ddRAD-seq alignment data for Unionid mussels

North American watersheds contain the world’s highest diversity of
freshwater mussels (Unionoida), and up to 40 species can co-occur in a
single riffle. They collectively exhibit little evidence for ecological
differentiation during the long-lived, benthic phase of their life
cycle.  In contrast, their brief parasitic larval phase
involves the infection of a wide diversity of fish hosts. Gravid female
mussels have evolved multiple methods for increasing the probability of
infecting a host fish. Some species use a passive broadcast strategy:
placing high numbers of larvae in the water column and relying on chance
encounters with potential hosts for infection. Most species have a
proactive strategy that entails the use of prey-mimetic lures to change
the behavior of the hosts, i.e., eliciting a feeding
response through which they become infected. Gravid females collectively
produce two main lure types: a mantle tissue lure (on the female’s body)
and a brood lure, containing infective larvae, that she releases into the
external environment. In this study, we used a phylogenomic approach
(ddRAD-seq) to place the diversity of infection strategies used by 54
North American lampsiline mussels into an evolutionary context. Ancestral
state reconstruction recovered evidence for the early evolution of mantle
lures in this clade, with brood lures and broadcast infection strategies
both being independently derived twice. The most common infection
strategy, occurring in our largest ingroup clade, is a mixed one in which
mimetic mantle lures are apparently the predominant
infection mechanism, but gravid females also release simple,
non-mimetic brood lures at the end of the season. This mixed
infection strategy clade shows some evidence of an increase in
diversification rate and most members use bass
(Micropterus & Ambloplites spp.) as
their predominant fish hosts. Broad linkage between infection
strategies and predominant fish host genera is also seen in other
lampsiline clades: worm-like mantle lures of Toxolasma
spp. with sunfish (Lepomis spp.); insect larvae-like brood lures
(Ptychobranchus spp.), or mantle lures
(Medionidus spp., Obivariaspp.), or mantle
lures combined with host capture (Epioblasma spp.) with a
spectrum of darter (Etheostoma & Percinaspp.)
and sculpin (Cottus spp.) hosts, and tethered brood lures
(Hamiota spp.) with bass
(Micropterus & Ambloplites spp.). Our
phylogenetic results confirm that discrete lampsiline
mussel clades exhibit considerable specialization in the primary fish host
clades their larvae parasitize, and in the host infection strategies they
employ to do so. They are also consistent with
the hypothesis that larval resource partitioning of fish
hosts is an important factor in maintaining species
diversity in mussel assemblages.  We conclude that
taking their larval ecology and host-infection mechanisms into account,
lampsiline mussels may be legitimately viewed as a cryptic adaptive
radiation.