Historical biogeography of the land snail Cornu aspersum: a new scenario inferred from haplotype distribution in the Western Mediterranean basin

Abstract

Background: Despite its key location between the rest of the continent and Europe, research on the phylogeography of north African species remains very limited compared to European and North American taxa. The Mediterranean land mollusc Cornu aspersum (= Helix aspersa) is part of the few species widely sampled in north Africa for biogeographical analysis. It then provides an excellent biological model to understand phylogeographical patterns across the Mediterranean basin, and to evaluate hypotheses of population differentiation. We investigated here the phylogeography of this land snail to reassess the evolutionary scenario we previously considered for explaining its scattered distribution in the western Mediterranean, and to help to resolve the question of the direction of its range expansion (from north Africa to Europe or vice versa). By analysing simultaneously individuals from 73 sites sampled in its putative native range, the present work provides the first broad-scale screening of mitochondrial variation (cyt b and 16S rRNA genes) of C. aspersum.

Results: Phylogeographical structure mirrored previous patterns inferred from anatomy and nuclear data, since all haplotypes could be ascribed to a B (West) or a C (East) lineage. Alternative migration models tested confirmed that C. aspersum most likely spread from north Africa to Europe. In addition to Kabylia in Algeria, which would have been successively a centre of dispersal and a zone of secondary contacts, we identified an area in Galicia where genetically distinct west and east type populations would have regained contact.

Conclusions: Vicariant and dispersal processes are reviewed and discussed in the light of signatures left in the geographical distribution of the genetic variation. In referring to Mediterranean taxa which show similar phylogeographical patterns, we proposed a parsimonious scenario to account for the "east-west" genetic splitting and the northward expansion of the western (B) clade which roughly involves (i) the dispersal of ancestral (eastern) types through Oligocene terranes in the Western Mediterranean (ii) the Tell Atlas orogenesis as gene flow barrier between future west and east populations, (iii) the impact of recurrent climatic fluctuations from mid-Pliocene to the last ice age, (iv) the loss of the eastern lineage during Pleistocene northwards expansion phases.

Figure 1

Sampling locations of C. aspersum throughout the western Mediterranean basin. Population numbers are those given in additional file 1. Sample site symbols and colours indicate the mtDNA lineage of the population: pink for east type, green for west type (including Z). Grey areas represent microplates drifted from Oligocene: GK: Great Kabylia, PK: Lesser Kabylia, Ri: Rif Cordillera, Be: Betic range, Ba: Balearic Islands, Sa: Sardinia, Co: Corsica, Si: Sicilia, Ca: Calabria. The dotted line throughout Europe shows the -1°C January isotherm line during the LGM [18]. Dotted curves in Iberia represent seven putative terrestrial refugia [69]. Putative geographical barriers (Moulouya River basin, Edough Peninsula, Atlas and Tell system) and contact zones (black arrows in opposite direction in Kabylia, Galicia and Italia) suggested in the present paper are also reported.

Figure 2

Alternative biogeographical hypotheses for the current distribution of C. aspersum in Western Mediterranean and consequences in terms of spatial differentiation and genetic diversity. (a) schematic representation of the West (B) and East (C) lineages defined in the native range of the species, (b) colonization routes following the two migration models tested (model 1 or MAE: migration from north Africa to Europe, model 2 or MEA: migration from Europe to north Africa) (K: Kabylia, A: Aegean route, G: Strait of Gibraltar, T: Tyrrhenian route, π: genetic diversity).

Figure 3

Phylogenetic relationships among cyt b sequences. (a) Fifty percent majority-rule consensus phylogram from the BI analysis. Branches without posterior probability values (values in italics) are supported by less than 50% of the sampled trees. Sequence labels are abbreviated as in additional file 1. Main subdivisions are indicated on the right side of the tree (B and C lineages), and inside each subdivision, the respective clades are presented. Schematic colored shells indicate morphometric (shell and distal genitalia) type of each subdivision (green for B or west type, pink for C or east type). (b) Schematic representation of ML topology. (c) Schematic geographic location of west (B, green) vs east (C, pink) haplotypes.

Figure 4

Phylogenetic relationships among 16S RNA sequences. (a) Fifty-percent majority-rule consensus phylogram from the BI analysis (see legend of Fig 3 for details). (b) Schematic representation of ML topology. (c) Schematic geographic location of west (B, green) vs east (C, pink) haplotypes.

Figure 5

Median-joining network for the cyt b mtDNA haplotypes of C. aspersum. H haplotypes define ancestral types resulting from the star contraction analysis. Inferred median vectors are switched off for clarity. Each circle represents a haplotype, and circle size is proportional to haplotype frequency. Colours indicate the subdivisions inside haplogroups: white for east (C) and grey for west (B) north African types as previously defined [15], black for west European types. Dashed lines delineate haplogroups defined from cyt b phylograms. Branch lengths are approximately equal to inferred mutational steps (m) (short lines perpendicular to branches for m<10, numbers in brackets for m ≥ 10). Haplotype codes according to those in additional file 1.

Figure 6

Mismatch distributions found in subdivisions (a) B1, (b) B2 and (c) C2 for cyt b sequences, (d) Ba, (e) Bb and (f) C for 16S RNA sequences. The continuous and interrupted (connecting circles) lines indicate the expected and observed distributions of pairwise differences obtained by fitting a model of sudden population expansion [89].

Figure 7

Bayesian skyline plots showing the historical demographic trends for both B and C lineages detected for cyt b (a and b) and 16S RNA (c and d) sequences. Along the y-axis is the expressed population size estimated in units of N_eμ (N_e: effective population size, μ: mutation rate per haplotype per generation). Solid lines are median estimates whereas shaded areas represent confidence intervals.