High levels of multiple paternity in a spermcast mating freshwater mussel

Polymorphism in the aggressive mimicry lure of the parasitic freshwater mussel Lampsilis fasciola

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.

Investigating population dynamics from parentage analysis in the highly endangered fan mussel Pinna nobilis

Understanding dispersal patterns is a major focus for conservation biology as it influences local survival and resilience in case of local disturbance, particularly for sessile species. Dispersal can be assessed through parentage analyses by estimating family structure and self-recruitment. This study documents the family structure of a pelagic spawner, Pinna nobilis, which is facing a major crisis that threatens its survival as most of its populations have been decimated by a parasite, Haplosporidium pinnae. In this context, we focused on a single population (Peyrefite, Banyuls-sur-mer, France) where 640 individuals were sampled in 2011, 2015, and 2018 and genotyped for 22 microsatellite markers. Genetic diversity was high and homogeneous among years, with mean allele numbers ranging between 13.6 and 14.8 and observed heterozygosities (H o) between 0.7121 and 0.7331. Low, but significant, genetic differentiations were found between 2011-2015 and 2015-2018. A parentage analysis described 11 clusters, including one prevailing, and revealed that 46.9% of individuals were involved in half-sib relationships, even between years, suggesting that source populations were recurrent year after year. There were few individuals resampled between years (30 in 2015 and 14 in 2018), indicating a rapid turnover. Considering the large number of half-sib relationships but the low number of relations per individual, we conclude that P. nobilis exhibit homogeneous reproductive success. Self-recruitment was not detected, making this population highly vulnerable as replenishment only relies on connectivity from neighboring populations. In the context of the pandemic caused by H. pinnae, these results will have to be considered when choosing a location to reintroduce individuals in potential future rescue plans.

Securing genetic integrity in freshwater pearl mussel propagation and captive breeding

Securing genetic integrity is of key importance in conservation-oriented captive breeding programs releasing juveniles into the wild. This is particularly true for species such as the endangered freshwater pearl mussel (Margaritifera margaritifera) for which a number of captive breeding facilities has been established in Europe. The core objective of this study was to compare the genetic constitution of 29 cohorts of captive-bred freshwater pearl mussels from five different breeding facilities in Austria, France, Luxembourg and Germany, with their original 14 source populations from nine major European drainages, based on microsatellite markers. Captive-bred mussels represented 11 different genetic clusters, suggesting an important contribution of the breeding stations to securing the genetic diversity of the species. In almost all cases, the cultured offspring closely resembled the genetic constitution of the source mussels as revealed from the STRUCTURE analysis and the generally high assignment of offspring to the original source populations. The majority of captive-bred cohorts had an increased inbreeding coefficient and decreased genetic variability compared to their source populations as measured by A_R and H_O. Highest numbers of deformed juveniles coincided with very low levels of H_O < 0.05. Since erosion of genetic diversity in captive breeding was mostly evident in individual year-cohorts, long-term breeding over multiple years can minimize such effects. The systematic selection of priority populations for conservation, effective breeding strategies avoiding effects of in- and outbreeding by genetically informed selection of parent individuals, and a network of collaboration among the different breeding facilities would be very useful to increase resilience and effectiveness.

Higher mortality of the less suitable brown trout host compared to the principal Atlantic salmon host when infested with freshwater pearl mussel (Margaritifera margaritifera) glochidia

The freshwater pearl mussel (Margaritifera margaritifera) is a highly host-specific parasite, with an obligate parasitic stage on salmonid fish. Atlantic salmon (Salmo salar) and brown trout (Salmo trutta f. trutta and Salmo trutta f. fario) are the only hosts in their European distribution. Some M. margaritifera populations exclusively infest either Atlantic salmon or brown trout, while others infest both hosts with one salmonid species typically being the principal host and the other a less suitable host. Glochidial abundance, prevalence and growth are often used as parameters to measure host suitability, with the most suitable host species displaying the highest parameters. However, it is not known if the degree of host specialisation will negatively influence host fitness (virulence) among different host species. In this study we examined the hypothesis that glochidial infestation would result in differential virulence in two salmonid host species and that lower virulence would be observed on the most suitable host. Atlantic salmon and brown trout were infested with glochidia from two M. margaritifera populations that use Atlantic salmon as their principal host, and the difference in host mortality among infested and control (sham infested) fish was examined. Higher mortality was observed in infested brown trout (the less suitable host) groups, compared to the other test groups. Genetic assignment was used to identify offspring from individual mother mussels. We found that glochidia from individual mothers can infest both the salmonid hosts; however, some mothers displayed a bias towards either salmon or trout. We believe that the differences in host-dependent virulence and the host bias displayed by individual mothers were a result of genotype × genotype interactions between the glochidia and their hosts, indicating that there is an underlying genetic component for this parasite-host interaction.

Spermcast mating with release of zygotes in the small dioecious bivalve Digitaria digitaria

Digitaria digitaria, a small astartid usually less than 10 mm in length, has a non-brooding behaviour in spite of its limited space for gonad development. This species lives in highly unstable environments with strong currents, which represent a challenge for fertilization and larval settlement. The studied population of D. digitaria from the Strait of Gibraltar area was dioecious, with significant predominance of females and sexual dimorphism, where females are larger than males. The reproductive cycle is asynchronous throughout the year, without a resting period, but with successive partial spawning events. The presence of stored sperm in the suprabranchial chamber and inside the gonad of some females, together with the release of eggs along the dorsal axis of both gills, points to internal oocyte fertilization. Bacteriocytes were found in the female and male follicle walls, but no bacteria were observed inside any of the gametes. Digitaria digitaria could represent a “missing link” between spermcast mating bivalves with brooded offspring and bivalves with broadcast release of eggs and sperm. The small size, limiting the oocyte production, together with the unstable environment could represent evolutionary pressures towards sperm uptake in D. digitaria.

Promiscuity and sex ratio in the terrestrial isopod Armadillidium vulgare and consequences on genetic diversity

In promiscuous mating systems, both males and females mate with several partners. While the benefits of multiple mating are well recognized for males, there are several non-mutually exclusive hypotheses to explain multiple mating for females. Promiscuity is widespread in terrestrial isopods. Here, we placed experimental populations of the terrestrial isopod Armadillidium vulgare under varying sex ratios to manipulate the number of available partners, and better characterise the mating system in this species by performing paternity tests using microsatellite markers. We observed that females usually mate multiply with up to 5 males in a single event of reproduction. A higher number of fathers in broods did not increase brood size nor heterozygosity, but increased allelic richness. Promiscuity seems to be asymmetric in this species, with females being less affected by changes in sex ratio than were males and would be an adaptive mating system maximising the offspring genetic diversity.