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Travelling in time with networks: Revealing present day hybridization versus ancestral polymorphism between two species of brown algae, Fucus vesiculosus and F. spiralis.

Moalic Y, Arnaud-Haond S, Perrin C, Pearson GA, Serrao EA - BMC Evol. Biol. (2011)

Bottom Line: Individual-centered networks were analyzed on the basis of microsatellite genotypes from North Africa to the Pacific American coast, through the North Atlantic.Two genetic distances integrating different time steps were used, the Rozenfeld (RD; based on alleles divergence) and the Shared Allele (SAD; based on alleles identity) distances.Intermediate individuals linking both clusters on the RD network were those sampled at the limits of the sympatric zone in Northwest Iberia.

View Article: PubMed Central - HTML - PubMed

Affiliation: IFREMER, Institut Français de Recherche pour l'Exploitation de la Mer, centre de Brest, BP70, 29280 Plouzané, France.

ABSTRACT

Background: Hybridization or divergence between sympatric sister species provides a natural laboratory to study speciation processes. The shared polymorphism in sister species may either be ancestral or derive from hybridization, and the accuracy of analytic methods used thus far to derive convincing evidence for the occurrence of present day hybridization is largely debated.

Results: Here we propose the application of network analysis to test for the occurrence of present day hybridization between the two species of brown algae Fucus spiralis and F. vesiculosus. Individual-centered networks were analyzed on the basis of microsatellite genotypes from North Africa to the Pacific American coast, through the North Atlantic. Two genetic distances integrating different time steps were used, the Rozenfeld (RD; based on alleles divergence) and the Shared Allele (SAD; based on alleles identity) distances. A diagnostic level of genotype divergence and clustering of individuals from each species was obtained through RD while screening for exchanges through putative hybridization was facilitated using SAD. Intermediate individuals linking both clusters on the RD network were those sampled at the limits of the sympatric zone in Northwest Iberia.

Conclusion: These results suggesting rare hybridization were confirmed by simulation of hybrids and F2 with directed backcrosses. Comparison with the Bayesian method STRUCTURE confirmed the usefulness of both approaches and emphasized the reliability of network analysis to unravel and study hybridization.

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Network topology of F. spiralis and F. vesiculosus individuals with the Shared Alleles Distance (SAD). Only links with value smaller than or equal to the percolation distance Dp = 0.5 are present. Nodes representing individuals are circles for F. spiralis and squares for F. vesiculosus. Colors correspond to geographical regions.
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Figure 2: Network topology of F. spiralis and F. vesiculosus individuals with the Shared Alleles Distance (SAD). Only links with value smaller than or equal to the percolation distance Dp = 0.5 are present. Nodes representing individuals are circles for F. spiralis and squares for F. vesiculosus. Colors correspond to geographical regions.

Mentions: The network based on Shared Alleles distance (SAD), only influenced by allele frequencies and not by their divergence, does not discriminate the species as clearly as the RD did (Figure 2). Its different topology reveals hierarchical clustering reflected already on the percolation profile (see methods and Figure A3 in additional file 3) where the single peak with RD is replaced by three peaks with the SAD. At the percolation threshold (Dp = 0.5), the segregation of the sister species is not as clear as with RD. A large number of pathways still connect the two species and the decrease of threshold does not lead to a clear separation of F_spi and F_ves. Indeed, the first disconnecting cluster corresponds to the Northern American F_ves (thr = 0.39, see Figure A4 A in additional file 4), followed by the cluster of F_ves from South Portugal and Morocco (thr = 0.33, see Figure A4 B in additional file 4) while the last cluster of F_ves from the sympatric zone of Northwest Iberia (i.e., Northwestern Spain and Northern Portugal) remains linked to the complete cluster of F_spi. These observations are consistent with a significant average clustering coefficient value inferior with SAD than RD: < CC > = 0.46 (< CCo > = 0.17 with σo = 0.1, after 10,000 random simulations).


Travelling in time with networks: Revealing present day hybridization versus ancestral polymorphism between two species of brown algae, Fucus vesiculosus and F. spiralis.

Moalic Y, Arnaud-Haond S, Perrin C, Pearson GA, Serrao EA - BMC Evol. Biol. (2011)

Network topology of F. spiralis and F. vesiculosus individuals with the Shared Alleles Distance (SAD). Only links with value smaller than or equal to the percolation distance Dp = 0.5 are present. Nodes representing individuals are circles for F. spiralis and squares for F. vesiculosus. Colors correspond to geographical regions.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3040711&req=5

Figure 2: Network topology of F. spiralis and F. vesiculosus individuals with the Shared Alleles Distance (SAD). Only links with value smaller than or equal to the percolation distance Dp = 0.5 are present. Nodes representing individuals are circles for F. spiralis and squares for F. vesiculosus. Colors correspond to geographical regions.
Mentions: The network based on Shared Alleles distance (SAD), only influenced by allele frequencies and not by their divergence, does not discriminate the species as clearly as the RD did (Figure 2). Its different topology reveals hierarchical clustering reflected already on the percolation profile (see methods and Figure A3 in additional file 3) where the single peak with RD is replaced by three peaks with the SAD. At the percolation threshold (Dp = 0.5), the segregation of the sister species is not as clear as with RD. A large number of pathways still connect the two species and the decrease of threshold does not lead to a clear separation of F_spi and F_ves. Indeed, the first disconnecting cluster corresponds to the Northern American F_ves (thr = 0.39, see Figure A4 A in additional file 4), followed by the cluster of F_ves from South Portugal and Morocco (thr = 0.33, see Figure A4 B in additional file 4) while the last cluster of F_ves from the sympatric zone of Northwest Iberia (i.e., Northwestern Spain and Northern Portugal) remains linked to the complete cluster of F_spi. These observations are consistent with a significant average clustering coefficient value inferior with SAD than RD: < CC > = 0.46 (< CCo > = 0.17 with σo = 0.1, after 10,000 random simulations).

Bottom Line: Individual-centered networks were analyzed on the basis of microsatellite genotypes from North Africa to the Pacific American coast, through the North Atlantic.Two genetic distances integrating different time steps were used, the Rozenfeld (RD; based on alleles divergence) and the Shared Allele (SAD; based on alleles identity) distances.Intermediate individuals linking both clusters on the RD network were those sampled at the limits of the sympatric zone in Northwest Iberia.

View Article: PubMed Central - HTML - PubMed

Affiliation: IFREMER, Institut Français de Recherche pour l'Exploitation de la Mer, centre de Brest, BP70, 29280 Plouzané, France.

ABSTRACT

Background: Hybridization or divergence between sympatric sister species provides a natural laboratory to study speciation processes. The shared polymorphism in sister species may either be ancestral or derive from hybridization, and the accuracy of analytic methods used thus far to derive convincing evidence for the occurrence of present day hybridization is largely debated.

Results: Here we propose the application of network analysis to test for the occurrence of present day hybridization between the two species of brown algae Fucus spiralis and F. vesiculosus. Individual-centered networks were analyzed on the basis of microsatellite genotypes from North Africa to the Pacific American coast, through the North Atlantic. Two genetic distances integrating different time steps were used, the Rozenfeld (RD; based on alleles divergence) and the Shared Allele (SAD; based on alleles identity) distances. A diagnostic level of genotype divergence and clustering of individuals from each species was obtained through RD while screening for exchanges through putative hybridization was facilitated using SAD. Intermediate individuals linking both clusters on the RD network were those sampled at the limits of the sympatric zone in Northwest Iberia.

Conclusion: These results suggesting rare hybridization were confirmed by simulation of hybrids and F2 with directed backcrosses. Comparison with the Bayesian method STRUCTURE confirmed the usefulness of both approaches and emphasized the reliability of network analysis to unravel and study hybridization.

Show MeSH