Unravelling the complexity of salt marsh ‘<i>Fucus cottonii</i>’ forms (Phaeophyceae, Fucales)

Genetic affiliation, nuclear DNA content, and gamete functioning were examined in small salt marsh <i>Fucus</i> from three localities in western Ireland. Individuals with small and dioecious receptacles were found at all localities, but production of germlings was only evident at Locality 1. Here, the <i>Fucus</i> vegetation formed a morphological cline from <i>F. vesiculosus</i> with bladders in the mid-intertidal to small <i>Fucus</i> individuals lacking bladders in the salt marsh of the upper intertidal. Measurements of nuclear DNA content ranged from 1–1.8 pg at this locality, with <i>F. vesiculosus</i> individuals in the lower range. At the two other localities, the small salt marsh <i>Fucus</i> consisted of distinct morphological entities. Microsatellite analyses revealed that individuals at Locality 2 were derived mainly from <i>F. vesiculosus</i>, whereas those from Locality 3 were hybrids between <i>F. vesiculosus</i> and <i>F. spiralis</i> with greatest affiliation to <i>F. spiralis</i>. While the small salt marsh <i>Fucus</i> forms from Locality 2 had high nuclear DNA content (<i>c</i>. 4 pg) and were probably octoploids, the small salt marsh <i>Fucus</i> from Locality 3 formed two groups: one with high (3.9–4.6 pg) and one with low (1.5–1.9 pg) nuclear DNA content. Nuclear DNA content measured in individuals from Locality 3 varied between 1.1–2.8 pg in <i>F. vesiculosus</i> and 2–3.5 pg in <i>F. spiralis</i>, and showed a more or less stepwise increase in both species, consistent with polyploidy. We hypothesize that the small salt marsh <i>Fucus</i> forms originate from genome size changes in the parental taxa.