Unraveling the evolutionary history of ivies

Hedera helix (common ivy) is the most representative European species of the ara
Hedera helix (common ivy) is the most representative European species of the araliaceae / Marina Coca de la Iglesia

UAM researchers have studied the evolution of the ivy family using massive genome sequencing techniques. The results, published in the Journal of Systematics and Evolution, show that the genera of this family, including ivy, arose in a very short time and as a result of hybridization between species.

The PlantBEE(Land Plant Biogeography, Ecology and Evolution) team at the Autonomous University of Madrid (UAM) has been studying the evolution of the ivy family, the Araliaceae, for more than a decade. Specifically, he studies the Asian Palmate Leaf Group ("AsPG"), which represents 50% of the diversity of araliaceae.

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

One of the enigmas to which the team has devoted the most time is to unravel the evolutionary history of the AsPG. To do this, the researchers have studied DNA, which provides the individual pieces (genes) that together make up the genome.

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

These results, published in the Journal of Systematics and Evolution, were obtained thanks to massive genome sequencing techniques.

From Sanger studies to massive sequencing

Phylogenies represent the evolutionary relationships between species by means of trees very similar to those showing kinship in family genealogies, but using the information contained in the genes of the genome. Traditionally, these genes were obtained by Sanger sequencing, a technique that does not allow sequencing many genes.

"We know that the complete genome of a plant can contain more than 100,000 genes. Therefore, the 2-10 genes that we used to analyze in Sanger studies often prevented us from completing the phylogeny puzzle. However, new massive sequencing techniques make it easy to obtain hundreds of genes or even the entire genome," the authors explain.

Thus, Sanger’s studies in the AsPG provided a very incomplete puzzle on which it was very difficult to elucidate the evolutionary history of the group. By massive sequencing in 2019, the team obtained a new phylogeny with the complete plastid genome (>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 aunts and uncles), the oldest kinship (relationships between grandparents, great-grandparents and great-great-grandparents) remained a mystery.

"This result was very stimulating, as it indicated that the oldest ancestors of the AsPG originated in a very short time, which we call ’radiation,’ a process of great evolutionary interest," the researchers note.

"Although we didn’t know what had caused this radiation, we had clues. In 2014 we hypothesized that the early ancestors of the AsPG must have originated from hybridization between species. If so, the 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 plastidial genome and see if hybridization had taken place".

Two different stories

The nuclear genome contains information from the mother and the father, as opposed to the plastidial genome, which only contains 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 probably tell different evolutionary histories.

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

"We also identified a genome duplication in this common ancestor that went from the 24 chromosomes that most araliaceae have to 48. Surprising as it may seem, duplications like this are very common in plants, where it has also been seen that after duplication, radiation is frequent. In other words, the same pattern that we observed in AsPG.

Although this is a matter of speculation," the researchers continue, "this duplication of the genome could have favored radiation, since the increase in genetic variability (double the number of genes and from different species) can provide a great capacity to adapt to environmental changes, generating new species in a short time".

"The question we now ask, given the uneven diversity of AsPG in the tropics and temperate zones, is whether climate may have played a role in this scenario," the authors conclude.

Nuclear and plastidial phylogenies of the Araliaceae, with special detail of the Asian Palmate Leaf Group (AsPG). The colored triangles indicate the living genera of the Araliaceae and the circles show the ancestors that originated them (now extinct), the violet the common ancestor to all the AsPG and the greens are the oldest ancestors of the AsPG that descend from the common ancestor and have originated all the diversity that we see today / Gallego Narbón et al. 2022.

Bibliographic reference:

Gallego Narbón, Á., Wen, J., Liu, J., Valcárcel, V. 2022. Hybridization and genome duplication for early evolutionary success in the Asian Palmate group of Araliaceae. Journal of Systematics and Evolution 60(6), 1303-1318. doi: 10.1111/jse.12906.

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