Oceanic islands are not sinks of biodiversity in spore-producing plants

Abstract

Islands have traditionally been considered as migratory and evolutionary dead ends for two main reasons: island colonizers are typically assumed to lose their dispersal power, and continental back colonization has been regarded as unlikely because of niche preemption. The hypothesis that islands might actually represent dynamic refugia and migratory stepping stones for species that are effective dispersers, and in particular, for spore-producing plants, is formally tested here, using the archipelagos of the Azores, Canary Islands, and Madeira, as a model. Population genetic analyses based on nuclear microsatellite variation indicate that dispersal ability of the moss Platyhypnidium riparioides does not decrease in the island setting. The analyses further show that, unlike island populations, mainland (southwestern Europe and North Africa) populations underwent a severe bottleneck during the last glacial maximum (LGM). Our results thus refute the traditional view of islands as the end of the colonization road and point to a different perception of North Atlantic archipelagos as major sources of biodiversity for the postglacial recolonization of Europe by spore-producing plants.biogeography | speciation | genetic diversity | island syndromes | isolation-with-migration I sland biogeography has historically been and remains a research area of prime importance for the advance of biology (1). It was in an island setting that Darwin (2) and Wallace (3) presented their theories of evolution by natural selection. It was also in this context that McArthur and Wilson (4) described the "equilibrium model," according to which biodiversity patterns can be explained by isolation from source populations (immigration) and area size (extinction). Oceanic islands hence appear as natural laboratories of evolutionary and biogeographic processes and Wallace (5) suggested that understanding evolutionary processes within an island context is a key to understanding the same mechanisms in the more complex continental settings.Theoretical work on population structure has used island models as paradigms for understanding how interactions between gene flow, effective population size, and genetic drift influence patterns of genetic variation. Although surprisingly few studies have explicitly tested models of population structure and diversity in island and mainland plant populations (6), theory predicts that island populations should be less variable and more genetically differentiated relative to source populations (7). Differentiation of island populations is further enhanced by one of the most striking syndromes displayed by island taxa, that is, their typical loss of dispersal power as a result of either absence of predation and/or competition in the island setting or counter selection against high dispersal ability associated with the risk of massive individual loss to surrounding sea water (8). This process can be fairly rapid. In the annual daisy Lactuca muralis, for example, a substantial and significant loss of dispersal power owing...

Is the sword moss (Bryoxiphium) a preglacial Tertiary relict?

“…by the fact that the Macaronesian islands were colonized at least twice from geographically remote ancestors originating from North America on the one hand and Southeast Asia on the other. This result, which challenges the taxonomic status of the Madeiran endemic B. madeirense, parallels previous evidence that, in contrast with angiosperms, wherein single colonization event of Macaronesian archipelagos are the rule (Silvertown, 2004), multiple archipelago colonization took place in bryophytes (Vanderpoorten et al, 2008;Laenen et al, 2011;Hutsemékers et al, 2011). Interestingly, ancestral area estimations identified Macaronesia as the area of origin from which Iceland was colonized, reinforcing the view of Macaronesia as a stepping-stone for bryophyte species in transit toward West Europe territories across the Atlantic (Patiño et al, 2015a;Patiño and Vanderpoorten, 2015).…”

Baker's law and the island syndromes in bryophytes

“…If the loss of dispersal power hypothesis holds true in the group, island bryophytes would be expected to shift reproductive strategies towards increased levels of asexual reproduction. Previous investigations failed to demonstrate a significant loss of dispersal ability on islands (Hutsem ekers et al 2011). Paradoxically, however, the number of individuals of clonal origin was higher in island populations as compared to continental ones (Hutsem ekers et al 2011), suggesting a shift in the prevalent reproductive strategy to favour SDD.…”

“…The application of Baker's law in the group is, however, challenged by the fact that selection for selfing is reduced when multiple founder events occur (Busch 2011), a condition that has been reported in several instances (Hutsem ekers et al 2011;Laenen et al 2011; but see Karlin et al 2011). Yet, two specific features of bryophyte biology suggest that Baker's law might still be relevant in the group.…”

Glacial refugia and post-glacial colonization patterns in European bryophytes

“…1) with refugia in Macaronesia (e.g. Radula lindenbergiana (Laenen et al 2011) and Platyhypnidium riparioides (Hutsemékers et al 2011)), southwestern European mainland (e.g. Grimmia montana (Vanderpoorten et al 2008)), south in the British Isles (e.g.…”

“…has one widespread haplotype group throughout the distribution range, making refugia hard to localize, while another haplotype group is more restricted to western Europe, indicating that it colonized fewer available areas after the LGM (Hedenäs 2008a). Hutsemékers et al (2011) compared genetic variation of island and mainland populations of the southern temperate moss Platyhypnidium riparioides (Hedw.) Dixon using Macaronesian, southwestern European, and north African populations to test if islands can act as source rather than sink to mainland.…”