E. Durant McArthur

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.

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.

Age Structure and Expansion of Piñon-Juniper Woodlands: A Regional Perspective in the Intermountain West

Numerous studies have documented the expansion of woodlands in the Intermountain West; however, few have compared the chronology of expansion for woodlands across different geographic regions or determined the mix and extent of presettlement stands. We evaluated tree age structure and establishment for six woodlands in four ecological provinces in the central and northern Great Basin. Since 1860, the area occupied by pinon and or juniper has increased 125 to 625 percent. The increase of trees was a result of infill into shrub-steppe communities with relatively open low density stands of trees and expansion of pinon and juniper into sagebrush-steppe communities that previously did not support trees. Woodland expansion in Oregon, Utah, and Nevada were similar, but began two to three decades earlier in Idaho. The majority of woodlands are still in the early to mid phases of stand closure, which means they often support an understory of shrubs and herbaceous vegetation. This has implications for future changes that will occur within these woodlands in the next 30 to 50 years. In the absence of disturbance or management, the majority of these landscapes will become closed woodlands resulting in the loss of understory plant species and greater costs for restoration.

Cytogeography and chromosome evolution of subgenus Tridentatae of Artemisia (Asteraceae)

The subgenus Tridentatae of Artemisia (Asteraceae: Anthemideae) is composed of 11 species of various taxonomic and geographic complexities. It is centered on Artemisia tridentata with its three widespread common subspecies and two more geographically confined ones. Meiotic chromosome counts on pollen mother cells and mitotic chromosome counts on root tips were made on 364 populations ( = 3.1 plants per population). These population counts are ∼60% of all Tridentatae counts. Some are first records for taxa. The Tridentatae are a polyploid complex (x = 9) with ploidy levels from 2x to 8x, but mostly 2x (48%) and 4x (46%). Polyploidy occurs in nine of the 11 species and in many subspecies as well. Supernumerary or b chromosomes are present only at a low frequency. In the principal species, A. tridentata, 2x plants are larger than 4x ones, which are adapted to drier conditions, probably in consequence of their slower growth rates. Gigas diploidy is a phenomenon shared by some other woody genera, but is in contrast to the gigas polyploid nature of many herbaceous genera. Polyploidy occurs within populations and is essentially autoploid. Hybridization sometimes occurs at taxa interfaces in stable hybrid zones. Stable Tridentatae hybrid zones coupled with the groups inherent propensity for polyploidization has led to the establishment of a geographically and numerically large and successful complex of species.

GERMINATION RESPONSE OF ARTEMISIA TRIDENTATA (ASTERACEAE) TO LIGHT AND CHILL: PATTERNS OF BETWEEN-POPULATION VARIATION

Artemisia tridentata Nutt. is the dominant shrub over much of semiarid North America. It consists of several subspecies and includes ecotypes that occur over a range of habitats from warm desert fringes to montane forest and meadow communities. Fifteen seed collections representing the three common subspecies and a spectrum of habitats were subjected to a series of laboratory light and chill treatments. Recently harvested seeds were mostly nondormant at 15 C but required light for full germination. Removal of the pericarp, afterripening in dry storage, and short chill treatments all resulted in a reduction in light requirement. When examined on a by-collection basis, seed germination response variables were significantly correlated with each other and with mean January temperature at the seed collection site, an index of winter severity. Collections from colder sites were 16%-36% dormant in the light at 15 C and nearly 100% light-requiring, while collections from warm desert fringes were nondormant in light and only 50%-70% light-requiring. Cold winter collections required up to 98 d to germinate to 50% at 1 C in light, whereas warm winter collections germinated to 50% in as little as 16 d. There was no clear relationship between germination behavior and subspecific identity. The observed climate-correlated variation in germination response to light and chill appears to be of adaptive significance, but a genetic basis for patterns of infraspecific variation in germination response in this species has not yet been demonstrated.

Fire Rehabilitation Using Native and Introduced Species: A Landscape Trial

Abstract Following the 1999 Railroad Fire in Tintic Valley, Utah, we initiated a large-scale fire rehabilitation study comparing a predominately introduced species seed mix used by the US Department of Interior–Bureau of Land Management (BLM), a mix of native and introduced species provided by the US Department of Agriculture–Agricultural Research Service (ARS), and 2 native seed mixes (high and low diversity). Mixes were seeded with a rangeland drill on the big sagebrush (Artemisia tridentata var. wyomingensis [Beetle & A. Young] Welsh) study area whereas the pinyon–juniper (Pinus edulis Engelm.–Juniperus osteosperma [Torr.] Little) woodland study area was aerially seeded followed by 1-way chaining. On drill-seeded plots and by the third year after seeding the native high-diversity mix (16.4 kg pure live seed [PLS]·ha−1) had the highest seeded species cover (11.5%) and density (14 plants·m−2). Both the BLM (9.3 kg PLS·ha−1) and ARS (9.1 kg PLS·ha−1) seed mixes had higher seeded species cover (BLM = 8.5%, ARS = 8.2%) and density (BLM = 8.4 and ARS = 7.2 plants·m−2) than plots seeded to the low-diversity native mix (8 kg PLS·ha−1, cover = 3.8%, density = 3.6 plants·m−2). Indian ricegrass (Achnatherum hymenoides [Roemer and J. A. Schultes] Barkworth ‘Nezpar’) in the native high-diversity mix was especially successful on the sandy soils of the drill site, whereas seeds of other species may have been buried too deep for optimum emergence. Aerially-seeded and chained plots had similar and successful seeded species frequency, cover, and density (third-year average = 10.6% cover, 17.2 plants·m−2) among all species mixes. All seeded plots had lower cover of annual species than unseeded plots, indicating that revegetation is necessary to reduce weed invasion following catastrophic wildfire in big sagebrush communities lacking residual perennial understory vegetation.

The influence of topography on male and female fitness components of Atriplex canescens

The influence of environmental heterogeneity on components of male and female fitness is examined using Atriplex canescens growing on steep slopes and alluvium at the slope base as a model system. Female fitness is estimated as the grams of fruit produced per plant and the grams of fruit per gram leaf tissue. Male fitness is estimated as the grams of stamens produced, the number of pollen grains dispersed to a given distance, and the potential number of grams of fruit sired taking into consideration the number and distribution of mates and competing pollen donors. The influence of increased plant size on male and female fitness components, the cost of reproduction (as measured by biomass, joules and nitrogen) on a gross level and a per offspring basis are also examined. The results indicate that the female function is more limited on the slope than the male function. The efficiency of pollen dispersal (the number of pollen grains per unit donor plant volume dispersed to a given distance) is enhanced by growing on slopes. Males become less efficient at dispersing pollen as they increase in size, while the efficiency of female reproduction (grams of fruit per volume or gram leaf tissue) is unaffected by increasing plant size. The cost to a male of siring a gram of fruit is about the same as the cost to a female of producing the gram of fruit. Implications for the evolution of sexual lability and dioecy are discussed.

Are trioecy and sexual lability in Atriplex canescens genetically based?: evidence from clonal studies

Prior studies have alternatively considered floral phenotypes in Atriplex canescens as trioecious (having three sexual genders) and/or dioecious and having a “leaky genetical switch.” Clones transplanted from three populations and grown in common gardens reveal the existence of two distinctly different genetic controls regulating gender expression. In some clones gender is fixed as male (staminate) or female (pistillate), while in other clones gender varies, ranging from a mixture of male and female ramets to simultaneous hermaphrodites with various proportions of male and female flowers. For clones which vary their sex expression, variation occurs within irrigation treatments, between treatments and over time, as a consequence of the combined effects of genotype plus environment. The magnitude of sex change is also a product of the interaction of genetics and environment. Some clones have been repeatedly examined for 20 years.

Variable responses of insects to hybrid versus parental sagebrush in common gardens

Both ecological and genetic mechanisms have been proposed to explain patterns of herbivore attack on interspecific plant hybrids, but distinguishing among them can be difficult in natural hybrid zones. We performed a common-garden experiment to evaluate four genetic hypotheses: dominance, additivity, elevated hybrid susceptibility, and elevated hybrid resistance. Censuses and cage experiments were used to compare insect responses to basin big sagebrush (Artemisia tridentata spp. tridentata), mountain big sagebrush (A. t. vaseyana), and their F2 progeny. After two growing seasons, hybrid shrubs resembled mountain big sagenbrush in size, but were more similar to basin big sagebrush in flower production. Censuses of naturally colonizing insects (the gall midge Rhopalomyia obovata, the bagworm moth Apterona helix, and the aphid Obtusicauda coweni) tended to support the dominance hypothesis: if the insect clearly discriminated between the two parents, its frequency on hybrids closely resembled that on one of the parents. Moreover, colonization of hybrids in all three cases suggested a dominance deviation toward the susceptible parent rather than toward the resistant parent. In contrast to the censuses, cage experiments involving two insects supported the hybrid-susceptibility hypothesis; both survival and growth of the grasshopper Melanoplus sanguinipes and growth of the leaf beetle Trirhabda pilosa were higher on hybrid shrubs than on either parent. Because many secondary compounds have been determined to occur at intermediate concentrations in F2 shrubs, dominance for susceptibility may indicate that insects respond to plant traits (e.g., oviposition stimulants and deterrents) in a threshold manner. Mechanisms underlying increased hybrid susceptibility are less clear, but our experimental design makes environmental explanations (e.g., the plant-stress hypothesis) unlikely. Although we eliminated several confounding factors, our results agree with the conclusion from natural hybrid zones that insect responses to hybrid plants are likely to be idiosyncratic; even congeneric species did not respond similarly to hybrid and parental plants.