Dioecious Grass Research Articles

Can't live with them, can't live without them? Balancing mating and competition in two-sex populations.

Two-sex populations are usually studied through frequency-dependent models that describe how sex ratio affects mating, recruitment and population growth. However, in two-sex populations, mating and recruitment should also be affected by density and by its interactions with the sex ratio. Density may have positive effects on mating (Allee effects) but negative effects on other demographic processes. In this study, we quantified how positive and negative inter-sexual interactions balance in two-sex populations. Using a dioecious grass (Poa arachnifera), we established experimental field populations that varied in density and sex ratio. We then quantified mating success (seed fertilization) and non-mating demographic performance, and integrated these responses to project population-level recruitment. Female mating success was positively density-dependent, especially at female-biased sex ratios. Other demographic processes were negatively density-dependent and, in some cases, frequency-dependent. Integrating our experimental results showed that mate-finding Allee effects dominated other types of density-dependence, giving rise to recruitment that increased with increasing density and peaked at intermediate sex ratios, reflecting tension between seed initiation (greater with more females) and seed viability (greater with more males). Our results reveal, for the first time, the balance of positive and negative inter-sexual interactions in sex-structured populations. Models that account for both density- and sex ratio dependence, particularly in mating, may be necessary for understanding and predicting two-sex population dynamics.

Potassium Nitrate Alters Buffalograss Bur Permeability

Buffalograss is a warm-season, dioecious grass species yielding burs, which are routinely used for field plantings. The pistillate plants bear the burs containing 1–5 caryopses. Isolated caryopses readily germinate, but caryopses contained within burs exhibit strong dormancy, suggesting that burs inhibit germination. Priming burs with low concentrations of potassium nitrate (KNO3) is used as an industry standard to improve germination. Seed dormancy and germination in many species are strongly influenced by endogenous hormone levels, principally abscisic acid (ABA) and gibberellic acid (GA). It follows that buffalograss seed dormancy might be induced or overcome by altering the ratio of ABA to GA. The objectives of this research were 1) to contrast the effects of priming with KNO3 or water on bur germination, 2) to study how these treatments affected hormone profiles, specifically the ABA/GA ratios in the burs, and 3) to quantify treatment effects on the water permeability of the burs. Hormone profiles were analyzed following four postharvest seed-soaking treatments (24-hour 0.05 m KNO3, 24-hour H2O, 48-hour 0.05 m KNO3, and 48-hour H2O). Water infiltration tests on nontreated, 24-hour H2O-treated, and 24-hour 0.05 m KNO3-treated seeds were also conducted. Inconclusive hormone profiling results did not support the hypothesis that KNO3 postharvest treatment raises GA levels to encourage germination. Instead, our data support changes in seed morphology following KNO3 postharvest seed treatments which alter water permeability of the seedcoat leading to increased germination.

Kin and sex recognition in a dioecious grass

Plants have evolved complex mechanisms to recognize and respond to the presence of neighboring plants, and the genetic identity of a neighbor has been shown to make a difference in this response. Studies have found that plants are able to differentiate among self- versus non-self and among sibling (kin) competitors. Here, we present data for the dioecious grass Distichlis spicata on seedling recognition of kin and sex. D. spicata exhibits extreme spatial segregation of the sexes (SSS) in the field, and previous work has shown that intra-sexual competition is less than inter-sexual competition in the field. In this experiment, we conducted experiments in the lab, exposing the seedlings to liquid media in which seedlings had been previously grown, rather than have the seedling physically contact one another. We found that inter-sexual interactions caused a decrease in the total dry weight and an increase in root/shoot ratio of the plants compared with intra-sexual interactions. These findings suggest that D. spicata plants can recognize and respond to plant sex and that inter-sexual competition contributes to SSS, even when additional interactions, such as mycorrhizal fungi are controlled, and physical interactions between plants are removed. In the kin recognition analysis, we found that plants paired with another plant from the same mother had significantly greater lateral root number and length than plants paired with non-kin, suggesting that in this highly clonal grass, kin recognition may be an important mechanism in competitive interactions.

Changes in sex ratios of a dioecious grass with grazing intensity: the interplay between gender traits, neighbour interactions and spatial patterns

Summary Dioecious plants frequently face sex‐related resource allocation trade‐offs. Differential traits of male and female plants can have important consequences that influence their direct and indirect interactions with neighbours and herbivores. Gender differences in traits have been considered to be inherently linked to sex‐biased ratios along environmental stress gradients. However, the mechanisms behind the variation in sex ratios with increasing biotic stress remain elusive. We linked the results of process‐based field experiments with fine‐scale spatial patterns of naturally established plants (both sexes) at different grazing intensities. We demonstrated that slow‐growing females were better defended against insects and small mammals but were more susceptible than males to competition from neighbours. In large‐herbivore exclosures, we found that sex‐biased spatial patterns at the microsite scale can be predicted from gender‐specific traits, with females more segregated from competitors (unpalatable grasses) than males. Traits related to growth/defence trade‐off in females, coupled with their higher competition costs than males, suggest that gender dimorphism in traits can directly influence species interactions and spatial organization. In the presence of large herbivores, female individuals were preferentially browsed despite their larger investment in antiherbivore compounds. The greater distance of females from unpalatable competitors was the main determinant of female‐biased consumption by domestic herbivores. Females growing close to neighbours successfully eluded browsing but also faced competition. Scaling up to a population level, population sex ratios drift from female‐ to male‐bias with increasing domestic grazing intensity. Synthesis: The sexual dimorphism in traits of a dioecious bunchgrass results in differential herbivory by natural enemies, differential competitive ability and differential spatial distribution of sexes. Domestic grazers disrupt this balance by shifting species interactions and their spatial organization. Changes in dioecious plant populations towards male‐biased sex ratios with grazing intensity could not be predicted solely by gender‐specific traits.

The Biological Flora of Coastal Dunes and Wetlands:Distichlis spicata(C. Linnaeus) E. Greene

Lonard, R.I.; Judd, F.W., and Stalter, R., 2013. The biological flora of coastal dunes and wetlands: Distichlis spicata (C. Linnaeus) E. Greene. Distichlis spicata (C. Linnaeus) E. Greene is a New World temperate-, subtropical-, and tropical-zone rhizomatous, dioecious grass. It is an important species in coastal salt marshes, brackish marshes, and tidal wetlands, where it is often a dominant species. Inland ecotypes of this taxon occur in wet alkaline sites throughout temperate regions of the United States, Mexico, and South America. Distichlis spicata often occurs in wetlands, where total soluble salts seasonally range from about 20 to 43 ppt and where the pH varies from 5 to 7. Higher salinities in the rhizome–root matrix reduce shoot growth more than root growth. Also known as salt grass, D. spicata is a clonal stress tolerator that spreads laterally into disturbed sites in the upper marsh, and it recovers quickly from burial by wrack debris after storms. A typical water level range for salt grass is from 10 to 15 cm below to 5 cm above the marsh surface, but it can tolerate tidal amplitudes ranging from 1.36 to 1.74 m for 1 hour on some coastlines. Pistillate plants typically dominate lower elevations in tidal marshes, whereas higher elevations in marshes support populations dominated by male plants. Herein, we present a review of the biology of this important species.

Testing the interaction between inter-sexual competition and phosphorus availability in a dioecious grass

Spatial segregation of the sexes (SSS) occurs in many dioecious plants, usually along an environmental gradient. However, little data is available on how male and female plants of species with SSS respond to environmental conditions along these gradients. We examined whether male and female plants of Distichlis spicata (L.) Greene, a dioecious grass species that exhibits extreme SSS along a phosphorus gradient, differed in their responses to phosphorus and competition treatments in the greenhouse. We found little evidence that phosphorus plays a role in SSS in D. spicata. However, supporting earlier work, we found that plants subject to inter-sexual competition were significantly more likely to have high root/shoot ratios than plants subject to intra-sexual competition, suggesting that competition is an important driver of SSS in this species.

Inter-sexual competition in a dioecious grass

Spatial segregation of the sexes (SSS) occurs in many dioecious angiosperms, but little data are available on the fitness advantages, if any, for males and females. We examined whether reciprocally transplanted male and female seedlings of Distichlis spicata, a dioecious grass species that exhibits extreme SSS, differed in their responses to microhabitats and competition treatments. Plants grown without conspecific competitors grew equally well in both male- or female-majority habitats, suggesting that male and female plants do not have differential resource needs at the juvenile life-history stage. However, plants subject to intra-sexual competition were significantly larger than plants subject to inter-sexual competition, suggesting that niche partitioning may occur in D. spicata.

Sex‐specific variation in the interaction between Distichlis spicata (Poaceae) and mycorrhizal fungi

Associations between mycorrhizal fungi and plants can influence intraspecific competition and shape plant population structure. While variation in plant genotypes is known to affect mycorrhizal colonization in crop systems, little is known about how genotypes affect colonization in natural plant populations or how plant sex might influence colonization with mycorrhizal fungi in plant species with dimorphic sexual systems. In this study, we analyzed mycorrhizal colonization in males and females of the wetland dioecious grass Distichlis spicata, which has spatially segregated sexes. Our results suggest that D. spicata males and females interact with mycorrhizal fungi differently. We discuss the implications for the role of this sex-specific symbiotic interaction in the maintenance of the within-population sex ratio bias of D. spicata.

Females make tough neighbors: sex-specific competitive effects in seedlings of a dioecious grass

If males and females of a species differ in their effect on intraspecific competition then this can have significant ecological and evolutionary consequences because it can lead to size and mortality disparities between the sexes, and thus cause biased population sex ratios. If the degree of sexual dimorphism of competitive effect varies across environments then this variation can generate sex ratio variation within and between populations. In a California population of Distichlis spicata, a dioecious grass species exhibiting extreme within-population sex ratio variation (spatial segregation of the sexes), I evaluated the intraspecific competitive effects of male and female D. spicata seedlings in three soil types. The sex of seedlings was determined using a RAPD-PCR marker co-segregating with female phenotype. Distichlis spicata seedlings, regardless of sex, were six times larger when grown with male versus female conspecific seedlings in soil from microsites where the majority of D. spicata plants are female, and this sexual dimorphism of competitive effect was weaker or did not occur in other soil types. This study suggests that it is not just the higher costs of female versus male reproduction itself that cause spatial segregation of the sexes in D. spicata, but that differences in competitive abilities between the sexes--which occur as early as the seedling stage--can generate sex ratio variation.

Effect of Fine‐Scale Spatial Variation of Soil Nitrogen on the Performance of the Sexes ofPoa ligularisin Patchy Ecosystems of Northern Patagonia

In patchy environments of arid Patagonia, males of the dioecious grass Poa ligularis dominate in N‐poor microsites, while females are more common in N‐rich microsites. In order to explore functional differences related to spatial segregation of the sexes, we analyzed biomass allocation and tissue N concentration in males and females of P. ligularis growing in a range of soil N concentrations (sN). Based on the general patterns of responses described for plants from N‐rich and N‐poor habitats, we hypothesized that (1) females, which dominate in N‐rich microsites, would increase biomass allocation with increasing sN, while males, frequent in N‐poor microsites, would show a limited response and (2) tissue N concentration would display a wider variation in males than females in response to changes in sN. At three sites in northern Patagonia, we randomly selected 15 plants of each sex of P. ligularis growing inside shrub patches and 15 in the interpatch areas and evaluated the biomass and N concentration of ab...

Spatial sex segregation in the dioecious grass Poa ligularis in northern Patagonia: the role of environmental patchiness

We examined the effect of environmental patchiness on the spatial segregation of the sexes in the dioecious anemophilus grass Poa ligularis. Because the species is sensitive to grazing, a better understanding of environmental factors that control its spatial distribution and abundance could improve conservation efforts. We hypothesized that (i) males and females are spatially segregated in the microenvironments created by plant patches as the result of sexual specialization in habitat and/or resources use, (ii) sexual specialization is related to different tolerance to competition and reproductive costs of males and females, and (iii) changes in patch structure affect the microenvironment and the intensity of spatial segregation of the sexes. We analyzed the spatial distribution of sexes at three sites with different plant and micro-environmental patchiness and performed a controlled competition experiment with different substitution of males and females. Our results showed that large plant patches created larger sheltered soil fertility islands than small patches. As patch size and their area of influence increased, the density and the spatial segregation of the sexes of P. ligularis also increased, resulting in biased habitat-specific sex ratios. In accordance with their higher reproductive costs, females were more frequent in sheltered (low air evaporative demand) and nitrogen-rich areas inside patch perimeters than males. Females were also better able to tolerate inter-sexual competition than males. In contrast, males tolerated low nitrogen concentration in soil and low sheltering, probably gaining advantage in pollen dispersal. Inter- and intra-sexual competition, however, affected the reproductive output of both sexes. From the point of view of conservation, environmental patchiness is important to the status of P. ligularis populations. The reduction of patch size limits the available microsites, biases the sex ratio towards males inside patches, increases inter- and intra-sexual competition, and it might be expected to decrease overall seed and pollen production and consequently potential recruitment.

Gender-Specific Selection during Early Life History Stages in the Dioecious Grass Distichlis spicata

Gender-Specific Selection during Early Life History Stages in the Dioecious Grass Distichlis spicata

Genetic diversity among Texas bluegrass genotypes (Poa arachnifera Torr.) revealed by AFLP and RAPD markers

Texas bluegrass Poa arachnifera Torr., is a vigorous sod-forming perennial, dioecious grass, tolerant to heat. It is native to the Southern Great Plains. Genetic relationships existing among 28 Texas bluegrass genotypes were investigated using amplified fragment length polymorphism (AFLP) and randomly amplified polymorphic DNA (RAPD). A total of 3756 AFLP markers were generated on the 28 genotypes of Texas bluegrass. A wide range of polymorphism (23.08–85.33%) was observed among primer combinations with a mean of 64.11%. Among 441 RAPDs assayed, 335 were polymorphic with a mean polymorphic rate of 73.71%. Unweighted pair-group method using an arithmetic average (UPGMA) cluster analysis using AFLP and RAPD data separated the 28 Texas bluegrass accessions into two broad groups. With a few exceptions, the females clustered with females and males with males. These results indicate that, it may be possible to discriminate between males and females using molecular markers. Principal coordinate analysis of AFLP and RAPD data also indicated two distinct groups and revealed genetic variability among and within the groups. Based on their genetic similarity indices, high correlation was observed between AFLP and RAPD markers.

EVOLUTION OF DIOECY IN BUCHLOE DACTYLOIDES (GRAMINEAE): TESTS FOR SEX‐SPECIFIC VEGETATIVE CHARACTERS, ECOLOGICAL DIFFERENCES, AND SEXUAL NICHE‐PARTITIONING

Buchloe dactyloides is a perennial dioecious grass in which male and female inflorescences are so strikingly dimorphic that they were originally assigned to different genera. The objective of this paper is to present the results of tests for sex‐specific vegetative characters, ecological differences, and sexual niche‐partitioning, combining them with prior information on the reproductive biology of Buchloe for an evaluation of the key factors leading to the evolution of dioecy and sexual dimorphism. Field and greenhouse data were collected from Oklahoma and Kansas populations on vegetative characters, allocation to reproduction, and relative growth and competitive success along resource gradients. Except for greater susceptibility to leaf rust by males, there were no significant differences between males and females in vegetative characters, total biomass, or reproductive effort. Field studies of spatial distributions of males and females failed to show any relation to soil, topography, or soil moisture. In a 45‐month greenhouse experiment starting at the seedling stage, the relative growth and competitive success of randomly paired individuals showed no evidence for differential competitive success or for niche‐partitioning of males and females. The “outcrossing advantage” and subsequent sexual specialization of the female inflorescence appear to be the major factors underlying this dimorphic system.

Late Quaternary Fossils of Poa fendleriana (Muttongrass): Holocene Expansions of Apomicts

Late Quaternary Fossils of Poa fendleriana (Muttongrass): Holocene Expansions of Apomicts

RELATIONSHIP BETWEEN SYNAPTOSPERMY AND DIOECY IN THE LIFE HISTORY STRATEGIES OF BUCHLOE DACTYLOIDES (GRAMINEAE)

Buchloe dactyloides (Nutt.) Engelm. is a perennial dioecious grass, exhibiting synaptospermy in that the 1–5 seeds in the dispersal unit (bur) are not released prior to germination. A major positive effect is that at least 50% of the burs contain both a male and female, making sexual reproduction possible and reducing founder effects upon colonization by one or a few burs. The objectives of this research were to examine the relationships between synaptospermy and dioecy in the life history strategies of Buchloe dactyloides, and to investigate whether the potential costs of synaptospermy would exceed its benefits if the species were not dioecious. An experiment that tested the effects of tightly clumping three seedlings vs. a uniform spacing of single seedlings at two densities and two moisture levels indicated only slightly negative effects of this aspect of synaptospermy. Although number of tillers and shoot biomass per plant were lower for clumped seedlings at all density and moisture combinations, only 50% of the possible uniform vs. clumped comparisons were significantly different. In addition, these effects, even at low moisture and/or high density, were not great enough to affect survivorship during the 16‐week growth period. Experiments under both optimal and low moisture conditions showed that seeds within burs germinated at a significantly higher percentage than isolated or “hulled” seeds; in contrast to the differential timing of germination from burs, almost all of the seedlings from hulled seeds were produced during the first of three germination periods. Although survivorship for these seedlings was quite high under optimal conditions, it was greatly reduced under low moisture, being only one‐fourth of that for seedlings arising from burs. The possible reduction in the wind dispersal of propagules due to their teeth and to their greater weight and size appears offset by more effective ungulate and water dispersal. Considering the evidence that the bur anchors the seedling in the soil, protects the seeds from fire, reduces precocious germination by requiring considerable moisture for saturation, and enhances seed longevity, the benefits of synaptospermy appear to outweigh the costs, even if those benefits related to dioecy are not considered.

Habitat assortment of sexes and water balance in a dioecious grass.

For a dioecious plant species in which males are associated with more xeric habitats and females with more mesic ones, (a) the xeric-mesic habitat difference was confirmed by measuring plant water potential, (b) and males and females had similar water balances and seemed to have no different adaptations to drought. There are slight differences in water potential between the sexes of dioecious plant species, but water balance can be more favorable in either the male or the female. On this account, we reject the "disruptive selection" hypothesis of Freeman et al. (1975) as an explanation for habitat assortment of sexes in dioecious plants. Alternative explanations, based upon parental determination of offspring sex ratios, or environmentally determined sex change, seem more likely.

SEX AND SUPERNUMERARY CHROMOSOMES IN THE DIOECIOUS GRASSSOHNSIA FILIFOLIA

A dioecious grass Sohnsia filifolia (Fourn.) Airy Shaw (Syn. Calamochloa filifolia Fourn.) from Mexico has been found to have 2n = 20 chromosomes in both male and female plants. The staminate plants have one chromosome much longer than the other chromosomes of the complement. One pistillate plant was found to have 30 chromosomes, among which the largest chromosome is quite similar to the largest component of the diploid male plant. The longest chromosome has been designated as the Y chromosome. An XY-mechanism of the Drosophilia type has been suggested for the sex determination system in this species. One small supernumerary chromosome was observed in the microsporocytes of some male plants, but was absent in roots.