Extensive Introgression among Strongylocentrotid Sea Urchins Revealed by
Gametic isolation is thought to play an important role in the evolution
of reproductive isolation in broadcast-spawning marine invertebrates.
However, it remains uncertain whether gametic isolation typically
develops early in the speciation process or accumulates after other
reproductive barriers are already in place. It is also unknown whether
gametic incompatibilities have effectively prevented introgression
during later stages of divergence. Here, we use phylogenomic approaches
to test whether the well-documented asymmetric gametic incompatibilities
between strongylocentrotid urchins have been effective in preventing
introgression. Despite a well-supported species tree, we found
considerable phylogenetic discordance that cannot be explained by
incomplete lineage sorting alone. There was strong support for
introgression between at least four pairs of extant taxa: S.
pallidus ↔ S. droebachiensis, S. intermedius ↔ S.
pallidus, S. purpuratus ↔ S. fragilis, and M.
franciscanus ↔ P. depressus. There was additional evidence for
introgression on internal branches of the phylogeny. Although gametic
incompatibilities may be important in species recognition and the
maintenance of species boundaries in strongylocentrotid urchins, gametic
isolation does not appear to have been an effective barrier to
introgression. The continued divergence in the face of widespread
introgression indicates that other reproductive isolating barriers
likely exist and may have been more critical in establishing
reproductive isolation early in speciation.