Conchita Alonso

Extreme intraplant variation in nectar sugar composition in an insect-pollinated perennial herb

Variation in nectar chemistry among plants, flowers, or individual nectaries of a given species has been only rarely explored, yet it is an essential aspect to our understanding of how pollinator-mediated selection might act on nectar traits. This paper describes variation in nectar sugar composition in a population of the perennial herb Helleborus foetidus (Ranunculaceae) and dissects it into components due to variation among plants, flowers of the same plant, and nectaries of the same flower. The proportions of sucrose, glucose, and fructose in single-nectary nectar samples collected at two times in the flowering season were determined using high performance liquid chromatography (HPLC). Sugar composition varied extensively among nectaries, and nearly all combinations of individual sugars were recorded. Population-wide variance was mainly accounted for by variation among flowers of the same plant (56% of total), nectaries of the same flower (30%), and only minimally by differences among plants (14%). In absolute terms, intraplant variation was similar to or greater than that ordinarily reported in interspecific comparisons. Results suggest that the prevailing notion of intraspecific constancy in nectar sugar composition may be unwarranted for some species and that more elaborate nectar sampling designs are required to detect and appropriately account for extensive within-plant variance. Within-plant variation in nectar sugar composition will limit the ability of pollinators to exert selection on nectar chemistry in H. foetidus and may be advantageous to plants by reducing the number of flowers visited per foraging bout by variance-sensitive, risk-averse pollinators.

Variation in herbivory within and among plants of Daphne laureola (Thymelaeaceae): correlation with plant size and architecture

1 Herbivory by noctuid moth larvae (Lepidoptera, Noctuidae) on plants of Daphne laureola L. (Thymelaeaceae) was studied under natural conditions in a south-eastern Spanish montane habitat. The main objective of the study was to determine how size and architectural features correlated with herbivory level (mean percentage leaf area removed by the end of the larval growth season) and herbivore load (mean number of caterpillars recorded per day) both among and within plants. 2 A significant correlation was found between herbivore load and herbivory level of individual plants. Herbivory levels differed widely (range = 0.1-12.8% leaf area) and were considerably smaller than those often used in artificial defoliation experiments. 3 Variation among plants in the incidence of noctuid larvae was directly related to the number of leaf whorls, and inversely to the mean basal diameter of stems. These responses to size and architectural traits may be explained by discrimination by ovipositing females. 4 Within plants, larvae preferentially selected leaf whorls having shorter supporting stems and lower branching orders. Movement costs may be reduced by larvae using plant architectural traits as cues for within-plant food selection. 5 The reasons for and potential implications of the different features used by adult noctuids discriminating between plants and by their larvae selecting leaf whorls are discussed.

Global DNA cytosine methylation as an evolving trait: phylogenetic signal and correlated evolution with genome size in angiosperms

DNA cytosine methylation is a widespread epigenetic mechanism in eukaryotes, and plant genomes commonly are densely methylated. Genomic methylation can be associated with functional consequences such as mutational events, genomic instability or altered gene expression, but little is known on interspecific variation in global cytosine methylation in plants. In this paper, we compare global cytosine methylation estimates obtained by HPLC and use a phylogenetically-informed analytical approach to test for significance of evolutionary signatures of this trait across 54 angiosperm species in 25 families. We evaluate whether interspecific variation in global cytosine methylation is statistically related to phylogenetic distance and also whether it is evolutionarily correlated with genome size (C-value). Global cytosine methylation varied widely between species, ranging between 5.3% (Arabidopsis) and 39.2% (Narcissus). Differences between species were related to their evolutionary trajectories, as denoted by the strong phylogenetic signal underlying interspecific variation. Global cytosine methylation and genome size were evolutionarily correlated, as revealed by the significant relationship between the corresponding phylogenetically independent contrasts. On average, a ten-fold increase in genome size entailed an increase of about 10% in global cytosine methylation. Results show that global cytosine methylation is an evolving trait in angiosperms whose evolutionary trajectory is significantly linked to changes in genome size, and suggest that the evolutionary implications of epigenetic mechanisms are likely to vary between plant lineages.

Pollination success across an elevation and sex ratio gradient in gynodioecious Daphne laureola

Gynodioecy is a dimorphic breeding system in which hermaphrodite and female individuals coexist in populations. Sex ratio and gender-relative lifetime seed production determine the stability of gynodioecy, and both genetic and ecological factors may influence these parameters. I analyzed the consequences of variation in population sex ratio and site elevation for the relative pollination success of female and hermaphrodite individuals of Daphne laureola in southern Spain, where previous studies failed to detect female fecundity advantages at two mid-elevation sites. Pollination success, estimated as stigmatic pollen loads, number of pollen tubes per style, and percentage of fertilized flowers, was higher for hermaphrodites than females in populations with 20-56% females. Furthermore, female quantitative disadvantage in pollination success increased with elevation, suggesting that the higher availability of pollen due to the increased proportion of hermaphrodites could not mitigate the negative effect that other factors associated with elevation apparently had on pollination. Supplemental hand pollinations showed that female seed production was pollen limited in populations with a proportion of females >50%, although both pollination success and natural fruit set of females in these sites were the highest recorded.

Seasonal variation in leaf characteristics and food selection by larval noctuids on an evergreen Mediterranean shrub

Despite year round availability of foliage, abundance of generalist noctuid larvae (Lepidoptera: Noctuidae) in evergreen- dominated Mediterranean forests has a narrow, distinct spring peak. This restricted larval period has been suggested to result in part from avoidance of the nutritionally poor mature foliage, and preference for nutritionally superior spring-produced young leaves. This study examines this hypothesis by (i) documenting differences in nutritional characteristics between expanding (April) and mature (June) young leaves of the evergreen Mediterranean shrub Daphne laureolaL. (Thymelaeaceae), and (ii) experimentally studying the feeding preferences of noctuid larvae for young leaves, old leaves (≥ 1 yr old), and developing fruits of this species in one south-eastern Spanish locality. Young leaves of D. laureola declined in nutrient concentration and specific dry mass from April to June. The responses of noctuid larvae, in terms of both relative preference and total consumption, to this seasonal variation in chemical and physical features of young leaves were also investigated. When noctuid larvae were simultaneously offered young leaves, old leaves and developing fruits, they exhibited similar preferences for young leaves and developing fruits, and rejected old leaves developed during the previous year. Noctuid larvae did not modify their consumption of young leaves relative to old leaves and developing fruits in response to seasonal changes. Food selection patterns exhibited by D. laureola noctuid herbivores, notably the rejection of old leaves in favour of young ones, are consistent with the hypothesis relating restricted larval periods of these generalist consumers with the low food value of the previous season leaves of evergreen Mediterranean plants.

Mating system, sex ratio, and persistence of females in the gynodioecious shrub Daphne laureola L. (Thymelaeaceae)

Although in gynodioecious populations male steriles require a fecundity advantage to compensate for their gametic disadvantage, southern Spanish populations of the long-lived shrub Daphne laureola do not show any fecundity advantage over hermaphrodites in terms of seed production and early seedling establishment. By using allozyme markers, we assess the mating system of this species in five populations differing in sex ratio, and infer levels of inbreeding depression over the whole life cycle by comparing the inbreeding coefficients at the seed and adult plant stages. Extremely low outcrossing rates (0.001<t<0.125) were consistently found for hermaphrodites in all populations, whereas, as expected, female progeny were entirely outcrossed. In most populations, offspring were much more inbred than their parents, and heterozygosity of adults was greater than expected from outcrossing rate estimates, with very few selfed progeny appearing to reproduce in the field. The combination of extensive selfing in hermaphrodites and a strong inbreeding depression expressed late in the life cycle (and thus, only estimable by indirect measures based on genetic markers) may explain the persistence and high frequency of D. laureola females in southern Spanish populations.

MSAP markers and global cytosine methylation in plants: a literature survey and comparative analysis for a wild-growing species

Methylation of DNA cytosines affects whether transposons are silenced and genes are expressed, and is a major epigenetic mechanism whereby plants respond to environmental change. Analyses of methylation‐sensitive amplification polymorphism (MS‐AFLP or MSAP) have been often used to assess methyl‐cytosine changes in response to stress treatments and, more recently, in ecological studies of wild plant populations. MSAP technique does not require a sequenced reference genome and provides many anonymous loci randomly distributed over the genome for which the methylation status can be ascertained. Scoring of MSAP data, however, is not straightforward, and efforts are still required to standardize this step to make use of the potential to distinguish between methylation at different nucleotide contexts. Furthermore, it is not known how accurately MSAP infers genome‐wide cytosine methylation levels in plants. Here, we analyse the relationship between MSAP results and the percentage of global cytosine methylation in genomic DNA obtained by HPLC analysis. A screening of literature revealed that methylation of cytosines at cleavage sites assayed by MSAP was greater than genome‐wide estimates obtained by HPLC, and percentages of methylation at different nucleotide contexts varied within and across species. Concurrent HPLC and MSAP analyses of DNA from 200 individuals of the perennial herb Helleborus foetidus confirmed that methyl‐cytosine was more frequent in CCGG contexts than in the genome as a whole. In this species, global methylation was unrelated to methylation at the inner CG site. We suggest that global HPLC and context‐specific MSAP methylation estimates provide complementary information whose combination can improve our current understanding of methylation‐based epigenetic processes in nonmodel plants.

Variation in herbivory by Yponomeuta mahalebella on its only host plant Prunus mahaleb along an elevational gradient

1. The effect of natural variation in abiotic conditions on the herbivory interaction between Prunus mahaleb (Rosaceae) and its monophagous folivore, larvae of Yponomeuta mahalebella (Lepidoptera, Yponomeutidae), was analysed for 2 consecutive years along an elevational gradient in Sierra de Cazorla, south‐east Spain.

Choosing a place to grow. Importance of within‐plant abiotic microenvironment for Yponomeuta mahalebella

The caterpillars of Yponomeuta mahalebella Latr. (Lepidoptera: Yponomeutidae) are monophagous on Prunus mahaleb (Rosaceae) leaves. Adult females deposit eggs in batches, which determines larval gregarious behaviour. Coupled with gregarious behaviour, caterpillars spin silk tents within they will feed until pupation. Distribution of tents in the field, their effect in microenvironmental larval growth conditions and the consequences for adult body mass and survival of larvae were studied.

Ecological plant epigenetics: Evidence from model and non-model species, and the way forward

Growing evidence shows that epigenetic mechanisms contribute to complex traits, with implications across many fields of biology. In plant ecology, recent studies have attempted to merge ecological experiments with epigenetic analyses to elucidate the contribution of epigenetics to plant phenotypes, stress responses, adaptation to habitat, and range distributions. While there has been some progress in revealing the role of epigenetics in ecological processes, studies with non-model species have so far been limited to describing broad patterns based on anonymous markers of DNA methylation. In contrast, studies with model species have benefited from powerful genomic resources, which contribute to a more mechanistic understanding but have limited ecological realism. Understanding the significance of epigenetics for plant ecology requires increased transfer of knowledge and methods from model species research to genomes of evolutionarily divergent species, and examination of responses to complex natural environments at a more mechanistic level. This requires transforming genomics tools specifically for studying non-model species, which is challenging given the large and often polyploid genomes of plants. Collaboration among molecular geneticists, ecologists and bioinformaticians promises to enhance our understanding of the mutual links between genome function and ecological processes.