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Decrease in multi-locus heterozygosity in suspension-cultured mussels (Mytilus edulis) through loss of the more heterozygous individuals
Abstract
The mean multi-locus heterozygosity (MLH) of suspension-cultured mussels has been reported to decrease within a few months after traditional sleeving in previous studies. Two experiments were carried out to determine if this decrease could be attributed to the more rapid migration of the more heterozygous mussels out of the mesh tubes after sleeving. Since these individuals occur in greater numbers at the periphery of the sleeves, they are more susceptible to fall-offs. In the first experiment, the mean MLH of the mussels emerging from the mesh tubes was significantly higher than the MLH of the spat used for sleeving two weeks before. In a second experiment with caged and free sleeves under field conditions, a significant decrease in MLH was observed, but only for the mussels from the free sleeves. This decrease occurred in late December, only one month after sleeving. It is hypothesized that the experimental cages limited turbulence and thus fall-offs during the weeks following sleeving. As a consequence, the slower moving homozygotes were able to catch up with the faster heterozygotes at the sleeve's periphery. In contrast, mussels in the free sleeves may have experienced more turbulent conditions leading to high fall-offs of the more heterozygous mussels at the sleeve's periphery during the crucial period following sleeving. These losses could impact mussel production since MLH explained 32% of the variation in individual dry tissue weight in this study. Furthermore, the total fresh weight of mussels with high MLH (5-7 heterozygous loci) was almost twice that of the mussels with low MLH (0-2 heterozygous loci) about one year after sleeving: 11.38 vs. 5.95 g. In addition, there were 3.7 times more commercial-sized mussels (>50 mm) with a high MLH than with a low MLH (80% vs. 21.5%). In contrast to traditional sleeving with plastic mesh tubes, a new method of mechanical sleeving provides continuous sleeves wrapped in cotton cloth. This biodegradable cloth helps retain the heterozygotes on the sleeves before it disintegrates.
... For more than 25 years, mussel farming in the Î les-de-la-Madeleine archipelago (Gulf of St. Lawrence, Québec, Canada) has exclusively relied on the wild spat supply (juveniles, mostly >15 mm) from the small lagoon of Bassin du Havre-Aubert (BHA) (Guillou et al., 2020). This management strategy was based on the demonstrated higher productivity of this mussel stock compared to others collected elsewhere in this archipelago (< 65 km from BHA) (Myrand et al., 2009a(Myrand et al., , 2009b. Furthermore, the BHA stock showed lower sensitivity to summer mortality episodes that occur during the warmest period of summers (> 20 • C) (Myrand and Gaudreault, 1995;Tremblay et al., 1998aTremblay et al., , 1998bMyrand et al., 2000). ...
... We suggest that the selective pressure could be more effective around metamorphosis because of the sensitivity of this stage (Pechenik, 2006;Rayssac et al., 2010;Jenewein and Gosselin, 2013;Gosling, 2015a). We needed to mimic mussel farming practises because mussel stock performance had been observed on spat settled on artificial collectors (Sénéchal et al., 2008;Myrand et al., 2009aMyrand et al., , 2009bGuillou et al., 2020). Therefore, we tested our hypothesis on spat sampled from similar collectors and then moved to controlled conditions in mesocosms. ...
... Shell growth improved by 22% in surviving mussels, suggesting that less performant mussels had been eliminated. This selection appeared to be the key prerequisite providing higher-quality mussels stock from BHA, which are characterized by different genetic characteristics, lower metabolic needs, and lower thermosensitivity along with higher resistance to stressful conditions, high productivity, and strong commercial performance (Myrand and Gaudreault, 1995;Tremblay et al., 1998aTremblay et al., , 1998bTremblay et al., , 1998cTremblay et al., , 1998dMyrand et al., 2002Myrand et al., , 2009aMyrand et al., , 2009bGuillou et al., 2020Guillou et al., , 2023. ...
Our previous work confirmed that the higher survival, growth, and commercial performance of a mussel stock collected from a specific lagoon have been maintained over the last 25 years at a microgeographic scale despite intensive transfers of its spat to other local mussel sites. The objective of this study was to identify potential selection mechanisms supporting the quality of this stock. We used a mesocosm approach to examine the effect of maximum seawater temperature measured in the field on the different ontogenic stages of Mytilus edulis to simulate similar patterns in the laboratory. We demonstrated that thermal selection did not affect the development of peri-metamorphic stages (< 1 mm) since no effects were observed on growth, survival, or prodissoconch II size under natural thermal variations. However, our experimental results clearly indicated the existence of a thermal selective pressure (24 °C for 85 h) on 4.5–7 mm juvenile mussels, with growth performance enhanced in surviving individuals. Mortality was 37% higher compared to the control group, suggesting the elimination of more sensitive and less performant mussels. Interestingly, surviving individuals showed improved shell and tissue growth rates, by 22 and 51%, respectively. Our results support the hypothesis that the thermal selection of highly performant stock is size-dependent. The selective pressure demonstrated in this study along with our previous work highlighting the physiological and genetic differences between local stocks constitute the key mechanisms leading to the observed stable production performance over decades at a microgeographic scale.
... These criteria were (i) a minimum of 35% of spat (size >6 mm) exceeding 15 mm in length on each collector, and (ii) production of at least 2 m of mussel socks (40 mm opening) from a single 1 m collector ("sock:collector ratio" > 2:1). These criteria lead to the usual commercial density of around 680 to 830 mussels m −1 [37][38][39]. Five socks (3 m in length) were sleeved per stock and suspended on long-lines in the lagoon (HAM) and offshore (BP) grow-out sites. ...
... The sock:collector ratio is a commercial indicator taking into account both spat density and size to determine the volume of commercial-sized spat that can be sleeved; this technique has been recommended in several studies conducted in eastern Canada [37][38][39]63,64]. In this context, the sock:collector ratio is a standardized way to compare the performance of different spat supplies. ...
... The performance of the BHA spat supply could probably be linked to the restricted configuration of this lagoon (3 km 2 , mean depth of 2-3 m), which leads to earlier, faster, and higher warming of the water column that stimulates earlier spawning events [23,33,34]. In addition, several studies have revealed specific metabolic and genetic characteristics of BHA mussels that provide better resistance to stress as well as higher growth and survival rates [18][19][20][21][22]38,39]. However, these advantages did not systematically improve the commercial performance of this stock. ...
Bivalve aquaculture is an important component of the economy in eastern Canada. Because of current social, environmental, economic, and resource constraints, offshore mussel cultivation seems to be a promising strategy. With the objective of optimizing farming strategies that support the sustainability and development of the mussel industry at a microgeographic scale, we evaluated, after a traditional two year production cycle, the commercial performance of spat from several mussel (Mytilus edulis) stocks originating from sites separated by less than 65 km and cultivated at two different grow-out sites (shallow lagoon and offshore waters). The spatiotemporal variation in spat performance was studied through a multiyear in situ “stock-site” spat transfer design. The spat supply originating from the Bassin du Havre-Aubert lagoon systematically exhibited a larger size at sleeving time when compared to other stocks, and a better productivity level when harvested. Nevertheless, an alternative strategy would be to collect spat from the Havre-aux-Maisons lagoon, mostly because of the important commercial volumes of spat that can be collected there. Commercial performance (net income) was three times higher in the deep offshore grow-out site than in the shallow lagoon site. This better productivity in the open sea confirms the highly valuable strategy of offshore mussel farming in this area, where it was hypothesized that the less stressful environmental conditions positively influence reproduction, survival, and growth trends.
... More heterozygous individuals had a higher protein synthesis efficiency and a higher scope for growth compared to more homozygous individuals [21] . Furthermore, Myrand et al. [22] showed that the total wet weight of M. edulis with high MLH were twice that of the mussels with low MLH, after one year of suspension culture. As the use of genetic characteristics, like MLH has been shown to be a reliable indicator of physiological fitness in M. edulis [23] , there is some interest in assessing this indicator/tool for the two species, within similar environment. ...
... The allozymes chosen for this study were polymorphic enzymes implicated in metabolic functions and already documented to be related to fitness parameters in M. edulis [22] (Myrand et al., 2002). Allozyme analyses were carried out on groups of M. edulis, M. trossulus or hybrid after individual species identification following the methods described below. ...
... Temp = temperature; picoCyano = pico-cyanobacteria; nanoCyano = nano-cyanobacteria; picoEuka = pico-eukaryotes; nanoEuka = nano-eukaryotes, totalBact = total heterotrophic bacteria. demonstrated lower basal metabolism and genetic characteristics of the BHA-fall stock could stimulate better energy allocation to growth compared to other local stocks (Tremblay et al., 1998d;Myrand et al., 2009a). In addition, culture sites had a great effect on growth as the offshore site improved by 27% the overall growth rate of the six stocks studied. ...
Bivalve aquaculture in nearshore sites has expanded worldwide. Mussel farming productivity is commonly assumed to depend on survival and growth performance, which can be highly variable among spat stocks originating from different sites, even at a microgeographical scale. Using a traditional two-year production cycle, we monitored the productivity of neighbouring blue mussel (Mytilus edulis) spat stocks that had been cultivated in such contrasted environments that are shallow lagoons and open sea sites. We demonstrated that mussels collected in the nearshore, Bassin du Havre-Aubert (BHA) lagoon, exhibited the best survival rate (≥ 90%) in both nearshore and offshore sites, while other stocks experienced mortality as high as 60% under the same conditions. At spat recovery, the BHA spat stock showed higher gene expression levels for catalase and heat shock protein 70 (HSP70), which are involved in antioxidant defenses and the cellular stress response, respectively. Offshore conditions clearly enhanced overall growth performance independently of stock origin, as shown by the higher condition indexes, higher energy lipids (triacylglycerol), and generally higher glycogen content in mussels at the end of the production cycle. These observations of improved performance at the more favourable open-water conditions, particularly during summer, confirmed the importance of site-related environmental factors for productivity. The observed survival (+ 23%) and growth (+ 27%) trends support the hypothesis that offshore cultivation represents a valuable strategy for mussel aquaculture development. Considering current social, environmental, economic, and resource constraints, offshore mussel cultivation may be a viable alternative strategy for aquaculture development.
... Allozymes chosen to determine levels of multi-locus heterozygosity were polymorphic enzymes implicated in metabolic functions and already demonstrated to be related to fitness parameters in the blue mussel, Mytilus edulis (Myrand et al., 2002(Myrand et al., , 2009). Allozyme analysis was carried out on a horizontal cellulose acetate plate, as described in Tremblay et al. (1998b). ...
In Atlantic Canada, oyster farmers rely on wild oyster spat collections for their main source of seed. In this study, the physiological fitness of oysters originating from four seed producing areas in eastern New Brunswick (NB) was compared. Oysters [50–70 mm shell height (SH)] from Caraquet, Miramichi, Richibucto and Bouctouche, NB were assessed in a laboratory setting using the following parameters: aerobic capacity (metabolic rate), multi-locus heterozygosity (MLH) and stress response (lysosomal destabilization). Field trials were carried out in different environments in the Richibucto estuary with oysters (∼65 mm SH) from Miramichi and Bouctouche. In the laboratory, significant difference in maximum metabolic rates (VO2max) was found for oysters from Caraquet and Bouctouche, compared to those from Richibuctou. The VO2max were 37 % and 45 % lower in Caraquet and Bouctouche oysters, respectively, compared to those from Richibucto. The Miramichi oysters did not differ significantly from the other oyster populations, including Richibucto (P = 0.063). Significant difference in standard metabolic rates (VO2min) also was found for oysters from Bouctouche, compared to Richibucto. However, aerobic capacity (VO2min:VO2max ratio) did not differ among oyster populations, and no significant difference was found in MLH levels or in lysosomal destabilization indices following the thermal stress event. Field trials (August-November 2012) showed low mortality (0–8%) after ∼100 days, regardless of the sampling period—September vs. November or rearing site—downriver vs. upriver and lysosomal destabilisation indices did not differ between Miramichi and Bouctouche oysters. Results suggest that oysters from the major seed producing areas in eastern New Brunswick (Caraquet, Miramichi, Richibuctou and Bouctouche) are similar in physiological performance and quality.
... One such strategy is open ocean aquaculture which represents a promising alternative for the sustainable cultivation of bivalve species (Polk 1996;Hesley 1997;Stickney 1998, Bridger andCosta-Pierce 2003). However, this activity is still in development stages (Hickman 1992), and main mussel culture techniques (i.e., rafts, pole racks and/or longline systems) have been established almost exclusively in protected near-shore waters (Pérez-Camacho et al. 1991;Myrand et al. 2009) or in estuarine habitats (Penney et al. 2002;LeBlanc et al. 2005;Drapeau et al. 2006). Therefore, few studies have reported successful biological and/or engineering results on open ocean mussel aquaculture to date (Langan 2001;Langan and Horton 2003;Buck 2007). ...
Mussel shell biometry, nutritional quality as well as consumer sensory evaluation of experimental open ocean cultured mussel Mytilus galloprovincialis (Lamarck 1819) were analyzed and compared to that of commercial mussels from Galician Rías available in the local market. Both mussel products were of the same commercial size. In this study, open ocean mussels were significantly higher and wider than those of Galician Rías. In addition, with the exception of ash content, both mussel products showed similar biochemical composition. Regarding fatty acid profiles, however, statistical differences were detected. These differences were not fully reflected in the sensory assessment. In terms of consumer acceptability, both mussel products were considered equally satisfactory.
... However, the mid-parent heterosis rates for instantaneous growth rates of shell length and body weight were completely different; the ratio value of mid-parent heterosis in instantaneous growth rate of shell length was negative (−11.04%), whereas that of body weight was positive (4.80%). This situation was also reported in studies on interpopulation hybridizations of shellfish (Mallet and Haley 1984;Cruz and Ibarra 1997;Hedgecock and Davis 2007;Myrand et al. 2009). ...
In the present study, intraspecific crossing experiments between Chinese (H) and Thai populations (T) of Babylonia areolata were conducted by using a complete diallel design (H, T, TH, and HT). A total of 45 pedigreed families were obtained and cultured under the same environmental conditions. Positive heterosis was observed for many developmental parameters and shell sizes for most stages from juvenile to adult. Hatching and metamorphosis occurred significantly earlier in the hybrid (HT) than in the parent groups and a crossed group (TH). Rate of metamorphosis was also higher in HT than in the other three groups. Heterosis in shell length and body weight of the crossed group (HT) from the nursery and grow-out stage to harvest time was high and significantly different from the H, T, and TH groups. For mortality rate from juvenile to adult, total and average heterosis of the crossed group (TH) was positive and significantly different from H, T, and HT. The results of the present study show that intraspecies crosses can be a potentially useful way to increase the economic traits for B. areolata and thus provide a promising method for genetic improvement of the species as a new aquaculture stock in southern China.
... It seems to be well established that increasing heterozygosity causes the reduction of the metabolic costs associated to protein deposition (Bayne and Hawkins, 1997;Hawkins et al., 1986Hawkins et al., , 1989 this being a physiological factor contributing to the maximization of energy budget in fast growing individuals. Although such association between faster growth and reduced rates of protein turnover would be expected to result in reduced basal or maintenance metabolic rates in fast growing specimens, available literature has provided contradictory results: for instances, Hawkins (1995), Hawkins and Day (1996); Garton et al. (1984), Myrand et al. (2009) found significantly lower maintenance metabolic rates in fast as compared to slow growers, however, such a relationship was not found in present study and many others (e.g., Bayne, 2000;Bayne et al., 1999b;Holey and Foltz, 1987;Pace et al., 2006;Tamayo et al., 2011Tamayo et al., , 2013Tamayo et al., , 2014Toro and Vergara, 1998). These studies were performed under different nutritional scenarios that ranged from hatchery conditions (food abundance) to natural conditions (more restrictive in terms of quantity and quality of available food). ...
... Intensive work has analyzed the differences in genotype and/or genetic expression underlying inter-individual differences in physiological performance. an early genetic approach based on multiple locus heterozygosity analysis used by many researchers (Koehn and gaffney 1984;Hawkins et al. 1986;Hawkins 1995;Hedgecock et al. 1995;Bayne and Hawkins 1997;Myrand et al. 2009) recognized an advantageous hybrid vigor: more heterozygous individuals tended to grow faster due to their diminished metabolic demand (garton et al. 1984;Hawkins 1995) which resulted from more efficient protein turnover (Hawkins et al. 1986(Hawkins et al. , 1989aHawkins and Day 1996;Bayne and Hawkins 1997;Morgan et al. 2000). recently, new techniques for massive Dna sequencing have been widely applied to the study of bivalves (Hedgecock et al. 2007;Jenny et al. 2007;Tanguy et al. 2008;Meyer and Manahan 2010;Zhang et al. 2012;Zhao et al. 2012;genard et al. 2013) and can help link physiological performance to quantitative or qualitative differences in genotype or gene transcription. ...
Juvenile oysters (Crassostrea gigas) (produced in November 2009) reared under uniform hatchery conditions for 4 months were selected for extreme growth rate differences by repeatedly taking larger and smaller individuals to achieve weight differences >30 × between fast (F) and slow (S) growers. The physiological basis of differential growth was analyzed in experiments in June 2010, where components of energy gain (clearance and ingestion rates and absorption efficiency), energy loss (metabolic rates) and resulting scope for growth (J h−1) were compared for groups of F and S oysters fed three different ration levels ( ≈0.5, 1.5 and 3.0 mg of total particulate matter L−1). In both F and S oysters, a higher food ration promoted asymptotic increases in energy gain rates through regulatory
adjustments to clearance rates, which maintained similar absorption efficiencies across the food concentrations. No significant differences were found between growth groups in mass-specific physiological rates (i.e., per unit of body mass). However, the scaling of these rates to a common size in both groups using allometric coefficients derived for C. gigas revealed higher energy gain rates coupled with lower metabolic costs of growth in fast growers. Thus, appropriate size-standardization is essential in accounting for observed differences in growth rate. Present results are in accordance with previous reports on other bivalve species on the physiological processes underlying endogenous growth differences, suggesting that the same interpretation can be applied to the extremes of these differences.
... Sekino et al., 2002;Lind et al., 2009;De La Cruz et al., 2010). In mussels, although no artificial fertilization of eggs is carried out (Gosling, 2003), loss of the mean multi-locus heterozygosity has been reported in suspension-cultured populations of M. edulis, in Iles-de-la-Madeleine (Quebec, Canada) (Myrand et al., 2009 and references therein). The phenomenon occurred mostly in the periphery of the sleeves and was attributed to fall-offs of heterozygous individuals. ...
Genetic variation of mussels Mytilus galloprovincialis in central-eastern Mediterranean Sea is investigated in this study. A total of 550 individuals sampled from two cultured and 11 wild populations from Italy, Croatia, Greece and Turkey were genotyped at 10 microsatellite loci. Significant deviations from Hardy-Weinberg expectations were observed in more than 75% of the tests performed. All populations showed extensive heterozygote deficits, which remained at significant levels even after correction for null alleles, providing evidence that null alleles were only partly responsible for deviations from Hardy-Weinberg equilibrium in these molluscs. Moreover, null alleles seem to have limited influence on the population genetic differentiation. Similar levels of multi-locus heterozygosity and allelic richness were observed in all populations, cultured and wild, implying the sustainability of the exploited populations. Lack of isolation by distance and markedly low genetic differentiation between the nine Greek sampling sites (shoreline > 1000 km) was revealed by Mantel tests, F-ST values, exact tests and analyses of molecular variance, indicating that mussels from these regions are either at or close to panmixia. Similarly, patterns of genetic homogeneity were also found between the two Italian samples, whereas the observed genetic differentiation of the populations from Turkey and Croatia probably reflects the specific topographic and oceanographic conditions of these regions.
... However, mussels could be subject to fall-off under turbulent conditions and/or through adjustment of densities (i.e. self-thinning) with time (Fréchette et al. 1996; Myrand et al. 2009a). At sleeving, mesh tubes are filled with juveniles that must then reorganize and attach themselves to the culture substratum (rope or mesh tube) to maintain their position and access to space and food (Mallet and Myrand 1995; Sénéchal et al. 2008). ...
Mussel culture relies on the mussel self-attachment capacity through byssus production. By doing so, no cages or containment devices are needed. It has been previously suggested that thread production requires a non-negligible part of the energy expenditure in blue mussels Mytilus edulis. Therefore our work investigates the relationships between byssal thread production, mussels energetic reserves and phospholipids profiles in the foot. The relationship between thread production and heterozygosity was also examined. The study was realized in a small lagoon of the Îles-de-la-Madeleine, southern Gulf of St. Lawrence, Canada, on four sampling dates between June and September encompassing the pre- to post-spawning periods. Our results show a significant relationship between the thread numbers and attachment strength. However, no correlations were found between energy reserves, phospholipids composition of the foot or heterozygosity level and byssal production. Our results suggest that mussels in suspension culture in this lagoon were not energy-limited so that the energy reserves did not influence the byssal thread production.
... However, mussels could be subject to fall-off under turbulent conditions and/or through adjustment of densities (i.e. self-thinning) with time (Fréchette et al. 1996;Myrand et al. 2009a). At sleeving, mesh tubes are filled with juveniles that must then reorganize and attach themselves to the culture substratum (rope or mesh tube) to maintain their position and access to space and food (Mallet and Myrand 1995;Sénéchal et al. 2008). ...
Mussel culture relies on the mussel self-attachment capacity through byssus production. By doing so, no cages or containment devices are needed. It has been previously suggested that thread production requires a non-negligible part of the energy expenditure in blue mussels Mytilus edulis. Therefore our work investigates the relationships between byssal thread production, mussels energetic reserves and phospholipids profiles in the foot. The relationship between thread production and heterozygosity was also examined. The study was realized in a small lagoon of the Îles-de-la-Madeleine, southern Gulf of St. Lawrence, Canada, on four sampling dates between June and September encompassing the pre- to post-spawning periods. Our results show a significant relationship between the thread numbers and attachment strength. However, no correlations were found between energy reserves, phospholipids composition of the foot or heterozygosity level and byssal production. Our results suggest that mussels in suspension culture in this lagoon were not energy-limited so that the energy reserves did not influence the byssal thread production.
Published information on the spatial organization of mussel culture suspensions (collectors, socks, droppers and poles) is reviewed. Despite the several hundred studies on mussel culture this type of information is very scarce but suggests that mussels self-organize into phyllotatic patterns similar to those widely studied in cylindrical/tubular biological structures and mineral crystals. I demonstrate that space regulated self-thinning (SST) models developed for plane substrates are not applicable to mussel suspensions and propose a new SST model based on the packing geometry of mussel shells around a rope. The robustness of this model is assessed by its application to a wide range of cultured populations. This comparison suggests that the carrying capacity of individual suspensions for commercial-sized mussels is equivalent to a single layer of tightly packed mussels around the rope, independently of the species, type of suspension, presence/absence of pegs, site (growth rate) and stocking density and size. The emergent mechanical properties of these porous, extremely rough and heterogeneous bodies are reviewed. The proposed SST model provides insights into the biomechanical processes of self-thinning and sloughing.
Differential loss of heterozygous individuals that move to the periphery of mussel aggregations, where they are at greater risk for dislodgement, has been proposed as an explanation for observed heterozygote deficiencies in blue mussels. To test the dislodgement hypothesis, correlations between heterozygosity and mussel motility, as well as characteristics of byssogenesis and byssal thread attachment strengths, were determined in a wild and a farmed population of blue mussels (Mytilus edulis) from New Hampshire, USA. Although both populations exhibited a heterozygote deficit as measured by three microsatellite loci, no relationship was found between heterozygosity and increased motility in either population. Similarly, no relationship was found between heterozygosity and byssogenesis or attachment strength. Hence, differential dislodgement is highly unlikely as a possible contributor to the loss of heterozygous individuals.
The present situation and future projections regarding shellfish culture in Chile are analysed. Production has increased exponentially, mainly of the 'Chilean mussel' Mytilus chilensis (Hupe) and the 'northern scallop' Argopecten purpuratus (Lamarck). At present, seven species are cultured, two of which were introduced. The total invertebrate production corresponding to cultures has increased from 1.6% to 38% over the last 15 years. The effects of the following factors on present and future levels of shellfish culture in Chile are discussed: disease, fouling, borers, red tide and environmental variations caused by the cultures themselves. Potential culture of approximately 70 commercially endemic species not only constitutes an economic option, but also favours their conservation. Biographical notes: Daniel A. López holds a PhD in biology. He is a Professor and Ex-Principal of the Universidad de Los Lagos, Osorno, Chile. He is the author of more than 50 publications in ISI journals on ecological aspects of aquatic resources (marine and fresh water), and aquaculture. In addition, he is a member of the national study committee and the postgraduate commission on animal health and production pertaining to the national scientific research commission, maximum organisation for the promotion, management and funding of science in Chile, and a member of the national aquaculture commission.
Mussels from a natural population of Mytilus edulis, which had settled onto ropes, were collected from Killary Harbour on the west coast of Ireland in November 1986 when they were approximately 5 mo old. Shell lengths in the cohort ranged between 13 and 33 mm and did not differ significantly from a normal distribution. Heterozygosity at five polymorphic loci (Gpi, Pgm, Lap, Odh, and Est-D) was determined for each measured individual. Highly heterozygous individuals did not achieve higher and more uniform growth rates than homozygous individuals of the same cohort. When the results for each locus were considered individually, there was a significant positive correlation between heterozygosity and growth rate for the Odh locus. Significant departures from Hardy-Weinberg expectations were observed at the Pgm, Lap and Odh loci, and the magnititude of the deficiency was marginally larger among smaller, slower growing mussels of the cohort. Overall, these results contrast with those observed by other investigators for a single cohort of M. edulis sampled from Long Island Sound (USA).
We present a method for estimating optimal stocking density (OSD), via the analysis of the relationship between yield (biomass, B) and population density (N) at harvest, using a B-N diagram (BND). This differs from the usual approach in aquaculture, in which yield is expressed as a function of initial population density, and B and N are analysed separately. Both methods allow estimation of OSD. In addition to OSD, however, the BND potentially allows: (1) assessment of the relative importance of competition-dependent and competition-independent mortality factors; (2) estimation of approximate OSD and maximum yield by extrapolation of results from short-term experiments; and (3) indentification of the nature of the factor regulating competition-dependent mortality.
Developmental homeostasis, measured as either fluctuating asymmetry or variance of morphological characters,increases with enzyme heterozygosity in many, but not all, natural populations. These results have been reported for Drosophila, monarch butterflies, honeybees, blue mussels, side-blotched lizards, killifish, salmonid fishes, guppies, Sonoran topminnows, herring, rufous-collared sparrows, house sparrows, brown hares, white-tailed deer, and humans. Because heterozygosity at a few loci can not predict heterozygosity of the entiry genome, these loci must be detecting localized zones that influence the developmental environment.
Houle (1994) showed that marker-associated heterosis due to general inbreeding depression could not be distinguished from direct overdominance at the marker locus by examining mean genotypic fitnesses, in the one-locus case. Indeed, both hypotheses equally fit the same regression model, referred to as the 'adaptive distance model' (Smouse 1986). I here extend the analysis to several loci and to the relationship between marker genotype and variance in fitness. Several predictions differ between the overdominance and inbreeding hypotheses: (1) all locus-specific effects are equal under inbreeding, whereas they are not under overdominance; (2) the adaptive distance model has an increasingly low fit when the number of loci increases, under inbreeding, whereas it always explains the whole variance in fitness under overdominance; (3) a negative relationship is predicted between mean fitness and the variance in fitness, under inbreeding, which is not predicted under overdominance. Some statistical tests are derived from these predictions, that help to identify the genetic basis of heterosis. Simulations show that the power of these tests allows their application to real datasets.
Fitness consequences of interrelations between genotype, protein metabolism and physiology were examined in 88 individual Mytilus edulis exposed to 150 pbb dissolved copper until death. Electromorphs heterozygous for the enzyme phosphoglucose isomerase (Pgi) survived longest, complementing previous findings, and suggesting an adaptive nature of specific Pgi genotypes. Time to death varied between 4 and 28 days, and did not correlate with tissue concentrations of copper at death. Instead, multiple regressions confirm separate positive effects both of filtration rate and protein turnover upon rate of copper accumulation, as well as separate negative effects of these same two processes upon time to death. Rate of filtration was the primary physiological determinant of survival; we discuss underlying relations whereby the faster breakdown and renewal of proteins also accompanied reduced survival during stress induced by exposure to high levels of dissolved copper.
Genotype-dependent changes in whole-body protein turnover are shown to
be of significance to metabolic efficiency, energetic maintenance
requirements and associated rates of dietary absorption in Mytilus
edulis. Faster growth rates were, on average, evident in more
heterozygous mussels. Such variation did not stem from different
efficiencies of nitrogen conversion, which remained high and independent
of either genotype or age. Rather, faster growth derived from decreased
energy requirements for maintenance. This decrease was associated with
greater efficiencies of protein synthesis [(net protein balance/gross
protein synthesis) × 100] and lower intensities of nitrogen
metabolism. Energy `saved' by virtue of higher metabolic efficiency was
used to effect increases of both ingestion and absorption efficiency.
Alternatively, reduced synthetic efficiencies among more homozygous
mussels necessitated elevated protein synthesis per unit deposition,
high efficiencies of nitrogen conversion being sustained by a greater
proportional recycling of breakdown products. Energetic costs were thus
enhanced, with a consequent reduction in growth. Efficiencies of protein
synthesis declined with age, whereas maintenance requirements grew.
These findings are discussed in relation to individual fitness, and
proposed as a mechanism by which genotype may mediate intraspecific
variations of growth among animals generally.
Two samples of one-year-old American oysters (Crassostrea virginica Gmelin) obtained from the same inlet two years apart and grown under identical conditions were analyzed for their weight distribution and electrophoretic profiles at several enzyme loci. Although growth in the two years was significantly different, the electrophoretic profiles of the two populations were remarkably similar. On both occasions heterozygosity was positively correlated with mean weight and negatively correlated with the coefficient of variation. In general there was a deficiency of heterozygotes. This deficiency was more pronounced in the slow-growers than in the fast-growers. These observations suggest that growth rate (a limiting factor in current oyster aquaculture) can be improved by genetic manipulation. Use of large scale electrophoretic survey in providing guidelines for genetic improvement are discussed.
A large population of oysters was established by sampling swimming larvae, 18 to 25 days old. When the population was one year old, the 200 largest and 200 smallest individuals were removed and assayed electrophoretically for five enzyme loci. The following results were obtained: The three genotypes at one locus (Got-1) had different average weights. There were no significant deviations from Hardy-Weinberg genotypic equilibrium for this locus. Homozygosity excess was observed at the other four loci (Lap-2, Pgi, Pgm, Est-3). It was further observed that classes of higher weight had higher average heterozygosities, and that individual heterozygosity was positively correlated with body weight. It was concluded that heterozygosity and growth rate are positively correlated but that this correlation is not necessarily a causal one.
The genetic characteristics of wild and suspension-cultured blue mussels from three lagoons of the Magdalen Islands were determined from the study of seven enzyme loci. Only 3.5% of mussels scored at the MPI* locus were identified as Mytilus trossulus. For M. edulis, no major differences could be detected in allelic frequencies of wild mussels within and among the lagoons, indicating that gene flow around the Magdalen Islands is high. High levels of gene flow are also inferred from the high Fst and Nem values. Temporal stability is suggested by homogeneity in allelic frequencies of mussels from different size classes (assumed to represent different cohorts) sampled within each lagoon. An important heterozygote deficiency was observed in wild mussels from the House Harbour and the Great Entry lagoons where the mean Ho was 0.34 and 0.32, respectively. In contrast, there were no significant heterozygote deficiencies or excesses at most loci in wild mussels from Amherst Basin whose Ho = 0.50. The degree of heterozygosity was significantly higher in the wild mussels from Amherst Basin than in those from the other two lagoons. Significant decreases in the degree of heterozygosity were observed in suspension-cultured mussels 1 and 2 y after their transfer as spat from Amherst Basin to grow-out sites in the Great Entry and the House Harbour lagoons. One possible explanation for the observed decreases in the degree of heterozygosity is that a substantial number of heterozygotes collected in Amherst Basin may be lost through fall-off during suspension-culture activities, as suggested by the lower mean heterozygosities of sleeved mussels at both grow-out sites (mean Ho = 0.35-0.38).
Previous studies have suggested that a large proportion of the more heterozygous mussels are lost while in
suspension culture. Such a reduction in multi-locus heterozygosity (MLH) could impact mussel production
because more heterozygous individuals have better growth and survival than more homozygous ones. The
present study followed the changes in MLH over a year in two different cohorts and found a reduction in MLH
during the culture cycle for both cohorts. This reduction did not occur during the early phases of the culture
operations, such as sleeving (= socking), or during the first weeks in suspension culture, but took place
during the following growing season starting in early summer for both cohorts. The MLH decrease was not
associated with different environmental conditions at the growing site. It is hypothesized that mussels
characterized by high MLH are more mobile than those with lower MLH and migrate out of the mesh tubes
faster after sleeving to get access to better growing conditions (space and food). However, being at the
periphery of the sleeves, these more heterozygous individuals are more vulnerable to fall-offs due to
turbulence and/or self-thinning. We suggest that the MLH decrease on the sleeves is not caused by the
selective mortality of more heterozygous individuals but rather by higher losses through fall-offs.
Mussels secrete byssal threads regularly to the substratum, and the strength of these threads can fluctuate with time. The present study examined weekly variations in the attachment strength of 2 yr old cultured mussels Mytilus edulis on submerged longlines in a semi-enclosed lagoon from late May to mid-October. Some possible factors influencing attachment strength were investigated: environmental factors (temperature, food availability, wind velocity and hydrodynamic conditions such as current velocity, turbulence and wave height) and reproductive condition were measured concurrently. Attachment strength was measured directly on cultured mussels using a dynamometer. Attachment strength varied 2-fold from summer to fall, a difference related not only to the number of byssal threads but also to their individual strengths. The hierarchical influence of each factor on attachment strength is discussed. Our results suggest that spawning seemed to be correlated with an important decrease (–32%) in attachment strength, and that water temperature (negative relationship) and turbulence (positive relationship) were the most important factors explaining the variation in mussel attachment strength. In contrast to previous studies carried out in intertidal zones, no trade-offs were observed between reproduction and attachment strength.
Suspended mussel culture entails loading high densities of juvenile mussels into mesh socks, and hanging them from floating longlines, often resulting in intraspecific crowding, reduced growth, and mussel yield. Despite this potential bottleneck in culture, there are few data on behavioral mechanisms that regulate juvenile density and growth rate. A field experiment was conducted with culture socks to examine the effects of stocking density (High 800 mussels/30.5 cm; Low 400 mussels/30.5 cm), blue mussel species (Mytilus edulis and M. trossulus) and environment on early development of the culture population. In situ photography and direct sampling were used to generate time series of mussel size, valve gape, siphon area, shell orientation, and emergence in experimental socks at a commercial farm in Ship Harbour, Nova Scotia, Canada. Moored CTD-current meters and water sampling were used to characterize the sites. Emergence from the culture socks required 1–2 mo, with faster initial emergence in M. trossulus. High densities generally did not affect mussel emergence rate or orientation, but higher current speeds produced a negative effect on M. edulis emergence rate. At the experimental densities used, emergence was complete and few mussels remained inside the culture sock. Nonetheless, interior mussels showed mechanical stress such as shell distortion and reduced growth compared with emerged individuals. Mussels exhibited largely horizontal orientation presumably in response to horizontal currents. Siphon area displayed a unimodal response with an optimum at the middle of the current speed range. No density-dependent effect on valve gape was detected, but a significant interaction between siphon area and stocking density suggests that optimal hydrodynamic response is sensitive to crowding. Growth of shell and tissue showed little negative density-dependence. Mussels are proposed to alleviate crowding via behavioral sorting. Although there is concern that growth studies of cultured mussels are often deficient because they use cages or other containers rather than socks, this study suggests that conditions of the sock environment such as crowding may be less important than macroenvironmental factors (e.g., current speed) in determining growth.
We examine several models that may account for the observation that in populations of marine molluscs in general, and of the American oyster (Crassostrea virginica) in particular, the growth of an individual is related to its degree of heterozygosity and, also, that the number of heterozygous individuals in the population is less than expected on the assumption of random mating and no selection. We classify these models into nonselective, selective, and mixed models. We conclude that mixed models are the most likely to apply to real populations, but cannot exclude selective models. Nonselective models appear least likely. Current evidence favors a model that assumes that heterozygotes enjoy a fitness advantage as adults, primarily because of their faster growth, and that the lower numbers of heterozygotes in the population result from some form of nonrandom fertilization. One possible source of nonrandom fertilization is variation in the time of spawning of individuals due to differences in body size.
The study of metabolism has traditionally focused upon factors that influence metabolic rate, at levels of both the metabolic pathway and the whole organism. This paper focuses on the cost, and thereby the efficiency, of metabolic processes. The genotype-dependent cost of enzyme turnover is proposed as a biochemical genetic mechanism for relating genetic variation at single genes to phenotypic variation in quantitative traits of energy metabolism. Decreased costs of maintenance metabolism can accompany artificial selection for increased production (e.g. growth, reproduction, etc.) and lower maintenance is correlated with multiple locus heterozygosity in outbred populations. In both cases, high production has been associated with lower rates of protein turnover. Several factors influence the ATP-equivalent cost of enzyme turnover. These factors are used to calculate the cost of turnover for a single enzyme. This cost can conservatively constitute up to several percent of the total daily mass-specific energy demands of maintenance metabolism. Genetic variants of an enzyme can differ in the cost of turnover. These differences can constitute the basis for metabolic changes associated with artificial selection for production and the metabolic differences that are associated with individual levels of heterozygosity. The metabolic and evolutionary significance of genotype-dependent turnover costs is a function of individual energy balance. The strength of selection against increases in cost will be an inverse function of individual energy balance and is therefore influenced by both environmental and genetic factors.
During the spring of 1986, a wild cohort of young Mytilus edulis L. (2–6 mm) was collected from the low shore near Beaumaris, North Wales, U.K., and was divided into two groups which were ongrown for several months at different densities in through-flow containers. After ongrowing, samples of mussels were subjected to electrophoresis at up to eight enzyme loci and their shell length was measured. A significant positive correlation between the number of heterozygous loci and the growth rate of individuals was observed in the M. edulis reared at high density. No such significant correlation was found in the M. edulis grown at low density. It is proposed that the density-related stress experienced by the M. edulis reared at high density accentuates an underlying relationship between heterozygosity and growth rate.
Growth comparisons were made involving mussels (Mytilus spp.) collected from five different localities in Britain in 1980–1981. Two of the localities, Mumbles, South Wales, and Bude, Southwest England, have pure populations of M. edulis and M. galloprovincialis, respectively. The other three, Whitsand Bay, Southwest England, Croyde Bay, Southwest England and Robin Hood's Bay, Northeast England, have hybrid populations with both M. edulis and M. galloprovincialis ancestry. To make growth comparisons, mussels from different populations were mixed in oyster nets and transplanted to three different localities. After periods of transplantation varying between several months and one year, growth was assessed by measuring increase in shell length or dry body weight. Starch-gel electrophoresis was used to assay variation in the transplanted mussels at three allozyme loci partially diagnostic for M. edulis and M. galloprovincialis. The results provide evidence of small growth differences between populations and between allozyme genotypes within populations. These differences accounted for no more than a few percent of the total variation in growth between mussels. Statistically significant results were obtained, but were frequently found not to be reproducible. There is no clear evidence of a growth difference between M. edulis and M. galloprovincialis. Allozyme heterozygotes appear to have growth rates intermediate between allozyme homozygotes; this study thus fails to provide evidence for overdominance with respect to growth rate.
Developmental homeostasis, measured as either fluctuating asymmetry or variance of morphological characters, increases with enzyme heterozygosity in many, but not all, natural populations. These results have been reported forDrosophila, monarch butterflies, honeybees, blue mussels, side-blotched lizards, killifish, salmonid fishes, guppies, Sonoran topminnows, herring, rufous-collared sparrows, house sparrows, brown hares, white-tailed deer, and humans. Because heterozygosity at a few loci can not predict heterozygosity of the entiry genome, these loci must be detecting localized zones that influence the developmental environment.
Studies of malate dehydrogenase in honeybees,Apis mellifera, and lactate dehydrogenase in killifish,Fundulus heteroclitus, revealed that developmental homeostasis varied with heterozygosity of individual loci. Heterozygotes differed from homozygotes in fluctuating asymmetry, morphological variance, and in correlations between morphological characters.
The protein loci in these studies code for enzymes, and therefore do not directly influence morphological characters. However, some enzymatic loci substantially influence metabolism, and contribute to variation in the amount of energy available for development and growth. This argument can be made most convincingly for the LDH polymorphism in killifish. LDH genotypes differ in enzyme kinetic properties that measure differences in physiological efficiency, and these differences produce measurable and predictable differences in physiology and development. Under environmental conditions which impose a stress upon development, genotypes at these loci may have different amounts of energy available for development, and consequently exhibit different levels of developmental homeostasis.
A single population of the hard clam Mercenaria mercenaria was examined for repeatability of correlations between multilocus heterozygosity and fitness-related traits. Clams were collected in June 1986. October 1987, and June 1988 from a site at Shark River Inlet, New Jersey. A large sample taken in 1988 gave the best representation of all age classes, particularly new recruits. Genetic variation was estimated using nine protein-encoding gene loci. All three samples were deficient in heterozygotes; however, deficiencies were heterogeneous across loci, providing no evidence for systematic inbreeding. Although multilocus heterozygosity was not associated with growth rate in any of the three samples, the large 1988 sample revealed a significant association with age. A resampling analysis of the 1988 data set revealed that detection of weak heterozygosity-age correlations was profoundly influenced by sample size and dependent upon adequate representation of young age classes in the population.
Our study examined whether the differences in susceptibility to summer mass mortality of two stocks of mussels from the Magdalen Islands (Québec, Canada) are related to bioenergetic and/or genetic factors. The relative importance of maintenance and maximal metabolic rates, scope for growth (SFG) and the O:N ratio were followed over time to assess whether the increased incidence of mortality in late summer reflects a decrease in bioenergetic status at this period. The stock of mussels which was more susceptible to summer mortality had higher values of VO2. Furthermore this stock had a more negative scope for growth and lower O/N ratio in early August. These parameters are likely to reflect unfavourable environmental conditions, which led the mussels to rely upon protein catabolism. We also observed a negative correlation between multiple-locus heterozygosity and standard VO2. The more susceptible stock of mussels had a lower degree of multiple-locus heterozygosity. Thus, we suggest that the periodic, but irregular, outbreaks of summer mortality are the result of a synergistic interaction involving dietary deficiencies, temperature, a possible post-spawning stress and the genetic characteristics of the stock. The higher metabolic demand associated with a reduced degree of heterozygosity will impose a supplementary stress and render such stocks more vulnerable to summer mortality. The results are in agreement with the hypothesis that high levels of heterozygosity are related with lower costs of maintenance.
Consistent patterns of genetic variation in the marine bivalve Macoma balthica (L.) were found after exposure to low levels of copper, starvation, and along geographic clines. The geographic clines were related to temperature and salinity. Genetic differences were primarily found in the LAP (Leucine aminopeptidase) locus; under stress the frequency of one specific allele, and thereby the heterozygosity, decreased strongly. The degree of changes depended on age and condition of the animals; adult animals showing stronger changes than juveniles. Low concentrations of copper yielded stronger genetic changes than high concentrations. It is concluded that genetic diversity and stress-sensitivity of species change predictably along a gradient in response to environmental harshness. Animals near the southern limit of their range were more sensitive to additional stress.
The locus-specific effects of heterozygosity upon individual growth rate were determined for 15 polymorphic enzymes among 1906 individuals from a single cohort sample of the marine bivalve Mulinia lateralis. Two measures of individual growth rate (total wet weight and shell length) were made at collection and after a period of growth in the laboratory. The correlation between heterozygosity and growth rate was independently determined for each locus using multiple linear regression, thereby providing a rank of individual locus effects; these differed significantly. The four estimated rankings of relative locus effects (initial length, initial weight, length added in the laboratory, and added weight) were not statistically different. That is, a locus with a large effect of heterozygosity on growth rate in nature had a similarly large effect on laboratory growth rate. The effect of a locus was not related to heterozygosity per se; some highly heterozygous loci had no detectable correlation with growth rate. The data contained two pairs of relatively tightly linked loci; in both cases one locus of a pair had significant effects on growth rate, while the other had no effect. Loci with large and significant correlations with growth rate synthesize enzymes which function in protein catabolism or glycolysis; heterozygosity in enzymes of the pentose shunt, redox balance, or other miscellaneous metabolic roles was not correlated with growth rate. Since the metabolic basis for the correlation is known to derive from individual differences in net energy status, particularly energetic costs of whole-body protein turnover, these data indicate that phenotypic effects (e.g., variation in growth rate) are determined by heterozygosity at the studied genes, not other linked loci.
We have used the polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) techniques to design two genetic markers for blue mussels in the Mytilus edulis species complex. Both of these markers target the gene encoding the mussel polyphenolic adhesive protein. The first marker, Glu-5', is highly differentiated among and can be used to identify the three blue mussel species, M. edulis, M. galloprovincialis and M. trossulus. The second marker, Glu-3', can identify M. edulis and M. galloprovincialis. Using these markers we have demonstrated that hybrid mussels from Whitsand Bay, UK carry alleles for this gene that are the products of intragenic recombination. The high frequency (10 per cent) of these recombinant alleles within the hybrid population suggests that recombination is fairly frequent within this gene or that hybridization between M. edulis and M. galloprovincialis is substantial and has been occurring over considerable evolutionary time. The two novel genetic markers, Glu-5' and Glu-3' will be invaluable in additional studies regarding the importance of hybridization among blue mussels.
Three mussel groups differing in mean multilocus heterozygosity (MLH) were used to examine the MLH-fitness relationship. Mussels were submitted to aerobic and anaerobic stressful conditions in the laboratory, and their LT(50) was measured. Mortality was not random in two of the three groups and affected the homozygous individuals more. This selective mortality caused a significant increase in the mean MLH of the survivors, but only for the two groups characterized by the lowest initial MLH and significant deficits in heterozygotes at the onset of the experiments. While these experiments were ongoing, the same two groups also suffered a 40% mortality rate in lantern nets under field conditions. This mortality also increased the mean MLH in survivors. All groups showed strong inverse relationships between MLH and standard metabolism. Our results suggest that the higher resistance of more heterozygous individuals is related to their lower metabolic needs.
Evolutionary relationships among fitness traits are considered in terms of the near-to-universal scenario of stressful environments leading to a resource-deficient and hence energy-deficient world. Fitness approximates to energetic (and metabolic) efficiency under this environmental model. When fitness is high, stress resistance (reducible to oxidative-stress resistance) and metabolic stability are maximal, and energy expenditure is minimal. Rapid development should then be favored followed by a long lifespan and high adult survival. Positive associations among diverse fitness or life-history traits are expected, controlled by stress-resistant 'good genotypes'. Heterozygotes tend to show higher energetic efficiency and hence higher fitness than do corresponding homozygotes under extreme environments, and to give parallel associations among life-history traits. Energy budgets under abiotic environments are pivotal for integrative evolutionary studies of life histories in natural populations.
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