Asymmetric Flowers Research Articles

Ancient asymmetries in the evolution of flowers

Dorsoventral asymmetry in flowers is thought to have evolved many times independently as a specialized adaptation to animal pollinators [1, 2]. To understand how such a complex trait could have arisen repeatedly, we have compared the expression of a gene controlling dorsoventral asymmetry in Antirrhinum with its counterpart in Arabidopsis, a distantly related species with radially symmetrical flowers. We found that the Arabidopsis gene is expressed asymmetrically in floral meristems, even though they are destined to form symmetrical flowers. This suggests that, although the flowers of the common ancestor were probably radially symmetrical, they may have had an incipient asymmetry, evident at the level of early gene activity, which could have been recruited many times during evolution to generate asymmetric flowers.

Effects of root hemiparasitic infection on host performance: Reduced flower size and increased flower asymmetry

We conducted two pot experiments to examine the relationship between hemiparasitic plant (Rhinanthus serotinus, Scrophulariaceae) infection and host (Linum usitatissimum, Linaceae and Brassica rapa ssp. oleifera, Brassicaceae) performance. We were especially interested in the effects of hemiparasitism on the size and shape asymmetry of host flowers, since neither subject has been studied before. We also conducted a field experiment to examine the effects of shape asymmetry of B. rapa ssp. oleifera flowers on pollination success. The shape of flowers produced by both L. usitatissimum and B. rapa ssp. oleifera plants grown without parasites was less asymmetric, and for both host species, the flowers of plants grown without parasites had significantly longer petals than flowers produced by hosts with parasites. Significantly more bumblebees visited the inflorescences of B. rapa with untreated flowers compared to inflorescences with highly asymmetrical flowers (two of the four petals clipped shorter), whereas no difference was found in the number of visits made by syrphid flies. Parasitism significantly reduced host biomass and reproduction in both host species.

Symmetry in Flowers: Diversity and Evolution.

This article traces research on floral symmetry back to its beginnings. It brings together recent advances from different fields that converge in floral symmetry and new unpublished material on diversity and development of floral symmetry. During floral development, symmetry may change: monosymmetric flowers may have a polysymmetric early phase; polysymmetric flowers may have a monosymmetric or even asymmetric early phase; more than one symmetry change is also possible. In Lamiales s.l. (comprising the model plant Antirrhinum, where the cycloidea gene produces monosymmetric flowers with the adaxial side of the androecium reduced), taxa also occur in which the androecium is reduced on both sides, adaxial and abaxial. As a trend in asymmetric flowers, enantiomorphy (with two mirror-image morphs) at the level of individuals seems to occur only in groups in which the flowers are predominantly of a relatively simple construction. In contrast, one morph is fixed at the level of species or higher taxa in groups with more complicated flowers. This is indicated by the apparent lack of enantiomorphy in corolla contortion in asterids but its predominance in rosids with contort flowers, or by the apparent lack of enantiomorphy in the pollination organs of asymmetric flowers in Faboideae but its presence in asymmetric flowers in Caesalpinioideae. To study the evolution of the diverse symmetry patterns, a concerted approach from different fields including molecular developmental genetics, pollination biology, and comparative diversity research is necessary.

Effect of floral symmetry on flower choice and foraging behaviour of bumble bees

Bumble bees are known to prefer symmetrical over asymmetrical flowers and bilateral over radial flower types. This study examined the basis of these preferences in three experiments with artificial flowers. The first experiment showed that flower-naive worker bees (Bombus impatiens) displayed no innate preference for symmetrical over asymmetrical or bilateral over radial flowers in choice tests. The second experiment tested whether bees showed a learning or memory bias for symmetrical over asymmetrical rewarding flowers when foraging on arrays of either bilateral or radial flower types. There was no evidence that bees learned or remembered symmetrical rewarding flowers better than asymmetrical rewarding flowers. The percentage of visits to rewarding flowers during test runs for bees foraging on arrays with bilateral flowers was consistently greater than on arrays with radial flowers. A third experiment examined the effect of nectar-guide symmetry on flower-handling and travel times. Bees were tested on homogeneous arrays with bilateral or radial flowers of either symmetrical or asymmetrical shapes and with symmetrical, asymmetrical, or no nectar guides. Guide symmetry had no effect on flower-handling or travel times. However, bees handled symmetrical flowers about 20% faster than asymmetrical flowers and bilateral flowers about 45% faster than radial flowers; travel times of bees on arrays with bilateral flowers were about 2.5 times faster than travel times of bees foraging on arrays of radial flowers.

Insect preference for symmetrical artificial flowers.

An insect preference for floral symmetry may be maintained because plants with symmetrical flowers, which are able to control developmental processes under given environmental conditions, also are able to provide more pollinator rewards than plants with asymmetrical flowers. Alternatively, insects may have an inherent preference for symmetrical structures and thereby impose selection for the maintenance of symmetry in flowers even in the absence of any pollinator rewards. We tested for an insect preference for radially symmetrical flowers by using horizontally placed units of four circular coloured flower models varying in size and symmetry. The shape and colour of the model flowers did not resemble any naturally occurring flowers in the environment. Insects and Hymenoptera, respectively (five species of Diptera and one species of Coleoptera) that visited the flower models clearly preferred symmetrical models over asymmetrical ones, and the ranking of visits to the models reflected a preference for large, symmetrical flowers. These results provide evidence for a preference for symmetrical flower models, even in the absence of pollinator rewards.

Some pollinators do not prefer symmetrically marked or shaped daisy (Asteraceae) flowers

We noted no significant pollinator preference for symmetry in daisy (Asteraceae) flowers. Asymmetry of markings on Gorteria diffusa did not negatively affect pollinator visitation by bee-flies. Nevertheless, the flowers of this species tend to be symmetrically marked. Beetles did not discriminate against artificial daisy flowers which had petals placed in asymmetrical positions. We are sceptical about Moller's (1995) view that the symmetry of flowers is due to pollinator discrimination against asymmetrical flowers.

Secondary and tertiary pollen presentation in Polygala myrtifolia and allies (Polygalaceae, South Africa)

In Polygala myrtifolia and probably other South African Polygala species with asymmetrical flowers, pollen is first presented in a very unusual manner; eight sausage-like pollen masses are attached by their ends to the crook-shaped tip of the style (secondary pollen presentation). Floral morphology suggests that when the asymmetrical keel of the flower is pressed down by a sufficiently strong bee, the pollen presenter springs out and the stigma and pollen hit the visitor dorsally on the right flank pleuro-nototribic. When the keel returns to its original position, the remaining pollen is moved into the rostrum of the keel. In subsequent visits pollen is squeezed out of there by a pocket at the tip of the style. This tertiary pollen presentation follows the piston mechanism and dusts the ventral side of the visitor (sternotribic).

Plant development: Making asymmetric flowers

The question of how positional information is assigned in floral meristems is beginning to be answered, aided recently by the cloning of a gene responsible for conferring dorsal fate in Antirrhinum majus flowers.

Pollinator Preference for Symmetrical Flowers and Sexual Selection in Plants

Floral characters are often subject to strong directional selection from pollinators, and this may disrupt developmental homeostasis in flowers that therefore develop large degrees of fluctuating asymmetry. Fluctuating asymmetry in floral traits may lead to sexual selection in plants if pollinators assortatively visit symmetrical flowers. We studied the relationship between pollination and floral asymmetry in ten different flowering plant species in Spain, Denmark, and Sweden. Pollinators preferentially visited symmetrical flowers independently of whether the plant species produced nectar. Symmetrical flowers of some species had higher pollinator rewards in terms of nectar content than asymmetrical flowers. Handling time of flowers was unaffected by asymmetry since insect visits to symmetrical flowers lasted as long as visits to asymmetrical flowers. Insect preferences for symmetrical flowers should have resulted in sexual selection with respect to symmetry. The evolution of pollinator preferences for symmetrical flowers and the potential fitness benefits to plants with symmetrical flowers due to this preference are discussed