3 Populations and Pathways
91
competition between related individuals and the increased mortality during dispersal
(Pechenik 1999).
Despite its biological significance, only a limited number of studies have
attempted to elucidate the mechanisms underlying the timing of acquisition of
competence in marine invertebrates. Although the metabolic pathways and the
substances inducing or preventing larval development, metamorphosis and settlement are studied in numerous species, the molecular and genetic bases of these
processes as well as their variations in response to selective pressures are still
poorly documented (Hadfield 1998). Until recently, genomic tools were poorly
available for non-model organisms. As pointed out by Medina (2009), the study
of non-model organisms can now provide lights on major issue about life-cycle
evolution, for instance, on-going studies based on micro-arrays and Q-PCR carried
out in the invasive gastropod, Crepidula fornicata (Taris et al. 2009) demonstrating the involvement of NO signalling pathway in larval metamorphosis. Another
major illustration comes from several recent studies by Degnan and collaborators
on gastropods of the genus Haliotis (Degnan et al. 1997, Jackson et al. 2005,
Williams et al. 2009). Through the identification of cDNA fragments representing
differentially-expressed genes, they obtained a suite of genes likely to be involved
in metamorphosis and competence in H. asinina (Jackson et al. 2005). Results of an
extensive study carried out by means of cDNA microarrays then suggested that pathways operating at larval metamorphosis may regulate the expression of novel genes
specific to abalone and molluscs metamorphosis (Williams et al. 2009). Analyses of
the conservation of these genes and of their associated polymorphisms should help
in understanding the selective processes acting on larval development pathways.
3.3.1.2 Genetic Basis of Adaptive Differentiation in High Gene Flow Species
Marine species often exhibit high gene flow through larval dispersal. Such dispersal
ability may prevent local adaptation in a heterogeneous environment in the absence
of spawning shifts between the inter-connected populations. On the other hand, both
the very high fecundities (allowing differential mortalities) and large population
sizes (enabling a high selection efficiency through the parameter 2 N e s) provide
opportunities for counteracting the homogenising effects of gene flow. The question
is to determine the actual extent of adaptive polymorphism, which accounts for the
presence of the same marine species in contrasting environments, and to examine
under which set of conditions such an adaptation may arise and persist.
Population genomics and more specifically genome scan analyses can provide
relevant tools to address these questions. In the case of divergent selection according to habitats, new beneficial alleles may sweep to fixation in a single habitat where
such an allele is advantageous. Subsequently, the frequency of some neutral alleles
at closely linked loci should be increased in that region, due to genetic hitchhiking. Therefore, the genetic divergence should be greater for neutral loci, which are
closely linked to the selected locus than for other neutral loci. This is the rationale for locating outlier loci and then candidate genes that may have been targets
of selection, with the various types of markers and techniques described above.
91
competition between related individuals and the increased mortality during dispersal
(Pechenik 1999).
Despite its biological significance, only a limited number of studies have
attempted to elucidate the mechanisms underlying the timing of acquisition of
competence in marine invertebrates. Although the metabolic pathways and the
substances inducing or preventing larval development, metamorphosis and settlement are studied in numerous species, the molecular and genetic bases of these
processes as well as their variations in response to selective pressures are still
poorly documented (Hadfield 1998). Until recently, genomic tools were poorly
available for non-model organisms. As pointed out by Medina (2009), the study
of non-model organisms can now provide lights on major issue about life-cycle
evolution, for instance, on-going studies based on micro-arrays and Q-PCR carried
out in the invasive gastropod, Crepidula fornicata (Taris et al. 2009) demonstrating the involvement of NO signalling pathway in larval metamorphosis. Another
major illustration comes from several recent studies by Degnan and collaborators
on gastropods of the genus Haliotis (Degnan et al. 1997, Jackson et al. 2005,
Williams et al. 2009). Through the identification of cDNA fragments representing
differentially-expressed genes, they obtained a suite of genes likely to be involved
in metamorphosis and competence in H. asinina (Jackson et al. 2005). Results of an
extensive study carried out by means of cDNA microarrays then suggested that pathways operating at larval metamorphosis may regulate the expression of novel genes
specific to abalone and molluscs metamorphosis (Williams et al. 2009). Analyses of
the conservation of these genes and of their associated polymorphisms should help
in understanding the selective processes acting on larval development pathways.
3.3.1.2 Genetic Basis of Adaptive Differentiation in High Gene Flow Species
Marine species often exhibit high gene flow through larval dispersal. Such dispersal
ability may prevent local adaptation in a heterogeneous environment in the absence
of spawning shifts between the inter-connected populations. On the other hand, both
the very high fecundities (allowing differential mortalities) and large population
sizes (enabling a high selection efficiency through the parameter 2 N e s) provide
opportunities for counteracting the homogenising effects of gene flow. The question
is to determine the actual extent of adaptive polymorphism, which accounts for the
presence of the same marine species in contrasting environments, and to examine
under which set of conditions such an adaptation may arise and persist.
Population genomics and more specifically genome scan analyses can provide
relevant tools to address these questions. In the case of divergent selection according to habitats, new beneficial alleles may sweep to fixation in a single habitat where
such an allele is advantageous. Subsequently, the frequency of some neutral alleles
at closely linked loci should be increased in that region, due to genetic hitchhiking. Therefore, the genetic divergence should be greater for neutral loci, which are
closely linked to the selected locus than for other neutral loci. This is the rationale for locating outlier loci and then candidate genes that may have been targets
of selection, with the various types of markers and techniques described above.