Pollination: a reproductive strategy
Plants need to exchange pollen in order to reproduce, and insects are the most efficient method. Currently there are about 200,000 pollinators, most of which are insects. Insect pollination —especially by butterflies, bees and flies— is a process that is always associated with flowering plants (angiosperms), of which there are currently more than 240,000 species. Thysanopterans are minute insects that were long considered low-efficient pollinators and that usually feed on plant tissues and pollen, generally on angiosperms.
They catch and transport the pollen more efficiently
The international research team found four female thysanopterans that had been enclosed in the amber in Álava for 105-110 million years, with their bodies covered with pollen of gymnosperms. One of the females became trapped in the resin when transporting 140 pollen grains, whereas another was transporting 137 grains. These insects are less than two millimetres long and exhibit highly specialized hairs with a ringed structure which had never been seen before and which increases their ability to collect and transport pollen grains. These hairs are very similar to the ones of bees, which have the same function. Males have also been found, but without these hairs and without pollen. Researchers describe these insects in a new genus (Gymnopollisthrips) comprising two new species G. minor and G. major. This extinct genus, Gymnopollisthrips, belongs to a family that currently exists, the Melanthripidae. All data, including the amount of grains transported by each female, indicates that Gymnopollisthrips were efficient pollinators, like the current most efficient angiosperms pollinators.
What kind of pollen did insects transport?
Pollination of gymnosperms by insects is a rare phenomenon. Current gymnosperms, such as pines, firs, araucarias and cycads, are pollinated by wind, which randomly transports pollen. About 110 million years ago, during the Cretaceous period, forests were composed primarily of gymnosperms, and angiosperms were a minority. In that period, resin gymnosperms produced a large amount of resin, which is currently found fossilized in the amber, for example in the piece found in Álava.
The study concludes that pollen, belonging to the Cycadopites group, is from a kind of cycad or ginkgo tree, which are currently relict groups. Only one species of ginkgo trees, Ginkgo biloba, currently survives, which is considered a living fossil. Pollen grains from this species are very small, having 20 thousandths of a millimetre, and they must have had a surface with some kind of adherent capacity, as it may be inferred from the groupings observed in the amber. These two features are characteristic of the pollen that is transported to other flowers by insects. On the other hand, there are recorded cycads which have been pollinated by beetles and thrips. The only known case of a genus of thrips that exclusively pollinates a group of cycads is in Australia. Interestingly the finding in the Spanish amber is not related with these Australian cases, contrary to what it may seem.
Why did insects from the Cretaceous collect pollen?
According to experts, ringed hairs to collect and transport pollen did not grow due to an evolutionary selection to pollinate. Pollination must have been accidental, although the process must have involved some selective pressure for the plant. The benefit for the thrips can only be explained by the possibility to feed their larvae with pollen. This suggests that these new species must have formed colonies with some social behaviour. This phenomenon, from sub-social to true social behaviour, has been described in some species of current thrips. However, the fossils preserved in amber that were found in the North of Spain do not provide further data on possible characteristics of the colonies. Still, it is likely that larvae lived in the ovules of some kind of ginkgo for shelter and protection. In the amber deposits in Spain many remains of leaves and ovulate organs from an extinct type of ginkgo are found.
A revolution in terrestrial ecosystems
The co-evolution of angiosperms and insects was a great evolutionary success and it determined the displacement of gymnosperms due to an intensive competence. This revolution in terrestrial ecosystems was just beginning when the resin that originated the amber in Spain was produced.
Only amber can preserve behavioural features like pollination in such rich detail over millions of years. This finding indicates that thrips might be one of the first pollinator groups in geological history, long before evolution turned some of them into angiosperms pollinators. The samples of the new study published in the PNAS belong to the collection of the Natural Sciences Museum of Álava. The investigation has been possible thanks to the support of the Diputación Foral de Álava and to an I&D project funded by the Spanish Ministry of Economy and Competitiveness.
