The Perils of Pollination

What am I looking at?

This is an image of several pollen grains infected with parasitic fungi sitting on the stigma of a mallow flower. The larger grains are from the mallow (1) and the smaller grains are from an aster flower (family Asteraceae; 2). The thin blue segmented strands wrapped around -- and even invading -- the pollen grains is a parasitic fungus, possibly from the genus Cladosporium or Alternaria (3). The larger blue and red tube-like structures are single-cell trichomes, or hairs, forming the sticky part of the stigma that the pollen is resting on (4). The colors you see in this image are produced by cellulose- and chitin-binding fluorescent dyes (Calcofluor White, and Congo Red). Autofluorescence (colors produced by when the laser from the microscope hits the sample) of the mallow pollen grain wall (sporoderm) is green.

Biology in the Background

Pollen is produced by flowers and designed to travel. Pollen grains are blown by the wind or picked up by animals known as pollinators, such as insects and birds. Once pollen is deposited on a flower of the same species, it initiates the process of creating a seed by passing its genetic material down through the flower and into the ovules (eggs) contained below the flower.  

Some fungi take advantage of this pollen distribution system to aid in their own dispersal and spread. These fungi wrap thin tendrils of hyphae (which make up the main “body” of a fungus) around pollen grains and invade at openings in the grain’s wall. Pollen provides a rich source of carbohydrates, proteins, and other nutrients that the fungus can use to grow. Then when the pollen grain is picked up by the wind or pollinators, these tendrils break away and travel with the pollen to its new destination, allowing the fungus to travel much farther than it could on its own. Once the pollen lands, the hyphae can grow and spread throughout the fungi’s new home, potentially blocking fertilization in its new host flower.  

Mallow pollen can grow up to 150 micrometers wide, roughly twice the width of a human hair. Daisy pollen is about 20 micrometers wide, roughly 4 times smaller than the width of a human hair. The hyphae of the fungus you see here can grow up to 6 micrometers wide, roughly 13 times smaller than the width of a human hair.

Technique

These images were created using confocal microscopy.