Seed dispersal syndromes are morphological characters of seeds correlated to particular seed dispersal agents. Dispersal is the event by which individuals move from the site of their parents to establish in a new area. A seed disperser is the vector by which a seed moves from its parent to the resting place where the individual will establish, for instance an animal. Similar to the term syndrome, a diaspore is a morphological functional unit of a seed for dispersal purposes. Characteristics for seed dispersal syndromes are commonly fruit colour, mass, and persistence. These syndrome characteristics are often associated with the fruit that carries the seeds. Fruits are packages for seeds, composed of nutritious tissues to feed animals. However, fruit pulp is not commonly used as a seed dispersal syndrome because pulp nutritional value does not enhance seed dispersal success. Animals interact with these fruits because they are a common food source for them. Although, not all seed dispersal syndromes have fruits because not all seeds are dispersed by animals. Suitable biological and environmental conditions of dispersal syndromes are needed for seed dispersal and invasion success such as temperature and moisture. Seed dispersal syndromes are parallel to pollination syndromes, which are defined as floral characteristics that attract organisms as pollinators. They are considered parallels because they are both plant-animal interactions, which increase the reproductive success of a plant. However, seed dispersal syndromes are more common in gymnosperms, while pollination syndromes are found in angiosperms. Seeds disperse to increase the reproductive success of the plant. The farther away a seed is from a parent, the better its chances of survival and germination. Therefore, a plant should select certain traits to increase dispersal by a vector (i.e. bird) to increase the reproductive success of the plant.
Evolution Seeds have evolved traits to reward animals to enhance their dispersal abilities. Differing foraging behaviours of animals can lead to selection of dispersal traits and spatial variation such as increase in seed size for mammal dispersal, which can limit seed production. Seed production is limited by some seed syndromes because of their cost to the plant. Therefore, seed dispersal syndromes will evolve in a plant when the trait benefit outweighs the cost. The seed dispersers themselves play an essential role in syndrome evolution. For example, birds put strong selection pressure on seeds for colour of fruits because of their enhanced vision. Illustrations of such colour evolution include green colour being produced because its photosynthesis abilities are less costly while red colour emerges as a byproduct for protection from arthropods. For visible characteristic differences to develop between dispersers and non-dispersers a few conditions need to be met 1. Specialization must increase dispersal success whether morphological, physiological or behavioural 2. Energy investment for dispersal will be taken from energy investment of other traits 3. Dispersal traits will benefit the dispersers over non-dispersers. Phenotypic (visible characteristics) differences in non-dispersers and dispersers can be caused by external factors, kin competition, intraspecific competition and habitat quality.
History In 1930, Ridley wrote an important book called The dispersal of plants throughout the world, which goes into detail about each form of dispersal; dispersal by wind, water, animals, birds, reptiles and fish, adhesion, and people. He details the morphology and traits for each dispersal method, which are later described as seed dispersal syndromes. This began the idea of seed trait selection being associated with a form of seed dispersal. Then in 1969 van der Pijl identified seed dispersal syndromes based on each mechanism of seed dispersal in his book Principles of Dispersal in Higher Plants. He is the pinnacle of seed dispersal syndromes and is cited by many scientists who study seed dispersal syndromes. He describes the morphology of interactions between fruits and flowers, and classifies dispersal in invertebrates, fish, reptiles, birds, mammals, ants, wind, water and the plant itself. Janson in 1983 continued the study on seed dispersal syndromes and classified seed dispersal syndromes of fruit by size, colour and husk or no husks in species of Peruvian tropical forest. He went in depth about the interaction between plants that have adapted to seed dispersal by birds and mammals. Willson, Irvine & Walsh in 1989 added more factors to the study of seed dispersal syndromes and looked at differing fleshy fruits and their correlation to moisture and differing ecological factors. They looked at bird-dispersal and mammal-dispersal and how the fruits differed in dispersal syndromes such as colour and size. These scientists began the theory and ideas behind seed dispersal syndromes that are crucial to the evolution of reproduction in plants.
Types and functions Dispersal syndromes have been previously classified by: size, colour, weight, protection, flesh type, number of seeds, weight and start time of ripening. Syndromes are often associated with the type of dispersal and morphology. Also chemical composition can influence the disperser's fruit choice. The following are types of seed dispersal and their syndromes.
Anemochory
Anemochory is defined as seed dispersal by wind. Common dispersal syndromes of anemochory are wing structures and brown or dull coloured seeds without further rewards. Van der Pijl named seeds for anemochory flyers, rollers, or throwers to represent the seed dispersal syndromes and their behaviour. Flyers are typically categorized as dust diaspores, balloons, plumed or winged. Dust diaspores are small flat structures on seeds that appear to be the transition to wing diaspores, balloons are inflated seed characteristics and plumes are hairs or elongation seed characteristics. Wings have evolved to increase dispersal distance to promote gene flow. Anemochory is commonly found in open habitats, canopy trees, and dry season deciduous forests. Wind dispersers mature in the dry season for optimum high long-distance dispersal to increase success of germination.
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