Polyploidy on Islands: Its Emergence and Importance for Diversification

Abstract

Whole genome duplication or polyploidy is widespread among floras globally, but traditionally has been thought to have played a minor role in the evolution of island biodiversity, based on the low proportion of polyploid taxa present. We investigate five island systems (Juan Fernández, Galápagos, Canary Islands, Hawaiian Islands, and New Zealand) to test whether polyploidy (i) enhances or hinders diversification on islands and (ii) is an intrinsic feature of a lineage or an attribute that emerges in island environments. These island systems are diverse in their origins, geographic and latitudinal distributions, levels of plant species endemism (37% in the Galapagos to 88% in the Hawaiian Islands), and ploidy levels, and taken together are representative of islands more generally. We compiled data for vascular plants and summarized information for each genus on each island system, including the total number of species (native and endemic), generic endemicity, chromosome numbers, genome size, and ploidy levels. Dated phylogenies were used to infer lineage age, number of colonization events, and change in ploidy level relative to the non-island sister lineage. Using phylogenetic path analysis, we then tested how the diversification of endemic lineages varied with the direct and indirect effects of polyploidy (presence of polyploidy, time on island, polyploidization near colonization, colonizer pool size) and other lineage traits not associated with polyploidy (time on island, colonizer pool size, repeat colonization). Diploid and tetraploid were the most common ploidy levels across all islands, with the highest ploidy levels (>8x) recorded for the Canary Islands (12x) and New Zealand (20x). Overall, we found that endemic diversification of our focal island floras was shaped by polyploidy in many cases and certainly others still to be detected considering the lack of data in many lineages. Polyploid speciation on the islands was enhanced by a larger source of potential congeneric colonists and a change in ploidy level compared to overseas sister taxa.

The genomic basis of the plant island syndrome in Darwin’s giant daisies

“…Indeed, this outstanding phenotypic and ecological variation has led authors to refer to this group as the ‘Darwin finches of the plant world’ 13 . All Scalesia species are ancestrally tetraploid (2 n = 4 x = 68) 14 , 15 , and the polyploid genetics may have provided the genetic grist for the diversification, as suggested for island floras 16 .…”

Genomic insights into rapid speciation within the world’s largest tree genus Syzygium

“…Species radiations—wherein perplexing amounts of diversity appear to have formed extremely rapidly—have featured prominently in the history of evolutionary theory 1 . Various underlying mechanisms for their formation have been proposed 2 , including adaptation 2 , non-adaptive processes 3 , 4 , hybridisation 5 , 6 , and polyploidy 7 , 8 , but the relative importance of these drivers remains incompletely understood. Species radiations on islands have been among the most prominently studied systems 9 – 12 .…”

The genomic basis of the plant island syndrome in Darwin’s giant daisies

“…The outstanding phenotypic and ecological variation has led previous authors to refer to it as the ‘Darwin finches of the plant world’ (Stöcklin 2009). All Scalesia species are ancestrally tetraploid (2 n =4 x =68) (Ono 1967; Uno Eliasson 1974), and the polyploid genetics may have provided the genetic grist for adaptive radiation, as has been speculated for other island floras (Meudt et al 2021).…”