sex ratios prompted movement off of the original host plant by reproductive females; this effect may have been due to harassment of ovipositing females by males.     

Resource quality and spatial variation in sex ratios of a free-living solitary sawfly,Dineura virididorsata (Hym., Tenthredinidae)

The sex ratio in final-instar larvae of a birch-feeding, free-living solitary sawfly, Dineura virididorsata, was investigated in Finnish Lapland. The prepupal proportion of females, pooled over ten sites, was 56%, and at four individual sites the sex ratio was significantly female-biased. Larval survival from egg to prepupae did not differ between the sexes. This suggests a femalebiased primary sex ratio in the field. The sex ratio varied among the sites but not among host trees within sites. Contrary to previous results with hymenopterans, we did not find that differences in the sex ratio depended on forage quality: site-specific or tree-specific sex ratios did not correlate with the average prepupal weight. A literature search indicated that female-biased sex ratios are also common in other free-living sawflies. We are unable to explain sex ratios of Dineura virididorsata or other free-living sawflies with existing general models.     

Differences in environmental response between the sexes of the dioecious shrub Baccharis halimifolia (Compositae)

Summary Baccharis halimifolia (Compositae) is a dioecious shrub which grows on the upland fringe of tidal marshes along the Atlantic and Gulf Coasts of North America. We examined the responses of the two sexes to variation in nutrient and moisture availability plant density, and defoliation. By growing plants from seedlings to flowering adults under various combinations of soil type, fertilization rate and plant density, we were able to establish different rates of plant growth and mortality. Plants grown at high density and low nutrient and water supply grew the least, incurrent the most mortality and showed a male-biased sex ratio (73% male). At low density with abundant nutrients and water, plants grew more, survived well, flowered frequently, and were female-biased (75% female). Changes in sex ratio were probably the result of sex-related mortality rather than sexual lability of the seedlings. While changes in sex ratio occurred under experimental conditions in the green-house, no evidence for differences in habitat utilization between the sexes were found in the field and the sex ratio (59% female) did not vary across habitats. In the marsh habitats we sampled where water and nutrients were apparently available, there was no evidence for differential mortality between the sexes. When defoliated (75% of leaf tissue), both sexes showed similar reductions in reproductive effort (number of flower heads/shoot). Our results indicate that differences between the sexes of Baccharis in their response to environmental growing conditions is an important ecological factor associated with the separation of male and female function into separate individuals.     

The Effects of Inflorescence Size and Flower Position on Biomass and Temporal Sex Allocation in <Emphasis Type="Italic">Lobelia sessiliflora</Emphasis>

To test the prediction of sex allocation theory that plants or flowers high in resource status emphasize the female function, we explored the variation in both biomass (the number of pollen grains and ovules) and temporal (male and female durations) sex allocation among and within plants of protandrous Lobelia sessilifolia in relation to plant size and flower position within plants. Among plants, the mean number of pollen grains and ovules per flower of a plant increased with plant size, whereas the mean P/O ratio (number of pollen grains/number of ovules ratio) decreased with plant size. The mean male duration, the mean female duration, and the mean ratio of male duration/flower longevity per flower of a plant were not correlated with plant size. Thus, large plants emphasized female function in terms of biomass sex allocation, which is consistent with the prediction of size-dependent sex allocation theory. The results for temporal sex allocation, however were inconsistent with the theory. Within plants, the mean number of pollen grains and ovules per flower at each position decreased from lower to upper flowers (early to late blooming flowers) and that of the P/O ratio increased from lower to upper flowers. The mean male duration and the mean female duration per flower decreased from lower to upper flowers, whereas the mean ratio of male duration/flower longevity increased from lower to upper flowers. The population sex ratio changed from male-biased to female-biased. Thus, later blooming flowers emphasized the male function in terms of both biomass and temporal sex allocation, consistent with the sex allocation theory, regarding the change in the population sex ratio.     

雌雄异熟植物露蕊乌头开花时间对雌雄功能期及表型性别的影响

开花时间决定了植物雌雄功能的交配机会, 最终影响繁殖成功。交配环境假说认为雌雄异熟植物开花时间的差异能引起植物表型性别的变异, 改变种群内的交配环境, 影响植物对雌雄功能的最佳性分配。为了研究开花时间对雌雄异熟植物的雌雄性别时期及表型性别的影响, 本文以毛茛科雄性先熟植物露蕊乌头(Aconitum gymnandrum)为实验材料, 记录了雄性和雌性功能期, 分析了植株开花时间、花的雌雄功能期和表型性别的关系。结果表明: 在植物同一花序内, 较晚开放的花有更长的雄性期和更短的雌性期, 性分配在时间上偏雄。雌雄功能期在时间上的相对分配随植物开花时间的变化表现出相似的趋势： 较晚开的花或较晚开花的个体, 花的雄性功能期相对于雌性功能期更长, 在时间上更偏向雄性功能。而且, 开花时间的差异影响种群内花的性比和植物个体的表型性别动态。随着开花时间由早到晚的变化, 种群内早期以雄花为主,末期以雌花为主, 种群内性别环境由偏雄向偏雌变化, 因此植株个体的平均表型性别则从偏雌转向偏雄。本文结果支持交配环境假说, 雄性先熟的露蕊乌头开花早期, 种群内花的性别比偏雄, 种群表型性别环境偏雄, 因而植物个体平均表型性别偏雌, 性别分配(即时间分配)偏向雌性功能, 而晚开花个体的平均性别偏雄, 更偏向雄性功能的分配。     

Sex ratios in natural populations ofAphelinus mali (Hym.: Aphelinidae) in relation to host size and host density

In an apple orchard at Armidale, the Northern Tablelands of NSW, population sex ratios ofAphelinus mali (Haldeman), an endoparasitoid of the woolly apple aphid,Eriosoma lanigerum (Hausmann) varied from 0.51 (proportion of males) at low host densities to female-biased at high host densities (proportion of males ranged from 0.35–0.39). This shift in sex ratio seems to be caused by the differences in allocation of sons and daughters to hosts of different sizes. In the fieldA. mali parasitizes all life stages (four nymphal instars and adult) of the woolly aphid upon encountering. According to Hughes'(1979) optimal diet model, such general host acceptance seems to be the best strategy. However, it allows the host nymphs or adults to continue to develop or reproduce until about to mummify (pupate). No mortality was observed when first or second-instar hosts were parasitized in the laboratory. Field collected small mummified hosts yielded male-biased sex ratios whereas large mummified hosts produced mainly females. In the laboratory, progeny from smaller hosts (first to third-instars) produced sex ratios which were not significantly different from 0.5 whereas progeny from larger hosts (third and fourth-instars) produced female-biased sex ratio. During winter (June–August) and early spring (September–October) when the host populations in the orchard were predominantly nymphs, the parasitoid tended to allocate equal resources to male and female offspring. In contrast, at peak population densities in summer and autumn (December–May) when larger hosts were available, the sex ratios were female-biased. The host size ofE. lanigerum andA. mali is, therefore, an important component in the dynamics of host-parasitoid interactions.     

Female common lizards (Lacerta vivipara) do not adjust their sex-biased investment in relation to the adult sex ratio

Sex allocation theory predicts that facultative maternal investment in the rare sex should be favoured by natural selection when breeders experience predictable variation in adult sex ratios (ASRs). We found significant spatial and predictable interannual changes in local ASRs within a natural population of the common lizard where the mean ASR is female-biased, thus validating the key assumptions of adaptive sex ratio models. We tested for facultative maternal investment in the rare sex during and after an experimental perturbation of the ASR by creating populations with female-biased or male-biased ASR. Mothers did not adjust their clutch sex ratio during or after the ASR perturbation, but produced sons with a higher body condition in male-biased populations. However, this differential sex allocation did not result in growth or survival differences in offspring. Our results thus contradict the predictions of adaptive models and challenge the idea that facultative investment in the rare sex might be a mechanism regulating the population sex ratio.     

Female mating behavior in the field cricket,Gryllus pennsylvanicus (Orthoptera: Gryllidae) at different operational sex ratios

The influence of operational sex ratio on the mating behavior of female field crickets,Gryllus pennsylvanicus, was investigated. Females were predicted to be more discriminating under conditions of high mate availability and show less selectivity when males were rare. Such selectivity was indicated in this study with the proportion of courtships leading to a mating changing with sex ratio. Females accepted almost 70% of all courtships at the female-biased sex ratio, but only about half of all courtships were successful at even or male-biased sex ratios. Females moved least at the female-biased sex ratio. There was also a trend for females to be guarded more under male-biased conditions. Female weight did not influence any of the behaviors examined.     

Gender plasticity in Sagittaria sagittifolia (Alismataceae), a monoecious aquatic species

 Aquatic plants are well known for their high degree of phenotypic plasticity in vegetative structures, particularly leaves. Less well understood is the extent to which their sexuality can be modified by environmental conditions. Here we investigate gender plasticity in the European clonal monoecious aquatic Sagittaria sagittifolia (Alismataceae) to determine how floral sex ratios may vary with plant size and inflorescence order. We sampled two populations from aquatic habitats in East Anglia, U.K. and measured a range of plant attributes including ramet size and the number of female and male flowers per inflorescence. The two populations exhibited similar patterns of phenotypic gender, despite contrasting patterns of total allocation to female and male flower number. Plants produced male-biased floral sex ratios but female flower number increased from the first to the second inflorescence whereas male flower number decreased. Size-dependent gender modification occurred in both populations, but the patterns of allocation to female flower production differed between the two populations. Our results are consistent with the view that monoecy is a sexual strategy that enables plants to adjust female and male allocation in response to changing environmental conditions. Received September 16, 2002; accepted October 23, 2002 Published online: March 20, 2003     

Delayed pollination,stigma length,sex expression,and progeny sex ratio in spinach,Spinacea oleracea (Chenopodiaceae)

Delayed outcrossed pollination of female spinach plants (Spinacea oleracea L.) resulted in increased stigma length and a male-biased progeny sex ratio. One group of females was outcrossed 10–14 d after anthesis, a second group was never outcrossed, and a third group, the control, consisted of females that were outcrossed as soon as stigmas appeared. Stigma length was significantly greater for plants in the delayed and never outcrossed groups compared to the control. Furthermore, stigmas of virgin flowers grew until they were either pollinated or the plants produced anthers. Plants that were never outcrossed produced their own anthers and self pollinated. The resulting progeny were all female. The sons of the delayed outcrossed group produced more stamens, on average, than sons of the control group. The observed male-biased sex ratio among the progeny of delayed outcrossed plants could be due to gametic selection. To test for this, plants were held virgin until their stigmas reached a length of at least 3 mm. These stigmas were then pollinated either distally or proximally. No significant difference was found between the progeny sex ratios of these two treatments. However, both sex ratios were more male biased than progeny of plants pollinated the day of anthesis (control group of the first experiment). We conclude that maternal factors, rather than gametophytic selection, may be responsible for the male-biased sex ratio observed in the first experiment.     

Opposing effects of mortality factors on progeny operational sex ratio may thwart adaptive manipulation of primary sex ratio

Despite extensive research on mechanisms generating biases in sex ratios, the capacity of natural enemies to shift or further skew operational sex ratios following sex allocation and parental care remains largely unstudied in natural populations. Male cocoons of the sawfly Neodiprion abietis (Hymenoptera: Diprionidae) are consistently smaller than those of females, with very little overlap, and thus, we were able to use cocoon size to sex cocoons. We studied three consecutive cohorts of N. abietis in six forest stands to detect cocoon volume‐associated biases in the attack of predators, pathogens, and parasitoids and examine how the combined effect of natural enemies shapes the realized operational sex ratio. Neodiprion abietis mortality during the cocoon stage was sex‐biased, being 1.6 times greater for males than females. Greater net mortality in males occurred because male‐biased mortality caused by a pteromalid parasitic wasp and a baculovirus was greater and more skewed than female‐biased mortality caused by ichneumonid parasitic wasps. Variation in the susceptibility of each sex to each family of parasitoids was associated with differences in size and life histories of male and female hosts. A simulation based on the data indicated that shifts in the nature of differential mortality have different effects on the sex ratio and fitness of survivors. Because previous work has indicated that reduced host plant foliage quality induces female‐biased mortality in this species, bottom‐up and top‐down factors acting on populations can affect operational sex ratios in similar or opposite ways. Shifts in ecological conditions therefore have the potential to alter progeny fitness and produce extreme sex ratio skews, even in the absence of unbalanced sex allocation. This would limit the capacity of females to anticipate the operational sex ratio and reliably predict the reproductive success of each gender at sex allocation.     

Variation in restorer genes and primary sexual investment in gynodioecious Plantago coronopus: the trade-off between male and female function

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.     

Behavioral and physiological female responses to male sex ratio bias in a pond-breeding amphibian

Introduction

The phenomenon of sexual conflict has been well documented, and in populations with biased operational sex ratios the consequences for the rarer sex can be severe. Females are typically a limited resource and males often evolve aggressive mating behaviors, which can improve individual fitness for the male while negatively impacting female condition and fitness. In response, females can adjust their behavior to minimize exposure to aggressive mating tactics or minimize the costs of mating harassment. While male-male competition is common in amphibian mating systems, little is known about the consequences or responses of females. The red-spotted newt (Notophthalmus viridescens) is a common pond-breeding amphibian with a complex, well-studied mating system where males aggressively court females. Breeding populations across much of its range have male-biased sex ratios and we predicted that female newts would have behavioral mechanisms to mitigate mating pressure from males. We conducted four experiments examining the costs and behavioral responses of female N. viridescens exposed to a male-biased environment.

Results

In field enclosures, we found that female newts exposed to a male-biased environment during the five-month breeding season ended with lower body condition compared to those in a female-biased environment. Shorter-term exposure to a male-biased environment for five weeks caused a decrease in circulating total leukocyte and lymphocyte abundance in blood, which suggests females experienced physiological stress. In behavioral experiments, we found that females were more agitated in the presence of male chemical cues and females in a male-biased environment spent more time in refuge than those in a female-biased environment.

Conclusions

Our results indicate that male-biased conditions can incur costs to females of decreased condition and potentially increased risk of infection. However, we found that females can also alter their behavior and microhabitat use under a male-biased sex ratio. Consistent with surveys showing reduced detection probabilities for females, our research suggests that females avoid male encounters using edge and substrate habitat. Our work illustrates the integrated suite of impacts that sexual conflict can have on the structure and ecology of a population.     

Sex ratio variation and spatial segregation of the sexes in populations ofRumex acetosa andR. acetosella (Polygonaceae)

Populations ofRumex acetosa andR. acetosella were studied during two growth seasons. The ramet sex ratios ofR. acetosa were always female-biased. InR. acetosella the sex ratios expressed more variation but were mostly female-biased. In both species the sex ratios commonly varied between subpopulations reflecting a partial spatial segregation of the sexes. No marked differences between sexes in vegetative vigour were detected in either species. Interactions between sex ratios, various soil characteristics and population densities were determined. Possible mechanisms for causing biased sex ratios and partial spatial segregation of the sexes are discussed.     

Experimental evolution under varying sex ratio and nutrient availability modulates male mating success in Drosophila melanogaster

Biased population sex ratios can alter optimal male mating strategies, and allocation to reproductive traits depends on nutrient availability. However, there is little information on how nutrition interacts with sex ratio to influence the evolution of pre-copulatory and post-copulatory traits separately. To address this omission, we test how male mating success and reproductive investment evolve under varying sex ratios and adult diet in Drosophila melanogaster, using experimental evolution. We found that sex ratio and nutrient availability interacted to determine male pre-copulatory performance. Males from female-biased populations were slow to mate when they evolved under protein restriction. By contrast, we found direct and non-interacting effects of sex ratio and nutrient availability on post-copulatory success. Males that evolved under protein restriction were relatively poor at suppressing female remating. Males that evolved under equal sex ratios fathered more offspring and were better at supressing female remating, relative to males from male-biased or female-biased populations. These results support the idea that sex ratios and nutrition interact to determine the evolution of pre-copulatory mating traits, but independently influence the evolution of post-copulatory traits.     

Sex allocation and clutch size in the gregarious larval endoparasitoid wasp, Cotesia glomerata

The ability of the gregarious larval endoparasitoid Cotesia glomerata L. (Hymenoptera: Braconidae) to adjust progeny sex ratio and clutch size was investigated. The sex ratios (proportion of males) of field clusters were diverse, but many (70%) were female-biased. Nearly 10% yielded males only, suggesting a low percentage of unmated females in the field. In over half of the clusters containing females, the sex ratio was below 0.3. Superparasitism was common in the field, and females were believed to increase progeny sex ratio when attacking previously-parasitized hosts. However, in a single oviposition bout, sex allocation was not precisely controlled both in the field and laboratory. In the laboratory, the number of eggs laid in a day tended to decrease with increasing female age. For females which were offered two hosts per day and for those offered three hosts per day, this value became nearly the same several days after the start of oviposition. The total number of hosts which a female could parasitize during her lifetime was often less than 40. Some of the old females which attacked more than 40 hosts produced male-biased clutches; this was due to sperm depletion, because sperm remained viable throughout a female's lifetime. The amount of sperm used in a single oviposition bout seemed fixed and was not dependent on the number of eggs laid. Females with much oviposition experience did not produce new eggs to compensate for deposited eggs, and the efficiency of egg use (deposited eggs/total eggs) was more than 80%.     

Sex ratio in populations of Silene otites in relation to vegetation cover,population size and fungal infection

Abstract. In contrast to populations of most dioecious Silene species (which usually are female-biased), populations of Silene otites have been frequently reported to be male-biased. We describe sex ratio variation in 34 natural S. otites populations in Central Germany in relation to vegetation cover, population size and fungal infection. The overall sex ratio was unbiased in 1994 and only slightly male-biased in 1995. Sex ratio varied among the populations from 26.6 % to 72.6 % females. The sex ratio of small populations varied strongly due to stochastic processes. Furthermore, we found that populations in habitats with high vegetation cover contained a higher percentage of females. Hermaphroditic plants, theoretically, could increase male bias as they only produce male or hermaphroditic offspring. Their frequency in the populations, however, was far too low to affect sex ratio. In 1994 12.1 % and in 1995 17.0 % of the plants were infected by the smut fungus Ustilago major. Disease incidence in the population was not related to sex ratio, suggesting equal susceptibility of males and females. The sex ratio of partially infected plants did not deviate from the population sex ratio, both under field conditions and in a greenhouse laboratory experiment. The results suggest that the frequently reported male bias in Silene otites populations is not a general pattern, but is mainly caused by environmental conditions.