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Spring 2013

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

May 21, 2013

Blackberry sexuality: It’s complicated

In May, the icy-white flowers of blackberry appear in sunny and partly shady open areas. If their arrival coincides with a cold snap, Washington experiences "blackberry winter," with the flowers resembling snow clumped at the ends of blackberry canes, a reminder that spring always has the potential to regress to a season of zipped jackets and raised thermostats.

The flowers open at the ends of branches sprouting from the blackberry's arching, second-year canes, or floricanes.

Poking up through the floricanes are rubbery, yellow-green shoots, or primocanes, emerging from the blackberry's underground stem. They will produce leaves and stems, but no flowers or fruits until next year. Before winter, primocanes will form buds and drop their leaves, the stems poised to become next year's floricanes.

This year's floricanes will die back by winter, but not before producing the succulent black fruits that birds, foxes and people enjoy in the summer.

The fruits are the result of the blackberry's versatile reproductive strategies. Like many flowering plants, blackberry flowers are hermaphrodites: A single flower contains both male and female

Rubus or Blackberry in flower, illustration by Patterson Clark

parts. Insects feeding on the flower's nectar can transfer pollen from one flower to another. Pollen contains sperm cells, which can make their way down into a blackberry flower's numerous ovaries to complete fertilization, leading to the formation of embryos contained within seeds.

But blackberries also practice apomixis, the ability to bypass fertilization by developing seeds asexually in the ovaries, without any help from pollen. Those blackberries sliding down the side of your ice cream may contain

seeds from both sexual and asexual origins.

Sometimes apomixis is used only as a backup. One species of bramble is exclusively sexual in its home range in Southeast Asia, but it relies mostly on apomixis in Madagascar, where it grows as an invasive plant.

Apomixis has evolutionary advantages: Asexual seeds essentially yield immaculate clones of the parent, passing on the parent's beneficial traits without interruption. Over long periods, this can lead to numerous

blackberry microspecies, about 400 of which are known to grow in eastern North America.

One problem with apomixis, however, is that plants grown from the asexual seed have diminished fertility. But if two genetically different apomictic blackberries manage to reproduce sexually, their hybrid offspring will tend to be much more fertile — and more likely to engage in sexual reproduction.

Sources: The Plant Cell, American Journal of Botany, Plant Systematics and Evolution, University of Michigan Herbarium