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.     

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.     

RESPONSE TO SELECTION ON AUTOGAMY IN PHLOX

Two cycles of artificial selection were performed to increase autogamous fruiting in two wild populations of the self-incompatible Phlox drummondii, to decrease autogamous fruiting in two wild populations of the self-compatible Phlox cuspidata, and to both increase and decrease autogamous fruiting in a cultivar of P. drummondii which is pseudo-self-compatible. The breeding systems were determined to be genetically quite flexible, independent of inbreeding depression and other genetic phenomena which could hinder a breeding system shift. This is especially true for increasing autogamy. Self-pollen-pistil compatibility seems to be the single character affected by selection. Based on the continuous variation in both autogamy and self-compatibility, we suggest that the change has been due to genes which modify the self-incompatibility reaction rather than to the simple segregation of alleles at the S-locus.     

REPRODUCTIVE CHARACTER DISPLACEMENT IN PHLOX

Plants of the red- and pink-flowered P. drummondii were introduced into a natural population of P. cuspidata. Their seeds were progeny-tested, and the percentage hybrid seed determined for each variant. Thirty-eight percent of the seed from the pink variant were hybrid compared to 13% of the red. The difference in hybrid production is best explained by corolla color. The self- and cross-compatibility of nine sympatric and nine allopatric populations were studied in the greenhouse. On the average, sympatric populations are more self-compatible than allopatric populations. Pollen germination is 13.7% vs. 7.3%, the difference being statistically significant. The self-compatibility differential was accompanied by a self seedset differential. On the average, sympatric populations are more cross-compatible with P. cuspidata than are allopatric populations. The germination of P. cuspidata pollen on sympatric P. drummondii stigmas was 13.3% vs. 9.8% on allopatric stigmas. However, the difference is not statistically significant. The presence of P. cuspidata has promoted reproductive character divergence in P. drummondii. The shift in corolla color and the increase in self-compatibility reduce the potential for gametic wastage and interspecific hybridization.     

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.     

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.     

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.     

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.     

Quantitative genetics of seed size variation inPhlox

Summary Because seed size is often associated with survival and reproduction in plant populations, genetic variation for seed size may be reduced or eliminated by natural selection. To test this hypothesis we assessed genetic sources of variation in seed size in a population ofPhlox drummondii to determine whether genetic differences among seeds influence the size they attain. A diallel cross among 12 plants from a population at Bastrop, Texas, USA allowed us to partition variance in the mass of seeds among several genetic and parental effects. We found no evidence of additive genetic variance or dominance genetic variance for seed mass in the contribution of plants to their offspring. Extranuclear maternal effects accounted for 56% of the variance in seed mass. A small interaction was observed between seed genotype and maternal plant. Results of this study support theory that predicts little genetic variation for traits associated with fitness.     

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).     

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.     

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.     

DYNAMICS OF SYNTHETIC PHLOX DRUMMONDII POPULATIONS AT THE SPECIES MARGIN

We compare the dynamics of artificial populations of the annual Phlox drummondii introduced along a transect extending from within to beyond the species margin southwest of San Antonio, Texas. The margin is defined by a shift from sandy, nutrient poor soils to clay, nutrient-rich soils. Fourteen sites were chosen along the 6.5-mi transect. In each site seeds were planted into 1.5-m-square cleared plots and uncleared 10-m transects. Transplants also were introduced into cleared plots. Survival, flower production and seed set were monitored over the course of the growing season. Individual fitnesses and the reproductive rates of artificial populations generally declined as the distance from the margin increased. The results indicate soil type per se is not limiting Phlox and that plants can successfully grow and reproduce, albeit at a reduced level, in sites beyond their natural margin.     

EFFECTS OF PARENTAL IDENTITIES AND ENVIRONMENT ON COMPONENTS OF CROSSING SUCCESS IN PHLOX DRUMMONDII

A controlled pollination study, involving a diallel crossing scheme among 15 Phlox drummondii individuals grown under either control, low water or low nutrient conditions, was performed. The results of this research demonstrate that, although plant size and flower number are responsive to the treatments, pollen germination percentage, the survival probability of developing seeds and the weight of mature seeds are buffered against water and nutrient deprivation. However, the identities of male and female parents did have significant effects on pollen germination and seed survival, and female identity had a significant effect on seed weight.     

Breeding systems in Clivia (Amaryllidaceae): late‐acting self‐incompatibility and its functional consequences

Late‐acting (ovarian) self‐incompatibility, characterized by minimal or zero seed production following self‐pollen tube growth to the ovules, is expected to show phylogenetic clustering, but can otherwise be difficult to distinguish from early‐acting inbreeding depression. In Amaryllidaceae, late‐acting self‐incompatibility has been proposed for Narcissus (Narcisseae) and Cyrtanthus (Cyrtantheae). Here, we investigate whether it occurs in the horticulturally important genus Clivia (Haemantheae) and test whether species in this genus experience ovule discounting in wild populations. Seed‐set results following controlled hand pollinations revealed that Clivia miniata and C. gardenii are largely self‐sterile. Self‐ and cross‐pollinated flowers of both species had similar proportions of pollen tubes entering the ovary, and those of C. gardenii also did not differ in the proportions of pollen tubes that penetrated ovules, thus ruling out classical gametophytic self‐incompatibility acting in the style, but not early inbreeding depression. Flowers that received equal mixtures of self‐ and cross‐pollen set fewer seeds than those that received cross‐pollen only, but it was unclear whether this effect was a result of ovule discounting or interactions on the stigma. The prevention of self‐pollination by the emasculation of either single flowers or whole inflorescences in wild populations did not affect seed set, suggesting that ovule discounting is not a major natural limitation on seed production. Flowers typically produce one to three large fleshy seeds from approximately 16 available ovules, even when supplementally hand pollinated, suggesting that fecundity is mostly resource limited. The results of this study suggest that Clivia spp. are largely self‐sterile as a result of either a late‐acting self‐incompatibility system or severe early inbreeding depression, but ovule discounting caused by self‐pollination is not a major constraint on fecundity. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175 , 155–168.     

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.     

EFFECTS OF HYBRIDIZATION AND INBREEDING ON FITNESS IN PHLOX

The effects of interpopulation hybridization, and self-fertilization and immigration on fitness in Phlox drummondii were analyzed in 5 natural field sites. Germination rates, survivorship to flowering, fecundity, and net reproductive rates (R_o) were determined for planted populations of natives, hybrids, aliens, and the products of one and two generations of self-fertilization. At all sites, seed germination was 36% for natives, 35% for hybrids and 28% for aliens. Survivorship for natives was 50%, compared to 49% for hybrids and 41% for aliens. The mean fecundity was 34, 35, and 39 seeds per plant for natives, hybrids, and aliens, respectively. The R_o of natives averaged 5.2 vs. 6.4 for hybrids and 4.2 aliens. At all the sites, germination averaged 25% for self-1 plants and 19% for self-2 plants compared to 24% for open-pollinated controls. Survivorship progressed from 50% in the controls to 46% in self-1 and 44% in self-2 plants. Seed production per plant averaged 35% in the control vs. 32% in the self-1 and 25% in the self-2 plants. The mean R_o of the control was 3.8 vs. 3.3 in the self-1 and 2.14 in the self-2 plants. Our results demonstrate that the genetic variable may have a substantial effect on plant fitness in the field.     

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.     

SIZES OF NATURAL MICROGAMETOPHYTE POPULATIONS IN PISTILS OF PHLOX DRUMMONDII

The numbers of microgametophytes in the stigmas of the outcrossing Phlox drummondii were determined in 19 natural populations. Microgametophyte numbers per stigma ranged from 0 to over 40 in each population. The number of flowers per plant, distance to the nearest-neighbor, and number of plants within 1 m of the focal plants explained about 23% of the variation in microgametophyte numbers within 2 of the populations. The mean number per stigma across all populations was 14.34; and the mean number per population varied from 7.77 to 19.52. Given that there are 3 ovules per flower, there was an average of 4.73 microgametophytes per ovule. Sixty-nine percent of all pistils contained more than 3 microgametophytes, thereby presenting an opportunity for intergametophytic competition.