D. Carl Freeman

Antisymmetry, directional asymmetry, and dynamic morphogenesis

Fluctuating asymmetry is the most commonly used measure of developmental instability. Some authors have claimed that antisymmetry and directional asymmetry may have a significant genetic basis, thereby rendering these forms of asymmetry useless for studies of developmental instability. Using a modified Rashevsky-Turing reaction-diffusion model of morphogenesis, we show that both antisymmetry and directional asymmetry can arise from symmetry-breaking phase transitions. Concentrations of morphogen on right and left sides can be induced to undergo transitions from phase-locked periodicity, to phase-lagged periodicity, to chaos, by simply changing the levels of feedback and inhibition in the model. The chaotic attractor has two basins of attraction-right sidedominance and left side dominance. With minor disturbance, a developmental trajectory settles into one basin or the other. With increasing disturbance, the trajectory can jump from basin to basin. The changes that lead to phase transitions and chaos are those expected to occur with either genetic change or stress. If we assume that the morphogen influences the behavior of cell populations, then a transition from phase-locked periodicity to chaos in the morphogen produces a corresponding transition from fluctuating asymmetry to antisymmetry in both morphogen concentrations and cell populations. Directional asymmetry is easily modeled by introducing a bias in the conditions of the simulation. We discuss the implications of this model for researchers using fluctuating asymmetry as an indicator of stress.

Developmental stability in plants: symmetries, stress and epigenesis

Plant developmental stability has received little attention in the past three or four decades. Here we review differences in plant and animal development, and discuss the advantages of using plants as experimental subjects in exploring developmental stability. We argue that any type of developmental invariant may be used to assess developmental stability and review the use of fluctuating asymmetry in studies of plant developmental stability. We also examine the use of deviations from translatory, radial, and self-symmetry as measures of developmental instability. The role of nonlinear dynamics and epigenesis in the production of the phenotype is also discussed.

Sexual specialization and inbreeding avoidance in the evolution of dioecy

Dioecy has evolved independently, many times, among unrelated taxa. It also appears to have evolved along two contrasting pathways: (1) from hermaphroditism via monoecy to dioecy and (2) from hermaphroditism via gynodioecy to dioecy. Most dioecious plants have close cosexual relatives with some means of promoting outcrossing (e.g., herkogamy, dichogamy, self-incompatibility, or monoecy). To the extent that these devices prevent inbreeding, the evolution of dioecy in these species cannot logically be attributed to selection for outcrossing. In these cases, the evolution of dioecy is, we believe, due to selection for sexual specialization. However, in other species, that lack outbreeding close relatives, dioecy may have evolved from gynodioecy (males and hermaphrodites) as an outbreeding device. Subsequent disruptive selection and selection for sexual specialization may have also shaped the evolution of dioecy from gynodioecy in these species, resulting in two genetically determined, constant sex morphs.Both pathways for the evolution of dioecy require the operation of disruptive selection, though the gynodioecy route involves more restrictive disruptive selection and a genetic designation of gender. In contrast, the monoecy route is not dependent on the genetic designation of two sex morphs, but, rather, allows the possibility of sexual intermediates and sexual lability. Both pathways produce one morph in which maleness is suppressed and another in which the female function is negligible or nonexistent—the reproductive mode recognized as dioecy.Evidence is presented here to support the thesis that instances of sexual lability, the presence of an array of sexual intermediates, sex-switching, and sexual niche segregation can be explained in terms of the pathway that was taken in the evolution of a particular dioecious species. In addition, the degree of sexual dimorphism seen in dioecious species is correlated with mode of pollination (insector wind-pollinated) and other ecological factors.

INFLUENCE OF ENVIRONMENT ON THE FLORAL SEX RATIO OF MONOECIOUS PLANTS

Both theoretical and empirical data suggest that sessile, simultaneous hermaphroditic individuals growing in patchy environments may experience unequal reproductive success via male and female gametes depending on whether they occupy favorable or marginal sites (Schaffner, 1935; Heslop-Harrison, 1972; Freeman et al., 1976, 1980; Charnov and Bull, 1977; Horovitz, 1978; Charnov, 1979a, 1979b; Willson, 1979). Under such conditions, differential tendencies toward maleness and femaleness should be selected for among individuals confined to favorable or unfavorable patches. Further, simultaneously hermaphroditic individuals should display large variations in the sex ratio of flowers on different individuals (Willson, 1979). Such variation might be expected to be controlled by both genetic and environmental cues, if the patches are small and/or the quality of any given patch varies significantly between breeding seasons (Heslop-Harrison, 1972; Freeman et al. 1976; Charnov and Bull, 1977; Frankel and Galun, 1977; Willson, 1979). In 1975, we began a series of studies to determine if the sex ratios of dioecious species differed on sites of differing water availability. We demonstrated that, in five dioecious species, males were proportionately more abundant on xeric sites than on mesic sites, while the females were over-represented on mesic sites (Freeman et al., 1976). Others have made similar observations (Pickett, 1915; Davey and Gibson, 1917; Schaffner, 1922; Lysova and Khizhnyak, 1975; Richards, 1975). We proposed that this pattern was due to disruptive selection acting on the differential success of male and female gametes on sites of differing quality. It seems likely that such forces may have been important in the evolution of the dioecious habit, at least in arid environments. Since disruptive selection appeared to alter the relative success of the male and female functions of dioecious species occupying a local array of sites that differed in the amount of available soil moisture during much of the growing season, it seemed likely that monoecious plants would also differentially emphasize maleness and femaleness on sites of unequal quality. In this note we examine the floral sex ratios of three monoecious species, and how these ratios differ on sites of differing water availability. Since our data (unpubl.) show that most temperate monoecious species are wind pollinated, we have restricted our studies to wind pollinated species.

Differences in native soil ecology associated with invasion of the exotic annual chenopod, Halogeton glomeratus

Various biotic and abiotic components of soil ecology differed significantly across an area where Halogeton glomeratus is invading a native winterfat, [ Krascheninnikovia (= Ceratoides) lanata] community. Nutrient levels were significantly different among the native, ecotone, and exotic-derived soils. NO3, P, K, and Na all increased as the cover of halogeton increased. Only Ca was highest in the winterfat area. A principal components analysis, conducted separately for water-soluble and exchangeable cations, revealed clear separation between halogeton- and winterfat-derived soils. The diversity of soil bacteria was highest in the exotic, intermediate in the ecotone, and lowest in the native community. Although further studies are necessary, our results offer evidence that invasion by halogeton alters soil chemistry and soil ecology, possibly creating conditions that favor halogeton over native plants.

Narrow hybrid zone between two subspecies of big sagebrush (Artemisia tridentata : Asteraceae) XII. Galls on sagebrush in a reciprocal transplant garden

Several species of gall-forming insects specialize on big sagebrush (Artemisia tridentata), a species that shows much clinal and subspecific variation throughout its geographic range. Two of those subspecies, basin big sagebrush (A. t. ssp. tridentata) and mountain big sagebrush (A. t. ssp. vaseyana), form a narrow hybrid zone at Salt Creek, Utah. Reciprocal transplant experiments have shown that the hybrid big sagebrush at Salt Creek are more fit than either parental subspecies, but only in the hybrid zone. Do genotype and environment influence the density and distribution of galls on big sagebrush? We counted galls on parental and hybrid big sagebrush in three reciprocal transplant gardens at Salt Creek. Gardens were in each of the two parental zones and in the hybrid zone. Transplanted seedlings came from five source populations: two parental and three hybrid populations. We identified seven kinds of gall-forming flies (Rhopalomyia midges and Eutreta fruitflies) that produced identifiable galls. Densities of galls varied among the three gardens and five source populations, and there was also a significant garden by source interaction in gall density. In general, variation in gall density among gardens (i.e., environments) was much greater than the variation among source populations (i.e., genotypes). Nevertheless, significant genotype-environment interactions were observed for five of the seven kinds of galls. Overall density of galls, mostly due to Rhopalomyia ampullaria, was greatest in the high-elevation (mountain) garden and least in the low-elevation (basin) garden. To the best of our knowledge, this is the first reciprocal transplant experiment addressing herbivore richness in a hybrid zone.

Ecology of plant dioecy in the intermountain region of Western North America and California

SummaryData from 44 plant communities of the arid zone of western North America and from the entire California flora suggest that the dioecious habit (separate sexed individuals) is best developed among wind pollinated woody plants. A rationale is presented for the ways in which wind pollination and large plant size have favored the development of the dioecious habit. Based upon the patterns examined, it is concluded that inbreeding depression alone is insufficient to account for the evolution of the dioecious habit in many temperate species.

THE INFLUENCE OF ENVIRONMENT ON THE SEX RATIO AND FITNESS OF SPINACH

Spinach, Spinacia oleracea var. americana, was grown under water-limited (DRY) and -abundant (WET) conditions in a number of controlled experiments. Four factors influenced the observed sex ratios of spinach: (1) precocious flowering of males, (2) WET and DRY conditions, (3) photoperiod, and (4) seed source. The reproductive potentials of male, female, and monoecious plants differed between WET and DRY environments, and environmental sex determination (ESD) operated as an adaptive response to a differential in fitness between the sexes. The data also indicated that compensation, differential movement of gametes between the sexes, partial niche separation, and ESD may have played a role favoring the dioecious state in spinach.

Shoulder outcome measures: a comparison of 6 functional tests.

Background Several shoulder function scores are used in research, with no universally adopted standard. This study compares 6 shoulder outcome scales. Hypothesis Correlations exist between shoulder outcome scales, allowing conversion between scales. Shoulder scales are correlated with age. Study Design Regression and correlation study. Methods Seventy subjects with shoulder pain completed 6 shoulder outcome scales. Pearson correlations were calculated between the total scores of the 6 instruments, between the components of the scales, and with age. Regression equations were calculated between scales. Results The range of r values for total scores was 0.495 [.lessequal] r [.lessequal] 0.770, P [.lessequal] .01. In general, a scales components were themselves highly correlated and added little new information to the scale (0.260 [.lessequal] r [.lessequal] 0.705, P [.lessequal] .05). Most of the scale scores were highly correlated with age (0.291 [.lessequal] r [.lessequal] 0.582, P [.lessequal] .05). Constants reported corrections for age reduced (from r = -0.582 to r = -0.250, P < .05) but did not eliminate age as a confounding variable. Conclusions Correlations exist between shoulder outcome scales, but existing shoulder scales are not equivalent in their assessments of function; they contain redundant information and, in some cases, may reflect a patients age better than his/her shoulder function. The utility of conversion equations is minimized as a result of low to moderate correlations between scales.

Ecological Effects of Ranching: A Six-Point Critique

Abstract Ranching is the dominant land use in much of the American West. Although a copious literature has examined the effects of various grazing practices on native ecosystems, we present here the idea that ranching has important impacts on the land independent of those caused by grazing itself. If biological conservation is to be successful on the western grasslands and shrublands, ranchers must be central to any plan. Focusing on the Great Plains of the United States, and on Wyoming in particular, we raise six points of concern that must be addressed before we can hope to restore or maintain native ecosystems on the range.