Véronique A. Delesalle

Temporal variation in the pollen:ovule ratios of Clarkia (Onagraceae) taxa with contrasting mating systems: field populations

Among plants, pairs of selfing vs. outcrossing sister taxa provide interesting systems in which to test predictions concerning the magnitude and direction of temporal changes in sex allocation. Although resource availability typically declines towards the end of the growing season for annual taxa, temporal changes in mating opportunities depend on mating system and should change less in selfing taxa. Consequently, given that the pollen:ovule (P:O) ratio of flowers reflects the investment in (and potential fitness pay‐off due to) male vs. female function, we predicted that the P:O ratio should also be less variable among and within selfers than in closely related outcrossers. To test these predictions, we measured temporal changes in sex allocation in multiple field populations of two pairs of sister taxa in the annual flowering plant genus Clarkia (Onagraceae). In the outcrossing Clarkia unguiculata and the selfing Clarkia exilis, ovule production declined similarly from early to late buds, whereas pollen production remained constant or increased in the outcrosser but remained constant or decreased in the selfer. Consequently, the P:O ratio increased within unguiculata populations but marginally increased or stayed constant in exilis populations. In all populations of the selfing Clarkia xantiana spp. parviflora and the outcrossing C. x. spp. xantiana, both ovule and pollen production per flower declined over time. The effects of these declines on the P:O ratio, however, differed between subspecies. In each xantiana population, the mean P:O ratio did not differ between early and late flowers, although individuals varied greatly in the direction and magnitude of phenotypic change. By contrast, parviflora populations differed in the mean direction of temporal change in the P:O ratio. We found little evidence to support our initial predictions that the P:O ratio of the selfing taxa will consistently vary less than in outcrossing taxa.

Stability of pollen–ovule ratios in pollinator‐dependent versus autogamous Clarkia sister taxa: testing evolutionary predictions

It has been proposed that natural selection should favor distinct temporal patterns of sex allocation in selfing vs pollinator-dependent taxa. In autogamous selfers in which pollen receipt is highly reliable, selection should favor genotypes that maintain low and stable pollen to ovule (P : O) ratios throughout flowering. By contrast, in outcrossers the optimum P : O ratio of an individuals flowers will depend on pollinator abundances and mating opportunities, both of which may vary over time. In this case, selection may favor temporal variation among flowers in the P : O ratio. An opposing prediction is that selfing taxa will be developmentally more unstable than outcrossers because of lower homeostasis caused by high homozygosity. We compared temporal changes in the P : O ratio in two pairs of sister taxa in the genus Clarkia. We examined hundreds of glasshouse-raised maternal families representing three wild populations each of the outcrossing, insect-pollinated Clarkia unguiculata, the facultatively autogamous Clarkia exilis and the outcrossing and selfing subspecies of Clarkia xantiana: ssp. xantiana and parviflora, respectively. Temporal change in the P : O ratio was significantly greater in both outcrossers than in their selfing sister taxa, although the proportional changes in the P : O ratio (relative to the first bud produced) did not differ significantly between sister taxa (0.07 < P < 0.10). Our results provide partial support for the hypothesis that the P : O ratio is more stable in selfing than in outcrossing taxa and reject the hypothesis that selfers are less stable.

Cluster M mycobacteriophages Bongo, PegLeg, and Rey with unusually large repertoires of tRNA isotypes

ABSTRACT Genomic analysis of a large set of phages infecting the common host Mycobacterium smegmatis mc2155 shows that they span considerable genetic diversity. There are more than 20 distinct types that lack nucleotide similarity with each other, and there is considerable diversity within most of the groups. Three newly isolated temperate mycobacteriophages, Bongo, PegLeg, and Rey, constitute a new group (cluster M), with the closely related phages Bongo and PegLeg forming subcluster M1 and the more distantly related Rey forming subcluster M2. The cluster M mycobacteriophages have siphoviral morphologies with unusually long tails, are homoimmune, and have larger than average genomes (80.2 to 83.7 kbp). They exhibit a variety of features not previously described in other mycobacteriophages, including noncanonical genome architectures and several unusual sets of conserved repeated sequences suggesting novel regulatory systems for both transcription and translation. In addition to containing transfer-messenger RNA and RtcB-like RNA ligase genes, their genomes encode 21 to 24 tRNA genes encompassing complete or nearly complete sets of isotypes. We predict that these tRNAs are used in late lytic growth, likely compensating for the degradation or inadequacy of host tRNAs. They may represent a complete set of tRNAs necessary for late lytic growth, especially when taken together with the apparent lack of codons in the same late genes that correspond to tRNAs that the genomes of the phages do not obviously encode. IMPORTANCE The bacteriophage population is vast, dynamic, and old and plays a central role in bacterial pathogenicity. We know surprisingly little about the genetic diversity of the phage population, although metagenomic and phage genome sequencing indicates that it is great. Probing the depth of genetic diversity of phages of a common host, Mycobacterium smegmatis, provides a higher resolution of the phage population and how it has evolved. Three new phages constituting a new cluster M further expand the diversity of the mycobacteriophages and introduce novel features. As such, they provide insights into phage genome architecture, virion structure, and gene regulation at the transcriptional and translational levels.

EVOLUTION OF MATING SYSTEM AND THE GENETIC COVARIANCE BETWEEN MALE AND FEMALE INVESTMENT IN CLARKIA (ONAGRACEAE): SELFING OPPOSES THE EVOLUTION OF TRADE-OFFS

Abstract Sex allocation theory has assumed that hermaphroditic species exhibit strong genetically based trade-offs between investment in male and female function. The potential effects of mating system on the evolution of this genetic covariance, however, have not been explored. We have challenged the assumption of a ubiquitous trade-off between male and female investment by arguing that in highly self-fertilizing species, stabilizing natural selection should favor highly efficient ratios of male to female gametes. In flowering plants, the result of such selection would be similar pollen:ovule (P:O) ratios across selfing genotypes, precluding a negative genetic correlation (rg) between pollen and ovule production per flower. Moreover, if selfing genotypes with similar P:O ratios differ in total gametic investment per flower, a positive rg between pollen and ovule production would be observed. In outcrossers, by contrast, male- and female-biased flowers and genotypes may have equal fitness and coexist at evolutionary equilibrium. In the absence of strong stabilizing selection on the P:O ratio, selection on this trait will be relaxed, resulting in independence or resource-based trade-offs between male and female investment. To test this prediction, we conducted artificial selection on pollen and ovule production per flower in two sister species with contrasting mating systems. The predominantly self-fertilizing species (Clarkia exilis) consistently exhibited a significant positive rg between pollen and ovule production while the outcrossing species (C. unguiculata) exhibited either a trade-off or independence between these traits. Clarkia exilis also exhibited much more highly canalized gender expression than C. unguiculata. Selection on pollen and ovule production resulted in little correlated change in the P:O ratio in the selfing exilis, while dramatic changes in the P:O ratio were observed in unguiculata. To test the common prediction that floral attractiveness should be positively genetically correlated with investment in male function, we examined the response of petal area to selection on pollen and ovule production and found that petal area was not consistently genetically correlated with gender expression in either species. Our results suggest that the joint evolutionary trajectory of primary sexual traits in hermaphroditic species will be affected by their mating systems; this should be taken into account in future theoretical and comparative empirical investigations.

Covariation among floral traits in Spergularia marina (Caryophyllaceae): geographic and temporal variation in phenotypic and among‐family correlations

Strong covariation among traits suggests the presence of constraints on their independent evolution due to pleiotropy, to linkage, or to selective forces that maintain particular trait combinations. We examined floral trait covariation among individuals, among maternal families within and across populations, and over time, in greenhouse‐raised plants of the autogamous Spergularia marina. We had three aims. First, since the phenotype of traits expressed by modular organs often changes as individuals age, estimates of the degree of genetic covariation between such traits may also change over time. To seek evidence for this, we measured weekly (for five weeks) an array of floral traits among plants representing ∼ 10 maternal families from each of four populations. The statistical significance of the phenotypic and among‐family correlations among traits changed over time. Second, we compared populations with respect to trait covariation to determine whether populations or traits appear to be evolving independently of one another. Differences observed among populations suggest that they have diverged genetically. Third, we sought correlations that might reflect constraints on the independent evolution of floral traits. Investment in another and ovule production per flower vary independently among maternal families; there was no evidence for a “trade‐off” between male and female investment. We propose that in autogamous taxa one should not find a negative correlation between pollen and ovule production per flower, as such taxa cannot evolve sexual specialization and should be under strong selection to maintain an efficient pollen:ovule ratio, preventing the evolution of male‐biased or female‐biased genotypes. We found that other pairs of floral traits, however, expressed highly signficant correlation coefficients, suggesting the presence of some evolutionary constraints, at least within some populations, although their strength depended on exactly when flowers were sampled.

Nutrient Levels and Salinity Affect Gender and Floral Traits in the Autogamous Spergularia marina

The effects of nutrient levels and salinity on prezygotic components of gender (number of ovules and of anthers per flower), on secondary accessory structures (number and size of petals), and on possible indicators of normal development (number of abnormal anthers and staminodes) were investigated for six maternal families of the highly autogamous Spergularia marina, cultivated under controlled conditions in hydroponic tubs. Mean ovule number and mean staminode number per flower were not affected by changes in nutrient levels and salinity, while mean anther number per flower decreased and mean petal size increased under the harsher growing conditions (low-nutrient level or salinity). Mean petal and abnormal anther numbers per flower were affected by nutrient levels but not by the salinity treatment. We observed few significant phenotypic correlations among the seven studied traits but observed strong phenotypic correlations between petal number and abnormal anther number under all growing conditions. The overall lack of treatment-specific correlations indicates that evolutionary trajectories in this species may not be strongly affected by the environmental conditions under which natural selection occurs. In contrast to other studies, we report that, for traits expressed prior to pollination, male reproductive traits (number of normal and abnormal anthers per flower) were more sensitive to growing conditions than were traits related to female reproduction (ovule number per flower). Finally, in partial opposition to our initial prediction, we found that the anther/ovule ratio was highly sensitive to salinity but not to nutrient level.

Size-dependent pollen:ovule ratios and the allometry of floral sex allocation in Clarkia (Onagraceae) taxa with contrasting mating systems

Multiple field populations of two pairs of diploid sister taxa with contrasting mating systems in the genus Clarkia (Onagraceae) were surveyed to test predictions concerning the effects of resource status, estimated as plant size, on pollen and ovule production and on the pollen:ovule (P:O) ratio of flowers. Most theoretical models of size-dependent sex allocation predict that, in outcrossing populations, larger plants should allocate more resources to female function. Lower P:O ratios in larger plants compared to smaller plants have been interpreted as supporting this prediction. In contrast, we predicted that P:O ratio should not vary with plant size in predominantly selfing plants, in which each flower contributes to reproductive success equally through male and female function. We found that, in all four taxa, both ovule and pollen production per flower usually increased significantly with plant size and that the shape of this relationship was decelerating. However, ovule production either decelerated more rapidly than or at the same rate as pollen production with plant size. Consequently,the P:O ratio increased or had no relationship with plant size. This relationship was population-specific (not taxon-specific) and independent of the mating system. Possible explanations for the increasing maleness with plant size are discussed.

Seed Mass and Morphology in Outcrossing and Selfing Species of Clarkia (Onagraceae): An SEM Study

Seeds from three pairs of outcrossing‐selfing sister taxa from the genus Clarkia (farewell‐to‐spring, Onagraceae)—Clarkia unguiculata, Clarkia exilis, Clarkia xantiana ssp. xantiana and ssp. parviflora, and Clarkia concinna ssp. concinna and ssp. automixa—were studied to assess the effects of contrasting mating systems on seed mass and seed morphology. For each outcrossing‐selfing comparison, the seed mass of the selfing taxon was less than that of the outcrossing taxon. Seed mass typically differed significantly among populations within a taxon. Scanning electron microscopy showed that the seeds from all these taxa share several characteristics: a bullet to shield shape, a reticulate exotesta pattern, presence of crystals in the seed coat, and a seed coat that varies in thickness over the length of the seed. No morphological feature reliably distinguished seeds of outcrossing taxa from those of selfing taxa. The lack of morphological differences in conjunction with the consistent differences in seed mass between selfing and outcrossing seeds in these taxa supports the hypothesis that evolutionary forces have acted only on seed mass and not on seed morphology.

Floral trait variation in Spergularia marina (Caryophyllaceae) : ontogenetic, maternal family, and population effects

Traits expressed by modular organisms present difficulties when estimating the genetic component to their variation if their phenotype changes as an individual ages, confounding ontogenetic and genetic sources of phenotypic variation. For such traits, it is necessary to control for ontogenetic effects in order to estimate accurately the degree of genetic variation in a trait. To measure the magnitude of ontogenetic change in floral traits and to determine whether it may obscure underlying genetic sources of floral trait variation in the autogamous annual, Spergularia marina (Caryophyllaceae), we monitored the expression of floral traits over a five-week period in greenhouse-raised plants from four California wild populations. From 130 individuals representing 8–10 maternal families per population, we sampled one flower per week to record the number of ovules, normal anthers (the number of which is positively correlated with pollen production per flower), abnormal anthers (those that grade phenotypically into petals), and petals; and the areas of a single petal and the entire corolla. All traits except the number of abnormal anthers exhibited strong temporal changes in phenotype, although the direction and magnitude of the change differed among traits. To determine whether populations appear to be evolving independently, we examined differences among them in floral trait means, in the magnitude of among-family variation, and in the degree to which they show temporal change in floral trait means. There were significant differences among population means for all traits except the number of ovules and petals per flower. In addition, populations differ in those traits that exhibit significant differences among maternal family means. For most traits, populations also differ in the magnitude of week-to-week changes in mean phenotype, suggesting the presence of genetic variation among populations in the expression of temporal change in floral traits. Finally, for most traits, the magnitude and significance of differences among populations and among maternal family means changed over time. Consequently, broad-sense heritability estimates, predictions of the response to selection, and measures of interpopulation divergence for floral traits are sensitive to the time of flower sampling in S. marina. The role of natural selection in moulding ontogenetic variation in floral traits has not been extensively studied in any species, but the presence of genetic variation among populations observed here suggests that this character is open to evolutionary change.

TEMPORAL INSTABILITY OF GENETIC COMPONENTS OF FLORAL TRAIT VARIATION : MATERNAL FAMILY AND POPULATION EFFECTS IN SPERGULARIA MARINA (CARYOPHYLLACEAE)

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