Flower colour variation across a hybrid zone in Antirrhinum as perceived by bumblebee pollinators

To assess if pollinators’ behaviour could explain the maintenance of hybrid zones between different flower colour morphs, we analyzed flower colour variation in an Antirrhinum hybrid zone using spectrometry and a model of bee perception. Some colours generated by hybridization were not observed in any Antirrhinum species and even appeared to be rare among angiosperms. Variation in flower colours within the hybrid zone was continuous; the most similar colours were predicted not to be discriminated from one another in natural foraging situations. However, when compared at a scale corresponding to bees’ foraging range, some flower colours could be discriminated from all colours displayed by neighbouring plants. This could affect pollinator behaviour and explain lower visitation rates within the centre of the hybrid zone. Behavioural studies involving bumblebees and plant mixtures of parental and hybrid flower colours carefully characterized with appropriate visual models will be necessary to test this hypothesis. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: Lars Chittka     

Evidence for cytoplasmic inheritance of a developmental organizer affecting growth habit and leaf shape in Antirrhinum majus

A cross between two distinct, true-breeding plants of Antirrhinum majus L. showed an unexpected pattern of inheritance of growth habit in the F2, which was extended to both growth habit and leaf shape in the F3 generation of all the plants traced further. All the F3 families, offspring of individual F2 plants, were very uniform for both growth habit and leaf shape traits but differed distinctly from each other in these respects. The backcrosses of selected F3 and F4 families to the original parents in the cross did not segregate for the distinctive family phenotypes. This led to the postulate that a cytoplasmic factor was involved in the regulation and/or integration of genetic information concerned with growth habit/leaf shape. The similarity of the reciprocal backcrosses of the F3 and F4 families led to the further postulate that the proposed cytoplasmic factor was specified by both the maternal and paternal parents to a similar degree. That the gene component was segregating normally was shown by the inheritance of four marker genes for flower colour, colour pattern and flower shape.     

Determining the contribution of epidermal cell shape to petal wettability using isogenic Antirrhinum lines

The petal epidermis acts not only as a barrier to the outside world but also as a point of interaction between the flower and potential pollinators. The presence of conical petal epidermal cells has previously been shown to influence the attractiveness of the flower to pollinating insects. Using Antirrhinum isogenic lines differing only in the presence of a single epidermal structure, conical cells, we were able to investigate how the structure of the epidermis influences petal wettability by measuring the surface contact angle of water drops. Conical cells have a significant impact on how water is retained on the flower surface, which may have indirect consequences for pollinator behaviour. We discuss how the petal epidermis is a highly multifunctional one and how a battery of methods, including the use of isogenic lines, is required to untangle the impacts of specific epidermal properties in an ecological context.     

Mutations perturbing petal cell shape and anthocyanin synthesis influence bumblebee perception of Antirrhinum majus flower colour

We wished to understand the effects on pollinator behaviour of single mutations in plant genes controlling flower appearance. To this end, we analysed snapdragon flowers (Antirrhinum majus), including the mixta and nivea mutants, in controlled laboratory conditions using psychophysical tests with bumblebees. The MIXTA locus controls petal epidermal cell shape, and thus the path that incident light takes within the pigment-containing cells. The effect is that mixta mutant flowers are pink in comparison to the wild type purple flowers, and mutants lack the sparkling sheen of wild type flowers that is clearly visible to human observers. Despite their fundamentally different appearance to humans, and even though bees could discriminate the flowers, inexperienced bees exhibited no preference for either type, and the flowers did not differ in their detectability in a Y-maze—either when the flowers appeared in front of a homogeneous or a dappled background. Equally counterintuitive effects were found for the non-pigmented, UV reflecting nivea mutant: even though the overall reflectance intensity and UV signal of nivea flowers is several times that of wild type flowers, their detectability was significantly reduced relative to wild type flowers. In addition, naïve foragers preferred wild type flowers over nivea mutants, even though these generated a stronger signal in all receptor types. Our results show that single mutations affecting flower signal can have profound effects on pollinator behaviour—but not in ways predictable by crude assessments via human perception, nor simple quantification of UV signals. However, current models of bee visual perception predict the observed effects very well.     

Allozymic differentiation of the Antirrhinum graniticum and the Antirrhinum meonanthum species groups

Forty populations from eight taxa belonging to six species of Antirrhinum were studied for electrophoretic differentiation at 14 allozyme loci. All the studied species show that marker alleles and genetic distances between conspecific populations are lower than for other species of the genus. Results permit the recognition of A. onubensis and A. boissieri at specific rank. The low level of genetic distance together with morphological characters and sympatric range, support the recognition of A. ambiguum as a new subspecies of A. graniticum, as A. graniticum ssp. ambiguum Mateu & Segarra stat. nov. The results of the study support separation of the studied taxa into two groups coinciding with subsections Streptosepalum and Antirrhinum, but disagreeing with the arrangement of species into them. According to the results of the study these groups consist of A. meonanthum and A. braun-blanquetii (Streptosepalum) and A. graniticum s.l., A. onubensis and A. boissieri (Antirrhinum). The status of A. meonanthum var. rothmaleri requires further clarification.     

Flower ontogeny in Capparis spinosa (Capparaceae) with special emphasis on symmetry expression

The flower ontogeny of Capparis spinosa L. was studied using scanning electron microscopy (SEM) in order to shed light on the early expression of symmetry in a representative member of Capparaceae. Sepals start decussately with the abaxial and adaxial sepals, followed by the lateral ones. Petal appearance proceeds uni‐directionally from the abaxial to the adaxial side. Stamens develop centrifugally on an androecial ring. Five to seven carpel lobes arise on a gynoecial ring. Monosymmetry is established in early development by the uni‐directional petal appearance and is further enhanced by the acquisition of an abaxial nectary appendage and differential development of perianth organs at anthesis. The early monosymmetric pattern found in C. spinosa differs from the early monosymmetric and early disymmetric pathways of Cleomoideae. Our findings are consistent with the hypothesis that the late establishment of monosymmetry is a reflective of symmetry in early development.     

Evolution of Allometry in Antirrhinum

Correlated variation in shape and size (allometry) is a major component of natural diversity. We examined the evolutionary and genetic basis for allometry using leaves and flower petals of snapdragon species (Antirrhinum). A computational method was developed to capture shape and size variation in both types of organ within the Antirrhinum species group. The results show that the major component of variation between species involves positively correlated changes in leaf and petal size. The correlation was maintained in an F2 population derived from crossing two species with organs of different sizes, suggesting that developmental constraints were involved. Identification of the underlying genes as quantitative trait loci revealed that the larger species carried alleles that increased organ size at all loci. Although this was initially taken as evidence that directional selection has driven diversity in both leaf and petal size, simulations revealed that evolution without consistent directional selection, an undirected walk, could also account for the parental distribution of organ size alleles.     

Cytologische Untersuchungen an Antirrhinum

Ohne ZusammenfassungMit 1 Textabbildung (8 Einzelbildern) und Tafel I und II.     

Antirrhinum majus microspore maturation and transient transformation in vitro

The male gametophyte of higher plants represents an excellent system to study gene regulation, cell fate determination and cellular differentiation in plants because of its relative simplicity compared to the sporophyte and its accessibility for cytological and molecular analysis. Unicellular plant microspores are single haploid cells, which can be isolated in large amounts at a defined developmental stage. Microspores cultured in vitro in a rich medium develop into mature pollen grains, which are fertile upon pollination in vivo. It is reported here that isolated Antirrhinum majus microspores when cultured in an optimal medium develop to form mature, fertile pollen. Their development closely resembled that of pollen formed in vivo. Isolated microspores were bombarded with Aquorea victoria Green Fluorescent Protein (GFP), Discosoma Red Fluorescent Protein (dsRFP) and beta-glucuronidase (GUS) reporter genes under the control of various promoters and transient expression was observed throughout pollen development in vitro. Bombarded and not bombarded in vitro-matured pollen grains were able to germinate both in vitro and on receptive stigmas and to set seed. The protocol of maturation, transient transformation and germination of Antirrhinum majus pollen in vitro described here provides a valuable tool for basic and applied research.     

Die Aurea-Sippen von Antirrhinum majus

Ohne Zusammenfassung