The common primrose (Primula vulgaris) is a favourite of biology teachers
because it illustrates enforced cross-pollination by insects. But the plant
has been puzzling ecologists for many years because a rare form that has
the advantage of being able to self-pollinate has not spread. Now, a group
of biologists in Britain has come up with an explanation that is precisely
the opposite of what anyone would expect: the rare type is unsuccessful
because its pollination is too efficient.
Flowers of primrose tend to come in one of two forms: the ‘pin’ and
the ‘thrum’. The pin type has a stigma at the top, just above the corrolla
tube. In the thrum type, the positions of he stigma and anthers are reversed.
According to the textbooks, insects get pollen on their bodies and transfer
it between pin and thrum flowers, or vice versa, to pollinate them.
The complication is that there is a rare third form of the primrose,
called the homostyle, which has its stigma and anthers at the same level.
It is self-fertile, as well as being able to pollinate pin flowers, and
it usually produces more seeds than the two other forms. the homostyle has
been observed for 40 years, but surprisingly it remains as rare as ever.
With its twin advantages of guaranteed pollination and high seed production,
it would seem reasonable to think that the homostyle form would spread rapidly
in the primrose population. Yet homostyles are found in significant numbers
only where they were discovered: in the Chiltern Hills and in parts of Somerset
and North Dorset between Shepton Mallet and Shaftesbury. Even in these locations,
the homostyle has not increased its numbers.
Mark Boyd, Jonathan Silvertown and Chris Tucker of the Open University,
Milton Keynes, think they may have found out why (Journal of Ecology, vol
78 p 799). Boyd watched primroses in the field for long periods. Though
some observers have reported bees and butterflies visiting the flowers,
he never saw pollination take place. These insects seemed to prefer other
flowers, such as violets, which were in bloom at the same time.
Boyd noticed that at night primroses are particularly visible and are
highly scented. He found that they are efficient at attracting moths, and
suggests that these might be their main pollinators of the flowers. This
possibility was suggested by Charles Darwin as long ago as 1877.
In the laboratory, Boyd confirmed that large moths are indeed effective
pollinators of both pin and thrum forms, but small moths can pollinate pins
only. He also found that homostyles set seed equally well with or without
moths. In the wild, therefore, homostyles produce large numbers of seeds
even when pollinators are scarce, as they often are. The other two forms
of primrose produce fewer, but larger, seeds.
Homostyles pollinate efficiently, so they produce many small seeds.
But larger seeds have an advantage when germinating, Boyd and his colleagues
suggest. So only a few of the homostyle seeds manage to germinate successfully.
Biologists usually consider the ability to self-pollinate an advantage for
a flower when pollinating insects are scarce. It seems that if the Open
University biologists are correct, some biology textbooks may have to be