92
M.S. Clark et al.
The outlier status of a locus having undergone a selective sweep has been demonstrated among 11 loci that are otherwise undifferentiated between two patches of
Mytilus edulis (Faure et al. 2008). Vasemägi et al. (2005) identified nine loci in
the Atlantic salmon, Salmo salar with highly significant deviations from the neutral expectations and proposed them as promising candidate genes for adaptation to
different habitats. A similar method was also implemented in the eelgrass Zostera
marina: using a genome-scan, over 25 anonymous or gene-linked microsatellite loci
appeared to be under selection in subtidal and intertidal populations of the eelgrass.
Oetjen and Reusch (2007a) identified three outlier loci, one of them being linked to
a nodulin gene involved in water transport regulation. This suggested a functional
significance in the genetic divergence observed at the outlier locus that may reflect
habitat differences.
3.3.1.3 Study of Hybrid Zones and the Speciation Processes
As in terrestrial ecosystems, ample opportunity for secondary contact between
differentiated gene pools also exists at sea. This creates hybridisation or tension
zones where the two genomes confront each other. These zones are maintained
by a balance between an afflux of parental genomes mediated mostly through larval dispersal and the removal of less fit hybrid gene combinations. Because of
the demographic characteristics of most marine species, which allows a very fine
selective sieving of the various genotypes, one expects that these zones settle precisely where the two differentiated gene pools (e.g. subspecies) display habitat
specificities. The combination of habitat specialisation genes (exogenous selection)
and under-dominant “speciation” genes (endogenous selection) gives rise to locally
coincident genetic clines where all the genes are in linkage disequilibrium and may
roughly resemble primary differentiation along an environmental gradient. These
clines are the result of the interaction of “congealed” genomes (i.e. genomes that
cannot completely remix with each other because of the existence of these “speciation” genes). One of the best studied examples of this is the mosaic zone of the blue
mussel Mytilus edulis and M. galloprovincialis (Bierne et al. 2002, 2003), but it is
likely that many other cases have gone unreported or misinterpreted. Recent surveys of Bathymodiolus populations using coalescence-based methods over multiple
genes both along the mid-Atlantic Ridge and the South East Pacific Rise indicated
that such secondary contact with hybridization are frequent occurrences in the deep
ocean with old introgression events that cover large portions of the oceanic ridges
(Faure et al. 2009). If genome scans are performed along those clines, many markers
will co-segregate, yet only a few may really reveal differential adaptation to varying
environmental conditions. This should not be underestimated when studying places
where many species show genetic clines such as at the mouth of the Baltic Sea
(Johannesson and André 2006). Ecological genomics represents a great potential
for disentangling the complex interactions of exogenous and endogenous selective
forces in the future. Both categories of genetic effects are pertinent to studying the
speciation process in the marine realm.
M.S. Clark et al.
The outlier status of a locus having undergone a selective sweep has been demonstrated among 11 loci that are otherwise undifferentiated between two patches of
Mytilus edulis (Faure et al. 2008). Vasemägi et al. (2005) identified nine loci in
the Atlantic salmon, Salmo salar with highly significant deviations from the neutral expectations and proposed them as promising candidate genes for adaptation to
different habitats. A similar method was also implemented in the eelgrass Zostera
marina: using a genome-scan, over 25 anonymous or gene-linked microsatellite loci
appeared to be under selection in subtidal and intertidal populations of the eelgrass.
Oetjen and Reusch (2007a) identified three outlier loci, one of them being linked to
a nodulin gene involved in water transport regulation. This suggested a functional
significance in the genetic divergence observed at the outlier locus that may reflect
habitat differences.
3.3.1.3 Study of Hybrid Zones and the Speciation Processes
As in terrestrial ecosystems, ample opportunity for secondary contact between
differentiated gene pools also exists at sea. This creates hybridisation or tension
zones where the two genomes confront each other. These zones are maintained
by a balance between an afflux of parental genomes mediated mostly through larval dispersal and the removal of less fit hybrid gene combinations. Because of
the demographic characteristics of most marine species, which allows a very fine
selective sieving of the various genotypes, one expects that these zones settle precisely where the two differentiated gene pools (e.g. subspecies) display habitat
specificities. The combination of habitat specialisation genes (exogenous selection)
and under-dominant “speciation” genes (endogenous selection) gives rise to locally
coincident genetic clines where all the genes are in linkage disequilibrium and may
roughly resemble primary differentiation along an environmental gradient. These
clines are the result of the interaction of “congealed” genomes (i.e. genomes that
cannot completely remix with each other because of the existence of these “speciation” genes). One of the best studied examples of this is the mosaic zone of the blue
mussel Mytilus edulis and M. galloprovincialis (Bierne et al. 2002, 2003), but it is
likely that many other cases have gone unreported or misinterpreted. Recent surveys of Bathymodiolus populations using coalescence-based methods over multiple
genes both along the mid-Atlantic Ridge and the South East Pacific Rise indicated
that such secondary contact with hybridization are frequent occurrences in the deep
ocean with old introgression events that cover large portions of the oceanic ridges
(Faure et al. 2009). If genome scans are performed along those clines, many markers
will co-segregate, yet only a few may really reveal differential adaptation to varying
environmental conditions. This should not be underestimated when studying places
where many species show genetic clines such as at the mouth of the Baltic Sea
(Johannesson and André 2006). Ecological genomics represents a great potential
for disentangling the complex interactions of exogenous and endogenous selective
forces in the future. Both categories of genetic effects are pertinent to studying the
speciation process in the marine realm.