AquaBreeding Project title: “Towards enhanced and sustainable use of genetics and breeding in the European aquaculture industry”




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Review on Breeding and Reproduction of European aquaculture species

Pacific oyster (Crassostrea gigas)



Pierre Boudry (Ifremer, France)


January 2008

1. General information on production and breeding

History of the domestication

Originating from the north eastern Asia, Crassostrea gigas is endemic to Japan. The species has now been introduced, mainly for aquaculture purposes, to a number of countries throughout the world [1]. It was introduced into France in the early 1970s following mass mortality of the Portuguese oyster Crassostrea angulata [2]. These two taxa are genetically close and their relative taxonomic status is still debated [3]. In European waters, C. gigas is cultured from Norway in the north to Portugal in the south, as well as in the Mediterranean Sea. Its biological characteristics make it suitable for a wide range of environmental conditions, although it is usually found in coastal and estuarine areas within its native range.


Production (Figure 1)

Oyster fisheries (i.e. exploitation of naturally occurring populations by gathering) were never very relevant to Crassostrea gigas since its introduction into Europe, and only account for the production of only a few tonnes/year. Aquaculture, on the other hand, currently provides most of the oysters marketed and should allow long term productivity of near shore marine and estuarine habitats. While farmers mainly grow seed collected from the wild, hatcheries provide a reliable supply of seed [4] and have allowed an increasing production of genetically improved oysters, through polyploidy and selective breeding (see sections below).



Figure 1: Distribution area of C. gigas in Europe.

According to the FAO, the Pacific oyster is grown in 27 countries and is the most highly produced mollusc species in the world. In 2004, China was the world leader with 3.75 million tonnes out of a total world production of 4.6 (i.e. 81% of world production). However, it is likely that other species of the genus Crassostrea were included in these Chinese C. gigas production statistics. European production is now around 120,000 tonnes/year, with France, Ireland, Spain, Ireland and the UK all major producers.


Biological features of interest for breeding practices

  • Main commercial data on weight and age at harvest and slaughtering performances

Due to the large spread of their cultivation worldwide, oysters are cultured using a broad variety of rearing techniques, from fully extensive to semi-intensive. Intensive culture is restricted to early stages (i.e. larvae in hatcheries and seed in nurseries) because large scale production of their food (i.e. phytoplankton) is not cost-effective at later stages. After a few months of rearing in open waters, wild oyster spat is either: removed from the spat collectors to be deployed on culture grounds (on bottom), on sticks, or into oyster bags on trestles, baskets, or suspensions; or else left on the collectors for pre-growing and therefore require thinning out or density decrease. Faster growth of hatchery seed in nurseries relative to wild seed often allows the production cycle to be shortened. Oyster density-stocking biomass is adapted to local carrying capacity and mesh size usually adapted to oyster size to maximize food availability, with the overall objective of reducing the length of the rearing cycle. Usually, oysters reaching market size (50-100g) are sorted, graded and stored in clean water before marketing, to remove mud and grit and operate a slight depuration. Maturation may represent a problem for meat quality during the reproductive season in summer due to a lowered quality of mature individuals and low meat content after spawning.


  • Generation interval for males and females

Under good growing conditions, oysters can produce gametes after just a few months so that a one-year generation time is feasible. However, generation time is usually 2-3 years to match age at market size.
Main diseases

Pathogens of oysters are known to include viruses, bacteria, fungi and protozoa. Some metazoans such as helminths, annelids and crustacea are considered to be only slightly or possibly even not pathogenic. The mass mortalities of C. angulata were associated with two irido-like viruses between the late 1960s and 1970s. More recently, high mortalities caused by Ostreid herpes virus 1 (OsHV-1) were also observed [5]. This virus affects larvae and juveniles of different bivalve species. Several diseases caused by bacteria have been described, mostly due to bacteria from the genera Vibrio, Pseudomonas, Aeromonas, Nocardia and Ricketssia. Bacteria that provoke high mortalities generally belong to the genus Vibrio, and mostly affect larval cultures and spat [6].




2. Genetic variability of the species

Wild genetic resources available

Pacific oyster population structure is strongly affected by stock transfers and introductions related to aquaculture. In combination with the large natural dispersal potential of planktonic larvae and large effective population sizes, this leads to very low population differentiation commonly observed over very large areas. Such results have been reported by most studies based on “neutral” markers such as allozymes [7], mtDNA or microsatellites [8]. A common garden experiment comparing progenies of French and Japanese broodstock suggested that the observed differences might be attributable to local adaptation of French stock since their introduction [9]. Polymorphism of presumed selected genes has also been proposed as an alternative method to investigate local adaptation to specific selective pressures such as pollutants [10].


Differences between wild and/or domesticated populations

Most cultured populations are based on collection of wild spat or wild broodstock, resulting to little or no genetic differentiation between wild and farmed populations. The European Research Training Network on Fisheries-induced Adaptive Changes in Exploited Stocks (www.iiasa.ac.at/Research/EEP/FishACE) is investigating the evolutionary consequences of exploitation-induced adaptive changes in French C. gigas populations. This study is based on the analyses of time series, so as to separate environmental and genetic effects on fitness-related traits.

Significant reduction of genetic variability, due to a limited number of broodstock individuals and/or high variance in individual reproductive success, is known to cause strong bottlenecks in hatchery-propagated stocks [11]. This is also the case in French hatcheries producing C. gigas seed [12].
Interaction between wild and domesticated stocks

Compared with fish species, very little is know about interactions between farmed and wild Pacific oyster populations. This is mainly because most farmed populations are not yet domesticated or selected. One of the concerns regarding the genetic impact of farmed oysters on natural populations is due to differences in effective population size of hatchery propagated stocks relative to wild/introduced populations. However, a negative impact remains to be demonstrated [13]. These questions are more relevant when dealing with the restoration of oyster reefs of naïve species using hatchery-propagated stocks, e.g. for the American oyster, Crassostrea virginica on the US east coast, or for the European flat oyster, Ostrea edulis.

A putative negative impact of farming triploid oysters is related to their partial fertility. Triploidy is not considered as a safe genetic confinement tool for oysters because some triploids can in fact breed. The potential impact of their incomplete sterility on wild populations is questionable as their progeny has been shown to be either diploid or aneuploid [14]. Another risk is the potential impact of escaped tetraploid broodstock, because these are fully fertile. The fate of tetraploids in the wild (i.e. their fitness relative to diploids and the impact of their breeding with diploids) is of concern in Europe where tetraploid broodstock is presently confined to prevent their release into the wild.

Inbreeding effects

In C. gigas, inbreeding depression is relatively well documented [15], notably due to the ease of comparing inbred versus outbred progenies and the relatively small generation time. Inbreeding depression is due to unmasking of recessive deleterious alleles (i.e. genetic load). Very high levels of genetic load, revealed by high and frequent distortion of marker segregation ratios, have been reported in oysters [16]. Culling, a common practice in Pacific oyster hatcheries and nurseries, can contribute to the removal of heterozygous genotypes that might otherwise offer better growth and survival characteristics [17].



3. Reproduction

Fecundity and main reproductive features

Gametogenesis is mainly driven by temperature [18]. It occurs once or twice a year in the wild according to environmental conditions. Conditioning under controlled hatchery conditions can be achieved throughout the year [18]. Spawning is induced by thermal (heat or cold shocks) or chemical (e.g., ammonium hydroxide, potassium chloride) stimuli. Strip-spawning is a common practice in cupped oysters. Fecundity is frequently very high and mature females often produce more than 50 million eggs [19].


Cryopreservation

Pacific oyster semen is characterized by a long duration of sperm motility (> 24 h). Different semen cryopreservation protocols have proved to be efficient [20]. Special attention is being paid to sperm of tetraploid broodstock [21]. Although results are often highly variable, due to variable gamete quality and/or sperm agglutination that hamper standardization [22], sperm freezing has become “routine” and research is now focusing on oocyte and larval freezing [23]. Some attempts have been made to constitute semen cryobanks of selected oyster lines, but only a limited quantity of semen is presently available. The use of cryopreserved gametes remains very limited for C. gigas.


Genetic and environmental sex determination, sexual dimorphism

Pacific oysters are alternate hermaphrodites. Synchronous hermaphrodites only occur extremely rarely and selfing is therefore likely to be very low. Sexual dimorphism is limited, although larger individuals are more frequently female than male. The endocrine and environmental factors influencing sex control are poorly known. Genetic determinants of protandric sex were investigated by [24].



4. Selection

Genetic variability per trait

Heritability estimates have been made for many traits in C. gigas. Estimates for larval and early stage traits are relatively low [25]. These traits are often regarding as being of limited aquacultural interest and genetic correlations between larval and post-larval traits are usually low. Realized heritability for yield (i.e. growth x survival) was estimated for several cohorts tested in different sites [26], and ranged from 0.01 ± 0.05 to 0.52 ± 0.16. More recently, Ward et al. (2005) provided estimates of heritabilities and genetic correlations for growth-related traits, ornamentation and shell colour, morphology and shape. The estimates of genetic parameters show a medium to high additive genetic basis for the studied traits, and no dominance has been observed for growth or the morphological parameters examined.

In France, particular study has been made of the heritability of resistance of spat to summer mortality. High heritability estimates (0.83 ± 0.40) [27] were confirmed by the response of this trait to divergent selection (Dégremont et al. in prep). Interestingly, broad-sense [28] and narrow-sense [27] heritability for growth are lower than for survival [29].

Other traits for which heritability estimates are available include plasticity of reproductive effort (between high and low food rations) [29] and heavy metal content [30].


Genetic correlations and undesirable side effects

Genetic correlations between survival, growth and reproductive effort were investigated by [29]. They reported a high plasticity of genetic correlations between reproductive effort and both survival and growth, underlining the importance of environmental conditions when estimating genetic correlations in C. gigas. Although no clear negative genetic correlations were observed between growth and survival at the spat stage [27], significant negative coheritability estimates were observed between adult mid-parent body weight and offspring survival [31].


G*E interactions

The magnitude of genotype x environment interactions has been examined in several studies [28, 32] and is limited in most cases. However, as mentioned above, environmental influences contribute to the high plasticity of genetic correlations estimated between reproductive effort and both survival and growth [29].


Genetic responses, progresses and control lines

To date, family-based selective breeding programs have been established in U.S.A. (Molluscan broodstock Program: http://hmsc.oregonstate.edu/projects/mbp/), Australia (Australian Seafood Industries: Thoroughbred oysters) and New Zealand (Cawthron Institute: http://www.cawthron.org.nz/aquaculture/selective-breeding.html), mainly to improve growth, yield and shell shape in C. gigas.


Dominance and intraspecific crossing

The use of non additive variance in breeding programs is being investigated in C. gigas [33]. Substantial evidence for the role of heterosis in yield of seed and adult oysters suggests that crossbreeding could make a useful contribution to the commercial improvement of C. gigas [34].



5. Polyploidisation and monosexing and hybrids

Triploid induction and performances

Following early studies on triploidy induction in the bivalves [35], optimum treatments (Cytochalasin-B or 6-DMAP) for the production of triploids were developed [36]. Growth performance of triploids is comparable or higher (in general by 20-30%) than that of diploids, whereas some qualitative traits can be superior (e.g. meat quality). Today, most triploid production is based on crosses between tetraploid and diploid genitors [37]. These so called “natural” triploids show superior performances relative to “chemically-induced” triploids (Eudeline et al. in prep).


Tetraploid induction and performances

Viable and fertile tetraploid oysters can be obtained by polar body 1 retention on gametes from triploid females [38, 39]. Such tetraploids can then be crossed with one another to increase tetraploid broodstocks. Genetic improvement of tetraploids is now being considered. It could notably be achieved by backcrossing improved diploid lines with tetraploids using the technique proposed by [40].


Gynogenesis, androgenesis and mitotic clone performances

Although gynogenesis can be successfully induced in C. gigas [41], gynogenetic oysters still remain to be phenotypically characterised and used in experimental or applied genetics. Androgenesis has also been investigated [42], though with limited survival success for the moment. Highly homozygous lines are sought for research, but the high genetic load in oysters makes this objective very challenging.


Interspecific hybridisation

Hybridizations between closely related Crassostrea taxa (e.g. C. gigas x C. angulata; C. gigas x C. sikamea) can be performed successfully, but have not led to any applications. Transfer of disease resistance between more distant taxa (e.g. C. gigas x C. virginica) is one of the main goals of such interspecific hybridization [43]. The combination of triploid induction and hybridization has also been proposed. Genetic markers are then essential to verify the hybrid status of progenies, as contamination is difficult to avoid in such crosses.



6. Genomics

The haploid genome of Pacific oyster consists of 10 chromosomes, weighs ~0.89 pg and contains 824 Mb. The first consensus linkage maps were constructed using 11-day-old larvae from three families and a total of 102 microsatellite DNA markers [44]. The microsatellites are distributed among 11-12 linkage groups, covering 616.1 and 770.5 cM for the male and female maps respectively. This young material was chosen because high segregation distortion is seen at later stages [16]. An AFLP map was also established [45]. The female framework map consisted of 119 markers in 11 linkage groups, spanning 1030.7 cM, with an average interval of 9.5 cM per marker. The male map contained 96 markers in 10 linkage groups, covering 758.4 cM, with 8.8 cM per marker. SNP markers have recently been developed and mapped (Sauvage et al., in prep). A first BAC library, containing a total of 73,728 clones with an average insert size of 152 kb and representing an 11.8-fold genome coverage, was made using an outbred individual [46]. A physical map based on BAC fingerprinting has now been constructed using a second BAC library of an inbred oyster (Gaffney, pers. com.). EST resources exceeding 50,000 sequences have been generated. Functional genomic analysis in relation to the reproductive biology, immunology, defence mechanisms and stress physiology is in progress. Following an initial phase targeting candidate genes (e.g. the amylase gene in relation to growth [47], the techniques of suppression subtractive hybridization (e.g. [48, 49]), microarrays [50], MPSS [51] and SAGE (Bachère, pers. com.) are now being used. These genomic resources will increasingly contribute to the genetic improvement of C. gigas in coming years [52].


Tools to evaluate population genetic variability

Analysis of the genetic structure of wild and farmed Pacific oyster populations previously relied on allozymes [53, 54], mtDNA [55] and microsatellites [8]. Today, with the development of oyster genomic resources, a large panel of markers is available [52].


Genetic markers for genealogical traceability

Microsatellite markers in C. gigas were first developed in the late 1990s [56] and their use for parentage assignment was illustrated by [57]. This approach allows breeding designs with communal rearing environments to be employed. However, though it has been successively applied both in adults and larvae [58], it has not yet been used in any breeding programs. Because of the high frequency of null alleles at microsatellite loci [59], which generate difficulties for parentage assignment, SNPs are seen as markers with greater potential.


QTL and Marker Assisted Selection

QTL mapping in oysters is in its infancy, but several studies are in progress, notably on resistance to summer mortality (Sauvage et al, in prep) and heterosis for growth-related traits (Hedgecock, pers. com.). Transcriptome studies will increasingly contribute to the identification of genes involved in traits of interest that could be used for Gene Assisted Selection (GAS) (e.g. [61]). Co-localization of some of these genes with QTLs for resistance to summer mortality is currently in progress.


Transgenesis

Although some research effort has been dedicated to genetic transformation, and the identification and isolation of appropriate promoters and constructs [62], no transgenic oysters have been produced up till now.



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