Within-plant variation in seed size and inflorescence fecundity is associated with epigenetic mosaicism in the shrub Lavandula latifolia (Lamiaceae)

Abstract

Background and aims: Sub-individual variation in traits of homologous structures has multiple ecological consequences for individuals and populations. Assessing the evolutionary significance of such effects requires an improved knowledge of the mechanisms underlying within-plant phenotypic heterogeneity. The hypothesis that continuous within-plant variation in some phenotypic traits can be associated with epigenetic mosaicism was examined.

Methods: Fifteen individuals of the long-lived, evergreen Mediterranean shrub Lavandula latifolia were studied. Five widely spaced 'modules', each consisting of a single inflorescence plus all its subtending basal leaves, were collected from each shrub. Genomic DNA was extracted from leaf samples and genome-wide cytosine methylation determined by reversed phase high-performance liquid chromatography (HPLC) with spectrofluorimetric detection. The number and mean mass of seeds produced were determined for each inflorescence. An assessment was made of whether (1) leaves from different modules in the same plant differed significantly in global DNA cytosine methylation, and (2) mosaicism in cytosine methylation contributed to explain variation across modules in number and size of seeds.

Key results: Leaves from different modules in the same plant differed in global DNA cytosine methylation. The magnitude of epigenetic mosaicism was substantial, as the variance in DNA methylation among modules of the same shrub was greater than the variance between individuals. Number and mean mass of seeds produced by individual inflorescences varied within plants and were quadratically related to cytosine methylation of subtending leaves, with an optimum at an intermediate methylation level (approx. 25 %).

Conclusions: The results support a causal link between global cytosine methylation of leaves in a module and the size and numbers of seeds produced by the associated inflorescence. It is proposed that variation in global DNA methylation within L. latifolia shrubs may result from the concerted action of plant sectoriality and differential exposure of different plant parts to some environmental factor(s) with a capacity to induce durable epigenetic changes.

Keywords: DNA methylation; Lavandula latifolia; epigenetic mosaicism; seed production; seed size; sub-individual variation.

Fig. 1.

Flowering Lavandula latifolia shrub (left) and idealized drawing of one of its modules, consisting of a single inflorescence plus its subtending leaves (right).

Fig. 2.

Variation in global DNA cytosine methylation across Lavandula latifolia plants (AGU01–TSE08) and between different modules from the same plant (five modules sampled per plant). Within each panel, modules were ranked in increasing order of mean cytosine methylation. Dots represent average values of the two independent replicates run for each module, and vertical segments extend ± 2 s.e. around the mean.

Fig. 3.

Model-fitted quadratic equations depicting the relationships between the number (log transformed) and mean mass of seeds produced by individual modules, and global cytosine methylation of genomic DNA from the modules’ leaves. Predicted relationships are shown for the sample as a whole (i.e. the fixed effects part in the mixed-effects model, thick black lines) and for each plant separately (i.e. the prediction for each random effect level, thin lines; plant colour codes as in Fig. 2).

Fig. 4.

Quadratic relationship between mean seed mass and mean global DNA cytosine methylation across the 15 Lavandula latifolia plants sampled (y = –28.392 + 2.317x – 0.045x²; F = 5.79, d.f. = 2, 12, P = 0.017, adjusted R² = 0.406). Plant means were obtained by averaging across modules within each plant. Colour codes for plants as in Fig. 2.