Fucus vesiculosus and spiralis species complex: a nested model of local adaptation at the shore level

Abstract

Intertidal rocky shores provide classic examples of habitat-driven divergent selection. We show that the species complex Fucus vesiculosus L./F. spiralis L. is composed of 3 distinct genetic entities that have evolved along different time scales. Using assignment tests based on microsatellite markers and performed on randomly sampled individuals in 2 separate geographic regions (Portugal and France), we reveal that F. spiralis consists of 2 genetic entities that have distinct vertical distributional patterns along the intertidal gradient of selective pressures. Individuals assigned to the cluster found higher on the shore are also morphologically different. They are smaller and bushy, with dichotomous ramifications and no sterile rime around receptacles. Patterns of genetic divergence suggest different times and pathways to reproductive isolation. Divergence between F. vesiculosus and the F. spiralis complex seems to have occurred first, coinciding with divergence in reproductive mode; dioecy versus selfing hermaphroditism. Later, in the hermaphroditic lineage, parallel evolution of 2 co-occurring genetic clusters may have been driven by natural selection and facilitated by high selfing rates in the F. spiralis complex.

Evolution and diversification within the intertidal brown macroalgae Fucus spiralis/F. vesiculosus species complex in the North Atlantic

“…Sampling method and original source (see Section 2.2) for F. spiralis: 1, this study; 2, this study; 3, Perrin et al (unpub. );4, Billard (2007) and Billard et al (2010 in the South) the transect was taken in the middle of the F. spiralis zone (this study). Method 3 utilized samples from two locations per site, separated by 100-200 m (see Engel et al, 2005) whose taxon identity had been previously confirmed by microsatellite genotyping (Perrin et al, unpub.…”

“…data, Engel et al, 2005 for Gris Nez). In Method 4, we selected individuals previously determined by microsatellite genotyping (in order to avoid hybrids) as F. spiralis High, F. spiralis Low (or as F. vesiculosus) that had been collected along a vertical transect on the shore or with random coordinates (a subset of individuals from Billard et al (2010) and also from Billard (2007) for Viana).…”

“…From a morphological standpoint, over 125 intraspecific taxa have been described for the nine most common species (Guiry and Guiry, 2010). From a genetic standpoint, several studies using molecular techniques have documented hybridization between the various species based on experimental evidence (Coyer et al, 2002a) or occurrence of intermediate genotypes in nature (Coyer et al, 2002b(Coyer et al, , 2006bWallace et al, 2004;Engel et al, 2005;Mathieson et al, 2006;Billard et al, 2010). Additionally, frequent establishment of successful reproductive hybrids has been noted Coyer et al, 2007).…”

“…spiralis High) and the other in the mid-upper region (F. spiralis Low) (Billard et al, 2010). Morphological differences exist between the entities and each is subjected to different regimes of desiccation resistance (Billard et al, 2010;Zardi et al, submitted for publication).…”

“…Morphological differences exist between the entities and each is subjected to different regimes of desiccation resistance (Billard et al, 2010;Zardi et al, submitted for publication). Although divergence of the habitat-specific ecotypes may have occurred by gradual adaptation of each entity to its respective micro-environment, the possibility of divergence of one entity from the other by hybridization/introgression with a co-occurring species cannot be discounted, especially as inter-specific gene flow is well documented within the genus Fucus (Wallace et al, 2004;Billard et al, 2005Billard et al, , 2010Engel et al, 2005;Coyer et al, 2006b;Mathieson et al, 2006;Perrin et al, 2007).…”