Inconsistency between different measures of sexual selection.

Measuring the intensity of sexual selection is of fundamental importance to the study of sexual dimorphism, population dynamics, and speciation. Several indices, pools of individuals, and fitness proxies are used in the literature, yet their relative performances are strongly debated. Using 12 independent common lizard populations, we manipulated the adult sex ratio, a potentially important determinant of the intensity of sexual selection at a particular time and place. We investigated differences in the intensity of sexual selection, as estimated using three standard indices of sexual selection-the standardized selection gradient (β'), the opportunity of selection (I), and the Bateman gradient (βss)--calculated for different pools of individuals and different fitness proxies. We show that results based on estimates of I were the opposite of those derived from the other indices, whereas results based on estimates of β' were consistent with predictions derived from knowledge about the species' mating system. In addition, our estimates of the strength and direction of sexual selection depended on both the fitness proxy used and the pool of individuals included in the analysis. These observations demonstrate inconsistencies in distinct measures of sexual selection and underscore the need for caution when comparing studies and species.

Linkage disequilibrium between a melanin-based colour polymorphism and tail length in the barn owl

A linkage disequilibrium between sexually selected and life history traits can be explained by three mutually non-exclusive mechanisms. Genes coding for two traits may be located close on the same chromosome, genes responsible for variation in one of the trait may pleiotropically alter the other, and non-random pairing with respect to two traits may generate a non-physical linkage disequilibrium between their genes. Knowledge of which of these three mechanisms is responsible for a covariation between two traits is of interest to understand why differently ornamented individuals differ in several phenotypic aspects. In Switzerland, barn owls Tyto alba mate randomly with respect to a colour polymorphism generating a large range of variants between reddish-brown and white, males being lighter coloured than females. Several studies have shown that plumage coloration is not neutral with respect to some life history components. To test whether coloration is genetically associated with body size, partial cross-fostering experiments were performed by exchanging some hatchlings between nests. These experiments showed that darker biological fathers produce longer-tailed offspring. This sex-specific pattern is consistent with the hypothesis of non-physical linkage disequilibrium. In line with this hypothesis, darker coloured males were mated with longer-tailed females, whereas female coloration was not associated with tail length of their mate. The finding that dark nestlings had a longer tail than their pale siblings also supports the physical linkage and pleiotropy hypotheses. Therefore, non-random pairing can generate or strengthen a genetic covariation between a secondary sexual character and a morphological trait.

Insularity and the evolution of melanism, sexual dichromatism and body size in the worldwide-distributed barn owl.

Abstract Island biogeography has provided fundamental hypotheses in population genetics, ecology and evolutionary biology. Insular populations usually face different feeding conditions, predation pressure, intraspecific and interspecific competition than continental populations. This so-called island syndrome can promote the evolution of specific phenotypes like a small (or large) body size and a light (or dark) colouration as well as influence the evolution of sexual dimorphism. To examine whether insularity leads to phenotypic differentiation in a consistent way in a worldwide-distributed nonmigratory species, we compared body size, body shape and colouration between insular and continental barn owl (Tyto alba) populations by controlling indirectly for phylogeny. This species is suitable because it varies in pheomelanin-based colouration from reddish-brown to white, and it displays eumelanic black spots for which the number and size vary between individuals, populations and species. Females are on average darker pheomelanic and display more and larger eumelanic spots than males. Our results show that on islands barn owls exhibited smaller and fewer eumelanic spots and lighter pheomelanic colouration, and shorter wings than on continents. Sexual dimorphism in pheomelanin-based colouration was less pronounced on islands than continents (i.e. on islands males tended to be as pheomelanic as females), and on small islands owls were redder pheomelanic and smaller in size than owls living on larger islands. Sexual dimorphism in the size of eumelanic spots was more pronounced (i.e. females displayed much larger spots than males) in barn owls living on islands located further away from a continent. Our study indicates that insular conditions drive the evolution towards a lower degree of eumelanism, smaller body size and affects the evolution of sexual dichromatism in melanin-based colour traits. The effect of insularity was more pronounced on body size and shape than on melanic traits.

Habitat fragmentation, ecology and sexual selection in forest bird species in Monteverde, Costa Rica

Abstract Forest fragmentation is often associated with local extinction and changes in species abundance patterns. The main topic of this thesis is the effect of forest fragmentation on birds in Monteverde, Costa Rica. This thesis also studies aspects of sexual selection and ecology of Long-tailed Manakins, Chiroxiphia Linearis. Chapter 1 investigates bird species assemblages in two degrees of forest fragmentation. It is shown that the distribution, abundance and diversity of forest bird species are strongly influenced by the amount of forest in the landscape matrix. Presence of cattle within the forest influences the presence of some bird species. The prevalence and intensity of ticks and blood parasites on birds in relation to fragmentation is described in Chapter 2. Overall tick prevalence is 3%. Understory birds are significantly more infested with ticks than species at intermediate heights. Tick prevalence on birds does not differ significantly between two degrees of forest fragmentation and individual tick loads tend to be higher in High- than in Low-fragmentation sites. Infestations by the blood parasites Haemoproteus sp. was low except in white-eared ground sparrow, Melozone leucotis, that is 28% and is significantly higher in High- than in Low-fragmentation sites. In chapter 3 results on the ecology and habitat movements of the Bare-necked Umbrellabird, Cephalopterus glabricollis, are presented. The abundance of umbrellabirds at high elevations during the breeding season coincides with the highest peak of fruit abundance. Birds leave the protected area during the non-breeding season moving to unprotected forest fragments. In chapter 4 ontogenetic changes in feather morphology through sexual maturity in Long-tailed Manakins are described. In adult males, rectrices length is positively correlated to testis volume. Changes in male morphology during ontogeny in the long-tailed manakin appear to be associated with their specific-display behaviours. Significant interpopulation differences in the morphology of Long-tailed Manakins are shown in chapter 5. These differences are more accentuated in morphological traits related to flight displays. A field experiment demonstrates that long rectrices impose flying costs for males and females. A reduction in flying ability was found to be strongest in males from a population presenting the highest degree of sexual dimorphism. Résumé La fragmentation des forêts est souvent associée avec des modifications dans l'abondance des espèces et des extinctions locales. Le thème principale de cette thèse est l'étude de l'effet de la fragmentation des forêts sur les oiseaux de Monteverde, Costa Rica. Elle décrit par ailleurs certains aspects de la sélection sexuelle et l'écologie du manakin à longue queue, Chiroxiphia linearis. Dans le Chapitre 1 je montre que la distribution, l'abondance et la diversité des assemblages d'oiseaux vivant dans la forêt sont fortement influencées pas le degré de fragmentation de celle ci. Par ailleurs, la présence ou l'absence de bétail dans les forêts influence la présence de certaines espèces d'oiseaux. Dans le chapitre 2 j'ai étudié la prévalence et l'intensité d'infestation par des tiques ainsi que la présence de parasites sanguins chez les oiseaux en relation avec la fragmentation des forêts. La prévalence globale de tiques est de 3 %, les oiseaux vivant au niveau du sol étaient plus souvent infectés par des tiques que les espèces se déplaçant à un niveau plus élevé. La prévalence de tiques sur les oiseaux n'était pas significativement différente entre les paysages avec différentes fragmentations. Les parasites sanguins du genre Haemoproteus sp. étaient présents à très basse fréquence à l'exception chez Melozone leucotis ou la prevalence était de 28% et significativement plus élevée chez les oiseaux vivant dans les forêts à forte fragmentation. Dans le Chapitre 3 je présente des résultats sur l'écologie et les mouvements entre habitats chez le "Bare-necked umbrellabird", Cephalopterus glabricollis. Cette espèce endémique du Costa Rica niche à haute altitude durant la période d'abondance des fruits et réalise une migration altitudinale vers des zones basses durant la saison de non reproduction. Dans le chapitre 4 je présente les changements ontogénétiques dans la morphologie du plumage des manakins à longue queue. Chez les mâles, les changements de morphologie semblent être associés avec leurs comportements de parade spécifiques. Dans le chapitre 5 je présente des différences morphologiques significative entre deux populations chez le manakin à longue queue et je montre que la capacité de vols chez les mâles est plus fortement influencée dans la population avec le degré de dimorphisme sexuel le plus prononcé.

On the fate of sexual traits under asexuality.

Environmental shifts and life-history changes may result in formerly adaptive traits becoming non-functional or maladaptive. In the absence of pleiotropy and other constraints, such traits may decay as a consequence of neutral mutation accumulation or selective processes, highlighting the importance of natural selection for adaptations. A suite of traits are expected to lose their adaptive function in asexual organisms derived from sexual ancestors, and the many independent transitions to asexuality allow for comparative studies of parallel trait maintenance versus decay. In addition, because certain traits, notably male-specific traits, are usually not exposed to selection under asexuality, their decay would have to occur as a consequence of drift. Selective processes could drive the decay of traits associated with costs, which may be the case for the majority of sexual traits expressed in females. We review the fate of male and female sexual traits in 93 animal lineages characterized by asexual reproduction, covering a broad taxon range including molluscs, arachnids, diplopods, crustaceans and eleven different hexapod orders. Many asexual lineages are still able occasionally to produce males. These asexually produced males are often largely or even fully functional, revealing that major developmental pathways can remain quiescent and functional over extended time periods. By contrast, for asexual females, there is a parallel and rapid decay of sexual traits, especially of traits related to mate attraction and location, as expected given the considerable costs often associated with the expression of these traits. The level of decay of female sexual traits, in addition to asexual females being unable to fertilize their eggs, would severely impede reversals to sexual reproduction, even in recently derived asexual lineages. More generally, the parallel maintenance versus decay of different trait types across diverse asexual lineages suggests that neutral traits display little or no decay even after extended periods under relaxed selection, while extensive decay for selected traits occurs extremely quickly. These patterns also highlight that adaptations can fix rapidly in natural populations of asexual organisms, in spite of their mode of reproduction.

Sexual dysfunction among young men : prevalence and associated factors

Les dysfonctions sexuelles représentent une préoccupation importante et parfois mésestimée dans la population masculine générale. L'éjaculation précoce et les dysfonctions érectiles faisant partie des dysfonctions sexuelles les plus fréquentes, nous avons choisi, dans le présent article, de nous concentrer sur ces deux aspects. Selon plusieurs études, la prévalence d'éjaculation précoce se situe entre 17 et 30%. Les associations décrites avec l'éjaculation précoce concernent le stress, la dépression et un jeune âge (<25 ans). La prévalence de dysfonction érectile, quant à elle, oscille entre 2 et plus de 80% dépendant principalement de l'âge. Beaucoup d'autres facteurs associés ont été décrits dans la littérature, tels que le diabète, les pathologies cardiovasculaires ou neurologiques, les médicaments, l'obésité ou l'inactivité physique. Tous ces facteurs étant plus fréquents au sein de populations vieillissantes, nous disposons de peu d'études concernant la dysfonction érectile chez de jeunes hommes. Notre étude propose donc d'évaluer la prévalence d'éjaculation précoce et de dysfonction érectile chez une population de jeunes hommes et d'étudier les associations possibles avec les consommations de substances, l'index de masse corporelle (IMC), l'activité physique ou la santé mentale. Chaque homme suisse entre 18 et 25 ans est appelé à participer à 2 journées de recrutement militaire. De septembre 2010 à mai 2011, les jeunes hommes recrutés à Lausanne et à Zürich (N= 9761) ont été invités à participer à l'étude C-SURF (Cohort Study on Substance Use Risk Factors). 5276 d'entre eux acceptèrent de répondre au questionnaire complet, et 73% (N=3886) le firent de manière effective. Le but de notre recherche étant d'étudier les dysfonctions sexuelles, nous nous sommes concentrés sur les participants sexuellement actifs (N=2507) et les avons divisés en 2 groupes, selon qu'ils présentaient ou non la dysfonction étudiée. Pour chaque dysfonction (éjaculation précoce et dysfonction érectile), nous avons utilisé une analyse bivariée afin de comparer les deux groupes (avec ou sans trouble) quant à d'éventuels facteurs associés. A cause de la nature transversale de l'étude, nous ne pouvions pas postuler de lien de cause à effet entre ces facteurs et les dysfonctions sexuelles, raison pour laquelle nous avons choisi d'utiliser des modèles log-linéaires pour mettre en exergue les associations significatives. Nos résultats montrent une prévalence d'éjaculation précoce de 11.4% avec comme associations principales la consommation de tabac et de drogues illégales (autres que le cannabis). La prévalence des dysfonctions érectiles est quant à elle proche du 30% et s'associe principalement avec l'usage de médicaments, ainsi qu'avec la santé physique et la santé mentale. Un jeune homme suisse sur trois souffre ainsi d'au moins une dysfonction sexuelle. Beaucoup de facteurs compromettants pour la santé y sont possiblement associés, raison pour laquelle les professionnels de la santé devraient saisir chaque occasion pour parler de sexualité avec leurs jeunes patients de sexe masculin.

Genetics of decayed sexual traits in a parasitoid wasp with endosymbiont-induced asexuality.

Trait decay may occur when selective pressures shift, owing to changes in environment or life style, rendering formerly adaptive traits non-functional or even maladaptive. It remains largely unknown if such decay would stem from multiple mutations with small effects or rather involve few loci with major phenotypic effects. Here, we investigate the decay of female sexual traits, and the genetic causes thereof, in a transition from haplodiploid sexual reproduction to endosymbiont-induced asexual reproduction in the parasitoid wasp Asobara japonica. We take advantage of the fact that asexual females cured of their endosymbionts produce sons instead of daughters, and that these sons can be crossed with sexual females. By combining behavioral experiments with crosses designed to introgress alleles from the asexual into the sexual genome, we found that sexual attractiveness, mating, egg fertilization and plastic adjustment of offspring sex ratio (in response to variation in local mate competition) are decayed in asexual A. japonica females. Furthermore, introgression experiments revealed that the propensity for cured asexual females to produce only sons (because of decayed sexual attractiveness, mating behavior and/or egg fertilization) is likely caused by recessive genetic effects at a single locus. Recessive effects were also found to cause decay of plastic sex-ratio adjustment under variable levels of local mate competition. Our results suggest that few recessive mutations drive decay of female sexual traits, at least in asexual species deriving from haplodiploid sexual ancestors.

Sex-dependent selection on an autosomal melanic female ornament promotes the evolution of sex ratio bias.

Sex-dependent selection often leads to spectacularly different phenotypes in males and females. In species in which sexual dimorphism is not complete, it is unclear which benefits females and males derive from displaying a trait that is typical of the other sex. In barn owls (Tyto alba), females exhibit on average larger black eumelanic spots than males but members of the two sexes display this trait in the same range of possible values. In a 12-year study, we show that selection exerted on spot size directly or on genetically correlated traits strongly favoured females with large spots and weakly favoured males with small spots. Intense directional selection on females caused an increase in spot diameter in the population over the study period. This increase is due to a change in the autosomal genes underlying the expression of eumelanic spots but not of sex-linked genes. Female-like males produced more daughters than sons, while male-like females produced more sons than daughters when mated to a small-spotted male. These sex ratio biases appear adaptive because sons of male-like females and daughters of female-like males had above-average survival. This demonstrates that selection exerted against individuals displaying a trait that is typical of the other sex promoted the evolution of specific life history strategies that enhance their fitness. This may explain why in many organisms sexual dimorphism is often not complete.

New genetic loci link adipose and insulin biology to body fat distribution.

Body fat distribution is a heritable trait and a well-established predictor of adverse metabolic outcomes, independent of overall adiposity. To increase our understanding of the genetic basis of body fat distribution and its molecular links to cardiometabolic traits, here we conduct genome-wide association meta-analyses of traits related to waist and hip circumferences in up to 224,459 individuals. We identify 49 loci (33 new) associated with waist-to-hip ratio adjusted for body mass index (BMI), and an additional 19 loci newly associated with related waist and hip circumference measures (P < 5 × 10(-8)). In total, 20 of the 49 waist-to-hip ratio adjusted for BMI loci show significant sexual dimorphism, 19 of which display a stronger effect in women. The identified loci were enriched for genes expressed in adipose tissue and for putative regulatory elements in adipocytes. Pathway analyses implicated adipogenesis, angiogenesis, transcriptional regulation and insulin resistance as processes affecting fat distribution, providing insight into potential pathophysiological mechanisms.

Heterologous desensitization of the glucagon-like peptide-1 receptor by phorbol esters requires phosphorylation of the cytoplasmic tail at four different sites.

Glucagon-like peptide-1 stimulates glucose-induced insulin secretion by binding to a specific G protein-coupled receptor that activates the adenylyl cyclase pathway. We previously demonstrated that heterologous desensitization of the receptor by protein kinase C correlated with phosphorylation in a 33-amino acid-long segment of the receptor carboxyl-terminal cytoplasmic tail. Here, we determined that the in vivo sites of phosphorylation are four serine doublets present at positions 431/432, 441/442, 444/445, and 451/452. In vitro phosphorylation of fusion proteins containing mutant receptor C-tails, however, indicated that whereas serines at position 431/432 were good substrates for protein kinase C (PKC), serines 444/445 and 451/452 were poor substrates, and serines 441/442 were not substrates. In addition, serine 416 was phosphorylated on fusion protein but not in intact cells. This indicated that in vivo a different PKC isoform or a PKC-activated kinase may phosphorylate the receptor. The role of phosphorylation on receptor desensitization was assessed using receptor mutants expressed in COS cells or Chinese hamster lung fibroblasts. Mutation of any single serine doublet to alanines reduced the extent of phorbol 12-myristate 13-acetate-induced desensitization, whereas substitution of any combination of two serine doublets suppressed it. Our data thus show that the glucagon-like peptide-1 receptor can be phosphorylated in response to phorbol 12-myristate 13-acetate on four different sites within the cytoplasmic tail. Furthermore, phosphorylation of at least three sites was required for desensitization, although maximal desensitization was only achieved when all four sites were phosphorylated.

Disease-causing mutations improving the branch site and polypyrimidine tract: pseudoexon activation of LINE-2 and antisense Alu lacking the poly(T)-tail.

Cryptic exons or pseudoexons are typically activated by point mutations that create GT or AG dinucleotides of new 5' or 3' splice sites in introns, often in repetitive elements. Here we describe two cases of tetrahydrobiopterin deficiency caused by mutations improving the branch point sequence and polypyrimidine tracts of repeat-containing pseudoexons in the PTS gene. In the first case, we demonstrate a novel pathway of antisense Alu exonization, resulting from an intronic deletion that removed the poly(T)-tail of antisense AluSq. The deletion brought a favorable branch point sequence within proximity of the pseudoexon 3' splice site and removed an upstream AG dinucleotide required for the 3' splice site repression on normal alleles. New Alu exons can thus arise in the absence of poly(T)-tails that facilitated inclusion of most transposed elements in mRNAs by serving as polypyrimidine tracts, highlighting extraordinary flexibility of Alu repeats in shaping intron-exon structure. In the other case, a PTS pseudoexon was activated by an A>T substitution 9 nt upstream of its 3' splice site in a LINE-2 sequence, providing the first example of a disease-causing exonization of the most ancient interspersed repeat. These observations expand the spectrum of mutational mechanisms that introduce repetitive sequences in mature transcripts and illustrate the importance of intronic mutations in alternative splicing and phenotypic variability of hereditary disorders.

Melanin pigmentation negatively correlates with plumage preening effort in barn owls

1. According to the handicap principle of sexual selection, colourful ornaments honestly signal absolute quality only if they entail fitness costs. The degree of melanism often covaries positively with aspects of individual quality, and hence melanin-based coloration should be costly to produce or to maintain in a good shape. This is, however, unlikely because melanin-based coloration is often strongly heritable and in birds the rate of feather wear decreases with the amount of melanin pigments packed in feathers. 2. The hypothesis that melanin pigments reduce the cost of maintaining colourful ornaments in a good shape predicts a negative correlation between the degree of melanism and both the size of the uropygial gland that produces preening secretions and the intensity of preening behaviour. 3. Using a correlative approach, I evaluated these two predictions in the barn owl Tyto alba in which the body underside varies from immaculate to heavily marked with black spots, a eumelanin-based trait, and from white to reddish-brown, a phaeomelanin-based trait. I correlated plumage traits with preening behaviour in nestlings and with the size and mass of the uropygial gland in dead adults. I also weighed nonornamental wing and tail feathers to assess whether the quality of nonornamental feathers is positively correlated with the degree of melanism of an ornamental plumage trait. 4. The degree of phaeomelanism was neither associated with preening behaviour nor with the size and mass of the uropygial gland. In line with the two predictions, individuals with more and larger black spots had a lighter uropygial gland and preened less frequently. Because nonornamental wing and tail feathers of spottier individuals were heavier per unit of surface area, the entire plumage of eumelanic individuals may be more robust and in turn require less care than the plumage of nonmelanic conspecifics. 5. In conclusion, the degree of eumelanism can be associated with aspects of individual quality even if eumelanic ornaments are neither costly to produce nor to maintain in a good shape. Document Type: Article

Modulatory Role of the Ovarian Function in Neuroimmunoendocrine Axis Activity.

The aim of this study was to evaluate the effect of ovariectomy on the acute-phase response of inflammatory stress. Ex vivo adrenocortical, peripheral mononuclear cell (PMNC) and adipocyte activities were studied in intact and ovariectomized mice. Endotoxemia was mimicked by intraperitoneal administration of bacterial lipopolysaccharide (LPS; 25 mg per mouse) to sham-operated and 21-day ovariectomized mice. Circulating corticosterone, tumor necrosis factor-alpha (TNFalpha) and leptin concentrations were monitored before and 30-120 min after the administration of LPS. Additionally, in vitro experiments were performed with isolated corticoadrenal cells, PMNCs and omental adipocytes from sham-operated and ovariectomized mice incubated with specific secretagogues. The results indicate that while ovariectomy enhanced TNFalpha secretion after in vivo administration of LPS, it reduced corticoadrenal response and abrogated LPS-elicited leptin secretion into the circulation. While the corticoadrenal sensitivity to ACTH stimulation was reduced by ovariectomy, the LPS-induced PMNC response was not affected. Exogenous leptin enhanced baseline PMNC function regardless of surgery. Finally, ovariectomy drastically reduced in vitro adipocyte functionality. Our data support the notion that ovariectomy modified neuroendocrine-immune-adipocyte axis function and strongly suggest that ovarian activity could play a pivotal role in the development of an adequate immune defense mechanism after injury.

Pretreatment and outcome correlates of past sexual and physical trauma in 118 bipolar I disorder patients with a first episode of psychotic mania.

OBJECTIVES: To assess the prevalence and correlates of childhood and adolescent sexual and/or physical abuse (SPA) in bipolar I disorder (BDI) patients treated for a first episode of psychotic mania. METHODS: The Early Psychosis Prevention and Intervention Centre admitted 786 first-episode psychosis patients between 1998 and 2000. Data were collected from patients' files using a standardized questionnaire. A total of 704 files were available; 43 were excluded because of a nonpsychotic diagnosis at endpoint and 3 due to missing data regarding past stressful events. Among 658 patients with available data, 118 received a final diagnosis of BDI and were entered in this study. RESULTS: A total of 80% of patients had been exposed to stressful life events during childhood and adolescence and 24.9% to SPA; in particular, 29.8% of female patients had been exposed to sexual abuse. Patients who were exposed to SPA had poorer premorbid functioning, higher rates of forensic history, were less likely to live with family during treatment period, and were more likely to disengage from treatment. CONCLUSIONS: SPA is highly prevalent in BDI patients presenting with a first episode of psychotic mania; exposed patients have lower premorbid functional levels and poorer engagement with treatment. The context in which such traumas occur must be explored in order to determine whether early intervention strategies may contribute to diminish their prevalence. Specific psychological interventions must also be developed.

Density-dependent pollen limitation and reproductive assurance in a wind-pollinated herb with contrasting sexual systems

1. Wind pollination is thought to have evolved in response to selection for mechanisms to promote pollination success, when animal pollinators become scarce or unreliable. We might thus expect wind-pollinated plants to be less prone to pollen limitation than their insect-pollinated counterparts. Yet, if pollen loads on stigmas of wind-pollinated species decline with distance from pollen donors, seed set might nevertheless be pollen-limited in populations of plants that cannot self-fertilize their progeny, but not in self-compatible hermaphroditic populations.2. Here, we test this hypothesis by comparing pollen limitation between dioecious and hermaphroditic (monoecious) populations of the wind-pollinated herb Mercurialis annua.3. In natural populations, seed set was pollen-limited in low-density patches of dioecious, but not hermaphroditic, M. annua, a finding consistent with patterns of distance-dependent seed set by females in an experimental array. Nevertheless, seed set was incomplete in both dioecious and hermaphroditic populations, even at high local densities. Further, both factors limited the seed set of females and hermaphrodites, after we manipulated pollen and resource availability in a common garden experiment.4. Synthesis. Our results are consistent with the idea that pollen limitation plays a role in the evolution of combined vs. separate sexes in M. annua. Taken together, they point to the potential importance of pollen transfer between flowers on the same plant (geitonogamy) by wind as a mechanism of reproductive assurance and to the dual roles played by pollen and resource availability in limiting seed set. Thus, seed set can be pollen-limited in sparse populations of a wind-pollinated species, where mates are rare or absent, having potentially important demographic and evolutionary implications.