Effects of mating system on adaptive potential for leaf morphology in Crepis tectorum (Asteraceae)

Background and Aims

A shift from outcrossing to selfing is thought to reduce the long-term survival of populations by decreasing the genetic variation necessary for adaptation to novel ecological conditions. However, theory also predicts an increase in adaptive potential as more of the existing variation becomes expressed as homozygous genotypes. So far, relatively few studies have examined how a transition to selfing simultaneously affects means, variances and covariances for characters that might be under stabilizing selection for a spatially varying optimum, e.g. characters describing leaf morphology.

Methods

Experimental crosses within an initially self-sterile population of Crepis tectorum were performed to produce an outbred and inbred progeny population to assess how a shift to selfing affects the adaptive potential for measures of leaf morphology, with special emphasis on the degree of leaf dissection, a major target of diversifying selection within the study species.

Key Results

Three consecutive generations of selfing had a minor impact on survival, the total number of heads produced and the mean leaf phenotype, but caused a proportional increase in the genetic (co)variance matrix for foliar characters. For the degree of leaf dissection, the lowest 50th percentile of the inbred progeny population showed a disproportionate increase in the genetic variance, consistent with the recessive nature of the weakly lobed phenotype observed in interpopulation crosses. Comparison of inbreeding response with large-scale patterns of variation indicates a potential for selection in a (recently) inbred population to drive a large evolutionary reduction in degree of leaf dissection by increasing the frequency of particular sibling lines.

Conclusions

The results point to a positive role for inbreeding in phenotypic evolution, at least during or immediately after a rapid shift in mating system.     

Phenotypic plasticity inCrepis tectorum (Asteraceae)

Two experiments were carried out using two different approaches to compare populations ofCrepis tectorum (Asteraceae). One was based on a comparison of means of various vegetative and reproductive characters and another was based on a comparison of response patterns of the same characters in a series of environments. Population divergence within two earlier recognized form series, one from weed habitats and one from alvar habitats on Baltic islands, resulted in a partially overlapping pattern in cluster analyses based on character means. However, the pattern revealed by a comparison of the direction and amount of plastic response suggested that populations within the two form series had more similar response patterns than other combinations of populations. It was concluded that patterns of plasticity may provide useful additional information on the overall similarity among taxa. An hypothesis that plants in weed populations should exhibit a greater phenotypic response to the environments than plants in alvar populations was rejected.     

Geographical variation and genetic analysis of leaf shape inCrepis tectorum (Asteraceae)

Canonical variate analysis of plants raised in a uniform environment was used to study the pattern of geographical variation in leaf shape ofCrepis tectorum (Asteraceae). The diversity in leaf shape was much greater among populations confined to areas with exposed bedrock in the Baltic region than among weed populations scattered throughout Europe and Canada. A Ward's clustering linked outcrop populations from the archipelago of SW. Finland and the islands of Öland (Sweden) and Saaremaa (Estonia) due to the deeply lobed leaves characterizing these populations, while outcrop populations along the coast of E. Sweden were grouped due to their weakly lobed, narrow and dentate leaves. Most of the weed populations were grouped together but there was no tendency for the variation in this group to be related to habitat or geographical location. A mosaic of variation reflected in sharp (random) differentiation among local populations was superimposed on the large-scale ecogeographical pattern.—Crossing data indicated that most of the variation in degree of leaf dissection is governed by one major gene with deeply lobed leaves dominant over weakly lobed leaves. I suggest that the simple pattern of inheritance may have favoured rapid evolutionary changes in leaf shape, particularly in the Baltic area which emerged relatively late from the sea. Genetic correlations may have constrained the pattern of variation at higher taxonomic levels, since some of the trait associations detected in a segregating F₂ generation were also found at the among-population level.     

The evolution of self-fertility inCrepis tectorum (Asteraceae)

The extent of self-fertility was examined in 16 populations ofCrepis tectorum. A hypothesis that a weedy habit favours autogamy was only partly supported. Low levels of self-fertility characterized non-weedy populations from calcareous grasslands (alvars) on the Baltic island in Öland. By contrast, plants in nearly all weed populations studied were more or less self-fertile. However, the trend towards autogamy may have occurred independently of the trend towards a weedy habit, as shown by moderately to high levels of self-fertility in alvar populations from two other Baltic islands. In the weed group, there was a tendency for plants from two field populations to be more autogamous than plants from more ruderal habitats. There was an association between self-fertility and small, inconspicious heads in the alvar group but the association was weaker when weed populations were also considered. The relatively wide heads characterizing the ruderal weed populations may, at least partly, be an indirect effect of increases in overall plant size and/or in the size of the fruit associated with each flower.     

Quantitative genetic variation in a population of Crepis tectorum subsp. pumila (Asteraceae)

Quantitative genetic variation was assessed in a population of Crepis tectorum subsp. pumila , a winter annual confined to calcareous grassland on the Baltic island of öland (SE Sweden). Plants from 40 maternal sibships were grown in a greenhouse and scored for a large number of traits representing all stages of the life cycle. The study included several characters that have been subject to ecotypic differentiation as well as traits known to be under current selection. Highly significant family differences were found for all but one character, suggesting that past selection was too weak to eliminate the genetic variability of characters presumed to be adaptive and there is a potential for further adaptive change in most traits. Two additional traits treated as qualitative were also found to differ significantly among families. A parallel cultivation experiment showed that the extent of population divergence in a quantitative trait was positively correlated with the amount of inter- family variation, implying stability of the relative variability for substantial periods of time, a possible reflection of phenotypic constraints being expressed both within and between populations. Additional data indicated that genetic trade-offs among traits under simultaneous selection, year-to- year variation in selection in combination with a long-lived seed bank, and genotype × environment interactions, could prevent the erosion of genetic variability in some characters connected with fitness.     

Morphometric differentiation,patterns of interfertility,and the genetic basis of character evolution inCrepis tectorum (Asteraceae)

Morphometric analyses of populations ofCrepis tectorum raised under uniform conditions support the earlier finding based on herbarium studies that extensive divergence has taken place in western Europe with a centre of diversity in the Baltic lowland area. A crossing experiment showed that subsp.pumila on the island of Öland has acquired a wide range of derived characters without the establishment of strong reproductive barriers, the only exception being a weak tendency for crosses with other populations on Öland to yield more fertile F₁ offspring than crosses with weedy and non-weedy populations in adjacent regions. Canadian weed populations showed a more uniform response in crosses with subsp.pumila than did morphologically similar weed populations scattered throughout Central Europe. While F₂ data suggest a monogenic basis of differences in the extent of leaf dissection, other distinctive traits appear to be governed by multiple genetic factors with individually slight effects.     

The relationship between seed dormancy,seed size and weediness,in Crepis tectorum (Asteraceae)

Summary I examined the germination characteristics of weed and outcrop populations of Crepis tectorum to test the hypothesis that the presumably more ephemeral weed habitat favors the highest levels of seed dormancy. The winter annual habit characterizing most plants of this species was reflected in a rapid germination of seeds sown in late summer. A slightly higher fraction of surface-sown seeds of weed plants delayed germination. Buried seeds of weed plants also survived better than seeds produced by plants in most outcrop populations, supporting the idea that weediness favors seed dormancy and a persistent seed bank. However, the differences in seed dormancy between the two ecotypes were small and not entirely consistent. Furthermore, high levels of seed dormancy were induced during burial in the outcrop group, suggesting that there is a potential for a dormant seed population in this habitat as well. Demographic data from one of the outcrop populations verified the presence of a large between-year seed bank. Possible environmental factors favoring seed dormancy in outcrop populations are discussed. The unusually large seeds of weedy Crepis contrasts with the relatively small difference in seed dormancy between the two ecotypes.     

Differential outcrossing rates in dispersing and non-dispersing achenes in the heterocarpic plant Crepis sancta (Asteraceae)

The association between mating systems and dispersal in plants has been studied mostly in cleistogamous species where, generally, seeds produced by cleistogamous (selfed) flowers are less dispersed than seeds produced by chasmogamous (potentially outcrossed) flowers. In heterocarpic Asteraceae, non-dispersing fruits (achenes) are produced at the periphery of the capitulum (outer florets) whereas dispersing achenes are produced by inner florets in the same capitulum. Since all the florets are protandrous, the outer floret developing first are in female phasis when anthesis of inner florets takes place. Thus, outer florets can be potentially selfed by the inner florets of the same capitulum whereas the latter must be pollinated by flowers of other capitula. Therefore outer florets should be more inbred than inner florets. To test this hypothesis, we measured the natural outcrossing rate in outer and inner florets using allozymes in three populations of the heterocarpic Crepis sancta. The results showed that the outcrossing rate was highest for non-dispersed achenes. Moreover, among the outcrossed achenes within a capitulum it was observed that the number of paternal parents of non-dispersing achenes was higher than for dispersing achenes. The pattern observed was therefore the opposite to the pattern of cleistogamous plants and contradicts the putative pollination mechanism we proposed for Asteraceae. The results agree with the predictions of sib competition theory which considers that outcrossing may minimize competitive interactions among relatives (sibs) falling near the mother plant. Higher outcrossing rate in outer florets could also occur because pollinators are more attracted to these florets.     

Nine polymorphic microsatellite markers in Crepis sancta (Asteraceae)

We describe nine new variable di‐ and trinucleotide microsatellites in the plant Crepis sancta, as well as conditions for multiplexing and simultaneous genotyping sets of loci. Their variability was assessed in four populations. Four to 33 alleles were detected per locus; two to 23 alleles per population. Significant deviations from Hardy–Weinberg equilibrium could be attributed either to a mating system involving partial selfing or to null alleles. Genetic differentiation over all loci was significant among the four populations studied. These markers will constitute an efficient tool for investigating dispersal patterns in fragmented landscapes. Cross‐species amplification provided few loci variability in three other Crepis species (C. pulcra, C. foetida and C. rubra).     

Population differentiation in Crepis tectorum (Asteraceae): patterns of correlation among characters

This study examines the relationship among traits distinguishing populations of C. tectorum and the extent to which existing trait associations reflect underlying (genetic) tradeoffs. Highly consistent trait associations were found in a comparison of 52 populations representing the western part of the geographical range of the species. In addition to a tight integration of traits reflecting plant stature and inflorescence development, there were consistent links between vegetative and reproductive traits; populations characterized by individuals with large leaves and tall stems with terminal branches usually had larger heads, flowers and fruits (achenes) than those whose individuals had small leaves and a short stem branched from the base. There was a weak negative relationship between the extent of leaf dissection and plant stature; short and compact plants had more deeply lobed leaves than tall plants with terminal branches. Few of these associations were present among families representing a single population of C. tectorum , but there was remarkable similarity between the correlations at the between-population level and those obtained in two segregating F2 progenies of crosses between contrasting populations. Hence, provided that the F2 correlations have a strong genetic basis, it appears that the course of population divergence has been constrained by the underlying correlation structure, although some trait associations may also be a result of selection operating in a correlated fashion on functionally related traits, perhaps leading to linkage disequilibrium of parental traits in the first segregating generation of a cross between ecologically differentiated populations.     

Ecological genetics of Bromus tectorum

Summary By incorporating demographic analyses of fitness components (e.g., survival and reproduction) within a reciprocal sowing design, we tested for 3 consecutive years whether local adaptation has occurred in the alien grass Bromus tectorum (cheatgrass) within 7 habitats along an environmental gradient from arid steppe to subalpine forest in the Intermontain Region of western North America. Patterns of emergence and survival were strongly influenced by the local environment. In terms of survival, expression of significant local adaptation in Tsuga heterophylla habitat varied among years. In contrast, relative differences in flowering time among seed sources were stable across sites and years. Populations from the arid steppe were the earliest to flower; flowering was latest in populations from the mesic Tsuga heterophylla habitat. In terms of net reproductive rate, evidence for local adaptation in B. tectorum was obtained in populations from habitats representing environmental extremes: an arid, saline site dominated by the shrub Sarcobatus vermiculatus and clearings within the cool, mesic Tsuga heterophylla forest habitat. Unlike the plants introduced from other sites, members of the resident population at the Sarcobatus site flowered and produced seeds before soil water became limiting. In contrast, net reproductive rates in other habitats were sometimes the lowest for populations in their home site. This lack of an advantage for local populations within more environmentally moderate sites suggests that limited dispersal may restrict the rate at which superior genotypes are introduced into a particular site.     

Life history patterns and ecology of macaque species

Macaques are found both in broadleaf evergreen forest and in more variable habitats. The former group might be expected to be subject to less variability in their environment and hence to suffer lower rates of density independent mortality. Life history evolution models predict that species in such conditions will have lower rates of development and breeding than those found in more variable habitats where density independent mortality is high. This prediction is tested here by comparing the breeding and development rates of nine species of macaque. Although measures of developmental rate are not found to vary in a predictable way with habitat, measures of breeding rate do correlate with habitat categories used. As predicted, species that are found in more variable habitats tend to have higher birth rates and a higher intrinsic rate of natural increase than do species in more stable, forest habitats. Contrary to prediction selection does not always act to produce an early age at first reproduction in macaques living in seasonal environments. This is discussed with relation to physiological and environmental constraints.     

Ecotypic divergence in Crepis tectorum (Asteraceae): inferring trait lability and correlational constraints from hormonally manipulated phenotypes

Despite long‐standing interest in the evolutionary role of plant hormones, relatively few studies have used hormonally manipulated phenotypes to address questions about phenotypic evolution in plants. In the present investigation, I subjected plants of Crepis tectorum subsp. pumila to simple gibberellin (GA) treatments under both field and greenhouse conditions to assess developmental lability and correlational constraints of phenotypic traits that distinguish this dwarf ecotype from conspecific populations of the much taller weed ecotype (subsp. tectorum). The hormonally manipulated plants largely phenocopied the weed ecotype in leaf shape, plant stature and branching habit, indicative of both high lability and tight integration of traits reflecting gross morphology. Floral size traits sometimes declined after GA application, especially under field conditions. The latter result conflicts with the positive correlations between floral and vegetative size traits seen in previous comparative analyses and point to a tradeoff that could act as a constraint on ecotype divergence. The response to GA was consistent in direction for most traits, as opposed to the magnitude of response, which varied depending on the trait, the population, the growing environment and the timing and level of hormone application. Taken together, the results highlight the potential for simple hormonal changes to cause large, plastic shifts in phenotype, but also illustrate the constrained nature of such influences.     

Life-history variation in a seed beetle: adult egg-laying vs. larval competitive ability

Summary Populations of the seed beetle Callosobruchus maculatus differ genetically in several traits that mediate intraspecific competition. This study examined competitive interactions between larvae from two strains that differed in their propensity to oviposit on occupied hosts. In a strain (S) where females avoided laying >1 egg/seed, larvae were highly competitive; if two larvae entered a small host simultaneously, only one adult emerged. In a strain (I) whose females were sloppier in their egg-laying decisions, more than half of the seeds bearing two larvae yielded two adults. If seeds contained one larva from each strain, only one adult emerged per seed, and 70% of these adults belonged to the more competitive S strain. A larva's probability of emergence could be increased if it entered the seed before its competitor. A two-day headstart was needed merely for I larvae to compete equally with S larvae. Competition also affected development time and adult weight, but its effect was highly strain- and sex-specific. Adult life-history traits also differed substantially between strains. Compared to I beetles, S beetles exhibited decreased longevity, lower fecundity, a truncated period of oviposition, and larger egg and body size. Fecundity was linearly related to body size in the I strain, but was largely independent of size in the S strain. When faced with a shortage of hosts, S females (whose progeny are highly competitive) withheld eggs and died without depositing 40% of their lifetime supply. In contrast, the fecundity of I females was independent of seed availability. Comparisons with previous studies suggest that both highly uniform egg-laying and strong interference among larvae may be a consequence of chronic association with a relatively small host. Results from the larval competition experiments were used as parameter estimates for a simple, game-theoretic model that postulates interference vs. exploitative strategies. Fitness comparisons suggest that a strategy employing interference competition cannot be invaded by a more exploitative form of competition in a small host.     

Floral reduction in Crepis tectorum (Asteraceae): tradeoffs and dominance relationships

The present investigation of Crepis tectorum examines the extent to which tradeoffs and patterns of dominance contribute to the association between flower size (head width) and degree of self-fertility among populations adapted to rock outcrops. Partial correlation analyses of phenotypic data from a F2 generation derived from a cross between two outcrop plants and of family means representing one of the parent populations indicate that small-flowered plants fail to reallocate resources to flower and fruit production, that small flower size offers little or no advantage in terms of autofertility, and that floral reduction has little influence on the rate of flower development. Hence, it may be necessary to invoke factors other than tradeoffs to explain the decline in flower size associated with the evolution of autogamy in C. tectorum. Comparison of parent and Fl hybrid means in the crossing experiment suggests incomplete dominance in the alleles for large flower size. Under the assumption that Fl hybrid means reflect the average degree of dominance across loci, I propose inbreeding depression as the basis for some of the reduction in floral morphology.     

Density-dependent natural selection does not increase efficiency

Summary Populations ofDrosophila melanogaster kept at high population density (K-selected) for 125 generations have higher larval viability than populations kept at low densities (r-selected) when both are raised under crowded conditions. In additionK-selected adults that emerge from crowded cultures are larger than theirr-selected counterparts. These differences cannot be explained by differences in efficiency of food use. The minimum food required for successful pupation is actually greater in theK-selected populations. I conjecture that there may be a trade-off between minimum food requirements and competitive ability, which has changed substantially in theK-selected populations. The possibility thatK-selected larvae can dig more more deeply and gain access to unused food is examined and rejected as a possible explanation of the viability differences. Evidence is provided supporting the hypothesis that the differences in viability may be due to an increased tendency of theK-selected larvae to pupate off the surface of the medium.     

C-Banding Patterns and Quantitative Karyotype Characteristics of Bulgarian Species of Crepis (Asteraceae)

Abstract: Giemsa C-banded idiograms were established from eight Crepis species of Bulgarian origin ( Crepis viscidula, C. paludosa, C. conyzaefolia, C. bithynica var. bithynica and var. fodorii, C. pulchra, C. sancta, C. setosa, and C. zacintha ) that had been previously studied for genome size and karyotype shape using classical methods. All taxa are diploids with descending chromosome numbers, x = 6, 5, 4, and 3. Satellites were always heterochromatic. Small but distinct centromeric bands were of general occurrence (except perhaps in C. paludosa ) and terminal bands were present at least in some chromosomes of the karyotypes. Higher amounts of heterochromatin in the form of terminal and intercalary bands were found in C. bithynica and C. zacintha, but banding patterns were not so specifically similar as to indicate common origin. There are a number of particular karyotypic characters which may turn out to be phylogenetically significant when corresponding information on many related taxa becomes available. Correlations of karyotypic parameters were calculated. In heterochromatin only percent heterochromatin, but not absolute heterochromatin amount, and genome size were significantly correlated. This is the result of a strong euchromatin variation but independent heterochromatin variation.     

Effects of competition on lifetime estimates of inbreeding depression in the outcrossing plant Crepis sancta (Asteraceae)

Inbreeding depression was studied in two populations of a Mediterranean allogamous colonizing species Crepis sancta. In order to test the hypothesis that the magnitude of inbreeding depression can be modified by successional processes, the growth and survival of individuals resulting from two generations of inbred crosses including selfing were analysed with interspecific competition (in natural vegetation) and without interspecific competition (by removing natural vegetation). Inbreeding depression was weak for seed production. Germination was little affected by inbreeding but mortality and the number of capitula showed inbreeding depression, especially in the presence of competition. This suggests that inbreeding depression is very sensitive to variations in environmental conditions such as interspecific competition. As a consequence, inbreeding depression cannot be considered as constant in natural conditions.     

Karyotype constancy and genome size variation in BulgarianCrepis foetida s. l. (Asteraceae)

Ten populations ofCrepis foetida from Bulgaria belonging to the three subspeciesfoetida, rhoeadifolia, andcommutata were analyzed karyologically using haematoxylin staining, Giemsa C-banding, fluorochrome banding, Ag-NOR staining, Feulgen cytophotometry (scanning densitometry and video-based image analysis), and propidium iodide flow cytometry. The quantitatively-evaluated karyotype structure was similar among all populations, with minor variation in a few intercalary sites only and in the amount of NOR-associated heterochromatin (satellites). In contrast to the karyotypic constancy the genome size ofC. foetida subsp.commutata was about 10% lower than those of the other two subspecies, which had similar genome sizes. The genome size measurements using three different methods resulted in highly correlated data. The genome size difference adds some weight to previous taxonomic opinions treatingC. foetida subsp.commutata at species level, asC. commutata. Prof. Dr. Stefan Kozhuharov (4 January 1933–24 August 1997).     

Recovery of dissected C-band regions in Crepis chromosomes

Summary Chromosome samples were prepared on a plastic coverslip covered with a polyester membrane and were subjected to the C-banding treatment. The C-band pattern was obtained after Giemsa staining. The C-band positive regions of the Crepis chromosomes were identified, dissected out by irradiation with a micro-laser beam and recovered in Eppendorf tubes.