Oxford University biologists have found a striking example of apparent on-going extinction in a European plant species. This confirms that biodiversity can be threatened when one type of plant species eradicates its relative by swamping it with incompatible pollen.
The findings will help predict the fate of plant species such as the wild daffodil Narcissus pseudonarcissus, celebrated by the poet William Wordsworth, and the potential impact of pollen flow from genetically modified crops.
Researchers at Oxford University's Department of Plant Sciences studied the plant Mercurialis annua (the annual mercury), a European species of weed, which has two types, one with separate male and female plants, the other with male and female flowers on the same plant (a hermaphrodite).
During the last ice age, males and females of the plant were restricted to the eastern Mediterranean Basin, while the hermaphrodites' ancestors occurred in southern Spain and present-day Morocco. Over time, when climates warmed, they moved towards each other and now occur side by side in northern Spain. Historical records show that, over the last 40 years, the males and females are rapidly displacing the hermaphrodites and moving south, further into Spain.
By allowing males, females and hermaphrodites to compete for mates in experimental populations, the researchers have now been able to explain the mechanism behind the rapid demise of the hermaphrodites: when hermaphrodites cross with either males or females, they produce sterile progeny (similar to a mule, which is a sterile offspring of a cross between donkeys and horses). This is because the two types of plant have a different number of chromosomes: the males and females have 16, while the hermaphrodites have 48 chromosomes.
The males have a large advantage because they produce much more pollen than hermaphrodites, and hold their flowers on an erect stalk above the leaf canopy. This leads to a process called 'pollen swamping,' by which the male plants father nearly all of the offspring in the population. Females, which have the same number of chromosomes as males, produce normal fertile progeny when pollinated by the males, but hermaphrodites, which have a different number, produce mainly sterile progeny. The result is the rapid extinction of hermaphrodite populations that the researchers have observed.
This finding has implications for other plant species. For example, in the English Lake District, the wild daffodil Narcissus pseudonarcissusis threatened by cultivated Narcissus 'Carlton', which has twice as many chromosomes. In the past it has been assumed that because the two species produce sterile hybrids, the wild species is not threatened. The results for the annual mercury suggest that the cultivated daffodil could in fact eliminate Wordsworth's daffodils.
Dr John Pannell, who co-authored the paper with his Oxford doctoral student Dr Richard Buggs, said: "It is clear that climate change and human interference can set plant populations on the move, with rapid extinction being one possible consequence when different species meet. Processes occurring in natural plant populations can teach us important lessons about what can happen when related but genetically incompatible species mate with one another."
The paper "Rapid displacement of a monoecious plant due to pollen swamping by a dioecious relative" by Richard J. A. Buggs and John R. Pannell will be published in Current Biology on 23 May 2006.
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