Cline coupling and uncoupling in a stickleback hybrid zone.

Area of Science:

• Evolutionary biology
• Population genetics
• Speciation

Background:

• Ecological selection can maintain genetic divergence between populations in different environments, even with hybridization.
• Linkage disequilibrium between genetic loci under selection can create a cohesive barrier to gene flow across hybrid zones.
• Strong selection on a single locus may decouple its geographic distribution (cline) from others, potentially weakening gene flow barriers.

Purpose of the Study:

• To investigate the phenomenon of "cline uncoupling" in a hybrid zone between stream-resident and anadromous sticklebacks.
• To examine the geographic distribution of genetic clines at two genes (Eda and ATP1a1) and five morphological traits known to be under divergent selection.
• To determine if strong ecological selection causes individual clines to diverge, indicating a weakening of gene flow barriers.

Main Methods:

• Studied a hybrid zone between stream-resident and anadromous stickleback populations.
• Analyzed allele frequencies at two nuclear genes (Eda and ATP1a1) and five morphological traits.
• Mapped the geographic distribution of genetic and phenotypic clines to assess their concordance and identify potential "cline uncoupling".

Main Results:

• All analyzed genetic and morphological clines were steep and geographically concordant, clustered at the upper end of the estuary.
• No evidence of "cline uncoupling" was detected, where a cline is pulled away from others due to strong divergent selection.
• A stepped shape, typically associated with steep concordant clines, was notably absent, raising questions about the uniformity of selection regimes.

Conclusions:

• The observed concordance of clines suggests that gene flow barriers are maintained in this stickleback hybrid zone.
• The absence of cline uncoupling indicates that strong ecological selection on individual loci has not significantly reduced overall linkage disequilibrium.
• While clines are concordant, the lack of a stepped shape implies that the selection pressures across these diverse loci (osmoregulation, armor, swimming) may not be identical, warranting further investigation.