Research shows that the black-lip pearl oyster Pinctada margaritifera (Linaeus 1758) can be successfully cultivated to form a pearl, after a specific period. Several farmers and scientists have performed these experiments and the process is common in several countries (Cartier et al., 2012). There are no pure artificial pearls in the world but 99% per cent of all the pearls in the billion-dollar industry are manufactured in farms. This process is arrived through rearing oysters in a domesticated fashion and manipulating them to produce cultured pearls in the same way that domestic animals produce farming products (Cartier et al., 2012). The process however involves two different oysters. The first begins the process of making pearls through manipulation and is called the donor oyster. Southgate and Lucas (2008), posit that the pearl production process involves two distinct processes, which are dependent on each other. Like in farming, there is always a need to improve production in quality and quantity (Aji, 2011; Taylor and Strack, 2008; Gervis and Smith, 1992). This experiment seeks to find out how, increasing the quality and quality of food given to the oysters can improve this process. However, the experiment focuses on the effect of temperature on donor shell growth (shell deposit rate) after one month at different temperatures preceding the graft operation. In the opposite, there was a clear signature of donor oysters that were fed with different algae concentration: significant differences in gene expression, more nacre in young pearl but overall no effect on pearl quality at 12-month post-graft operation. The results clearly showed that quality of food and temperature of environment on the donor oyster had an actual impact in the entire process. The same however did not reflect with regard with the recipient oyster (Kumar & Steenkamp, 2013; Ky et al, 2015). However, these impressive results were not reflected in the nature of the eventual cultured pearls. This is based on the parameters normally used in the commercial evaluation of pearls.
Introduction
The black-lip pearl oyster Pinctada margaritifera (Linaeus 1758) is cultivated to produce pearl, a unique gem built by a living organism, in several countries from tropical and subtropical regions. In French Polynesia, pearl production is a major economic force with the exportation of pearl products reaching 73 billion Euros in 2014 (ISPF, 2015). The production sites are located in the Society, Gambier, and Tuamotu archipelagos whose pearl production accounts for more than 95% of the world’s black cultured pearls (Cartier et al., 2012). As reported by Southgate and Lucas (2008), pearl production involves four different phases: pre-operative oyster conditioning; donor oyster selection and nucleus insertion; post-operative care; and oyster culture and pearl harvest. The pre-operative conditioning consists of a weakening process for 28-40 days with the aim of reducing the pearl oysters metabolism and gametogenic activity prior to the grafting operation (Aji, 2011; Taylor and Strack, 2008; Gervis and Smith, 1992). Various environmental considerations including water temperature, crowding of pearl oysters in particular places, and tropical and oxygen levels are important in this experiment (Aji, 2011; Taylor and Strack, 2008). Nevertheless, to our knowledge, no study has examined the influence of environmental parameters during pre-operative conditioning on pearl biomineralization process.
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Following the pre-operative conditioning, a surgical operation called “grafting” is carried out by skilled technicians. A piece of mantle, the tissue responsible for shell mineralization, is cut in the nacre producing part from a donor oyster. This small piece of mantle is inserted, together with a spherical nucleus made of mollusk shell or synthetic material, into the gonad of another pearl oyster referred to as the recipient oyster (Kishore and Southgate, 2014; Taylor and Strack, 2008). The external epithelial cells of the graft proliferate and cover the nucleus to form a pearl sac. The latter is complete after approximately thirty days of post-grafting (Cochennec-Laureau et al., 2010). Even though the graft is cut in the nacre-producing part of the donor oyster, the lower parts of the pearl layer are far from being homogeneous (Cuif et al., 2008). The authors reported a high variability in the thickness and composition of the pearl initial layers as well as unusual association of organic and mineral materials. From the very first secretion of the pearl sac, at 21 days post-grafting, the basal layer of the pearl is usually composed of thin organic layers mostly composed of proteins with the presence of mineral materials as dispersed microgranules of aragonite and calcite (Cuif et al., 2005). At 2 months of post-grafting, radial microstructures perpendicular to the surface of the nucleus appear due to the formation of organic envelopes. These microstructures form prismatic calcite or aragonite. The prismatic aragonite has never been observed in a shell. The establishment of regular and parallel nacreous layer composed of aragonite tablets during the pearl formation then appears but the production may occur directly on the organic layer as well as some month delay (Kumar & Steenkamp, 2013; Ky et al, 2015).
Pearl biomineralization depends on complex molecular processes. The pearl sac epithelium secretes shell matrix proteins (SMP), which play a major role in pearl biomineralization. Numerous SMP have been characterized and some genes encoding those proteins have been identified in pearl oysters (Joubert, Marie, Miyamoto, Suzuki, 2014). The proteins are thought partly to regulate the formation of the prismatic and nacreous shell layers (Marie et al., 2014). Some notable examples regarding the nacreous layer are: the gene Pif177 known to specifically bind to aragonite crystals (Suzuki et al., 2009); MSI60 involved in the formation of aragonite crystal (Sudo et al., 1997) and Pearlin exhibiting calcium- and chitin-binding properties (Montagnani et al., 2011). As for the prismatic layer, Aspein is involved in the calcite precipitation process (Isowa et al., 2012); Prismalin14 plays an important role in the regulation of calcification of the prismatic layer (Suzuki et al., 2007; Suzuki et al., 2004). Some proteins such as Nacrein are important for shell formation and are implied in both the aragonitic nacreous layer and the calcitic prismatic layer mineralization processes (Miyamoto et al., 2005).
The early activity of the pearl sac may be decisive for the pearl quality; environmental parameters during the pre-operative phase of pearl oysters culture may be of high influence (Joubert et al, 2014. Kvingedal et al, 2010). Therefore, giving the lack of critical attention paid to the purpose of our study was to examine the consequence of two environmental restrictions before embedding process on both donor and receiver oysters on the biomineralization dimensions from the black-lipped pearl oyster Pinctada margaritifera. Shell growth was measured in donor oysters. Pearl weight and quality were assessed. The expression levels of 6 biomineralization-related genes were measured in the mantle from donor oysters as well as in the pearl sacs from recipient oysters at different harvesting time (Kumar & Steenkamp, 2013).
Nonnacreous basal layer formation is typically and process which takes a very short time; during this time, the pearl-sac mineralizing actions change into the expected pearly nacreous material (Cuif et al, 2008). Two variables are very critical during this process; the thickness of the basal pearl layers and the structure utilized in the period of the experiment. These variables may work within the entire pearl formation or within only one pearl (Cuif et al. 2008)
The early development stages of the pearl layer are critical since they influence the grafting process and the preceding formation of the pearl-sac. Uncommon molecular formations could be as a result of a disruption of the genes formation within the cells during the mineralizing period (Cuif et al, 2008). Considering the quality of pearl and the control mechanisms in the pearl industry, the level of the process of recovery to new production of regular nacreous material should be the first component to be considered. The grafting sequence in this stage is significant in the process of developing the pearl-sac (Cuif et al., 2008).
Discussion
Bivalves physiological process and metabolism are mostly controlled by two environmental factors: temperature and food (Schöne et al. 2005; Laing 2000; Thébault et al. 2008; Schöne et al. 2002; Kanazawa and Sato 2008). In pearl oysters, those two parameters influence growth, reproduction, as well as biomineralization (Joubert et al. 2014; Teaniniuraitemoana et al., 2015; Southgate and Lucas 2011). Shell growth: after a one-month experiment, food concentration and temperature both influence pearl oysters shell growth with higher shell growth rate at higher temperature and higher food concentration. In agreement with Joubert et al. (2014), authors found, that shell growth rate increase with temperature and food concentrations (comments some of their results relating to shell growth) (Kumar & Steenkamp, 2013). Nevertheless, no significant differences in gene expression level of the mantle of donor oysters were found in our study. After a two-month experiment, Joubert et al. (2014) showed a higher expression of Pif177 at high food concentration and a down-regulation of MSI60. Our results are in agreement with Article Le Moullac et al (2016): no difference between 22 and 30°C for nine genes (Pif177, Pearlin, MSI60, MRNP34, Shematrin9, Prismalin14, Aspein, PUSP6, Nacrein). In their study, the modeled thermal optimum for biomineralization ranged between 21.5 and 26.5°C.
The environmental pre-conditioning did not affect the nuclei retention. The pre-conditioning did not influence the process of production of cultured cells in the creation of the initial nucleus by the donor and the creation of the resultant cultured pearl upon grafting (Alagarswami, 1983). The nature, size and rate of nucleus creation is crucial to the entire process of cultured pearl production. If this process is flawed, there will be no mantle tissue to graft and therefore no resultant cultured pearls. Further, a faster shell rate results in more mantle tissue and thus a higher cultured pearl production (Alagarswami, 1983). This retention rate, the rejection rate and the mortality were in the same range than the ones showed by Ky et al (2014).
Regarding the gene expression level in the pearl sac of recipient oysters, there is a molecular signature of donor oysters’ food pre-grafting operation (Kumar & Steenkamp, 2013). The pearl oysters that were grafted with donor oysters fed with higher concentration of microalgae showed significant over-expression of the three genes studied that are related to nacre formation (MSI60, Pif177 and Pearlin) and a down regulation of one gene related to prismatic layer formation (Prismalin14) at 45 days post grafting (Tangelder et al, 2015).
The post-grafting deposit made towards the creation of the actual pearl at the states of 3-12 month reflect the quality of the mantle tissue used. This is crucial to the rate of nacre material deposit. This is reflected by the presence of more material in the experiment where more food and higher temperatures were given to the donor oyster. The better conditions reflected in the mantle tissue produced and grafted a fact that agrees with McDougall et al, (2013), Ky et al, (2014) and Kishore, (2015). These results together with nacre deposition may suggest that feeding donor oysters with higher algae concentration may favor the pearl formation with nacre deposition faster (Nagai, 2013).
However, concerning the temperature pre-conditioning, no significant effect was observed either in the gene expression level of pearl sac or nacre deposition on young pearl or pearl quality. This phase involves the elementary art of establishing the commercial value of the resultant product through physical measurement of the cultured pearls. It also involves the establishment of the texture and luster and shape of the pearl (Hänni & Cartier, 2013; Kishore & Southgate, 2015B). None of this process are scientific in nature (Hiramatsu et al, 2013; Webster, 2014).
In the two experiments, there were no effects of recipient oysters conditioning; only effect of donors on food level experiment. Hence, pearl quality may be influenced by the donor and not the recipient (McGinty et al, 2010).
As evident in a xenograft experiment using P. maxima and P. margaritifera. McGinty et al. (2011), the donor and not the recipient oyster influence the pearl quality features (McGinty et al. 2010. This is seen in the experiment where the donor oyster is taken as the significant contributor to the pearl culture process following the mineralization technique. The high level of biomineralization abilities could also be a as a result of countless nacre deposition. This can also be drawn from the P. maxima and P. margaritifera experiment (Kono et al. 2000; Müller 1997; Strack 2006).
Tayale et al. (2012) in a recent study demonstrated that uninhabited donor oysters, often called the P. margaritifera, could influence the section of the gene of the oyster and the quality characteristics. Genetic dispositions, regarded as the ‘best’ could influence the breeding process and hinder the results of the experiment (Tayale et al., 2012). This fact is reiterated by Ky et al. (2013) who posit that sib selection becomes a simple process of the effect of the pearl characteristics cause the ‘family effect.’ Consequently, using pearl oysters, which have massive deposits of nacre could be advantages to the experiment and to the pearl industry as a whole (Ky et al., 2013). Considering the high levels of nacre proportions the study observes that laboratories are currently considering molecular tools, which can accurately select a good gene of the nacre pearl, than can be used to yield the best grade of pearl. Such a characteristic cannot be easily identified using the phenotypical method of selecting wild molluscs (Blay et al., 2016). There should be a consideration when handling the process above because nacre deposition can influence other cultured characteristics of the pearl (Tayale et al., 2012).
Defects in the surface of cultured pearl and the lustre of pearl influence the quality of the pearl and the grade expected. Results involving non-defective pearl surfaces indicate presence of heavy and thick nacre, which were of high quality. There are diverse ways in which the quality of the pearl can be influenced. One method is utilizing the fast nacre deposition criteria to smoothen the surface and reduce the amount of defects on the surface (Alagarswami, 1987C; Sato et al., 2013). The results above can also be explained using the observation of the environmental changes, during the experiment period. Since we did our experiment within 18 months, which coincides with the study by Blay et al. 2016, we found that period of culture and the location can influence pearl quality. A single location would be desirable and the same grafter utilized to ensure that the operations within the nuclei are kept constant. Different between the weight and the thickness of the pearl within the observable 874 pearls indicate the presence of genetic element in the donors. McGinty et al. (2010), confirmed the concept of genetic importance in nacre deposition, by using the xenograft study. Cuif et al. (2011) perceived that the materials of nacreous nature found on the surface of the nucleus could be the leadings causes of indiscretions present in the future reaped pearls. If the first model is not captured appropriately, it can be concluded that for the same degree of deposition of nacre, the pearls harvested would be effective by a ligger percentage (McGinty et al. 2010). Concisely, P. maxima mantle tissue-implantation could produce pearls with smoother surfaces. By extension, this will produce pearls of Grade A in P. maxima and less in quality in P. margaritifera donor tissue (McGinty et al. 2010). The difference evident in the two tissues is caused by the higher and fast deposition proportion (determined by the increase in weight and size of the pearl) in P. margaritifera tissue. This process defines the process of obtaining the best donors (Zhao et al., 2012; Wang et al., 2009; Ky et al, 2013; Demmer, Cabral & Ky, 2015).
Our results indicate that the lustre of cultured pearl did not exhibit major changes in the thickness and weight. Regardless of the high weight and longer length, the lustre did not show major changes. This is an indication of no direct relationship between lustre and weight/height. Pearl lustre is thought to affect the deposition of nacre a few weeks before the yield is determined. Winter time is considered by farmers as the best for harvesting pearls because deposition of nacre is slow at this time and therefore the process of deposition cannot be negatively influenced. Farmers in Gambier archipelago, have marked the best time for them to harvest the pearl. Apart from interference, the winter season is best for producing pearls, which have the best lustre (Marie et al., 2012; Fang et al., 2011). On the other hand, the summer season is not the best for harvesting pearl, because they are at the peak of deposition process; this process affects the lustre of pearl in significant ways. This issue is reported in North Tuamotu Archipelago. Because of the interr-season lagoon, variations in water temperature remain low (Marie et al., 2012; Fang et al., 2011).
A strong consideration should be made regarding genetic correlations to circumvent inadvertent collection of traits, which are not within the experimental process. Scientific literature have established that the negative correlation between lustre and nacre deposition can result when no specific consideration is done on the genetic dispositions of the pearl (Alagarswami 1987; Snow et al. 2004). Snow et al. (2004) posits that pearls with a bright lustre are formed by reliable and steady crystal development.
Numerous genes are involved in the nacre-based crystal formation. The process involves a long series of steps often manifesting into a complex biomineralisation procedure. The lustre of pearl is also affects by environmental factors, which influence the amount and quality of food they get (Snow et al. 2004). Other factors include cultural practices and the cleaning process of the oyster.
It is however, challenging to locate the recommended environmental settings for the thriving of physiological and molecular mechanisms, which influence the formation of the shell through the biomineralization technique (Joubert et al. 2014). The difficulty is heightened by the processes, which have not been standardized. Regulation of molecular organizations is known exactly known and the genes for classification have not be identified by scientists (Joubert et al. 2014). Conclusions
During the interaction with the nucleus, the pearl layer would develop superior postgrafting mineralization on the lower parts of its layer. There is a notable different between the pearl sac obtained in the postgrafting mineralization and the normal nacreous material coming out of the graft, before the animal mantle separated from it (Cuif et al., 2008). The experiment above focused on the effect of temperature on donor shell growth (shell deposit rate) after one month at different temperatures preceding the graft operation. In the opposite, there was a clear signature of donor oysters that were fed with different algae concentration: significant differences in gene expression, more nacre in young pearl but overall no effect on pearl quality at 12-month post-graft operation (Farn, 2013; Kishore & Southgate, 2014). However, a process of influencing the natural process of creating them was developed and has now been perfected.
Nacreous also known as mother pearl is the targeted outcome in the process of making cultured pearls (Alagarswami, 1987B) (Arnaud-Haond et al., 2006). This is a complex composite material that makes the mains substance of cultured pearls (Kishore, 2015). The instant part of the experiment is exponentially simpler and more straight-forward that the genetic part above as it involves the physical measure of rate of production of nacreous on the nucleus of the resultant cultured pearl, during the formative stages after it is grafted into the recipient oyster. It is also very important to note the difference between this and the earlier discussed experiment, involving the deposit of shell material within the donor shell (Farn, 2013; Kishore & Southgate, 2014).
The nature of the experiment combined the simple and the very complex scientific concept relating to cultured pearl production. The simple regards the effects of quantity and quality of food as well as temperature on the metabolic processes of an animal (Inoue et al., 2010; Inoue et al. 2010B; Fang et al., 2011). The complex entails how this process affects the extremely complicated process of cultured pearl production. This was gauged from the perspective of the donor oyster and the recipient oyster. Whereas similar experiments have been done before, this was unique in that the variables regarded only the food and temperature condition before the grafting process (Hänni & Cartier, 2013; Kishore & Southgate, 2015B). This limits the variables to the condition of the mantle tissue on the one part and the recipient oyster on the other at the instance of grafting.
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