Résumé
Diverging semi-isolated lineages either meet in narrow clinal hybrid
zones, or have a mosaic distribution associated with environmental
variation. Intrinsic reproductive isolation is often emphasized in the
former and local adaptation in the latter, although both can contribute to
isolation. Rarely these two patterns of spatial distribution are reported
in the same study system. Here we report that the long-snouted seahorse
Hippocampus guttulatus is subdivided into discrete panmictic entities by
both types of hybrid zones. Along the European Atlantic coasts, a northern
and a southern lineage meet in the southwest of France where they coexist
in sympatry with little hybridization. In the Mediterranean Sea, two
lineages have a mosaic distribution, associated with lagoon-like and
marine habitats. A fifth lineage was identified in the Black Sea. Genetic
homogeneity over large spatial scales contrasts with isolation maintained
in sympatry or close parapatry at a fine scale. A high variation in
locus-specific introgression rates provides additional evidence that
partial reproductive isolation must be maintaining the divergence.
Surprisingly, fixed differences between lagoon and marine populations in
the Mediterranean Sea belong to the most differentiated SNPs between the
two Atlantic lineages, against the genome-wide pattern of structure. These
parallel outlier SNPs cluster on a single chromosome-wide island of
differentiation. Since Atlantic lineages do not match the lagoon-sea
habitat variation, genetic parallelism at the genomic island suggests a
shared genetic barrier contributes to reproductive isolation in
contrasting contexts -i.e. spatial vs. ecological. We discuss how a
genomic hotspot of parallel differentiation could have evolved and become
associated either with space or with a patchy environment in a single
study system. Recommended in Yaniv Brandvain (2018) Genomic parallelism in
adaptation to orthogonal environments in sea horses. Peer Community in
Evolutionary Biology, 100056. doi:10.24072/pci.evolbiol.100056