Search

Unionoid freshwater mussels (Bivalvia: Unionidae) are free-living apart from a brief, obligately parasitic, larval stage that infects fish hosts, and gravid female mussels have evolved a spectrum of strategies to infect fish hosts with their larvae. In many North American species, this involves displaying a mantle lure: a pigmented fleshy extension that acts as an aggressive mimic of a host fish prey, thereby eliciting a feeding response that results in host infection. The mantle lure of Lampsilis fasciola is of particular interest because it is apparently polymorphic, with two distinct primary lure phenotypes. One, described as "darter-like", has "eyespots", a mottled body coloration, prominent marginal extensions, and a distinct "tail". The other, described as "worm-like", lacks those features and has an orange and black coloration. We investigated this phenomenon using genomics, captive rearing, biogeographic, and behavioral analyses. Within-brood lure variation and within-population phylogenomic (ddRAD-seq) analyses of individuals bearing different lures confirmed that this phenomenon is a true polymorphism. The relative abundance of the two morphs appears stable over ecological timeframes: the ratio of the two lure phenotypes in a River Raisin (MI) population in 2017 was consistent with that of museum samples collected at the same site six decades earlier. Within the River Raisin, four main "darter-like" lure motifs visually approximated four co-occurring darter species (Etheostoma blennioides, E. exile, E. microperca, and Percina maculata), and the "worm-like" lure resembled a widespread common leech, Macrobdella decora. Darters and leeches are typical prey of Micropterus dolomieui (smallmouth bass), the primary fish host of L. fasciola. In situ field recordings of the L. fasciola "darter" and "leech" lure display behaviors, and the lure display of co-occurring congener L. cardium, were captured. Despite having putative models in distinct phyla, both L. fasciola lure morphs have largely similar display behaviors that differ significantly from that of sympatric L. cardium individuals. Some minor differences in the behavior between the two L. fasciola morphs were observed, but we found no clear evidence for a behavioral component of the polymorphism given the criteria measured. Discovery of discrete within-brood inheritance of the lure polymorphism implies potential control by a single genetic locus and identifies L. fasciola as a promising study system to identify regulatory genes controlling a key adaptive trait of freshwater mussels. [ABSTRACT FROM AUTHOR]

North American watersheds contain a high diversity of freshwater mussels (Unionoida). During the long-lived, benthic phase of their life cycle, up to 40 species can co-occur in a single riffle and there is typically little evidence for major differences in their feeding ecology or microhabitat partitioning. In contrast, their brief parasitic larval phase involves the infection of a wide diversity of fish hosts and female mussels have evolved a spectrum of adaptations for infecting host fish with their offspring. Many species use a passive broadcast strategy: placing high numbers of larvae in the water column and relying on chance encounters with potential hosts. Many other species, including most members of the Lampsilini, 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. Two main lure types are collectively produced: mantle tissue lures (on the female’s body) and brood lures, containing infective larvae, that are released 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 centrarchids (Micropterus & Lepomis 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 (Lepomisspp.); insect larvae-like brood lures (Ptychobranchus spp.), or mantle lures (Medionidus spp., Obovaria spp.), or mantle lures combined with host capture (Epioblasma spp.) with a spectrum of darter (Etheostoma & Percina spp.) and sculpin (Cottus spp.) hosts, and tethered brood lures (Hamiota spp.) with bass (Micropterus 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 an adaptive radiation. [ABSTRACT FROM AUTHOR]

Understanding genetic diversity across large spatial scales helps to reveal patterns of population structure. Mitochondrial DNA sequences and microsatellite loci were used to analyze the phylogeography of a common unionid species (Lasmigona costata) from the Laurentian Great Lakes and historically connected river drainages. Phylogeographic patterns were assessed to determine colonization routes into the Great Lakes following glacial recession. A suite of seven microsatellite loci were genotyped and a fragment of the mitochondrial gene COI was sequenced. Multiple analyses using microsatellite allele frequencies suggest at least two distinct genetic populations for L. costata. A total of seven hypothesized post-glacial dispersal scenarios were compared using isolation by distance to test the various dispersal models. Evidence was strongest for two post-glacial dispersal routes into the Great Lakes: one utilizing a connection between the Wabash and Maumee River watersheds, and one utilizing a connection between the Wisconsin River and Green Bay watersheds. A highly differentiated and monophyletic population of L. costata was identified in the Ozark Highlands, which may constitute a unique taxonomic entity. [ABSTRACT FROM AUTHOR]