Convergent adaptation to a marginal habitat by homoploid hybrids and polyploid ecads in the seaweed genus Fucus

Abstract

Hybridization and polyploidy are two major sources of genetic variability that can lead to adaptation in new habitats. Most species of the brown algal genus Fucus are found along wave-swept rocky shores of the Northern Hemisphere, but some species have adapted to brackish and salt marsh habitats. Using five microsatellite loci and mtDNA RFLP, we characterize two populations of morphologically similar, muscoides-like Fucus inhabiting salt marshes in Iceland and Ireland. The Icelandic genotypes were consistent with Fucus vesiculosus x Fucus spiralis F1 hybrids with asymmetrical hybridization, whereas the Irish ones consisted primarily of polyploid F. vesiculosus.

Figure 1

mtDNA characterization and detection of hybrids by Structure. The top portion of each panel indicates type of mtDNA as determined by RFLP: white, F. spiralis; stippled, F. vesiculosus. The lower portion of each panel is a plot in which each individual is represented by a vertical bar partitioned into shaded segments with a length proportional to the individual's membership in the K clusters. Each colour (black, dark grey, light grey) represents a cluster. (a) Iceland, K=2, ln likelihood=−234.1; (b) Ireland, K=2, ln likelihood=−526.2; (c) Ireland, K=3, ln likelihood=−457.7. Fucus spiralis (Fsp), F. vesiculosus (Fv), muscoides-like Fucus (m-lF); sample numbers in parentheses.

Figure 2

Microsatellite traces for loci (a) B3 and (b) L20. Duplication events could not be detected in the remaining three loci as each was characterized by only two alleles. The multiple-peak pattern was repeatable with multiple PCR reactions and consistent with at least partial genome duplication, strongly suggesting polyploidy. Both individuals revealing polyploidy at locus B3 possessed alleles 141, 147, 151, whereas nine of the 11 polyploid individuals for locus L20 had allele sizes of 160, 163, 169, one with 155, 163, 224 and another with 160, 167, 224. All alleles for both loci were common in the non-polyploid individuals; therefore spurious amplification of other genome regions was unlikely. Multiple peaks could be a result of chimaeras, but such fusion has never been demonstrated in Fucus despite repeated attempts. In contrast, none of the Fv individuals showed more than two alleles for these loci.