Seed Morphology in Species from the Silene mollissima Aggregate (Caryophyllaceae) by Comparison with Geometric Models

Seed shape and size of Silene latifolia, differences between sexes, and influence of the parental genome in hybrids with Silene dioica

Introduction

Plants undergo various natural changes that dramatically modify their genomes. One is polyploidization and the second is hybridization. Both are regarded as key factors in plant evolution and result in phenotypic differences in different plant organs. In Silene, we can find both examples in nature, and this genus has a seed shape diversity that has long been recognized as a valuable source of information for infrageneric classification.

Methods

Morphometric analysis is a statistical study of shape and size and their covariations with other variables. Traditionally, seed shape description was limited to an approximate comparison with geometric figures (rounded, globular, reniform, or heart-shaped). Seed shape quantification has been based on direct measurements, such as area, perimeter, length, and width, narrowing statistical analysis. We used seed images and processed them to obtain silhouettes. We performed geometric morphometric analyses, such as similarity to geometric models and elliptic Fourier analysis, to study the hybrid offspring of S. latifolia and S. dioica.

Results

We generated synthetic tetraploids of Silene latifolia and performed controlled crosses between diploid S. latifolia and Silene dioica to analyze seed morphology. After imaging capture and post-processing, statistical analysis revealed differences in seed size, but not in shape, between S. latifolia diploids and tetraploids, as well as some differences in shape among the parentals and hybrids. A detailed inspection using fluorescence microscopy allowed for the identification of shape differences in the cells of the seed coat. In the case of hybrids, differences were found in circularity and solidity. Overal seed shape is maternally regulated for both species, whereas cell shape cannot be associated with any of the sexes.

Discussion

Our results provide additional tools useful for the combination of morphology with genetics, ecology or taxonomy. Seed shape is a robust indicator that can be used as a complementary tool for the genetic and phylogenetic analyses of Silene hybrid populations.

Curvature Analysis of Seed Silhouettes in Silene L

The application of seed morphology to descriptive systematics requires methods for shape analysis and quantification. The complexity of lateral and dorsal views of seeds of Silene species is investigated here by the application of the Elliptic Fourier Transform (EFT) to representative seeds of four morphological types: smooth, rugose, echinate and papillose. The silhouettes of seed images in the lateral and dorsal views are converted to trigonometric functions, whose graphical representations reproduce them with different levels of accuracy depending on the number of harmonics. A general definition of seed shape in Silene species is obtained by equations based on 40 points and 20 harmonics, while the detailed representation of individual tubercles in each seed image requires between 100 and 200 points and 60-80 harmonics depending on their number and complexity. Smooth-type seeds are accurately represented with a low number of harmonics, while rugose, echinate and papillose seeds require a higher number. Fourier equations provide information about tubercle number and distribution and allow the analysis of curvature. Further estimation of curvature values in individual tubercles reveals differences between seeds, with higher values of curvature in S. latifolia, representative of echinate seeds, and lower in S. chlorifolia with rugose seeds.

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Comparison of Seed Images with Geometric Models, an Approach to the Morphology of Silene (Caryophyllaceae)

Seed morphological description is traditionally based on adjectives, which originated from the comparison with other shapes, including geometric figures. Nevertheless, descriptions based on this feature are not quantitative and measurements giving the percentage of similarity of seeds with reference figures are not available in the literature. Lateral views of Silene seeds resemble the cardioid and cardioid-derived figures. Dorsal views, nonetheless, resemble ellipses and derivatives, allowing seed shape quantification by comparison with defined geometric figures. In this work, we apply already-described models as well as new models to the morphological analysis of 51 Silene species. Our data revealed the existence of a link between lateral and dorsal models. Lateral models closed in the hilum region (models LM2 and LM4) were associated with those convex models of the dorsal seed views (DM1-DM4, DM10). Lateral models more open around the hilum region adjusted to seeds characterized as dorso canaliculata type better, i.e., to those geometric models with partial concavities in their dorsal views. The relationship between lateral and dorsal models, as well as between the models to their utility in taxonomy, is discussed.

The Outline of Seed Silhouettes: A Morphological Approach to Silene (Caryophyllaceae)

Seed morphology is an important source of information for plant taxonomy. Nevertheless, the characters under study are diverse, and a simple, unified method is lacking in the literature. A new method for the classification of seeds of the genus Silene based on optical images and image analysis has recently been described on the basis of morphological measurements of the lateral seed views. According to the outline of their silhouettes, seeds from 52 species (49 of Silene and three related species) were classified in three groups: smooth, rugose and echinate, revealing remarkable differences between these groups. This methodology has been applied here to 51 new species, making a total of 100 species of Silene analyzed so far. According to our data, a new group was described, termed papillose. The results showed morphological differences between the four mentioned seed groups, with reduced values of circularity for dorsal and lateral seed views in the papillose and echinate groups and reduced values of solidity in the papillose seeds. The method was applied to the analysis of individual as well as to average seed silhouettes and some of the differences between groups were maintained in both cases.