The evolution and maintenance of monoecy and dioecy in Sagittaria latifolia (Alismataceae)

Plant species rarely exhibit both monoecious and dioecious sexual systems. This limits opportunities to investigate the consequences of combined versus separate sex function on mating patterns and genetic variation and the analysis of factors responsible for the evolution and maintenance of the two sexual systems. Populations of the North American clonal aquatic Sagittaria latifolia are usually either monoecious or dioecious and often grow in close geographic proximity. We investigated mating patterns, genetic structure, and relationships between the two sexual systems using allozyme variation in populations from southern Ontario, Canada. As predicted, selfing rates in monoecious populations (n = 6, mean = 0.41) were significantly higher than in dioecious populations (n = 6, mean = 0.11). Moreover, marker-based estimates of inbreeding depression (delta) indicated strong selection against inbred offspring in both monoecious (mean delta = 0.83) and dioecious (mean delta = 0.84) populations. However, the difference in selfing rate between the sexual systems was not reflected in contrasting levels of genetic variation. Our surveys of 12 loci in 15 monoecious and 11 dioecious populations revealed no significant differences in the proportion of polymorphic loci (P), number of alleles per locus (A), or observed and expected heterozygosity (H(o) and H(e), respectively). Strong inbreeding depression favoring survival of outcrossed offspring may act to maintain similar levels of diversity between monoecious and dioecious populations. Despite geographical overlap between the two sexual systems in southern Ontario, a dendrogram of genetic relationships indicated two distinct clusters of populations largely corresponding to monoecious and dioecious populations. Reproductive isolation between monoecious and dioecious populations appears to be governed, in part, by observed differences in habitat and life history. We suggest that selfing and inbreeding depression in monoecious populations are important in the transition from monoecy to dioecy and that the maintenance of distinct sexual systems in S. latifolia is governed by interactions between ecology, life history, and mating.     

Sex determination and the evolution of dioecy from monoecy in Sagittaria latifolia (Alismataceae)

The role of mutations of small versus large effect in adaptive evolution is of considerable interest to evolutionary biologists. The major evolutionary pathways for the origin of dioecy in plants (the gynodioecy and monoecy-paradioecy pathways) are often distinguished by the number of mutations involved and the magnitude of their effects. Here, we investigate the genetic and environmental determinants of sex in Sagittaria latifolia, a species with both monoecious and dioecious populations, and evaluate evidence for the evolution of dioecy via gynodioecy or monoecy-paradioecy. We crossed plants of the two sexual systems to generate F1, F2 and backcross progeny, and grew clones from dioecious populations in low-and high-fertilizer conditions to examine sex inconstancy in females and males. Several lines of evidence implicate two-locus control of the sex phenotypes. In dioecious populations sex is determined by Mendelian segregation of alleles, with males heterozygous at both the male- and female-sterility loci. In monoecious populations, plants are homozygous for alleles dominant to male sterility in females and recessive to female sterility in males. Experimental manipulation of resources revealed sex inconstancy in males but not females. These results are consistent with predictions for the evolution of dioecy via gynodioecy, rather than the expected monoecy-paradioecy pathway, given the ancestral monoecious condition.     

Pollinator responses to variation in floral display and flower size in dioecious Sagittaria latifolia (Alismataceae)

In animal-pollinated plants with unisexual flowers, sexual dimorphism in floral traits may be the consequence of pollinator-mediated selection. Experimental investigations of the effects of variation in flower size and floral display on pollinator visitation can provide insights into the evolution of floral dimorphism in dioecious plants. Here, we investigated pollinator responses to experimental arrays of dioecious Sagittaria latifolia in which we manipulated floral display and flower size. We also examined whether there were changes in pollinator visitation with increasing dimorphism in flower size. In S. latifolia, males have larger flowers and smaller floral displays than females. Visitation by pollinators, mainly flies and bees, was more frequent for male than for female inflorescences and increased with increasing flower size, regardless of sex. The number of insect visits per flower decreased with increasing floral display in males but remained constant in females. Greater sexual dimorphism in flower size increased visits to male inflorescences but had no influence on the number of visits to female inflorescences. These results suggest that larger flower sizes would be advantageous to both females and males, and no evidence was found that females suffer from increased flower-size dimorphism. Small daily floral displays may benefit males by allowing extended flowering periods and greater opportunities for effective pollen dispersal.     

Sex in advertising: dioecy alters the net benefits of attractiveness in Sagittaria latifolia (Alismataceae)

The flowers and inflorescences of animal-pollinated dioecious plants are generally small and inconspicuous in comparison with outcrossing cosexual species. The net benefits of an attractive floral display may be different for dioecious compared to cosexual populations because dioecious species experience a more severe reduction in pollen delivery when pollinators forage longer on fewer individuals. Here, we develop a model that predicts the decrease in pollen delivery in dioecious relative to cosexual populations from female-female, female-male and male-male visit sequences as the number of individuals visited varies. To evaluate the predictions of our model we conducted a common garden experiment with dioecious and monoecious (cosexual) arrays of the insect-pollinated herb Sagittaria latifolia. We find that, although increasing the advertisements of floral rewards (i.e. increasing floral display) attracts more pollinators to individuals, the probability that these pollinators subsequently deliver pollen to neighbouring plants depends on sexual system. Because the number of individual plants visited per foraging trip did not increase significantly with floral display, the relative pollination success of dioecious versus monoecious populations decreases with increased floral display. We propose that this could explain why dioecy is strongly correlated with reduced floral display among angiosperm species.     

慈姑属植物的生态及进化生物学研究进展

慈姑属(Sagittaria L.)隶属于泽泻科,是世界广布的水生植物,其生境多样、叶形和繁殖表型复杂,进化地位较特殊,是生态和进化生物学研究较典型的材料。笔者在查阅相关慈姑属研究的大量文献的基础上,对前人研究的物种、探讨的问题及研究结果进行了归纳与总结:慈姑属物种存在广布种和濒危种,且个别物种的濒危现状可能与其生境条件相关;慈姑属的个别种类成为稻田入侵杂草,其竞争能力因慈姑种类及水稻栽培品种的不同而各异;环境对慈姑属植物有很强的塑造作用,不同环境还会造成慈姑所在的水生植物群落结构发生变化;慈姑属植物具有较高的遗传多样性;在繁殖方面,慈姑属植物体现出不同繁殖方式和两性功能的权衡,以及在繁育系统上从雌雄同株到雌雄异株的进化途径。本文还提出了相关研究存在的不足、研究中应注意的科学问题,并对慈姑属植物未来的研究方向提供了新思路。     

Isolation and characterization of 11 microsatellite markers from Sagittaria latifolia (Alismataceae)

We developed 11 microsatellite loci for Sagittaria latifolia, an aquatic plant common to wetlands of North America. From an (AG)-enriched library, we identified 66 unique microsatellite sequences for which primers could be designed. Twenty-two loci reliably amplified a clear single band of expected size, and 11 loci were scoreable and polymorphic. For these 11 loci, we genotyped a monoecious and a dioecious population, yielding four to 14 alleles per locus. Three loci exhibited significant linkage disequilibrium leaving eight independent variable loci. Eight loci also amplified in four other Sagittaria species. These microsatellite loci will be useful to compare genetic structure among monoecious and dioecious populations of S. latifolia.     

嘉陵江下游蛇鮈的两性异形与雌性个体生殖力

为了解蛇鮈雌雄间是否存在显著的外部形态差异及雌性个体生殖力情况, 在繁殖期对嘉陵江下游(合川江段)共76尾蛇鮈样本的两性异形、性比及雌性个体生殖力进行分析.结果表明: 嘉陵江下游蛇鮈繁殖群体的性比接近1∶1,且蛇鮈两性的体型大小相同,但局部特征(如头部和躯干部等)呈现出显著的两性异形,如成体雄性蛇鮈的头部、胸鳍和腹鳍均较雌性蛇鮈大,而躯干部的体宽、体高和躯干长则是雌性蛇鮈大于雄性蛇鮈,这可能是性选择长期作用的结果.主成分分析显示,前3个主成分的累积贡献率达75.2%,但雌雄个体间形态特征相互重叠,无法将两者截然分开;利用判别函数对蛇鮈性别进行回判,综合判别准确率为92.1%.蛇鮈雌性个体绝对生殖力在979～19979粒;且与体长和去内脏体质量均呈显著正相关.同历史资料相比,本研究中嘉陵江蛇鮈的生殖力增大显著,这可能是蛇鮈对种群资源量下降和水环境变化主动适应的结果.     

Chloroplast haplotype variation among monoecious and dioecious populations of Sagittaria latifolia (Alismataceae) in eastern North America

Aquatic plants commonly have extensive geographical distributions, implying few restrictions to dispersal. Here we investigate the postglacial history of an aquatic plant with contrasting sexual systems (monoecy and dioecy), which are predicted to affect dispersal ability. We examined the distribution of cpDNA haplotypes using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) among 76 populations (32 monoecious, 38 dioecious, two mixed and four undetermined populations) of Sagittaria latifolia sampled throughout eastern North America. We also use these data to investigate the polarity of the evolutionary transition between monoecy and dioecy. Using PCR-RFLP, we identified eight cpDNA haplotypes. All haplotypes were found in unglaciated areas of the species' range, clustered primarily in the southeastern United States, providing evidence that glacial refugia probably occurred in this area. Genetic diversity (hT) was more than six times greater among monoecious compared to dioecious populations. All seven of the haplotypes for which the sexual system could be determined were represented among monoecious populations. In contrast, only four haplotypes were detected in dioecious populations and 94% of individuals from dioecious populations possessed a single haplotype. Monoecious populations possessing this widespread haplotype were restricted to the southern portion of the range, indicating that dioecy probably originated in this region and then spread northwards. The distribution of cpDNA haplotypes in dioecious populations represents a subset of the variation found in monoecious populations, a pattern expected if dioecy has evolved from monoecy in S. latifolia.     

Immune suppression and the cost of reproduction in the ground cricket, Allonemobius socius

One of the most common life history trade-offs in animals is the reduction in survivorship with increasing reproductive effort. Despite the prevalence of this pattern, its underlying physiological mechanisms are not well understood. Here we test the hypothesis that immune suppression mediates this phenotypic trade-off by manipulating reproductive effort and measuring immune function and mortality rates in the striped ground cricket, Allonemobius socius. Because A. socius males provide females with a hemolymph-based nuptial gift during copulation, and many structural components of immunity reside in the hemolymph, we also predicted that sexual selection may differentially affect how disease resistance evolves in males and females. We found that an increased mating effort resulted in a reduced immune ability, coupled with an increased rate in age-specific mortality for both sexes. Thus, immune suppression appears to be a link between reproductive effort and cost in this system. In addition, males and females appeared to differentially invest in several aspects of immunity prior to mating, with males exhibiting a higher concentration of circulating hemocytes and a superior bacterial defense capability. This pattern may be the result of previously established positive selection on gift size due to its affect on female fecundity. In short, female choice for larger gifts may lead to a sexually dimorphic immune ability.     

Phenotypic plasticity of hermaphrodite sex allocation promotes the evolution of separate sexes: an experimental test of the sex-differential plasticity hypothesis using Sagittaria latifolia (Alismataceae)

Separate sexes can evolve under nuclear inheritance when unisexuals have more than twice the reproductive fitness of hermaphrodites through one sex function (e.g., when females have more than twice the seed fertility of hermaphrodites). Because separate sexes are thought to evolve most commonly via a gynodioecious intermediate (i.e., populations in which females and hermaphrodites cooccur), the conditions under which females can become established in populations of hermaphrodites are of considerable interest. It has been proposed that resource-poor conditions could promote the establishment of females if hermaphrodites are plastic in their sex allocation and allocate fewer resources to seed production under these conditions. If this occurs, the seed fertility of females could exceed the doubling required for the evolution of unisexuality under low-, but not high-resource conditions (the sex-differential plasticity hypothesis). We tested this hypothesis using replicate experimental arrays of the aquatic herb Sagittaria latifolia grown under two fertilizer treatments. The results supported the sex-differential plasticity hypothesis, with females having more than twice the seed fertility of hermaphrodites under low-, but not high-fertilizer conditions. Our findings are consistent with the idea that separate sexes are more likely to evolve under unfavorable conditions.     

Gender variation of sequential inflorescences in a monoecious plant Sagittaria trifolia (Alismataceae)

In protogynous plants, female flowers of early blooming plants are at a reproductive disadvantage because they cannot set fruit due to the lack of available pollen. To study this phenomenon, gender expression of the monoecious herb Sagittaria trifolia was investigated over the entire flowering season in two field and two cultivated populations in Hubei and Hunan Provinces, China. In racemes of S. trifolia, flowers open sequentially from bottom to top, with female flowers opening first followed by male flowers. This creates a temporal separation of sexes in the species. Under field conditions small plants are often male, with production of both male and female flowers increasing with plant size. Femaleness increased among sequential inflorescences since female flower production increased whereas male flower production did not. Seed production was greater in large inflorescences because they contain more female flowers, and the number of ovules increased in female flowers at basal positions within the raceme. A consistent pattern of high seed set was observed in flowers from both field and cultivated populations. About 1 % of unfertilized ovules resulted from no pollination and 2 % of the seeds produced were only partly developed due to resource limitation. In the first inflorescence of the six experimental populations, 6.7-40.0 % of individuals produced only male flowers, and female flowers of 1.9-6.5 % individuals were aborted. The occurrence of male flowers in early blooming inflorescences could be an adaptive strategy to conserve resources and enhance pollination of female flowers in protogynous S. trifolia.     

Consequences of sexually dimorphic timing of emergence and flowering in Silene latifolia

1 Males and females of dioecious plant species often differ in a variety of secondary characteristics, such as size, flower number and flowering time, suggesting that dioecious species have sex-specific selection histories. However, a potential source of these dimorphic traits is age differences between males and females. By sowing 3000 seeds of the dioecious perennial, Silene latifolia (Caryophyllaceae), on a single day, we were able to assess the contribution of sex differences in emergence time to the development of sexually dimorphic adult traits.
2 Females emerged before males in our experimental field population, but on average males flowered first. Age differences between males and females did not therefore cause the earlier flowering of males.
3 The consequence of the detected differences in emergence and phenology between males and females was explored by path analysis. Emergence time had a strong direct effect on flower production as well as an indirect effect through flowering time. The regression coefficient of flowering time on emergence time was significantly larger for male plants.
4 A phenotypic selection analysis revealed that seedlings emerging early suffered greater mortality than those emerging later. Seedlings emerging early, however, developed into plants with more flowers, indicating that there was a trade-off between survivorship and reproductive performance. Seeds with intermediate emergence times had the highest total fitness, indicating the presence of stabilizing selection. Despite the strong mortality selection against early emergers, we detected no shift in the sex ratio compared with the sex ratio of seeds matured under low-mortality greenhouse conditions.     

蒲河流域河流生境质量综合评价及其与水质响应关系

河流生境是河流生物赖以生存的环境,是维持河流生态完整性、维护河流健康的重要因素。本文结合蒲河流域环境特点,运用层次分析法建立了蒲河流域河流生境质量评价体系,对流域内25个河段的生境质量状况进行了综合评价,并进一步分析了河流生境综合评价指数与河流水质的相关关系。结果表明:(1)蒲河流域25个河段的生境质量状况差异显著,其中1个河段的生境质量等级为好,5个河段为较好等级,16个河段为一般等级,3个河段为较差等级。(2)生境综合评价指数与TP、NH4+-N、CODCr呈负相关,生境质量评价体系中其他指标也与水质指标具有相关性,表明生境质量是影响水质的重要因素。     

Genetic covariances promote climatic adaptation in Australian Drosophila*

Evolutionary potential for adaptation hinges upon the orientation of genetic variation for traits under selection, captured by the additive genetic variance-covariance matrix (G), as well as the evolutionary stability of G. Yet studies that assess both the stability of G and its alignment with selection are extraordinarily rare. We evaluated the stability of G in three Drosophila melanogaster populations that have adapted to local climatic conditions along a latitudinal cline. We estimated population- and sex-specific G matrices for wing size and three climatic stress-resistance traits that diverge adaptively along the cline. To determine how G affects evolutionary potential within these populations, we used simulations to quantify how well G aligns with the direction of trait divergence along the cline (as a proxy for the direction of local selection) and how genetic covariances between traits and sexes influence this alignment. We found that G was stable across the cline, showing no significant divergence overall, or in sex-specific subcomponents, among populations. G also aligned well with the direction of clinal divergence, with genetic covariances strongly elevating evolutionary potential for adaptation to climatic extremes. These results suggest that genetic covariances between both traits and sexes should significantly boost evolutionary responses to environmental change.     

Agonistic interactions with asymmetric body size in two adult-age groups of the annual killifish Millerichthys robustus (Miller & Hubbs, 1974)

In this study, the author evaluated two adult age groups of the Mexican rivulus Millerichthys robustus with body size asymmetries to determine the strategies used by an annual killifish during agonistic interactions of different ontogenetic stages. To achieve this goal, the author first characterized the ethogram of agonistic interactions of M. robustus composed of seven behavioural units in males and five behavioural units in females. The author then analysed agonistic interaction strategies used by males and females with body size asymmetries in two groups of different adult ages that represent different ontogenetic stages: (a) just after sexual maturity was reached, at 5 weeks of age, and (b) near natural death, at 24 weeks of age. The agonistic behaviour patterns of M. robustus were compatible with the logic of mutual assessment. Large males had an advantage during their interactions in both age groups, winning all of the encounters. Nonetheless, there was more aggression in 5-week-old fish encounters. In addition, small 24-week-old fish were more aggressive than small 5-week-old fish. These changing strategies may be because of the cost–benefits required during a fight at each ontogenetic stage. In the female encounters, size did not predict winners, as both small and large fish won a similar number of encounters, and some contests remained unresolved regardless of age group. There was a tendency for small females of any age to risk more than males in fights to maintain reproductive fitness.     

Trade-offs between clonal and sexual reproduction in Sagittaria latifolia (Alismataceae) scale up to affect the fitness of entire clones

? Many plants combine sexual reproduction with vegetative propagation, but how trade-offs between these reproductive modes affect fitness is poorly understood. Although such trade-offs have been demonstrated at the level of individual shoots (ramets), there is little evidence that they scale up to affect genet fitness. For hermaphrodites, reproductive investment is further divided between female and male sexual functions. Female function should generally incur greater carbon costs than male function, which might involve greater nitrogen (N) costs. ? Using a common garden experiment with diclinous, clonal Sagittaria latifolia we manipulated investment in reproduction through female and male sex functions of 412 plants from monoecious and dioecious populations. ? We detected a 1?:?1 trade-off between biomass investment in female function and clonal reproduction. For male function, there was no apparent trade-off between clonal and sexual reproduction in terms of biomass investment. Instead, male function incurred a substantially higher N cost. ? Our results indicate that: trade-offs between investment in clonal propagation and sexual reproduction occur at the genet level in S.?latifolia; and sexual reproduction interferes with clonal expansion, with investment in female function limiting the quantity of clonal propagules produced, and investment in male function limiting the nutrient content of clonal propagules.     

Effects of different nutrient sources on plasticity of reproductive strategies in a monoecious species,Sagittaria graminea (Alismataceae)

Varied nutrient sources can influence the plasticity of reproductive strategies in monoecious species differently. We examined the plasticity of sexual and clonal reproductive components in distinct nutrient sources in a monoecious species, Sagittaria graminea Michx. The results showed that for aboveground reproductive components, in rich-phosphorus and low-nitrogen conditions, the species produced more male flowers, whereas female flowers did not significantly increase in number compared to low-phosphorus and low-nitrogen conditions (control). In rich-nitrogen and low-phosphorus conditions, the species produced more flowers, particularly female, and more seeds, compared to the control conditions. In the rich-nitrogen and rich-phosphorus conditions, plants increased male flowers, female flowers, total flower number, synchronously, as well as seed production. For the belowground reproductive components, in the rich-phosphorus conditions, the plants produced bigger corms than in the rich-nitrogen and control conditions, which further enhanced their competitiveness against companion species. In rich-nitrogen conditions, the plants produced more medium and small corms, and relatively more and longer stolons, which were useful in expansion and invasion of more space. Furthermore, the species displayed trade-off relationships between the aboveground and belowground reproductive components in unbalanced nutrient conditions (add-N or add-P). However, in low nutrient levels (control) or in nutrient abundant (add-N + P) conditions, there were no significant trade-off relationships between the aboveground and belowground reproductive components, based on quantities.     

Gender variation and the evolution of dioecy in Wurmbea dioica (Liliaceae)

A major obstacle for empirical tests of hypotheses concerning the evolution of dioecy in flowering plants is the limited number of species that possess both cosexual and dioecious populations. Wurmbea dioica (Liliaceae) is a diminutive, fly-pollinated geophyte native to temperate Australia. Marked geographical variation of floral traits is evident, particularly with respect to sex expression. A survey of phenotypic gender in 45 populations from Western Australia (WA), South Australia (SA), Victoria (Vic) and the Australian Capital Territory (ACT) revealed two contrasting patterns. Populations in SA, Vic, and ACT were uniformly dimorphic for gender, containing female and male plants, whereas populations in WA were either monomorphic or dimorphic. In most dimorphic populations varying numbers of male plants produced hermaphrodite flowers (male inconstancy). There was a significant negative relationship between female frequency and the proportion of inconstant male plants. Depending on region and population, male plants produced more flowers of larger size than females. In WA monomorphic populations often occurred on rich, moist soils at high density, whereas dimorphic populations were more commonly found at lower density on shallow soils in drier areas. In an area of sympatry, plants with contrasting sexual systems flowered at different times and were ecologically differentiated. The patterns of gender variation in W. dioica indicate that dioecy has evolved via the gynodioecious pathway. The spread of females in monomorphic populations may be favoured where ecological conditions result in increased selfing and inbreeding depression in hermaphrodites.