The secret of the blue flower

Researchers led by Dirk Albach from the Institute of Biology and Environmental Sciences are on the trail of the secret of the success of the widespread plant genus Veronica. In doing so, they are addressing one of the most topical questions in biology: What actually is a species?

Prof Dr Dirk Albach's treasure is located in a windowless room on the ground floor of building W04 at the university. The biologist and director of the Botanical Garden keeps around 15,000 herbarium specimens in two blue metal cabinets: pressed plants, preferably from root to flower. The dried plants are carefully attached to sheets of light-coloured paper with thin strips of adhesive tape. Each sheet is wrapped in brown paper and collected together with the specimens of related plants in thick, green folders.

Around a thousand of the Oldenburg herbarium specimens belong to the genus Veronica, or speedwell in German - a group with which Albach is very familiar. The key characteristic of the approximately 450 species of speedwell known worldwide is their blue flower. All members of the genus have four mostly round petals, technically known as petals, and two stamens. This is how botanists refer to the thin stamens and anthers containing the pollen. In terms of the shape and texture of the leaves, the size and arrangement of the flowers, however, the speedwell shows an astonishing variety: some stems are hairy, others smooth, some species grow creeping and have flowers only millimetres in size, others grow several metres high and adorn themselves with candle-shaped inflorescences. There are alpine species and those that have adapted to dry areas, riparian strips or pine forests. In Eurasia, Veronica species can be found from the British Isles to Japan, they grow in the Himalayas, on Greenland, in North America and New Zealand - and, of course, on the Oldenburg campus.

How do you define a species?

What particularly interests Albach about this widespread genus is its astonishing ability to hybridise, i.e. to form mixed forms: There is so much hybridisation among the European representatives that some specimens can hardly be assigned to a specific species. Some characteristics that were originally used for identification actually exist as a continuous spectrum. Apparently, plants have been interbreeding for thousands of years, so it is sometimes difficult to say which species are "stable" - i.e. do not constantly change through cross-breeding. "We want to understand why some hybrids survive and develop into new species and others don't," says the head of the Biodiversity and Evolution of Plants working group.

Albach is leading two research projects dealing with the history of the Veronica genus, which probably began around 15 million years ago somewhere in Asia. Since 2016, he has been working on a project funded by the Volkswagen Foundation together with researchers from Russia and Ukraine to investigate variants of the species Veronica spicata, the spiked loosestrife, which occurs in the Siberian Altai Mountains as well as in the steppes of Eurasia and Central Europe. He is also involved in the TaxonOMICS priority programme of the German Research Foundation (DFG), which investigates the question of what constitutes a biological species. Albach heads the sub-project "Type specimens and genomes - resolving a conflict using the example of frequently hybridising species of the genus Veronica". He and his colleagues isolate DNA samples from decades-old herbarium specimens. They go on excursions to locations from which so-called type specimens originate - i.e. specimens that form the basis for the description of a species - and they compare the appearance and genetic material of new plants collected in the field.

Evolution during the Ice Age

The results so far show that the Veronica species in the Altai Mountains are easier to distinguish from each other than in Central Europe. Although hybrids also occur in the remote mountain massif on the border between Kazakhstan, Siberia, Mongolia and China, these can be more easily traced back to the original species. The Veronica species in Ukraine, on the other hand, still puzzle Albach. "The pattern of hybridisation in Ukraine is very complex, we haven't understood it yet," he reports. Next summer, the researcher is planning an excursion to neighbouring southern Russia. In this area, which is rarely visited by botanists, he wants to unravel the mystery of why the species in the Altai Mountains are more stable than further west.

His suspicion: In Central Europe, the hybrids could have emerged during the last ice age, which began around 100,000 years ago and ended 10,000 years ago, whereas in the Altai they only emerged more recently. During the ice ages, large parts of the continent were covered by glaciers and barren tundra. Plants such as the speedwell retreated to refuges with milder temperatures. The plants continued to develop separately in small populations. "When the ice age ended, the plants from the various refugia spread across the continent again," explains Albach. He surmises that the Veronica genus quickly managed to adapt to new habitats through hybridisation. This enabled the hybrids to quickly colonise the deserted continent after the glaciers retreated. "Perhaps it is their ability to hybridise that makes the genus so successful," says the botanist.

Traces of foreign species in the genome

The versatile speedwell thus sheds light on one of the most topical questions in biology: What is a species anyway? According to the classic definition, a biological species is a population of individuals that reproduce together, produce fertile offspring and are genetically separated from other populations. "Various species concepts developed in biology over the course of the 20th century, most of which were based on newly developed methods," reports Albach. In the 1930s, for example, this was the observation of chromosomes. Later, electron microscopes were used to examine the shape of pollen, and DNA analyses became popular in the 1990s.

In recent years, however, it has become increasingly clear that even the genetic material DNA cannot always clearly show which species a specimen belongs to. This is because living organisms exchange genes with each other much more frequently than had long been assumed. There are even traces of foreign species in the human genome, such as the Neanderthal man or the Denisova man native to Asia.

"There is now a trend in evolutionary biology towards a more holistic approach," says Albach. In order to define a species or recognise that two populations are drifting apart, you have to look at various characteristics. Even in the age of molecular genetics, one thing is still crucial for evolutionary biologists: knowing the organism under investigation in detail. Some Veronica species, for example, differ only in the finest details - such as slightly differently shaped hairs or barely visible indentations in the leaves.