## LINZ Simone

*School of Computer Science, University of Auckland, Auckland, New Zealand*- Design and analysis of algorithms, Evolutionary Biology, Graph theory
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*12 Oct 2023*

### When Three Trees Go to War

#### Bounding the reticulation number for three phylogenetic trees

*Recommended by*

**Simone Linz**based on reviews by Guillaume Scholz and Stefan GrünewaldReconstructing a phylogenetic network for a set of conflicting phylogenetic trees on the same set of leaves remains an active strand of research in mathematical and computational phylogenetic since 2005, when Baroni et al. [1] showed that the *minimum number of reticulations h(T,T')* needed to simultaneously embed two rooted binary phylogenetic trees *T* and *T' *into a rooted binary phylogenetic network is one less than the size of a maximum acyclic agreement forest for *T* and *T'*. In the same paper, the authors showed that *h(T,T')* is bounded from above by *n-2*, where *n* is the number of leaves of *T* and *T'* and that this bound is sharp. That is, for a fixed *n*, there exist two rooted binary phylogenetic trees *T* and *T' *such that *h(T,T')=n-2*.

Since 2005, many papers have been published that develop exact algorithms and heuristics to solve the above NP-hard minimisation problem in practice, which is often referred to as *Minimum Hybridisation* in the literature, and that further investigate the mathematical underpinnings of Minimum Hybridisation and related problems. However, many such studies are restricted to two trees and much less is known about Minimum Hybridisation for when the input consists of more than two phylogenetic trees, which is the more relevant cases from a biological point of view.

In [2], van Iersel, Jones, and Weller establish the first lower bound for the minimum reticulation number for more than two rooted binary phylogenetic trees, with a focus on exactly three trees. The above-mentioned connection between the minimum number of reticulations and maximum acyclic agreement forests does not extend to three (or more) trees. Instead, to establish their result, the authors use multi-labelled trees as an intermediate structure between phylogenetic trees and phylogenetic networks to show that, for each *ε>0*, there exist three caterpillar trees on *n* leaves such that any phylogenetic network that simultaneously embeds these three trees has at least *(3/2 - ε)n* reticulations. Perhaps unsurprising, caterpillar trees were also used by Baroni et al. [1] to establish that their upper bound on* h(T,T')* is sharp. Structurally, these trees have the property that each internal vertex is adjacent to a leaf. Each caterpillar tree can therefore be viewed as a sequence of characters, and it is exactly this viewpoint that is heavily used in [2]. More specifically, sequences with short common subsequences correspond to caterpillar trees that need many reticulations when embedded in a phylogenetic network. It would consequently be interesting to further investigate connections between caterpillar trees and certain types of sequences. Can they be used to shed more light on bounds for the minimum reticulation number?

**References**

[1] Baroni, M., Grünewald, S., Moulton, V., and Semple, C. (2005) "Bounding the number of hybridisation events for a consistent evolutionary history". J. Math. Biol. 51, 171–182. https://doi.org/10.1007/s00285-005-0315-9

[2] van Iersel, L., Jones, M., and Weller, M. (2023) “When three trees go to war”. HAL, ver. 3 peer-reviewed and recommended by Peer Community In Mathematical and Computational Biology. https://hal.science/hal-04013152/