Botany Curator Heidi Meudt and colleagues have published a paper on what is special about the diversification of plants on islands, based on an investigation of the five best-studied island archipelagos, including New Zealand. Read on to find out more about their findings on the role of whole genome duplication – also known as polyploidy – in these island floras.
Comparing the floras of five archipelagos
Islands are special places, and their remarkable native floras – and the evolutionary forces that have shaped them – are of particular interest to biologists. On islands, polyploidy may facilitate long-distance dispersal, the survival of small populations, the evolution of novel traits, increased genetic diversity, and species diversification.
We aimed to test the role of polyploidy in contributing to the diversification of plants on islands by focusing on five island systems: Hawai‘i, New Zealand, Galápagos Islands, Canary Islands and Juan Fernández Islands.
The five archipelagos vary in area, distance to the nearest continent and number of islands. By compiling available data for all native and endemic plant species and genera over these five archipelagos, we found that the islands show a 5-fold difference in the number of native genera, a 10-fold difference in the number of native species, and a 20-fold difference in the number of endemic species.
For example, the Juan Fernández Islands have the smallest flora whereas New Zealand has the largest. New Zealand and the Canary Islands have the highest prevalence of polyploidy and also the most data available for further analysis.
Testing our hypothesis on island diversification
We then came up with a conceptual model to statistically test how species diversity in island lineages varies with polyploidy and extrinsic colonization history.
Our main hypothesis is that polyploidy has played an important role in the diversification of island floras by facilitating dispersal and establishment of plants to islands, and also by generating additional diversity through varying ploidy levels.
We assembled a final dataset comprising 150 lineages representing 1,805 endemic species over four of the fie island systems.
Based on data availability of published dated phylogenies and chromosome numbers, the majority of the 150 lineages analysed were from New Zealand (66%) followed by the Canary Islands and Hawai‘i (15% each), and Juan Fernández (4%). No lineages from the Galápagos Islands could be included due to a lack of data.
Polyploidy promotes endemic diversity on islands
Overall, our statistical analysis supported the hypothesis that polyploidy contributes significantly to dispersal, colonization, and diversification of island lineages on all islands studied here. In summary, we found:
- Greater levels of polyploidy directly promoted endemic diversity on islands.
- Island lineages whose original colonizers have sister groups with many species, that had already undergone polyploidization after diverging from their sister group, and that showed increasing ploidy levels over time, had more endemic island species.
- Our results overturn the “chromosomal stasis” model for islands, as at least 12% of lineages have undergone polyploidization after arrival on the island.
- There is a surprising lack of basic data for some “classic” island systems, like the Galápagos Islands. Generating such fundamental data for additional lineages and islands is important and should be prioritised.
Acknowledgements
This research was funded by Te Apārangi Royal Society of New Zealand Marsden Fund for the project, “Whole-genome duplication in plants: what is the pathway to success?” which was granted to PI Bill Lee (Manaaki Whenua – Landcare Research) in 2017. This international collaboration has also supported other published papers including student research such as Liddell et al. (2021) and Thomas et al. (2021).
Thanks to my collaborators on this paper: Dirk C. Albach (University of Oldenburg, Germany), Andrew J. Tanentzap (University of Cambridge, UK), Javier Igea (University of Cambridge, UK), Sophie C. Newmarch (Massey University, New Zealand), Angela J. Brandt (Manaaki Whenua – Landcare Research, New Zealand), William G. Lee (Manaaki Whenua – Landcare Research, New Zealand) and Jennifer A. Tate (Massey University, New Zealand).
References
- Liddell LG, Lee WG, Dale EE, Meudt HM, Matzke NJ. 2021. Pioneering polyploids: the impact of whole-genome duplication on biome shifting in New Zealand Coprosma (Rubiaceae) and Veronica (Plantaginaceae). Biology Letters 17(9):2021.0297.
- Meudt HM, Albach DC, Tanentzap AJ, Igea J, Newmarch SC, Brandt AJ, Lee WG, Tate JA. 2021. Polyploidy on Islands: Its Emergence and Importance for Diversification. Frontiers in Plant Science 12:637214.
- Thomas AE, Igea J, Meudt HM, Albach DC, Lee WG, Tanentzap AJ. 2021. Using target sequence capture to improve the phylogenetic resolution of a rapid radiation in New Zealand Veronica. American Journal of Botany 108(7):1289-306.
When it is stated that “island lineages whose original colonizers have sister groups with many species, that had already undergone polyploidization after diverging from their sister group, and that showed increasing ploidy levels over time, had more endemic island species.” one might ask what is the nature of this ‘colonization’? This is answered to some extent in the paper cited by Meudt et al 2021 “Polyploidy on Islands” and some comments are offered below:
“Island biotas are generally the net outcome of immigration (dispersal and establishment), local diversification, and extinction (Carlquist, 1974), and these processes are known to be influenced
by specifics of the island, such as age, area, distance from nearest potential source floras”.
This is a truism that is applicable also to habitats within continents for example. What is missing is the historical geological context for the origins and ‘arrival’ of ‘island’ biotas. For some reason the tectonic history of islands is left out in this chance dispersal model of ‘island biogeography’.
“For lineages with dated phylogenies, we extracted the mean time of their divergence from both sister lineages and their most recent common ancestor”
I might have missed it, but I did not see any reference to how the phylogenies were dated. If these dates used fossil calibrations then all of them are minimum age estimates only, and they cannot impose an upper age limit.
“The Hawaiian Islands are the island system farthest away from a continent, and thus its genera are mostly considered to have colonized the island system only once”
This is rather selective citation. While its general are “mostly’ considered to have colonized the island system (from the mainland), this is not the only model. Fore example, Heads (2012 et.) provided extensive evidence for a vicariance origin for the Hawaiian island biota and metapopulation survival in the region. The current geographic isolation of Hawaii shows little relation to its biotic composition or origin. In science its not the majority view that makes science, but the manner of evidence.
Galapagos flora “like its fauna, is considered to be derived from South America”.
Well, that depends on who you refer to. Heads & Grehan (2021) provided extensive tectonic and biogeographic evidence to support a vicariance origin for endemic Galapagos plants and animals. How this paper was overlooked is quite a surprise (and also seeming to also overlook the vicariance biogeography of the Canary Islands).
“Our comparative study was limited to data that were available for the island systems under study and
we were surprised that some of these “classic” island systems remain poorly known chromosomally and phylogenetically, especially the Galápagos.”
But they are well enough known for some plants and animals to provide excellent examples of tectonic correlation that are consistent with a vicariance origin for the Galapagos biota involving former island arcs in the eastern Pacific.
“Because island radiations are often young and/or rapid on an evolutionary timescale”
This may be often said, but ‘island’ radiations may in fact often be ‘old’, at least as old as the tectonic events involved in the geological history of these islands.
“Newer methodologies that take advantage of next-generation sequencing methods should be helpful in this regard”
Technical or technological advances are not the same as conceptual advances. All the ‘next-generation’ (its always the ‘next-generation’) technical innovations in the world may not matter if they are mired in conceptually inadequate biogeographic models.