terrestrial

Coevolution of angiosperms and insects

One of the reasons for the massive diversity of angiosperms today is that they evolved an extraordinary mechanism for dispersing their genes.

Insects have long been allied closely to plants, and it may be that angiosperms radiated so rapidly because of their association with specific insect pollinators. Cretaceous flowers often show features associated today with insect pollination: stamens with small anthers and low pollen production (minimising pollen waste), pollen grains often covered with a pollenkitt-like material which are larger than those adapted to wind dispersal.

There is one distinct advantage of insect pollination: the spread of genetic material over a wide area; something which wind-pollinated plants cannot achieve to the same extent. Insect pollination of flowers probably began with insects feeding on the plants they later came to work for, leading us to expect bite marks on early plants. Fossil evidence, however, is ambiguous: the records of insect feeding behaviour do not particularly coincide with the angiosperm radiation.

However, all is not lost. There is still some synchrony between the appearance of flowers and certain advanced pollinator groups: some Hymenoptera, including a group of wasps that are a sister group to bees, and Lepidoptera (butterflies and moths, see right), coincide roughly with the first appearance of angiosperms. However, other early pollinator groups, such as craneflies and fungus gnats), Coleoptera and other Hymenoptera (e.g. the saw flies) pre-date the record of angiosperms, and did not change much when they appeared. Meanwhile, the group of insects so closely associated with flowers, the honeybees, have only been around for about 100 million years.

To further analyse the possibility of coevolution, we can study extant pollinator behaviour. One of the problems faced with the pollination of flowers is that there is a conflict of interest between the flowers and the animals. It is in the plant’s interest for the pollinator to travel between widely spaced individuals, so pollen is transferred to a distantly related plant. However, the pollinator should favour a mixed diet, unless one type of flower provides a much higher reward than any other.

The two diagrams below show the organisms believed to have particularly influenced the Mid-Cretaceous radiation.

In both the images above (Labandeira et al., 2001), showing the insects in decreasing order of importance in their effects on angiosperm diversity, we can look at their respective diversities at strategic points in the fossil record. Angiosperms begin to appear frequently in the fossil record around the line reading ‘Albian’: Early Mid Cretaceous. Almost all the major pollinators were already present in the fossil record by the time the angiosperms made a proper appearance.

Aspects of the flower that may have coevolved

Scent: A far-reaching method of communication between plants and their pollinators. Insects have a number of uses for fragrances from plants: distance judgement, approaching, locating the best spot to land and to feed. Scent molecules are simple and small, and all volatile to a point.

Colour: Different pollinators respond differently to colour. Insects respond to long wavelengths of blue and ultraviolet, whereas birds, like humans, are more responsive to bright reds. Plants use colouration for two methods of transport: for pollen and for seeds. Flowers are coloured specifically for their pollinators. A plant that will predominantly be pollinated by birds will have bright reds and oranges, while insects will pollinate purples, blues and whites more readily. Different coloured lines and patterns on the flowers also serve as landing strips for insects, directing them immediately to the nectar reward.

Fruit: Fruit is generally fed upon by larger animals than insects, and is coloured for the eye of the disperser. Berries eaten by birds are generally red or black, and will change colour to signal this fact: an unripe berry will be an unappetising green colour. Animals swiftly learn which colours are good to eat: it is in the interests of plants to attract fruit-eaters only when their seeds are ready to be transported.

Mimicry: Some flowers mimic objects that might be attractive to their potential pollinators. Famously, some flowers look like, and even smell like, females of different insect species, and males insects are inevitably attracted.

Bee orchid - Andrew N Gagg's PHOTOFLORA It is astonishing to imagine how this kind of reproductive mimicry might have arisen. Probably a particular flower had a part that looked faintly similar to a female, or a particular scent similar to a female pheromone. The insect attempted copulation, pollen was attached and deposited when the insect moved to the next plant of the same species. Over generations of plants this mimicry was refined until it became the amazing trickery it is today.

Text Box: The bee orchid (left) is a brilliant example of a plant mimicking an insect:
the plants’ colouring and shape emulates exactly in the eye of a bumble bee, a fellow bee
pollinating a flower, offering a reproductive reward at no extra cost to the orchid.
Evolutionary duplicity at its finest.

The study of fossil pollen spores is palynology. Exines, the outer part, are the most resistant part of the grain and consequently, the most abundant and indeed ubiquitous plant organ in the fossil record. Pollen has two parts to the exine: the outer and the inner. The area separating these two also serves to distinguish between simple and more complex pollen types. The more primitive have a granular interspace, as with magnoiid types and gymnosperms, while more derived forms have little columns, which get progressively longer with flower complexity.

Illustration of the differences in pollen exine structure, Doyle (1978), http://www.unifiedworlds.com/cornet/Why02/why.htm

Doyle78c.jpg (77454 bytes)

Key features of angiosperm pollen include the tectate (tegillate) wall, and multiple germinal exits, probably to make germination much faster on the stigma, to release recognition substances on to the stigma, making sure that germination occurs fast. Both were adopted early, with analogous versions in the gymnosperms.