Plant tissues preserved for genetic studies
Sometimes described as "emergent", this collection was set up in 2010. It is linked with a particularly rapid method for drying plant material, using silica-gel, so that the DNA can be extracted and genetic studies can be carried out later on.
Today, the collection includes around 12,000 samples representing many groups of vascular plants (including Angiosperms, Gymnosperms and Pteridophytes) and coming from many regions of the world where the Muséum’s scientists undertake collecting field trips (in particular in New Caledonia, Madagascar and the neighbouring islands of the Indian Ocean, and South-East Asia).
This collection is based on a growing awareness of the benefits of combining morpho-anatomical observations, which have been carried out by botanists for a long time, with more recently developed genetic studies. This complementarity is exploited with the aim of distinguishing and classifying species, as well as studying their evolution; this is known as "integrative" systematics.
Collecting leaf samples and preserving them in silica gel began in the 2000s. In the field, a "modern" botanical collection now always consists of a dried plant, any complementary objects (e.g. flower, fruit, wood), various information (locality, date, etc.), but also a leaf fragment that will be quickly dried using silica gel to absorb humidity from the sample. The leaf fragment (a few cm2) is placed in a folded coffee filter to prevent the leaf from falling out, and placed in a pot filled with silica gel and hermetically sealed.
Traditional drying techniques are slow, causing the DNA to deteriorate and making any genetic study difficult or even impossible. Silica gel will speed up the drying process (2 to 3 days for most plants), so the sample’s DNA can be preserved for later studies.
When back to the laboratory, the samples are individually placed in small bags, bar-coded and stored in hermetically sealed boxes at -20°C (to maintain zero humidity and thus ensure their long-term preservation). The samples are then registered in a computer database, in connection with the corresponding herbarium specimen (known as the "reference specimen" or "voucher"). Thus the "leaf sample - herbarium specimen" pair will allow us to compare the results of morpho-anatomical and genetic studies at any time.
The main aim of storing samples in silica gel is to preserve the plant’s DNA. Thus the collection is mainly used for genetic studies, often based on DNA sequencing, and conducted by the Muséum’s researchers or other institutions that request material to the Muséum. In this case, only part of the sample kept at the Muséum is sent.
In parallel with these samples preserved specifically for genetic studies, we can also attempt to extract DNA from herbarium specimens (dried traditionally and therefore more slowly). Extraction is then more difficult, and the obtained DNA is often degraded.
Every year, the Herbarium sends over 1000 samples to researchers all over the world, and this number is in constant growth. The studies are usually aimed at defining and classifying species through the reconstruction of molecular phylogenies. Less commonly, the collection may also be used to measure the size of a plant’s genome, using the flow cytometry technique.