They unravel the evolutionary history of ivy

An international team has studied the evolution of the ivy family using massive genome sequencing techniques.

The PlantBEE (Land Plant Biogeography, Ecology and Evolution) group at the Autonomous University of Madrid (UAM) in Spain has been studying the evolution of the Araliaceae family of ivy for more than a decade. Specifically, it studies the Asian Palmate-Leaf Group (“AsPG”), which represents 50% of the diversity of Araliaceae.

AsPG is distributed on all continents except Antarctica. Curiously, the maximum diversity is in the tropical latitudes of Asia and America (16 genera, 900 species), while in temperate latitudes diversity is very low (6 genera, 55 species).

One of the puzzles the team has spent the most time on is unraveling the evolutionary history of AsPG. To do this, a team that includes Angélica Gallego Narbón and Virginia Valcárcel, both from the UAM, has studied DNA, which provides loose pieces (genes) that together make up the genome.

In a recent work, which is part of Gallego’s doctoral thesis (supervised by professors Virginia Valcárcel and Mario Fernández-Mazuecos), the team has discovered that the current genera of AsPG emerged in a very short time as a result of hybridization between species.

These results were obtained thanks to massive genome sequencing techniques.

Hedera helix (common ivy) is the most representative European species of the araliaceae. (Photo: Marina Coca de la Iglesia)

From studies with Sanger to massive sequencing

The phylogenies represent the evolutionary relationships between the species by means of trees very similar to those that show the kinship in the family genealogies, but using for this the information contained in the genes of the genome. Traditionally these genes were obtained with Sanger sequencing, a technique that does not allow many genes to be sequenced.

“We know that the complete genome of a plant can contain more than 100,000 genes. Thus, the 2-10 genes that we used to analyze in the Sanger studies often prevented us from completing the phylogeny puzzle. However, the new massive sequencing techniques easily allow hundreds of genes or even the complete genome to be obtained”, Gallego and Valcárcel detail.

Thus, Sanger’s studies on the AsPG provided a very incomplete puzzle on which it was very difficult to clarify the evolutionary history of the group. Through massive sequencing, in 2019, the team obtained a new phylogeny with the complete genome of the plastid (>200 genes). The plastid is one of the three genomes into which DNA is organized in plant cells.

Although this phylogeny clarified the closest kinship relationships (similar to relationships between siblings, cousins, parents, and uncles), the oldest kinship (relationships between grandparents, great-grandparents, and great-grandparents) remained a mystery.

“This result was very encouraging, since it indicated that the oldest ancestors of AsPG originated in a very short time, in what we call ‘radiation’, a process of great evolutionary interest”, the researchers point out.

“Although we didn’t know what had caused this radiation, we had clues. In 2014 we hypothesized that the first ancestors of AsPG must have originated by hybridization between species. If so, radiation could be explained by these hybridizations, since hybridization is a phenomenon that generates new species in a very short time. We then needed to study the nuclear genome to compare it with the plastid and see if there had been hybridization”.

two different stories

The nuclear genome contains information from the mother and the father, unlike the plastid that only has information from the mother. Therefore, if there is hybridization and mother and father are different species, it is very important to compare both genomes, since they will most likely tell different evolutionary histories.

Indeed, the two new phylogenies (nuclear and plastid) suggested very different histories, confirming the hybridization hypothesis. This hybridization not only affected the origin of the first ancestors within the AsPG, but also that of the common ancestor of the entire group.

“We also identified a duplication of the genome in this common ancestor that went from the 24 chromosomes that most Araliaceae have to 48. As surprising as it may seem, duplications like this are very common in plants, where it has also been seen that after the duplication is common for radiation to occur. That is, the same pattern that we observed in the AsPG”.

“Although this is now in the realm of speculation,” the researchers continue, “this duplication of the genome could have favored radiation, since the increase in genetic variability (twice as many genes and coming from different species) can provide a great capacity for adaptation.” before environmental changes generating new species in a short time”.

“The question we are now asking ourselves, given the unequal diversity of AsPG in the tropics and in temperate zones, is whether climate could have had something to do with this scenario,” conclude Gallego and Valcárcel.

The study is titled “Hybridization and genome duplication for early evolutionary success in the Asian Palmate group of Araliaceae”. And it has been published in the academic journal Journal of Systematics and Evolution. (Source: UAM)