Parallel speciation or long‐distance dispersal? Lessons from seaweeds (Fucus) in the Baltic Sea

Abstract

Parallel evolution has been invoked as a forceful mechanism of ecotype and species formation in many animal taxa. However, parallelism may be difficult to separate from recently monophyletically diverged species that are likely to show complex genetic relationships as a result of considerable shared ancestral variation and secondary hybridization in local areas. Thus, species' degrees of reproductive isolation, barriers to dispersal and, in particular, limited capacities for long-distance dispersal will affect demographical structures underlying mechanisms of divergent evolution. Here, we used nine microsatellite DNA markers to study intra-and interspecific genetic diversity of two recently diverged species of brown macroalgae, Fucus radicans (L. Bergstr€ om & L. Kautsky) and F. vesiculosus (Linnaeus), in the Baltic Sea. We further performed biophysical modelling to identify likely connectivity patterns influencing the species' genetic structures. For each species, we found intraspecific contrasting patterns of clonality incidence and population structure. In addition, strong genetic differentiation between the two species within each locality supported the existence of two distinct evolutionary lineages (F ST = 0.15-0.41). However, overall genetic clustering analyses across both species' populations revealed that all populations from one region (Estonia) were more genetically similar to each other than to their own taxon from the other two regions (Sweden and Finland). Our data support a hypothesis of parallel speciation. Alternatively, Estonia may be the ancestral source of both species, but is presently isolated by oceanographic barriers to dispersal. Thus, a limited gene flow in combination with genetic drift could have shaped the seemingly parallel structure.

Past climate changes and strong oceanographic barriers structured low‐latitude genetic relics for the golden kelp Laminaria ochroleuca

“…For marine organisms, ocean currents influence relative isolation and connectivity of populations (e.g., Coleman, ; Buonomo et al., ; Lourenço et al., ; Pereyra et al. ), keeping or diluting the signatures left by past climate changes (Lourenço et al., ). Dispersal predictions are increasingly compared with genetic estimates, but not along species ranges (e.g., Alberto et al., ; Billot, Engel, Rousvoal, Kloareg, & Valero, ; Buonomo et al., ; Coleman, ).…”

Reciprocal transplants support a plasticity-first scenario during colonisation of a large hyposaline basin by a marine macro alga

“…Fucus radicans is endemic in the Baltic Sea. It is sympatric with F. vesiculosus over large parts of the Bothnian Sea and in Estonia [15, 21]. Both species are dioecious with both males and females being capable of asexual reproduction.…”

“…We investigated this by testing two predictions from the model: (i) individuals of the original population have high enough plasticity to tolerate the marginal environment, and (ii) individuals of the marginal population show evidence of local adaptation. The Baltic Sea populations of F. vesiculosus and F. radicans both descend from a common F. vesiculosus lineage originating in the eastern part of the North Sea, close to the entrance of the Baltic Sea [21]. Thus we used individuals from a population in this area to represent the ancestors from which the current Baltic Sea individuals of both species have derived.…”

Tolerance to climate change of the clonally reproducing endemic Baltic seaweed, Fucus radicans: is phenotypic plasticity enough?

“…), and the two species are clearly separated genetically (Pereyra et al. ). The main factor that determines macroalgal distribution is the salinity gradient, which shapes all biotic interactions; for example, the detrimental effects of idoteid grazers on algae are ameliorated with decreasing salinity (Bergstrom et al.…”

“…) are widespread in the Bothnian Sea, covering more than 550 km 2 , while the Estonian populations mostly reproduce sexually (Pereyra et al. , Ardehed et al. ).…”