Towards A Species Concepts For Plants

Species_Concept Peter Nick
What is a "species" in plants? The question is not academic, but of great relevance for the valorisation of plant biodiversity.
The model of bisexuality, familiar to us is rather rare in the plant kingdom. The Ginkgo follows this model. We have developed a sex test to tell in the seedling stage, whether plants are male or female.

Why do we need a new species concept?

In animals species are defined according to Ernst Mayr by genetics - everybody able to yield fertile progeny is member of the same species. Horse and Donkey are of different species, because the Mule is sterile. With plants, this species concept is not working, because, here, primarily sterile hybrids can undergo spontaneous chromosome duplication and regain fertility (allopolyploidy). Many crop plants arose this way, for instance Wheat or Rapeseed. While animals chose their mating partners, plants rely on the help of pollinators. When these are not discriminating species, there will be merges.

How do we proceed?

Propagation barreers are leaky criteria for a plant species concept. We go, therefore for a more processual concept - a species is not a fixed entity, but an activity. Actually, we would need to talk about "speciation" rather than about "species". Any process leading to the formation of genetically separated species would fit. A chemical modification attracting different pollinators, such that this chemotype would turn so different that one day it cannot fuse with its "species fellows" would meet the criterion for such a species concept (even, if the process just has started). To follow speciation in the run, we work with Lamicacea, one of the most species-rich families within the flowering plants. Combining developmental biology, molecular evolution and chemical analytics, we try to understand, how new species are born. This leads to real-world applications. Many of these species are used by humans, without a clear understanding, what they are, leading to surrogation and even intentional fake in global trade. Thus, our work also addresses the issue of consumer protection in times of globalisation.

Sage as model

With more than a thousand described species, Sage belongs to the most diverse genera in the plant kingdom. Many of these species occur together - why do they not merge? The reason is a sophisticated lever mechanism, where the anthers place the pollen on different sites on the pollinating bee or bumblebee. In the next flower, the style picks up the pollen from the right site. Here, speciation is a matter of flower geometry. We focus, therefore on the genes of flower development and detected a duplication of the gene GLOBOSA that explains, how Sage entered the New World and switched from bees to humming-birds as pollinators. Actually, GLOBOSA defines the type of floral organ that will form from a primordium. In Sage, this gene conveys a second function during the subsequent morphogenesis of these floral organs.

Gender research in plants

Actually, most land plants are hermaphroditic and should be prone to self fertilisation. This would undermine the advantage of sexuality, the promotion of diversity. In fact, plants have developed many tricks to intercept self pollination (only in times of need, when there are no mating partners, they rely on it). This toolbox is of astounding diversity. Plants that change their gender depending on daytime as Avocado, plants with more than two genders, and even plants that use the model of two genders familiar to us. This binary model seems to be rather the exception than the rule, though. This research leads to real-world applications. for instance, it would be useful to recognise the sex of a Ginkgo tree when it is still young. The female trees shed later ovules that are "scented" with the smell of decaying butter, which is not really appreciated in cities. Here, we were able to develop a sex test for Ginkgo, where a simple PCR can tell in the seedling stage, whether the individual is male or female.