Hatchery-produced sandfish (Holothuria scabra) show altered genetic diversity in New Caledonia

Facing an alarming continuing decline of wild sea cucumber resources, management strategies were developed over the past three decades to sustainably promote development, maintenance, or regeneration of wild sea cucumber fisheries. In New Caledonia (South Pacific), dedicated management efforts via restocking and sea ranching programs were implemented to cope with the overharvesting of the sandfish Holothuria scabra and the recent loss of known populations. In order to investigate genetic implications of a major H. scabra restocking program, we assessed the genetic diversity and structure of wild stocks and hatchery-produced sandfish and compared the genetic outcomes of consecutive spawning and juvenile production events. For this, 1358 sandfish collected at four sites along the northwestern coasts of New Caledonia, as well as during five different restocking events in the Tiabet Bay, were genotyped using nine polymorphic microsatellite markers. We found that wild H. scabra populations from the northwestern coast of New Caledonia likely belonged to one panmictic population with high level of gene flow observed along the study scale. Further, this panmictic population displayed an effective size of breeders large enough to ensure the feasibility of appropriate breeding programs for restocking. In contrast, hatchery-produced samples did suffer from an important reduction in the effective population size: the effective population size were so small that genetic drift was detectable over one generation, with the presence of inbred individuals, as well as more related dyads than in wild populations. All these results suggest that dedicated efforts in hatcheries are further needed to maintain genetic diversity of hatchery-produced individuals in order to unbalance any negative impact during this artificial selection

Processus micro-évolutifs chez l'espèce marine invasive <i>Crepidula fornicata</i>

not availableLa première partie de cette thèse visait à tester l’hypothèse de processus de sélection suite à l’introduction de C. fornicata en dehors de son aire d’origine. Une approche par scan génomique a été menée sur 22 populations des aires d’origine et d’introduction. Une forte diversité génétique et une faible structure génétique entre les deux types d’aires indiquent une forte pression en propagules et de multiples introductions de différentes sources. Aucune signature de sélection n’a été identifiée entre aires native et introduites ; en revanche, huit locus « outliers », séparant les populations le long des côtes de la Floride, ont été identifiés, indiquant la probable évolution sur le long terme d’adaptations locales dans l’aire d’origine de la crépidule. Dans une seconde partie, les effets à une échelle locale de la dispersion naturelle et de la dérive génétique ont été analysés afin de tester l’hypothèse de « loterie familiale » (i.e. un nombre limité de familles assure le renouvellement complet de la population), à partir d’échantillonnages répétés de différents stades du cycle de vie (adultes, larves et juvéniles) au sein de la baie de Morlaix. Les variations de composition génétique des pools larvaires et des juvéniles suggèrent une variance du succès reproducteurs des adultes. En outre, de façon inattendue, des larves et juvéniles fortement apparentés sont observés. Enfin, les tailles efficaces estimées à partir de l’analyse des juvéniles (échantillonnés durant 9 ans) sont inférieures à celle de la population in situ. Ces résultats sont en faveur de l’hypothèse de loterie familiale chez cette espèce

Processus micro-évolutifs chez l'espèce marine invasive Crepidula fornicata

La première partie de cette thèse visait à tester l hypothèse de processus de sélection suite à l introduction de C. fornicata en dehors de son aire d origine. Une approche par scan génomique a été menée sur 22 populations des aires d origine et d introduction. Une forte diversité génétique et une faible structure génétique entre les deux types d aires indiquent une forte pression en propagules et de multiples introductions de différentes sources. Aucune signature de sélection n a été identifiée entre aires native et introduites ; en revanche, huit locus outliers , séparant les populations le long des côtes de la Floride, ont été identifiés, indiquant la probable évolution sur le long terme d adaptations locales dans l aire d origine de la crépidule. Dans une seconde partie, les effets à une échelle locale de la dispersion naturelle et de la dérive génétique ont été analysés afin de tester l hypothèse de loterie familiale (i.e. un nombre limité de familles assure le renouvellement complet de la population), à partir d échantillonnages répétés de différents stades du cycle de vie (adultes, larves et juvéniles) au sein de la baie de Morlaix. Les variations de composition génétique des pools larvaires et des juvéniles suggèrent une variance du succès reproducteurs des adultes. En outre, de façon inattendue, des larves et juvéniles fortement apparentés sont observés. Enfin, les tailles efficaces estimées à partir de l analyse des juvéniles (échantillonnés durant 9 ans) sont inférieures à celle de la population in situ. Ces résultats sont en faveur de l hypothèse de loterie familiale chez cette espècePARIS-BIUSJ-Biologie recherche (751052107) / SudocROSCOFF-Observ.Océanol. (292393008) / SudocSudocFranceF

Weird genotypes? Don't discard them, transmissible cancer could be an explanation

International audienceGenetic chimerism is rarely considered in the analysis of population genetics data, because assumed to be an exceptionally rare, mostly benign, developmental accident. An unappreciated source of chimerism is transmissible cancer, when malignant cells have become independent parasites and can infect other individuals. Parasitic cancers were thought to be rare exceptions, only reported in dogs (Murgia et al. However, the recent simultaneous report of four new contagious leukemias in marine mollusks (Metzger et al., Nature, 2016, 534, 705) might change the rules. By doubling up the number of naturally occurring transmissible cancers, this discovery suggests they may essentially be missed because not sufficiently searched for, especially outside mammals. We encourage population geneticists to keep in mind infectious cancer when interpreting weird genotypes in their molecular data. It would then contribute in the investigation of how widespread contagious cancer could really be in the wild. We provide an example with our own data in Mytilus mussels, a commercially important shellfish. We identified genetic chimerism in a few mussels that suggests the possible occurrence at low prevalence in European M. edulis populations of a M. trossulus contagious cancer related to the one described by Metzger et al. (Nature, 2016, 534, 705) in populations of British Columbia. K E

Data from: Unexpected collective larval dispersal but little support for sweepstakes reproductive success in the highly dispersive brooding mollusk Crepidula fornicata

In many marine invertebrates, long-distance dispersal is achieved during an extended pelagic larval phase. Although such dispersal should result in high gene flow over broad spatial scales, fine-scale genetic structure has often been reported, a pattern attributed to interfamilial variance in reproductive success and limited homogenization during dispersal. To examine this hypothesis, the genetic diversity of dispersing larvae must be compared with the post-dispersal stages, i.e. benthic recruits and adults. Such data remain however scarce due to the difficulty to sample and analyze larvae of minute size. Here we carried out such an investigation using the marine gastropod Crepidula fornicata. Field sampling of three to four larval pools was conducted over the reproductive season and repeated over three years. The genetic composition of larval pools, obtained with 16 microsatellite loci, was compared with that of recruits and adults sampled from the same site and years. Conversely to samples of juveniles and adults, large genetic temporal variations between larval pools produced at different times of the same reproductive season were observed. In addition, full- and half-sibs were detected in early larvae and post-dispersal juveniles, pointing to correlated dispersal paths between several pairs of individuals. Inbred larvae were also identified. Such collective larval dispersal was unexpected given the long larval duration of the study species. Our results suggest that each larval pool is produced by a small effective number of reproducers but that, over a reproductive season, the whole larval pool is produced by large numbers of reproducers across space and time

Data from: Paternity and gregariousness in the sex-changing sessile marine gastropod Crepidula convexa: comparison with other protandrous Crepidula species

In sex-changing animals with internal fertilization, gregarious behavior may increase mating opportunities and the frequency of multiple paternity, thus increasing maternal reproductive success. Crepidula convexa is a direct-developing protandrous gastropod characterized by only modest gregarious behavior compared with previously studied members of the genus: females are frequently found isolated. Using 6 microsatellite markers, we analyzed paternity profiles in 10 broods (25 embryos per mother). The number of assigned fathers varied among families from 1 to 4 fathers per brood. Interestingly, polyandry was not detected in solitary females but only in females grouped with conspecific individuals. Overall, we found an average of 1.8 fathers per brood, but this increased to 2.6 fathers per brood when considering only the nonisolated females. Among 18 unambiguously identified fathers, only 5 were collected in our samples, suggesting substantial male mobility. Comparison with previous paternity analyses in Crepidula fornicata and Crepidula coquimbensis revealed that polyandry appears as a common trait of these sex-changing gastropods despite their different grouping behaviors and life histories. As expected, the level of polyandry was nevertheless lower in the modestly gregarious C. convexa

Current hypotheses to explain genetic chaos under the sea

International audienceChaotic genetic patchiness (CGP) refers to surprising patterns of spatial and temporal genetic structure observed in some marine species at a scale where genetic variation should be efficiently homogenized by gene flow via larval dispersal. Here we review and discuss four mechanisms that could generate such unexpected patterns: selection, sweepstakes reproductive success, collective dispersal, and temporal shifts in local population dynamics. First, we review examples where genetic differentiation at specific loci was driven by diversifying selection, which was historically the first process invoked to explain CGP. Second, we turn to neutral demographic processes that may drive genome-wide effects, and whose effects on CGP may be enhanced when they act together. We discuss how sweepstakes reproductive success accelerates genetic drift and can thus generate genetic structure provided gene flow is not too strong. Collective dispersal is another mechanism whereby genetic structure can be maintained regardless of dispersal intensity, because it may prevent larval cohorts from becoming entirely mixed. Theoretical analyses of both the sweepstakes and the collective dispersal ideas are presented. Finally, we discuss an idea that has received less attention than the other ones just mentioned, namely temporal shifts in local population dynamics

Data from: Contrasting patterns of genome-wide polymorphism in the native and invasive range of the marine mollusk Crepidula fornicata

Selection processes are believed to be an important evolutionary driver behind the successful establishment of non-indigenous species, however evidence is still scarce. Genome-scans have often identified loci with atypical patterns of genetic differentiation (i.e. outliers) indicative of selection processes. Using microsatellite- and AFLP-based genome-scans, we looked for evidence of post-introduction selection in the mollusk Crepidula fornicata, native from the northwestern Atlantic and introduced in the northeastern Atlantic and northeastern Pacific during the 19th and 20th centuries. We examined 683 individuals from 7 native and 15 introduced populations spanning the latitudinal introduction and native ranges of the species. Our results showed the high genetic diversity in all populations with little genetic structure between the two ranges, a pattern typical of marine invaders. Analyzing 344 loci, no outliers were detected between introduced and native populations or within introduced populations. The genomic sampling may have been insufficient to reveal selection especially if it acts on traits determined by a few genes. Eight outliers were however identified within the native range, underlining a genetic singularity congruent with a well-known biogeographic break along the Florida. Our results call into question the relevance of AFLP genome-scans in detecting adaptation on the time-scale of biological invasions: genome-scans often reveal long-term adaptation involving numerous genes throughout the genome but seem less effective in detecting recent adaptation from pre-existing variation on polygenic traits. This study advocates other methods to detect selection effects during biological invasions – on phenotypic traits, although genome-scans may remain useful for elucidating introduction histories

MEC-17-0564

MEC-17-056