SHORT COMMUNICATION Functional divergence in heat shock response following rapid speciation of Fucus spp. in the Baltic Sea

In the Baltic Sea, the broadly distributed brown alga Fucus vesiculosus coexists in sympatry over part of its range (south west Gulf of Bothnia) with the Baltic endemic F. radicans sp. nov, while further north in colder and lower-salinity areas of the Baltic F. radicans occurs alone (north west Gulf of Bothnia). F. radicans appears to have arisen via rapid speciation from F. vesiculosus within the recent history of the Baltic (ca. 7500 BP). Possible functional divergence between the two species was investigated by comparing stress-responsive gene expression in a common-garden experiment. The experiment used two allopatric populations of Fucus vesiculosus from the Skagerrak (North Sea) and Central Baltic, as well as F. radicans from the same Central Baltic site. The two species in sympatry displayed divergent heat shock responses, while F. vesiculosus populations from allopatric sites did not. F. radicans was more sensitive to heat shock at 25"C, either alone or together with high irradiance and desiccation, than Baltic or Skagerrak F. vesiculosus. The results indicate that rapid functional divergence in the inducible heat shock response has occurred between sympatric species on a timescale of thousands of years. Introduction The brackish Baltic Sea is a marginal environment for marine species, both geographically and environmentally (Johannesson and André 2006). Although its current postglacial marine-brackish phase is recent (ca. 7500 BP (Ignatius et al. 1981; Donner 1995), isolation near the geographical and physiological limits of a species’ distribution may result in periods of accelerated evolution (Orr and Smith 1998; Thompson 1998). These characteristics make the Baltic extremely interesting for the study of adaptive responses of populations to environmental change (Pearson et al. 2000; Gabrielsen et al. 2002; Riginos and Cunningham 2005) and rapid speciation (van Oppen et al. 1995; Pereyra et al. 2009). A new species, endemic to the Baltic, was recently described (Bergström et al. 2005) as Fucus radicans, previously considered a dwarf morphotype of F. vesiculosus, together with which it occurs in sympatry in the SW Gulf of Bothnia. Further north in the Gulf of Bothnia (northern Baltic) at salinities of\5 PSU F. radicans is the only fucoid present (Tatarenkov et al. 2005; Pereyra et al. 2009). Species status for F. radicans is based on genetic and morphological differentiation from sympatric F. vesiculosus, and a greater capacity for, and predominance of, clonal reproduction (Bergström et al. 2005; Tatarenkov et al. 2005). Although unresolved by phylogenetic analysis of mitochondrial sequences (Coyer et al. 2006), microsatellite analysis has recently demonstrated reproductive isolation and very recent divergence time estimates between F. radicans and F. vesiculosus, possibly within hundreds rather than thousands of years BP (Pereyra et al. 2009). Endemic F. radicans therefore represents a case of rapid speciation within the Baltic Sea during the recent postglacial Littorina Sea period (ca. 7500 BP) (Ignatius et al. 1981; Russell 1985). Communicated by T. Reusch. A. Lago-Lestón ! C. Mota ! G. A. Pearson (&) Centro de Ciências do Mar, CIMAR-Laboratorio Associado, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal e-mail: [email protected] L. Kautsky Department of Botany, Stockholm University, 106 91 Stockholm, Sweden 123 Mar Biol (2010) 157:683–688 DOI 10.1007/s00227-009-1348-1