Fine‐ and regional‐scale genetic structure of the exotic ascidian Styela clava (Tunicata) in southwest England, 50 years after its introduction

Abstract

Styela clava, an ascidian native to the northwest Pacific, was first recorded in the Atlantic at Plymouth, southwest England, in 1953. It now ranges in the northeast Atlantic from Portugal to northern Denmark, and has colonized the east coast of North America. Within the region of first introduction, we aimed to characterize current genetic diversity in the species, elucidate the respective roles of human-aided vs. natural dispersal, and assess the extent of larval dispersal by looking for genetic differentiation at very small scales. Eight sites, mostly marinas, were studied along c. 200 km of coast in southwest England encompassing Plymouth. Five microsatellite loci were genotyped in 303 individuals to analyse gene flow at regional (among sites) and fine (within sites) scales. F-statistics and assignment tests were used to investigate regional genetic structure. At the fine scale, deviation from mutation-drift equilibrium was tested, and isolation by distance and genetic clustering analyses were undertaken. Significant genetic differentiation existed between sites, unrelated to geographical separation; migration between geographically distant marinas was inferred, highlighting the likely importance of human-mediated dispersal in range expansion and occupancy by S. clava. Fine-scale population structure was present within at least four sites, which may be explained by the limited dispersal ability of this ascidian and recruitment from differentiated pools of larvae. Populations in enclosed marinas had higher self-recruitment rates than those in open sites. Some marinas might therefore function as reservoirs of propagules for subsequent spread, whereas others might be sinks for migrants.

Bottlenecks and loss of genetic diversity: spatio-temporal patterns of genetic structure in an ascidian recently introduced in Europe

“…Similar patterns of population genetic differentiation have been ob served for other introduced ascidian species (e.g. , Rius et al 2008, Dupont et al 2009, Pineda et al 2011, but see Zhan et al 2010, Bock et al 2011.…”

Population genomics of the introduced and cultivated Pacific kelp Undaria pinnatifida: Marinas—not farms—drive regional connectivity and establishment in natural rocky reefs

“…Other marine species have shown differences amongst populations inhabiting different habitat types. For example, differences were found between natural reefs and hatcheries in the Manila clam Ruditapes philippinarum (Xing, Gao, & Li, 2014), between enclosed and open marinas in the invasive tunicate Styela clava (Dupont, Viard, Dowell, Wood, & Bishop, 2009), or between fixed and free‐floating populations of the invasive red seaweed Gracilaria vermiculophylla (Krueger‐Hadfield et al., 2016). Here, farmed populations were highly distinct from the other types of populations because, for a given sampling year, they displayed the lowest genetic diversity, and showed no inbreeding signal (negative F IS , Figure 2a).…”

“…pinnatifida at a regional scale. Similar chaotic connectivity patterns have been found in other marine NIS, such as the tunicate Styela clava (Dupont et al., 2009; Goldstien, Schiel, & Gemmell, 2010), as well as in native species established in marinas like the ascidian Ciona intestinalis (Hudson, Viard, Roby, & Rius, 2016), associated with boating activities. Floating pontoons and leisure boats, docking in the marinas, etc., are providing new habitats to many and diverse NIS taxa (e.g., bryozoans (Marchini, Ferrario, & Minchin, 2015), caprellids (Ros et al., 2013)) and are pathways of NIS spread (Clarke Murray, Pakhomov, & Therriault, 2011; Mineur, Johnson, & Maggs, 2008).…”

Ascidians as models for studying invasion success

“…On the other extreme, invasive ascidian populations that are unvaryingly genetically diverse have also been reported, as in the case of M. squamiger, S. clava, or Styela plicata (e.g., Dupont et al 2009;David et al 2010;. Indeed, this has been a comparatively much more common finding.…”

“…In this context, patterns of population genetic connectivity have been used to formulate hypotheses regarding the contribution of environmental variables to colonization dynamics (e.g., David et al 2010), the importance of sexual versus asexual propagules of dispersal (e.g., Bock et al 2011), and the relative roles of alternative pathways and vectors of post-establishment spread (e.g., Dupont et al 2009;Goldstien et al 2010;Bock et al 2011). Unfortunately, only a few studies have attempted to verify hypotheses formulated from genetic data with independent "SSR" denotes nuclear microsatellites, "mtDNA" refers to mitochondrial DNA sequences, "nuDNA" refers to non-ribosomal nuclear DNA sequences, "rDNA" denotes nuclear ribosomal DNA sequences, "I" indicates populations sampled in the invaded range, while "N" indicates populations sampled in the native range measures of population connectivity obtained in the field.…”