Phylogeography and molecular rates of subterranean aquatic Stenasellid Isopods with a peri‐Tyrrhenian distribution

Abstract

The subterranean Isopods belonging to the genus Stenasellus have an interesting disjunct distribution in the peri-Tyrrhenian area with morphologically closely related taxa occurring in Sardinia, Corsica, Tuscany and in the Pyrenees phreatic and interstitial waters. Because the dispersal capacities of these organisms are limited, their distribution has been associated traditionally with the tectonic events leading to the separation of the Sardinia-Corsica microplate from the Pyrenees and its subsequent movement towards the Italian peninsula. We sequenced a fragment of the mtDNA cytochrome oxidase I gene (COI) for multiple populations of the S. racovitzai species-group (Corsica, Sardinia, Tuscany) and S. virei (Pyrenees). We found that multiple phylogenetic analyses always gave the same topology, which is consistent with the genetic relations found using allozyme data, and with the palaeogeography of the area. The molecular data suggest that a combination of vicariance and dispersal events explain most effectively the present distribution pattern of these organisms. We also calculated COI rates and calibrated them against absolute time, taking advantage of the availability of two geologically based time estimates. Rates on all substitutions are similar to those published for other crustaceans for the same COI fragment, including taxonomically and ecologically distant groups. Rates on third codon positions or on transversions are generally lower than those found in other crustaceans.

Twenty Years of Molecular Biogeography in the West Mediterranean Islands of Corsica and Sardinia: Lessons Learnt and Future Prospects

“…Aquatic isopods of the genus Stenasellus are highly adapted to subterranean life, they spend their whole life cycle in subsurface freshwaters and active dispersal can only happen when such a habitat is continuous [33]. Given these characteristics, it was not unexpected to find molecular relationships (regardless of the markers employed) to be remarkably in agreement with the palaeogeography of the area.…”

“…Subsequent within-island evolution has been documented molecularly but very often it did not produce appreciable morphological differences. Illustrative is the case of the subterranean isopod Stenasellus [33,40,56]. The few known populations of this crustacean are virtually indistinguishable morphologically and yet they are deeply divergent from one another at both mitochondrial and nuclear loci.…”

Phylogeography of Semiterrestrial Isopod,Tylos granuliferus, on East Asian Coasts

“…(Drummond et al, 2011). It has been reported that the evolutionary rates of the COI genes in the Crustacea vary from 1.66%/Ma to 3.0%/Ma (Ketmaier et al, 2003), although a rate of approximately 2.0%/Ma is broadly used for the molecular clock of vertebrate mitochondrial cytochrome b (e.g., Rocha et al, 2005;Fernandes et al, 2012;Rapson et al, 2012). Because there is no reliable temporal calibration point directly applicable to our data, we used a rate of 2.5%/Ma as a representative value, which was reported to be the COI divergence rate in a stenasellid isopod (Ketmaier et al, 2003).…”

Genetic variation of the spiny spider crab Maja brachydactyla in the northeastern Atlantic

“…Inferred expansions were dated following the expression T = τ/2μk, where τ is the estimated number of generations since the expansion, μ is the mutation rate per site per generation and k is the sequence length. We assumed a mutation rate of 1% per lineage per million years (average estimate reported in Ketmaier et al 2003) and a mean generation time of 2 yr (see Freire et al 2002).…”