INBREEDING DEPRESSION IN PARTIALLY SELF-FERTILIZING PHLOX

Inbreeding depression was measured in terms of the relative survivorship of self versus cross seed in 14 Phlox drummondii cultivars and 10 populations of P. cuspidata. The cultivars are predominantly outcrossing; P. cuspidata is predominantly self-fertilizing. The relative survivorship of self versus cross seed averages 0.84 in the cultivars and 0.99 in P. cuspidata. In wild P. drummondii, the average is 0.83. The mean number of lethal equivalents per zygote averages 0.69 in the cultivars and 0.05 in P. cuspidata, versus 0.79 in the wild P. drummondii. There is a significant negative correlation between the amount of automatic self-seed produced by a cultivar and the number of lethal equivalents it carries (r = ?0.60). Phlox cuspidata has a lower level of cross-seed abortion than P. drummondii, which probably is not due to a difference in the incidence of lethal genes in their populations.     

QUANTITATIVE VARIATION IN PHLOX: COMPARISON OF SELFING AND OUTCROSSING SPECIES

Variation of 20 quantitative characters was examined within and among 10 populations of the predominantly outcrossing Phlox drummondii and 4 populations of the predominantly selfing P. cuspidata grown in a greenhouse. Multivariate analysis of variance, considering all characters simultaneously, indicated that there were significant differences among populations in both species while analysis of individual characters demonstrated that there were significant population differences for 19 characters in P. drummondii and 13 characters in P. cuspidata. On average, 16% of the total phenotypic variation in P. drummondii occurred among populations compared to less than 4% of the total variation in P. cuspidata. In addition, P. drummondii exhibited significant differences among families within populations more frequently than P. cuspidata. Most observed variation in both species occurred within families where environmental and genetic sources of variation could not be partitioned. There was a trend for P. drummondii to have higher heritabilities than P. cuspidata for most characters even when assumptions about breeding systems were relaxed. Thus, the outbreeding species exhibited larger genetic differences among populations and among families within populations than the selfing species in the greenhouse environment. These data suggest that P. drummondii has the greater evolutionary potential of the two species and are consistent with the hypothesis that differences in population structure result from differences in the breeding systems of the two species.     

Breeding systems and hybridization inPetrorhagia sect.Kohlrauschia (Caryophyllaceae)

In the diploid speciesP. prolifera, the evolution of autogamy has resulted in the presence of distinct selfing and outcrossing races. The change in breeding system toward autogamy is associated with a reduction in the size of floral features and a lower pollen production. The outbreeding populations have only been found in Greece, while the selfing populations have colonized much of Europe. The two races appear to be reproductively isolated from one another. —Petrorhagia sect.Kohlrauschia contains four species.P. glumacea is outbreeding and shows unilateral incompatibility with the small-flowered race ofP. prolifera and interfertility with the large-flowered race; althoughP. glumacea is sympatric with the large-flowered race, they do not appear to hybridize in the wild. Amongst the allopatric species, internal breeding barriers are also found. These may take the form of hybrid sterility or seed incompatibility.P. velutina is autogamous and reproductively isolated from all the other species of the section regardless of whether they are sympatric or allopatric. It is suggested that the evolution of autogamy has been an important factor in the spread ofP. velutina, the tetraploidP. nanteuilii, and the small-flowered race ofP. prolifera in Europe.     

AUTOGAMY AND INBREEDING DEPRESSION IN MOUNTAIN LAUREL,KALMIA LATIFOLIA (ERICACEAE)

We compared breeding systems, inbreeding depression, and pollination limitation between two populations of Kalmia latifolia in Virginia and Rhode Island. Plants were autogamous in Virginia but not Rhode Island. Although autogamy with selling is hypothesized to reduce levels of inbreeding depression, both populations showed similarly high inbreeding depression manifested as reduced fruit set. Autogamy may be uncorrelated with selling rate because autogamy is not obligate and because geitonogamy is likely. Autogamy in the Virginia population seems most likely to have evolved for reproductive assurance under competition for pollinator service. Fruit set was pollination limited in the Virginia population apparently because bumblebee pollinators were more attracted to a coflowering species, Vaccinium erythrocarpum.     

Mating system and the hybridization between self-compatible Phlox cuspidata and self-incompatible Phlox drummondii

Mating system can impact the frequency of hybridization and therefore the maintenance of species diversity. I evaluate the effects of weak self-incompatibility (SI) in Phlox cuspidata and SI in Phlox drummondii on mating success within species and on hybridization dynamics between species under controlled conditions. The effects of SI on hybridization frequency were assessed by manipulating the relatedness of conspecific pollen and the relative timing of pollen deposition in mixed-donor interspecific pollinations. Selfing as opposed to outcrossing increased hybridization by 16% in P. cuspidata maternal plants and by 48% in P. drummondii maternal plants because self pollen did not compete as well against heterospecific pollen. The relative timing of conspecific versus heterospecific pollen deposition also impacted hybridization. In self-compatible P. cuspidata, the deposition of self pollen 5 h earlier than heterospecific pollen decreased hybridization by 28%. In self-incompatible P. drummondii, a 5 h delay in the deposition of compatible conspecific pollen increased hybridization by 32%. In this hybrid system, early self-pollination can decrease hybridization (but increase inbreeding) by P. cuspidata maternal plants, and SI may increase hybridization by P. drummondii maternal plants.     

PHENOTYPIC PLASTICITY IN PHLOX. I. WILD AND CULTIVATED POPULATIONS OF P. DRUMMONDII

We investigated the changes in amounts and patterns of phenotypic plasticity which have arisen in the Texas annual Phlox drummondii during domestication. Character means and plasticities were compared for five populations: a wild population, three cultivated varieties (a Tall cultivar and two Dwarf cultivars), and a population of an escaped Tall cultivar naturalized in Texas. To measure plasticity, we scored the responses of 10 characters to six treatments and analyzed both the amount and direction of plastic response. Wild plants are phenotypically distinct from the Tall and Escaped cultivar and from the two Dwarf cultivars. Despite its substantial phenotypic divergence from the Wild population, the Tall cultivar's plasticity has changed little during domestication. Traits most strongly correlated with fitness show the least change in their plasticities. The two Dwarf varieties have very similar plasticities, despite strong phenotypic divergence from the Tall population and despite the fact that they were derived from different Tall lines. This suggests that indirect selection on phenotypic plasticity related to selection for the Dwarf habit has resulted in the characteristic plasticity of the Dwarf lines. The Escaped cultivar has substantially different plastic responses from those of the Wild or cultivated populations.     

Life Cycle of Autogamous Strains of Euplotes minuta*

SYNOPSIS. The life cycles of 5 autogamous strains of Euplotes minuta are reported. Interautogamic intervals (measured as number of fissions) are quite variable among clones belonging to the same strain, while their variability is much reduced (15 fissions) among sublines of the same clone. By selecting the clone with the shortest immature period to start successive autogamous generations, it has been found that all clones undergo autogamy almost synchronously and have a very short period of immaturity at the 5th autogamous generation. Both conjugation and autogamy, however, are consistently followed by immature periods in all autogamous strains examined. Mating capacity as well as competence for autogamy are reached almost simultaneously in all clones of the strains studied with the exception of about 1/3 of the A-31 clones in which autogamy occurs significantly earlier than conjugation. The results are discussed from the genetic point of view and in relation to the sexual mechanisms operating in nature within different populations of the species.     

HYBRIDIZATION BETWEEN ANNUAL SPECIES OF PHLOX: POPULATION STRUCTURE

Phlox cuspidata (n = 7) and P. drummondii subsp. drummondii (n = 7) are closely related annuals which are indigenous to eastern and central Texas. The species typically occupy different ecological niches but may form contiguous or confluent populations in disturbed habitats and hybridize therein. On the basis of correlative interpretations of exomorphic, chromatographic and fertility information, hybridizing populations can be segregated into three distinct classes: (1) highly fertile plants with the morphological and phenolic attributes of P. drummondii; (2) highly fertile plants with the morphological and chemical attributes of P. cuspidata; (3) sterile plants with manifestly intermediate morphology and complementary chromatographic patterns. These data strongly suggest that hybridizing populations of P. drummondii and P. cuspidata are tritypic, being composed of “pure” or essentially “pure” parental species and a group of plants which has all of the attributes characteristic of an F₁ hybrid.     

The breeding systems of six species of Arabis (Brassicaceae)

The ability of organisms to produce genetic variation for any trait, including resistance to pathogens, is partially determined by breeding system. I used enzyme electrophoresis, crossing experiments, and cytology to assess the breeding systems of cooccurring Arabis species that are often infected by rust fungi. The Arabis holboellii surveyed were pseudogamous apomicts with relatively high population-level ailozyme variation, and variable chromosome numbers (2n, 3n, 4n). Arabisgunnisoniana (3n) and Arabis lignifera (2n) were also pseudogamous but showed no allozyme variation either at the population level, or within progeny arrays. Arabis hirsuta may be an autogamous polyploid or it may be a pseudogamous apomict; more work is needed to clarify the breeding system of this species. Arabis drummondii and Arabis crandallii were sexual, but exhibited little genetic variability due to a predominance of self-fertilization. The use of several techniques was necessary to evaluate these breeding systems. Insect exclusion indicated when pollen was necessary for seed set, but could not differentiate between sexual reproduction and pseudogamy. Electrophoresis yielded information on the degree of selling (as evidenced by homozygosity) and apomixis (fixed heterozygosity), but could not differentiate between autogamy and apomixis in polyploids without ailozyme variation. Pseudogamy was confirmed when crosses between dissimilar genotypes yielded only the maternal genotype, and cytologically by irregular meiosis.     

Molecular signatures of selection on reproductive character displacement of flower color in Phlox drummondii

Character displacement, which arises when species diverge in sympatry to decrease competition for resources or reproductive interference, has been observed in a wide variety of plants and animals. A classic example of reproductive character displacement, presumed to be caused by reinforcing selection, is flower‐color variation in the native Texas wildflower Phlox drummondii. Here, we use population genetic analyses to investigate molecular signatures of selection on flower‐color variation in this species. First, we quantify patterns of neutral genetic variation across the range of P. drummondii to demonstrate that restricted gene flow and genetic drift cannot explain the pattern of flower‐color divergence in this species. There is evidence of extensive gene flow across populations with different flower colors, suggesting selection caused flower‐color divergence. Second, analysis of sequence variation in the genes underlying this divergence reveals a signature of a selective sweep in one of the two genes, further indicating selection is responsible for divergence in sympatry. The lack of a signature of selection at the second locus does not necessarily indicate a lack of selection on this locus but instead brings attention to the uncertainty in depending on molecular signatures to identify selection.     

DYNAMICS OF A HYBRID ZONE IN PHLOX: AN EXPERIMENTAL DEMOGRAPHIC INVESTIGATION

The dynamics of a narrow hybrid zone between Phlox drummondii ssp. drummondii and ssp. mcallisteri was investigated using an experimental demographic approach. Three populations of each subspecies as well as those from the hybrid zone were involved. Experimental plots containing seeds from each of the nine populations were established in each of the eight sites. Germination, survivorship to reproduction, and fecundity were monitored at 4–6 wk intervals. Finite rates of increase were estimated from these data. There were significant site effects for each demographic variable and rate of increase as well as significant population effects for germination and finite rate of increase. Although there were population effects, no significant differences were found between plant types in sites of the subspecies or the hybrid zone. We reject the hypothesis that the narrow width of the hybrid zone is the result of hybrid disadvantage. We propose that the narrow width is a result of restricted gene flow and discuss the supporting evidence.     

Pleospora drummondii spec. nov. und ihr Anamorph Stemphylium drummondii spec. nov.

Pleospora drammondii spec. nov. and its anamorph Stemphylium drammondii spec. nov. The causal organism of a leaf spot disease on Phlox drummondii is described as Pleospora drummondii spec. nov. and its anamorph Stempbylium drummondii spec, nov. — in contrast to earlier reports from the USA, in which the fungus was identified as S. botryosum.     

Alcohol dehydrogenase activity in diploid and autotetraploid Phlox

The effect of polyploidy on the activity of alcohol dehydrogenase was examined in a series of diploid and synthetic autotetraploid Phlox drummondii. In most cases autotetraploids had about twice as much activity as the corresponding diploids, in two cases autotetraploids had about 1.5 times more activity, and in one wild seed pair the activity of the tetraploid was somewhat lower.This study was supported in part by NSF grants (DEB 76-19914, DAL; 4392, AMT, BMS 75-19621, ML).     

Morphological Drift Accompanying Nascent Population Differentiation in the Ciliate Euplotes vannus

A rare phenomenon can occur in ciliated protists of the genus Euplotes, which can undergo genetic recombination by the normal outbreeding process of conjugation following mild starvation. Occasionally, the dominant mutation for the autogamy trait arises. Individuals possessing the trait show obligate self-fertilization upon mild starvation. This yields, after normal asexual division, a population of individuals that are reproductively isolated from the parental outbreeding strain. A morphometric analysis of sympatric autogamous and non-autogamous populations of Euplotes vannus from Somalia demonstrates that there has been morphological drift in gross body proportions in the autogamous populations. However, the positional patterns of the locomotory organelles on the ventral surface remain unchanged. The changes in body proportions in the autogamous populations are relevant to the mechanics of the conjugation process, which involves fusion of the oral regions of paired cells belonging to complementary mating types.     

Pollen competition and environmental effects on hybridization dynamics between Phlox drummondii and Phlox cuspidata

Pollen competition between species strongly influences hybridization dynamics in plants. By performing single- and mixed-donor pollinations, we show that soil Ca alters the outcome of interspecific pollen competition in the annual Phlox hybrid system of Phlox cuspidata and P. drummondii. In the absence of interspecific pollen competition, heterospecific pollen siring success of both species was influenced most strongly by the maternal growth environment, such that hybridization was facilitated when heterospecific pollen was deposited on stigmas of maternal plants growing in high Ca soils. When heterospecific pollen was forced to compete against conspecific pollen, however, the maternal growth environment did not influence hybridization, but the environmental origin of heterospecific pollen did, and this effect depended on the maternal species. Pollen of P. drummondii was more effective at outcompeting P. cuspidata pollen and preventing hybridization in P. drummondii dams when P. cuspidata pollen was derived from low Ca. Pollen competition within pistils of P. cuspidata was unaffected by pollen Ca environment. In situations in which P. cuspidata grows in lower soil Ca than P. drummondii, as has been documented in one population, these results suggest that the competitive ability of heterospecific pollen will be diminished by environmental effects of soil Ca. Thus, the environment in which pollen develops can influence interspecific pollen competition and hybridization frequency.     

NONFRUITING HERMAPHRODITIC FLOWERS OF CALOCHORTUS LEICHTLINII (LILIACEAE): POTENTIAL REPRODUCTIVE FUNCTIONS

The breeding system, flowering display, and fruiting pattern of Calochortus leichtlinii Hook. (Liliaceae) was studied in the eastern Sierra Nevada. This species is capable of both autogamy and insect-mediated outcrossing. Either of these two means of pollination alone could affect full seed set; the populations studied were not pollination limited in 1983. Most individuals produced two flowers but set only one fruit. Flowers are not selectively matured on the basis of the quantity or purported quality of the pollen they receive; the first flower to open was matured in all experiments. Nonfruiting flowers have at least three potentially important reproductive functions: disseminating pollen, allowing plants to produce more seeds in response to an abundance of soil moisture, and setting seeds if the first flower fails to develop fruit.     

Variation in context‐dependent foraging behavior across pollinators

Pollinator foraging behavior has direct consequences for plant reproduction and has been implicated in driving floral trait evolution. Exploring the degree to which pollinators exhibit flexibility in foraging behavior will add to a mechanistic understanding of how pollinators can impose selection on plant traits. Although plants have evolved suites of floral traits to attract pollinators, flower color is a particularly important aspect of the floral display. Some pollinators show strong innate color preference, but many pollinators display flexibility in preference due to learning associations between rewards and color, or due to variable perception of color in different environments or plant communities. This study examines the flexibility in flower color preference of two groups of native butterfly pollinators under natural field conditions. We find that pipevine swallowtails (Battus philenor) and skippers (family Hesperiidae), the predominate pollinators of the two native Texas Phlox species, Phlox cuspidata and Phlox drummondii, display distinct patterns of color preferences across different contexts. Pipevine swallowtails exhibit highly flexible color preferences and likely utilize other floral traits to make foraging decisions. In contrast, skippers have consistent color preferences and likely use flower color as a primary cue for foraging. As a result of this variation in color preference flexibility, the two pollinator groups impose concordant selection on flower color in some contexts but discordant selection in other contexts. This variability could have profound implications for how flower traits respond to pollinator‐mediated selection. Our findings suggest that studying dynamics of behavior in natural field conditions is important for understanding plant–pollinator interactions.     

Natural hybridization between an outcrossing and a selfingPhlox (Polemoniaceae): The maternal species of F1 hybrids

Natural F₁ hybrids between the outcrossingPhlox drummondii and the predominantly selfingP. cuspidata were examined to ascertain the proportion of hybrid individuals mothered by each species. Species-specific restriction fragment patterns (both nrDNA and cpDNA) were established as markers, and synthetic hybrids of known parentage were utilized to determine that the chloroplast genome is maternally inherited. Of 89 mature natural hybrids examined, approximately two thirds were mothered byP. drummondii, the outcrosser. That the outcrosser should mother most hybrids is expected since it is dependent upon incoming pollen for fertilization, and hybrids may result when heterospecific pollen is received. The fact that the highly selfingP. cuspidata mothered nearly one third of the hybrids is surprising, and may be related to both pre-zygotic and post-zygotic factors. Which species mothers hybrids has important implications for the potential for introgression as well as its direction.     

Genotype-environment interactions in determining fitness in dense,artificial populations of Phlox drummondii

Summary Survivorship and growth of individuals of the annual plant Phlox drummondii were examined in artificial, density-stressed populations composed of five genetically differentiated varieties sown in monocultures and in five pairwise mixtures. All experiments were replicated under two treatments of nutrient availability.Varieties differed significantly in competitive ability, and relative competitive success was shown to be highly habitat (treatment) dependent. In 19 of 20 mixed populations, the relative rankings of pairs of competitors were consistent with a priori expectations based on the morphological and/or historical differences between seed sources, but were not predictable from relative tolerances to density stress in monoculture.Estimated selection coefficients based on relative competitive abilities ranged from 0.04 to 0.94. Despite these large fitness differences among competing varieties, most of the variance in the absolute fitness of individuals resided within varieties. This variation may be mostly environmental.In the high nutrient treatment, where competition for light was most intense, variation in individual plant size was lower in mixed culture as compared to monocultures. Genetic variation for competitive ability may actually lead to greater size uniformity among survivors by promoting an early initiation of thinning, thereby reducing the density stress on survivors.