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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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Microsatellite detection of admixture (introgression) using the program STRUCTURE (Pritchard et al., 2000). Each individual is represented in the figure by a vertical bar and its colors indicate the proportional membership in each of k = 2 clusters, thereby providing a quantitative illustration of introgression. Red represents F. spiralis, green F. vesiculosus.
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Figure 5: Microsatellite detection of admixture (introgression) using the program STRUCTURE (Pritchard et al., 2000). Each individual is represented in the figure by a vertical bar and its colors indicate the proportional membership in each of k = 2 clusters, thereby providing a quantitative illustration of introgression. Red represents F. spiralis, green F. vesiculosus.

Mentions: In order to highlight the potential differences existing between the two methods and to depict their advantages and disadvantages in hybridization assessment, we compared the networks with the software STRUCTURE [16], that assigns genotypes proportionally to clusters defined based on minimizing linkage and Hardy-Weinberg disequilibria. The results obtained with STRUCTURE for two clusters (k = 2) are coherent with mainly two categories of individuals in agreement with the morphological determination of the natural dataset of all individuals into the two species (Figure 5A). Nevertheless, 21 intermediate individual genotypes (more than 10% of ancestry coming from one of the 2 species) are counted: 3 F_spi and 18 F_ves. It appears that only few of them (n = 5, 2 F_spi and 3 F_ves; ns = 1+1) are among the 17 putative hybrids detected with the SAD network and relocated at the end of the dataset (Figure 5B). It is also noticeable that although most of the individuals (n = 13, ns = 3) considered as admixed in STRUCTURE are in the sympatric zone of Northwest Iberia, some (n = 8, ns = 4) do not have a clearly intermediate position on the SAD network, with even one coming from an allopatric area (Southwest Portugal).


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)

Microsatellite detection of admixture (introgression) using the program STRUCTURE (Pritchard et al., 2000). Each individual is represented in the figure by a vertical bar and its colors indicate the proportional membership in each of k = 2 clusters, thereby providing a quantitative illustration of introgression. Red represents F. spiralis, green F. vesiculosus.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 5: Microsatellite detection of admixture (introgression) using the program STRUCTURE (Pritchard et al., 2000). Each individual is represented in the figure by a vertical bar and its colors indicate the proportional membership in each of k = 2 clusters, thereby providing a quantitative illustration of introgression. Red represents F. spiralis, green F. vesiculosus.
Mentions: In order to highlight the potential differences existing between the two methods and to depict their advantages and disadvantages in hybridization assessment, we compared the networks with the software STRUCTURE [16], that assigns genotypes proportionally to clusters defined based on minimizing linkage and Hardy-Weinberg disequilibria. The results obtained with STRUCTURE for two clusters (k = 2) are coherent with mainly two categories of individuals in agreement with the morphological determination of the natural dataset of all individuals into the two species (Figure 5A). Nevertheless, 21 intermediate individual genotypes (more than 10% of ancestry coming from one of the 2 species) are counted: 3 F_spi and 18 F_ves. It appears that only few of them (n = 5, 2 F_spi and 3 F_ves; ns = 1+1) are among the 17 putative hybrids detected with the SAD network and relocated at the end of the dataset (Figure 5B). It is also noticeable that although most of the individuals (n = 13, ns = 3) considered as admixed in STRUCTURE are in the sympatric zone of Northwest Iberia, some (n = 8, ns = 4) do not have a clearly intermediate position on the SAD network, with even one coming from an allopatric area (Southwest Portugal).

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