Aerial seed bank dynamics and seedling emergence patterns in two annual Mediterranean Asteraceae

Question: We explored the functional significance of seasonal aerial seed banks in two coexisting, heterocarpic annual Asteraceae with dormant (Chrysanthemum coronarium) and non‐dormant (Anacyclus radiatus) achenes. We hypothesised that the plant achene pool is a significant component of total seed reserves, and that within‐season seedling emergence timing is shaped by achene release patterns. Location: SW Spain. Methods: In an observational study, we established temporal achene release patterns. We also quantified the aerial and soil achene pools throughout the release season, and assessed seedling emergence timing. Sowing experiments were used to explore the influence of achene release dynamics on emergence timing, and to establish achene morph‐specific patterns of between‐year distribution of germination. Results: Achene release extended from late spring to late autumn (Chrysanthemum), or from early autumn to early winter (Anacyclus). Within species, achene morphs differed in release timing. Only in Chrysanthemum, a small achene fraction seemed to persist in the soil, and between‐year germination distribution differed among morphs. In coexisting populations, the Anacyclus plant achene pool was an order of magnitude higher than the soil pool throughout the release season, whereas in Chrysanthemum both pools were of the same magnitude during autumn. Within‐year seedling emergence was significantly staggered beneath parent plants compared with the pattern resulting solely from the germination response in soil, with the exception of Chrysanthemum in one of the two study years. Conclusions: Results suggest that seasonal aerial seed banks are effective within‐season, risk‐reducing traits in ruderal Mediterranean habitats characteristic of the study species.     

A mathematical model for wing dimorphism in maleCardiocondyla ants

Several species ofCardiocondyla ants have dimorphic males: wingless (ergatoid) and winged (alate) males, while otherCardiocondyla species includingC. nuda have only wingless males. We made an evolutionarily stable strategy model for explaning the male polymorphism and the ratios of wingless males in the genusCardiocondyla. Wingless males emerge earlier than winged males in each reproductive season. Females (F ₁) which have emerged before winged males copulate only with wingless males, and females (F ₂) which emerge after winged males copulate with both wingless and winged males. Wingless males have a lower copulation ability (b _n) than winged males (b _w). The reproductive success of females which copulate at the early stage (v ₁) is assumed to be larger than that of females which copulate at the late stage (v ₂). The model predicts that there are 3 different evolutionarily stable states: 2 monomorphic states of wingless and winged males, and a dimorphism of the 2 types of males. In the dimorphic state, the rate of wingless males increases as the survival rate of wingless males (s) increases,v ₁/v ₂ increases,F ₁/F ₂ increases andb _n/b _w increases. For dimorphism to exist,s b _n/b _w<1 must be satisfied, and this condition corresponds to the value of observed data. The value ofv ₁/v ₂ would be difficult to be obtained by actual data, but we can estimate this value with the model.     

Inbreeding effects on fitness traits in the heterocarpic herb Leontodon autumnalis L. (Asteraceae)

Heterocarpic plants are characterized by the production of distinct types of fruits that usually differ in their ecological behavior. In the Asteraceae, differences are mainly found between peripheral non-dispersal and central dispersal achenes (single-seeded fruits). Inbreeding depression is considered as an evolutionary force as it may reduce several fitness traits, and in the case of heterocarpic plants, it could influence fitness traits (e.g., seed set, germination rate, growth rate) of each fruit morph, which may have important ecological and evolutionary consequences. In particular, differential effects on fitness traits and dispersal of selfed and outcrossed progeny can strongly determine the viability of extant populations and the potential to colonize new habitats. We conducted a hand-pollination experiment in greenhouse conditions to test whether inbreeding affects the fitness of achene morphs in the heterocarpic herb Leontodon autumnalis (Asteraceae). Results show that achene morphs significantly differ in their ecological behavior, peripheral achenes germinating more and faster than central achenes. The significant interaction between pollination treatment and achene morph for germination probability might indicate a link between dormancy and mating system in L. autumnalis: germination was higher for outcrossed achenes in central achenes whereas the opposite pattern was exhibited by peripheral achenes. Selfing dramatically reduced seed set, probably as a consequence of strong self-incompatibility mechanisms rather than inbreeding effects. Inbreeding depression significantly affected late life-cycle traits, such as growth rate and biomass at flowering. Overall, results suggest that inbreeding depression seems to be an important selective force maintaining outcrossing in L. autumnalis.     

一年生短命植物疏齿千里光果实异形性的生态学意义

疏齿千里光(Senecio subdentatus)是分布在新疆北部古尔班通古特沙漠中的一种具异形瘦果的菊科一年生短命植物。将野外观测与室内实验相结合, 对该物种异形瘦果的形态、扩散和萌发特性, 以及异形瘦果产量与植株大小的关系进行了研究, 并对其生态学意义进行了探讨。结果表明: 疏齿千里光果序中的外围果和中央果均为柱形, 但前者为淡黄色, 后者为褐色, 且二者在大小、冠毛长度及果皮微形态等方面均存在明显差异。两种瘦果均以单个果实为扩散单元, 且在静止空气中的降落速度和在1 m·s^-1与2 m·s^-1风速下的扩散距离无显著差异, 说明虽然果实大小和冠毛长度对瘦果扩散具有不同的影响, 但对其整体扩散能力无明显影响。在各温变周期(5/2、15/2、20/10、25/15和30/15 ℃)处理中, 淡黄色外围果的萌发率均高于褐色中央果的, 且不同温度间两种瘦果的萌发率均存在显著差异, 但光照条件对其无显著影响。果序中的中央瘦果数明显多于外围果的, 且植株中外围果所占比例与植株大小间呈显著负相关关系, 而中央果所占比例与植株大小间呈显著正相关关系。这些特点说明, 该物种的小植株倾向于产生较多较易萌发的外围果, 大植株倾向于产生较多不易萌发的中央果。在古尔班通古特沙漠不可预测的极端环境中, 疏齿千里光可通过异形瘦果间的萌发差异及调节其不同大小植株中异形瘦果的比例, 来减少同胞后代之间的竞争, 增加其对不同微环境条件的生态适应性。     

Variability and Cryptic Heteromorphism of Ambrosia artemisiifolia Seeds: What Consequences for its Invasion in France?

BACKGROUND AND AIMS: Ambrosia artemisiifolia is a ruderal weed introduced from North America to Europe. It produces large amount of achenes which are highly heterogeneous in size. Due to the preponderant role of propagules in invasive plant processes, the achene mass variability related to germination, dispersal strategy and life history traits of offspring were investigated within this species. METHODS: The variability in achene mass was quantified among six populations sampled in different habitats. The effects of achene mass variation on germination were studied. The percentages of floating and non-floating achenes were evaluated in the studied populations. The consequences of floatability on the growth and traits of the offspring were studied. KEY RESULTS: Mean achene mass ranged from 1.72 to 3.60 mg, depending on the populations, and was highly variable. Variation among achenes within plants accounted for 63.9 % of the variance, whereas variances among plants within each population (22.2 %) and among populations (13.9 %) were lower. Achene masses were also positively correlated to the total germination percentage for four populations out of six. Two kinds of achenes were distinguished: floating and non-floating. The majority of floating achenes (90 %) sank 24 h after water immersion. Whatever the population, floating achenes were lighter, more dormant and germinated faster than non-floating achenes. Plants which issued from floating achenes had better growth than those from non-floating achenes. CONCLUSIONS: The capacity of A. artemisiifolia to be invasive in Europe appears to be high, possibly due to its huge plasticity in seed mass which may help it to cope with a wide range of conditions and to establish in disturbed habitats. Furthermore, the recent invasion of southern France by A. artemisiifolia could be partially explained by water dispersal of achenes through rivers and has pinpointed its colonization potential along French rivers.     

POPULATION DYNAMIC CONSEQUENCES AND EVOLUTION OF SEED TRAITS OF HETEROSPERMA PINNATUM (ASTERACEAE)

We summarize research on variation in achene morphology of Heterosperma pinnatum. Each flowering head has a broad range of achene morphologies. There is no between-year seed bank. There are no ecologically relevant differences in achene size, seedling size, or seedling growth rates of the different morphs. Achenes located centrally in the heads lose dormancy earlier during the period between autumn achene production and the onset (May-July) of the rainy season. This results in relatively more early emergence of central achenes. Awned central achenes also have greater adhesive dispersibility. Emergence, survival, and fecundity were measured for achenes sown into natural populations in central Mexico. When harsh conditions result in few seeds surviving to reproduce, the advantage tends to go to the more “conservative” peripheral achenes. Greenhouse experiments show a strong genetic component to the determination of achene proportions within and among populations and a variety of genetic correlations. Populations with a high proportion of awned achenes tend to be found in vegetation types where they are likely to be ephemeral. A high proportion of central achenes tend to be found in sites with little pre-rainy season precipitation. These patterns are interpreted in terms of selection for dispersal and the within-year timing of germination.     

Morph-specific differences in metabolism related to flight in the wing-dimorphic Aphis gossypii

Abstract The cotton aphid, Aphis gossypii Glover, is a wing-dimorphic species, which causes globally important agricultural losses. In this present study, we compared the biochemical basis of wing polymorphism in A. gossypii with respect to trade-off of energy resources, including glycogen, trehalose, lipids (total lipid, triglyceride and phospholipid), free fatty acids, and soluble protein between dispersal and reproduction morphs during the wing-bud nymph and adulthood. Total lipid, triglyceride and free fatty acids were significantly higher in winged versus wingless morphs at 12 h of adulthood, the period during which alates are able to fly. By contrast, the wingless morph contained more glycogen than the winged morph from the 4th nymphal stage to adulthood. Trehalose content in the wingless morph was also higher than that in the winged morph during the 3rd and 4th nymphal stages, but vice versa at 12 h of adulthood. Finally, soluble protein content increased from nymphs to adults and was higher during adulthood in aptera versus alate. Whole-body water content in 12-h adults was significantly higher in apterae than that in alatae. These results indicate significant physiological differences between morphs related to specialization for flight.     

Germination,reproduction and interference in the amphicarpic annual Emex spinosa (L.) Campd

Summary A comparison was made of the dimorphic subterranean and aerial achenes of the annual Emex spinosa (L.) Campd. Subterranean achenes were less dormant, had a higher percentage of viability, germinated at a faster rate, were less temperature-dependent in germination and produced larger seedlings than aerial achenes. Subterranean achenes germinated, equally well in light or dark but aerial achenes had a strong light requirement. There was a linear relationship between numbers of weight of aerial achenes and plant size in the field. More resources were allocated to reproduction in plants restricted in size because of environmental conditions. Subterranean achene production was less affected than aerial by density, nitrogen levels, varying field conditions and interference between plants grown, from subterranean and aerial achenes. In mixtures of such plants, those from subterranean achenes had larger leaf area and stems and more aerial achenes. These differences were not apparent in monocultures by the time of harvest. Seed dimorphism is more likely in short-lived fugitive species and the different roles associated with dimorphism (persistence in situ versus dispersal in E. spinosa) are seen as an ecological advantage in such species.     

Effects of wing polyphenism, aphid genotype and host plant chemistry on energy metabolism of the grain aphid, Sitobion avenae

Wing dimorphism has been proposed as a strategy to face trade-offs between flight capability and fecundity. In aphids, individuals with functional wings have slower development and lower fecundity compared with wingless individuals. However, differential maintenance costs between winged and wingless aphids have not been deeply investigated. In the current study, we studied the combined effect of wing dimorphism with the effects of aphid genotypes and of wheat hosts having different levels of chemical defences (hydroxamic acids, Hx) on adult body mass and standard metabolic rates (SMR) of winged and wingless morphs of the grain aphid, Sitobion avenae. We found that wingless aphids had higher body mass than winged aphids and that body mass also increased towards host with high Hx levels. Furthermore, winged aphids showed a plastic SMR in terms of Hx levels, whereas wingless aphids displayed a rigid reaction norm (significant interaction between morph condition and wheat host). These findings suggest that winged aphids have reduced adult size compared to wingless aphids, likely due to costs associated to the development of flight structure in early-life stages. These costs contrast with the absence of detectable metabolic costs related to fuelling and maintenance of the flight apparatus in adults.     

Ecology of achene dimorphism in Leontodon saxatilis

BACKGROUND [corrected] AND AIMS: Leontodon saxatilis produces two morphologically distinct achenes (morphs) in a single capitulum: one row of dark brown achenes without a pappus lies at the edge ('peripheral achenes'; 0.74 +/- 0.18 mg) while the inner ones are light brown with a pappus ('central achenes'; 0.38 +/- 0.07 mg). The hypothesis that achene heteromorphism in L. saxatilis widens its ecological amplitude was tested. KEY RESULTS: Achenes of both morphs germinated over the same range of temperatures (6-33 degrees C) but the central achenes showed significantly higher germination percentages, and the two also differed significantly in their annual dormancy cycle, with the peripheral achenes showing greater dormancy for part of the year. Seedlings from the two morphs did not differ significantly in total biomass after 2 and 4 weeks of growth, neither did they differ significantly in root and shoot weight and root:shoot ratio. Plants from both morphs growing at different regimes of soil moisture, nutrients and competition did not differ significantly in their number of achenes per capitulum. While the number of central achenes varied, that of peripheral achenes remained constant at approx. 13. Drier soil led to an increase in the number of central achenes in plants from both morphs. CONCLUSIONS: The peripheral achenes can replace the mother plant in the following growing season, whereas the central achenes are well adapted for wind dispersal and thus for colonization of new sites. However, the extent of variation in germination characteristics was similar to that found in seed populations of homomorphic plants, which suggests that germination percentage and other growth characteristics do not contribute to widening the ecological amplitude. The habitat of most heteromorphic species, the morphs of which differ greatly in germination and/or growth characteristic, are deserts or highly disturbed areas where such differences are highly advantageous, but in the moderate habitat of L. saxatilis the differences may prove a disadvantage.     

Morph‐specific differences in biochemical composition related to flight capability in the wing‐polyphenic Sitobion avenae

Flight performance at various times after emergence in the alate morph and age‐dependent changes in biochemical composition of winged and wingless morphs were evaluated in the wing‐polyphenic aphid Sitobion avenae (Fabricius) (Hemiptera: Aphididae). Alates exhibited the highest flight activity at 18–36 h after adult emergence. Throughout the nymphal and adult development, the whole‐body content of total lipid was significantly higher in the winged vs. wingless morph, whereas the content of water, soluble sugar, glycogen, phospholipid, and soluble protein showed significantly higher levels in the wingless vs. winged morph. There were no significant differences in the content of triglyceride and free fatty acid during nymphal and adult stages in both morphs. However, triglyceride content was significantly higher in the winged vs. wingless morph during adulthood. Differences in biochemical composition between morphs indicate that there is an energetic cost of flight capability. Our results from S. avenae adults showed that total lipid and triglyceride for the winged morph accumulated significantly to a maximum, and water content decreased significantly to a minimum, on days 1 and 2 after the final molt, exactly when the highest flight activity was reached. This study suggests that flight activity is associated with triglyceride and water content.     

The role of the soil seed bank in the unpredictable habitat of Anthemis chrysantha J. Gay,a critically endangered species

The ability of Anthemis chrysantha to form a soil seed bank (SB) was investigated in order to understand better the adaptation of this rare annual species to its arid and unpredictable natural habitat. The natural seed bank population was analyzed in five consecutive plant cohorts (2006–2010) by sampling at two different times: in May, after the germination period, and in October, after the first dispersal episodes due to the beginning winter rainfall. In addition, to determine the persistence in the soil of the two achene morphs of this species’ (white and dark achenes), an artificial SB was created where achenes were buried and exhumed successively after each season, during two years. In all the cohorts studied, seedling emergence from May samples indicated the existence of a persistent seed bank (PSB). Moreover, the dark achenes were largely responsible for the permanent fraction because, after two years of burial in the artificial SB, 85.0% of them remained apparently healthy without having germinated, versus 9.9% of white achenes. Both types of buried achenes exhibited an annual conditional dormancy/non-dormancy cycle, induced by low winter temperatures. The PSB dynamics appeared to oscillate between the minimum values at the end of the germination period in spring (up to 2000 achenes per square meter) and the maximum values of the dispersal episodes in early autumn (up to 6000 achenes per square meter), with fluctuations of achene density due to the variability in annual rainfall. Hence, the SB showed a decline due to the failure of fruiting in the 2008 cohort, caused by drought, although the low value of ca. 560 achenes per square meter was able to establish the following population. Our study highlights the importance of the PSB, which, in “bad” years, may be critical for the persistence of this species.     

Effects of pollen donors on seed formation inEspeletia schultzii (Compositae) populations at different altitudes

The influence of different pollen donors on seed formation was investigated in three populations ofEspeletia schultzii that differ in environmental conditions and life history characteristics. Self pollen and pollen from different donors (< 15m apart) within each population was used in a diallel design in order to test the genetic base of seed set variation. Three measures of seed formation were used: (1) achene number; (2) proportion of filled achenes (fruits) that distinguishes between achenes with seeds and empty achenes; (3) proportion of aborted seeds that distinguishes between viable and aborted seeds. Self-pollinations resulted in empty achenes. Achene number did not vary between the different pollen donors. A bimodal pattern of filled achenes was found in two populations in two consecutive years. On the other hand, a unimodal pattern was found in crosses between more distant donors (> 30m). These patterns seems to be the results of a sporophytic incompatibility system. Seed abortion was highest at the higher elevations and seems to be correlated with elevation rather than with any genetic effect.     

Changes in seed dormancy of Rosa multibracteata Hemsl. & E. H. Wilson with increasing elevation in an arid valley in the eastern Tibetan Plateau

Variations in levels of seed dormancy among conspecific populations are poorly understood. We aim to determine the variations in the level of dormancy in Rosa multibracteata along an elevational gradient, and to analyze the mechanisms underlying seed dormancy. The study was conducted in an arid valley in the eastern Tibetan Plateau at five elevations: 1700, 1900, 2100, 2300 and 2500 m. Achene traits were measured and physiological levels of dormancy were determined by measuring germination percentages of achenes treated with H₂SO₄ scarification, warm stratification and cold stratification and combinations of those treatments. Achene size, mass and pericarp thickness increased with increasing elevation, but embryo width decreased with increasing in the arid valley. The level of dormancy generally increased with increasing elevation. The level of dormancy of the rose achenes is intermediate from sites at elevations of 1700, 1900 and 2100 m and deep at elevations of 2300 and 2500 m. Comprehensive analysis suggests that the elevational pattern of level of seed dormancy could be ascribed to intrinsically thicker pericarp and deeper physiological dormancy in the embryo, and extrinsically decreased temperature, increased precipitation and soil moisture at the higher elevations. The great variations of achene traits and levels of seed dormancy across elevations suggest a phenotypic differentiation exists within this rose species. Thus, habitat conditions and achene traits should be considered when selecting roses as materials used to propagate seedlings for ornamental purposes or plant restoration.     

Stress grows wings: environmental induction of winged dispersal males in Cardiocondyla ants

Dispersal is advantageous, but, at the same time, it implies high costs and risks. Due to these counteracting selection pressures, many species evolved dispersal polymorphisms, which, in ants, are typically restricted to the female sex (queens). Male polymorphism is presently only known from a few genera, such as Cardiocondyla, in which winged dispersing males coexist with wingless fighter males that mate exclusively inside their maternal nests. We studied the developmental mechanisms underlying these alternative male morphs and found that, first, male dimorphism is not genetically determined, but is induced by environmental conditions (decreasing temperature and density). Second, male morph is not yet fixed at the egg stage, but it differentiates during larval development. This flexible developmental pattern of male morphs allows Cardiocondyla ant colonies to react quickly to changes in their environment. Under good conditions, they invest exclusively in philopatric wingless males. But, when environmental conditions turn bad, colonies start to produce winged dispersal males, even though these males require a many times higher investment by the colony than their much smaller wingless counterparts. Cardiocondyla ants share this potential of optimal resource allocation with other colonial animals and some seed dimorphic plants.     

Dormancy and dispersal in dimorphic achenes of tansy ragwort,Senecio jacobaea L. (Compositae)

Summary Marginal and central florets of the capitula of tansy ragwort Senecio jacobaea yield different kinds of fruit. The central (disk) achenes are lighter (x±SE=199±5g), more numerous (x±SE=58±0.6 achenes per head), and are equipped with a pappus aiding wind transport and rows of trichomes aiding animal transport. The marginal (ray) achenes are heavier (x±SE=286±7g), less numerous (virtually invariant at 13 achenes per head), and lack dispersal structures. Whereas disk achenes are relased shortly after they mature, ray achenes are retained by the parent for a period of months following maturity.Germination at constant temperature (20°C) and with alternating light (12 h light: 12 h dark) demonstrated that disk and ray achenes exhibit different germination syndromes. Germination percentage increases linearly with achene fresh weight in both types; for a given weight, disk achenes have a higher germination percentage than ray achenes. Germination time decreases with increasing achene weight in disk achenes, but increases with achene weight in ray achenes.The germination percentages and germination times for disk and ray achenes diverge progressively with increasing achene weight. The divergence in behavior is a result of diverging patterns of dry matter allocation in the two achene types. Increase in the size of disk achenes favors the embryo fraction, thereby speeding germination while reducing protection. Increase in the size of the ray achenes favors the pericarp fraction, thereby increasing protection while delaying germination.Reduced germination percentage and germination speed of the ray achenes were shown by experimental manipulation to be caused by physical inhibition by their thicker pericarps.Dimorphism in ragwort likely speards germination out in space and time, thereby increasing the number of safe sites an individual parent can exploit in disseminating offspring. The syndrome in other heteromorphic composites resembles that of ragwort, generally combining reduced dispersal-delayed germination in the outer achenes and distance dispersal-quick germination in the central achenes. The outer achenes are generally less numerous and larger. Dispersal traits (large numbers, early release and light wieght) are the direct opposite of dormancy traits (small numbers, delayed release and heavier weight). Thus conflicts between the properties determining dormancy and dispersal appear to require separate dormancy and dispersal phenotypes.     

The Relationship Between Achene Weight,Embryo Weight and Germination inTaraxacumApomicts

Apomicts provide the opportunity to investigate the variation in achene size and performance that is due to factors other than genetic differences between achenes. Achene characteristics of a single capitulum from each of 15 biotypes of apomicticTaraxacumwere investigated. Within a capitulum, achene fresh weight ranged from 4.45-fold to 13.74-fold. Overall, 59.8% of achenes germinated. The probability of any one achene germinating was influenced both by its source and by its weight and ranged from 0.049 to 0.902 between capitula and from 0.028 to 0.873 between weight classes. An investigation of the relationship between achene parts in three biotypes identified a linear trend for embryos to acquire a greater proportion of total achene weight as achene weight increases. Although the heaviest achenes showed the best germination, it is predicted that, for a given investment of maternal resources, seedling number would be maximized by producing achenes of a single intermediate weight.     

EVOLUTION OF TRADE-OFFS BETWEEN SEXUAL AND ASEXUAL PHASES AND THE ROLE OF REPRODUCTIVE PLASTICITY IN THE GENETIC ARCHITECTURE OF APHID LIFE HISTORIES

Life‐history theory postulates that evolution is constrained by trade‐offs (i.e., negative genetic correlations) among traits that contribute to fitness. However, in organisms with complex life cycles, trade‐offs may drastically differ between phases, putatively leading to different evolutionary trajectories. Here, we tested this possibility by examining changes in life‐history traits in an aphid species that alternates asexual and sexual reproduction in its life cycle. The quantitative genetics of reproductive and dispersal traits was studied in 23 lineages (genotypes) of the bird cherry‐oat aphid Rhopalosiphum padi, during both the sexual and asexual phases, which were induced experimentally under specific environmental conditions. We found large and significant heritabilities (broad‐sense) for all traits and several negative genetic correlations between traits (trade‐offs), which are related to reproduction (i.e., numbers of the various sexual or asexual morphs) or dispersal (i.e., numbers of winged or wingless morphs). These results suggest that R. padi exhibits lineage specialization both in reproductive and dispersal strategies. In addition, we found important differences in the structure of genetic variance–covariance matrices ( G ) between phases. These differences were due to two large, negative genetic correlations detected during the asexual phase only: (1) between fecundity and age at maturity and (2) between the production of wingless and winged parthenogenetic females. We propose that this differential expression in genetic architecture results from a reallocation scheme during the asexual phase, when sexual morphs are not produced. We also found significant G × E interaction and nonsignificant genetic correlations across phases, indicating that genotypes could respond independently to selection in each phase. Our results reveal a rather unique situation in which the same population and even the same genotypes express different genetic (co)variation under different environmental conditions, driven by optimal resource allocation criteria.     

Ecological correlates of achene mass variation in Chrysothamnus nauseosus (Asteraceae)

Chrysothamnus nauseosus (Asteraceae) is a complex polymorphic shrub species widely distributed in western North America. In a study of 86 populations belonging to 15 subspecies, achene mass varied among populations over a sevenfold range. Achene mass was closely correlated with capitulum size at the subspecies level, varied little between wild-growing and common-garden-grown members of a population, and was under strong genetic control. Subspecies with the heaviest achenes are restricted to specialized edaphic environments (dunes and badlands) or late seral montane riparian communities, while subspecies that are widely distributed and that occur in early seral habitats have less heavy achenes. Selection on achene mass has apparently been a notable feature of the adaptive radiation of Chrysothamnus nauseosus into the wide array of habitats it currently occupies. Within a wild population, achene mass was greater for plants fruiting in midautumn than for plants fruiting early or late in the autumn, and this same trend was observed within individual plants in garden populations, indicating environmental control, perhaps through resource limitation. Highly significant between-plant differences in achene mass were found in both wild and garden populations, suggesting that within-population genetic variation is sufficient to permit continuing selection.