Large-scale genetic patchiness among NE Atlantic populations of the brittle star Ophiothrix fragilis

Abstract

The brittle star Ophiothrix fragilis (Abilgaard, 1789) constitutes a heterogeneous morphological group that can be subdivided into 4 varieties. The species is also characterized by high demographic variability. The present study explores the possibility of genetic patchiness arising due to admixture of varieties or recruitment heterogeneity. We sequenced a portion of the mitochondrial cytochrome oxidase subunit I (mtCOI) gene and genotyped 7 allozymes from 21 populations of the 2 most common varieties. While mtCOI analyses showed that all the varieties grouped together with virtually no divergence among them (<1%), a clear divergence (18.6%) was evident between the northern Atlantic populations and individuals collected in the Mediterranean and along the Galician coast. Conversely, canonical analysis indicated that classification by variety explained a substantial part of the allozyme variance. Within O. f. pentaphyllum, no clear pattern of geographic structure was observed around the British Isles, with neighbouring populations often showing more genetic differentiation with one another than with distant populations. This either reflects admixture of cryptic lineages/species, diversifying selection due to contrasting habitats, or spatial and/or temporal admixture of genetically differentiated cohorts produced from isolated populations. The latter hypothesis is reinforced by the fact that significant haplotype frequency differences were recorded between larvae, juveniles and adults within the same locality. Providing that time-isolated clades co-occur within populations, the variation in the number and intensity of recruitment events must impact local genetic structure extensively and induce chaotic genetic patchiness at a large spatial scale.

Phylogeography of the sea star Marthasterias glacialis (Asteroidea, Echinodermata): deep genetic divergence between mitochondrial lineages in the north-western mediterranean

“…Moreover, this gene has successfully been used in echinoderm species making our results comparable with those previously observed for other species (see e.g. Baric and Sturmbauer 1999;Lessios et al 2003;Uthicke and Benzie 2003;Muths et al 2009). …”

“…The absence of population subdivision at both sides of the Gibraltar Strait strongly contrasts with population differentiation patterns across the AtlantoMediterranean transition described for a number of marine invertebrate species (see Borsa et al 1997;Patarnello et al 2007). Although discrepancies between population genetic patterns for high dispersal echinoderms between Atlantic and Mediterranean sea suggest that similar biological features do not necessarily imply similar population structure (Calderon et al 2008;Muths et al 2009;Zulliger et al 2009), a number of the marine invertebrate species analysed demonstrated either a common pattern of ''genetic divergence'' at both sides of this Strait or isolation by distance (see e.g. Quesada et al 1995;Zane et al 2000;Pérez-Losada et al 2002).…”

Species identification of echinoderms from the North Sea by combining morphology and molecular data

“…For example, for the deep-sea dwelling A. loveni, the North Sea individuals showed high divergences to those from Canada, resulting in two different BINs. A phylogenetic pattern and possible indication for cryptic species was found for O. fragilis with a high level of haplotype diversity and COI divergences of up to 18.6 % between North Atlantic and Mediterranean/Galician coast populations [77]. Several very divergent mitochondrial and nuclear lineages were also found for the cosmopolitan brittle star A. squamata [78,79].…”

Swarms of diversity at the gene cox1 in Antarctic krill

“…1 (Lin et al, 2008), the brittle star Ophiothrix fragilis (Muths et al, 2009) and the intertidal gastropods Nerita plicata and Nerita albicilla (Crandall et al, 2008). Of these species, only N. plicata has consistently high haplotype diversity over multiple population samples, such as in our case with Antarctic krill.…”

“…The nucleotide diversity of cox1 was also consistently high within all krill swarms (p ¼ 0.010275-0.011537, Table 1). To our knowledge, only one previous study has found consistently high nucleotide diversity across populations (Cyamus ovalis; Kaliszewska et al, 2005), whereas several studies have reported much higher nucleotide diversity for individual populations, usually from samples of cryptic species or the products of incipient speciation followed by secondary contact (for example, brittle stars, Muths et al, 2009). The high Antarctic krill diversity values reported in this study are unlikely to be an artefact of sequencing error or the presence of numts, as our sequence reproducibility assessment and tests for numts effectively ruled out these possibilities.…”