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Lecture 11

BIO153 Lecture 11.pdf

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University of Toronto Mississauga
Christoph Richter

2009 BIO153: Lecture 11 Coevolution of Plants and Pollinators February 23, 2009 The history of the seed plants pre-dates the diversification of terrestrial animals; thus, the delivery of pollen from an anther to a stigma originally depended on something other than a pollinator. Ancestral condition = wind pollination: anemophily ▯ all gymnosperms; some angiosperms ▯ inefficient: requires massive overproduction of pollen because most is wasted (does not reach the target) ▯ this inefficiency results in low rates of outcrossing Disadvantages to a plant: ▯ there is a considerable metabolic cost to pollen production ▯ doesn’t work in wet environments: pollen needs to be dry to be transported; not enough wind However, it can be advantageous: ▯ works well in low diversity stands (pollen has a good chance of making it to a conspecific) ▯ works well in open areas (lots of wind) ▯ works well in areas of low – moderate rainfall (doesn’t get wet & weighed down) ▯ works well when there is a short, unpredictable growing season (dependence on an animal pollinator can be a risky strategy) ▯ don’t need to invest energy in making flowers Some wind-pollinated angiosperms evolved from insect-pollinated ancestors (benefits outweighed the costs) ▯ flowering plants that no longer need a showy flower: small, drab flowers; no nectar ▯ don’t need to attract a pollinator ▯ e.g. grasses (adapted to dry, wide-open places) 1 ▯ most pollen allergens are from anemophilous angiosperms: ragweed, birches, grasses, etc. Pollen structure reflects the mode of pollination: wind-pollinated = lightweight, small, smooth, winged; animal-pollinated = often sticky; barbed Animal pollination: zoophily (entomophily = insect pollination) ▯ by end-Cretaceous (~65 mya), angiosperms began a rapid diversification due to interaction with animals (pollination; herbivory; parasitism, etc.) Animal pollination evolved from wind pollination ▯ now a mutualistic relationship (mutualism: both parties benefit) but likely evolved from exploitation (herbivory) The first step in animal pollination: from herbivory ▯ mutualism ▯ ovules exude sticky sap to capture wind- borne pollen, which attracted insects (probably beetles) ▯ carpel probably evolved originally as a structure to prevent herbivory ▯ plants evolved an alternative reward (nectar) to attract beetles, but keep them from damaging the ovule How do plants benefit? ▯ free from wind dependence: permitted the colonization of new environments ▯ e.g. rainforest: individuals widely dispersed (high species diversity); no wind in canopy ▯ direct delivery of gametes: lower pollen/ovule ratio ▯ can succeed in lower population densities ▯ promotes outcrossing How do pollinators benefit? 1. nutritive rewards: ▯ pollen: lipid, starch, protein 2 ▯ nectar: sugar, amino acids, lipids, vitamins ▯ edible flower parts, oils, resins ▯ flower may harbour prey 2. non-nutritive rewards: ▯ substances for making pheromones ▯ shelter ▯ heat But there are costs on both sides: plant: ▯ production of reward costly ▯ floral display, scent ma
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