Frequency of chimerism in populations of the kelp Lessonia spicata in central Chile

Abstract

Chimerism occurs when two genetically distinct conspecific individuals fuse together generating a single entity. Coalescence and chimerism in red seaweeds has been positively related to an increase in body size, and the consequent reduction in susceptibility to mortality factors, thus increasing survival, reproductive potential and tolerance to stress in contrast to genetically homogeneous organisms. In addition, they showed that a particular pattern of post-fusion growth maintains higher genetic diversity and chimerism in the holdfast but homogenous axes. In Chilean kelps (brown seaweeds), intraorganismal genetic heterogeneity (IGH) and holdfast coalescence has been described in previous research, but the extent of chimerism in wild populations and the patterns of distribution of the genetically heterogeneous thallus zone have scarcely been studied. Since kelps are under continuous harvesting, with enormous social, ecological and economic importance, natural chimerism can be considered a priceless in-situ reservoir of natural genetic resources and variability. In this study, we therefore examined the frequency of IGH and chimerism in three harvested populations of Lessonia spicata. We then evaluated whether chimeric wild-type holdfasts show higher genetic diversity than erect axes (stipe and lamina) and explored the impact of this on the traditional estimation of genetic diversity at the population level. We found a high frequency of IGH (60–100%) and chimerism (33.3–86.7%), varying according to the studied population. We evidenced that chimerism occurs mostly in holdfasts, exhibiting heterogeneous tissues, whereas stipes and lamina were more homogeneous, generating a vertical gradient of allele and genotype abundance as well as divergence, constituting the first time “within- plant” genetic patterns have been reported in kelps. This is very different from the chimeric patterns described in land plants and animals. Finally, we evidenced that IGH affected genetic differentiation among populations, showed lower levels of FST index when we compared holdfast than lamina samples. In the light of this, future studies should evaluate the significance of chimeric holdfasts in their ability to increase kelps resilience, improve restoration and ecosystem service.

Coral chimerism as an evolutionary rescue mechanism to mitigate global climate change impacts

“…This is most likely the reason why natural fusions among coral conspecifics are so frequently recorded in coral species (the literature cited above) and why we find that natural fusion rates may go as up as 40%-90% under controlled laboratory settings (Barki et al, 2002). Recorded chimeric rates are around 5% in field studies (Puill-Stephan et al, 2009), while actual rates of chimerism are presumably higher, as the (González & Santelices, 2017).…”

“…It is also possible that chimerism in corals is left undetected due to faulty sampling, as various parts of the coral colony may present different proportions of the partners' DNA. This reminiscences chimeric kelp algae where plastic DNA repertoire was recorded in the form of a vertical gradient (from holdfast to lamina) of genetic consistency (González & Santelices, ).…”

Nonclonal coloniality: Genetically chimeric colonies through fusion of sexually produced polyps in the hydrozoanEctopleura larynx

“…, 2016, ), and not multiple distinct genotypes as reported here (Table ). The level of chimerism discovered in E. larynx colonies approaches that of some red seaweeds, that are known to derive ecological benefits from chimerism and also appear able to limit the level of chimerism in certain tissues, perhaps reducing the burden of genetic conflict (González and Santelices ; Santelices et al. ).…”

“…). In coalescing red algae, which exhibit high levels of genetic chimerism from fusion (González and Santelices ), differences between growth rates between lineages of cells may serve to segregate the lineages into different axes, thus reversing chimerism in upright axes or branches after fusion as one cell lineage outcompetes the others (Santelices et al. ).…”

Intraorganismal genetic heterogeneity as a source of genetic variation in modular macroalgae

“…This genetic variability can arise by two different processes: somatic mutations (mosaicism) and/or through chimerism due to the coalescence of two or more conspecific, genetically different individuals (Santelices ). Recent measurements in natural populations of the brown macroalgae Lessonia spicata using microsatellites were able to localize the origin of IGH, showing a high frequency of 60%–100%, of which 33.3%–86.7% arose from chimerism (González and Santelices ). This suggests that the occurrence of IGH is more common than previously thought.…”