Abstract
Speciation in oceanic islands has attracted the interest of scientists since the 19th century. One of the most striking evolutionary phenomena that can be studied in islands is adaptive radiation, that is, when a lineage gives rise to different species by means of ecological speciation. Some of the best-known examples of adaptive radiation are charismatic organisms like the Darwin finches of the Galapagos and the cichlid fishes of the great African lakes. In these and many other examples, a segregation of the trophic niche has been shown to be an important diversification driver. Radiations are known in other groups of organisms, such as lichen-forming fungi. However, very few studies have investigated their adaptive nature, and none have focused on the trophic niche. In this study, we explore the role of the trophic niche in a putative radiation of endemic species from the Macaronesian Region, the Ramalina decipiens group. The photobiont diversity was studied by Illumina MiSeq sequencing of the ITS2 region of 197 specimens spanning the phylogenetic breadth and geographic range of the group. A total of 66 amplicon sequence variants belonging to the four main clades of the algal genus Trebouxia were found. Approximately half of the examined thalli showed algal coexistence, but in most of them, a single main photobiont amounted to more than 90% of the reads. However, there were no significant differences in photobiont identity and in the abundance of ITS2 reads across the species of the group. We conclude that a segregation of the trophic niche has not occurred in the R. decipiens radiation.
Highlights
Oceanic islands, which are of volcanic origin and have never been connected to continental land masses, have attracted the interest of scientists since the 19th century, using them as natural laboratories in which to study the origin of biodiversity and its evolution (Whittaker and Fernández-Palacios, 2007; Warren et al, 2015)
The most common photobionts found in this study were amplicon sequence variants (ASVs) 1 and 2, both belonging to Trebouxia sp
Regarding the Cape Verde archipelago, the main photobionts from all samples corresponded to one of the five ASVs placed in the Trebouxia clade C25
Summary
Oceanic islands, which are of volcanic origin and have never been connected to continental land masses, have attracted the interest of scientists since the 19th century, using them as natural laboratories in which to study the origin of biodiversity and its evolution (Whittaker and Fernández-Palacios, 2007; Warren et al, 2015). Radiations can be adaptive or non-adaptive (Gittenberger, 1991; Schluter, 2000; Rundell and Price, 2009). Non-adaptive radiations are those in which diversification is not accompanied by niche differentiation and usually gives rise to ecologically similar allopatric species (Gittenberger, 1991; Rundell and Price, 2009). In adaptive radiations, ecological speciation is the driver of diversification, and niche differentiation among radiation-originating species allows sympatric patterns (Schluter, 2000). The best-studied examples of adaptive radiation have focused on charismatic organisms, such as the Darwin finches of the Galápagos Islands (Grant and Grant, 2008), the silversword alliance of Hawaii (Landis et al, 2018), or the Anolis lizards of the Greater Antilles (Losos, 2009). The number of studies that have investigated the adaptive nature on radiations of other groups of less charismatic organisms is still very limited (Gaya et al, 2015; Mao et al, 2018)
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