The effects of in breeding on production traits of the southern bay scallop Argopecten irradians concentricus.
KEY WORDS: southern bay scallop, Argopecten irradians concentricus, self-fertilized families, production traits, inbreeding depression
The bay scallop Argopecten irradians is a hermaphroditic marine bivalve native to the east coast of the United States. Two of its geographical subspecies, A. irradians irradians and A. irradians concentricus, were introduced to China as alternative species for aquaculture (Zhang et al. 1986, Zhang et al. 1997, Zheng et al. 2006). After being first introduced in 1991 (He & Zhang 1998), culture of the southern bay scallop A. irradians concentricus was quickly extended, because of the short growth period, low production costs, and high economic return. Now scallop culture is gaining momentum and has become one of the pillar industries in Beibu Bay in the South China Sea (Liu et al. 2007a). With the rapid development of bay scallop aquaculture, however, some problems such as high mortality, slow growth, and diseases appeared (Liu et al. 2007b). Zhang et al. (2000) suggested that genetic degradation as a result of inbreeding was one of the main problems facing the bay scallop aquaculture industry.
For small and isolated populations, genetic drift or mating between relatives may increase homozygosity, and then results in a reduction of the mean phenotypic value of traits associated with fitness, such as inbreeding depression (Charlesworth & Charlesworth 1987, Falconer & Mackay 1996). Genetic study with simple sequence repeat markers has demonstrated that the average heterozygosity and allelic diversity in cultured populations of bay scallop from China were lower than that of the wild populations from the east coast of America (Wang et al. 2007). All finite populations experience some degree of inbreeding (Falconer & Mackay 1996, Pante et al. 2001). Moreover, the bay scallop is a simultaneous hermaphrodite that releases successively male and female gametes during the spawning process. Like other functional hermaphrodites, self-fertilization is a common occurrence during spawning, which can result in inbreeding (Zheng et al. 2006). Mass spawning is used by farmers to produce scallop seeds, and the use of limited parents increases the chances of inbreeding.
Inbreeding reduces heterozygosity, limits the potential response to selection in subsequent generations, and increases the potential for inbreeding depression (Bentsen & Olesen 2002). Inbreeding depression has been documented during long-term breeding in aquatic animals (Jarne & Charlesworth 1993, De Donato et al. 2005, Neira et al. 2006). However, information on the effect of inbreeding in marine bivalve molluscs is limited and conflicting. In the American oyster Crassostrea virginica, for example, poor larval survivorship and growth were found in inbred groups (Longwell & Stiles 1973). For the same species and for the same inbreeding coefficient, no inbreeding effects were detected on larval growth or survival, although there was a 6-10% inbreeding depression on spat growth (Mallet & Haley 1983). With respect to the Pacific oyster, Crassostrea gigas, Beattie et al. (1987) found that inbreeding can reduce growth of 2-y-old individuals, but Lannan (1980) did not find effects of inbreeding on survival to spat. In the hermaphroditic scallop Argopecten purpuratus, no differences in survival or growth were found between families produced by self-fertilization and pair mating at either the larval or juvenile stages (Winkler & Estevez 2003). Ibarra et al. (1995) found lower growth rate and survivorship in larvae of the scallop Argopecten circularis produced by self-fertilization than that produced by outbreeding. In the tropical scallop Euvola ziczac, no effects of self-fertilization were detected on growth or survivorship (Betancourt et al. 1994). Therefore, not all inbred populations experience inbreeding depression (Falconer & Mackay 1996, Beattie et al. 1987). The magnitude of inbreeding depression may vary considerably depending on the species and the traits examined (Bentsen & Olesen 2002, Bondari & Dunham 1987), and it is imperative to assess the effects of inbreeding on production traits of any species targeted for breeding.
The current study was designed to determine the effects of inbreeding on production traits of a population of the southern bay scallop A. irradians concentricus cultured in China. A better understanding of this information will benefit the design of selective breeding programs for southern bay scallops.
MATERIALS AND METHODS
Parental scallops used in this study were descendants from the initial introduction from Florida in 1991, which has been successively cultured in Beibu Bay, South China Sea, for 16 y (about 16 generations). In December 2007, adult scallops were transported to a scallop seed hatchery in Zhanjiang and conditioned in a 20-[m.sup.3] concrete tank. The broodstock were fed with Isochrysis zhanjiangensis and Platymonas subcordiformis. Water was changed 30-50% every day, and the water temperature was maintained at 23[degrees]C until the breeders reached ripeness (stage IV).
Experimental Design and Treatments
Two groups, one using self-fertilization and the other using mass mating, were produced. Scallops with turgid, well-colored gonads were chosen as parents and induced to spawn using temperature and UV radiation shock. For the self-fertilized group, 12 individuals randomly selected from the adult parents were placed individually in an 80-L bucket and allowed to spawn to produce self-fertilized larvae. Thus, 12 self-fertilized families were produced using sperm and eggs from the same individuals. Fertilized eggs of each self-fertilized family were separated into 3 replicates, and then each replicate was placed in 1 200-L concrete tank. For the mass-spawned group, 50 individuals randomly selected from the adult parents were placed in a 10,000-L concrete tank and allowed to mass spawn. Partial self-fertilized eggs randomly selected from the 10,000-L concrete tank were separated into 3 replicates, and each replicate was placed in a 200-L concrete tank.
At about 30 h after fertilization, larvae from each of the 39 replicates were collected using a 20-[micro]m-mesh sieve and placed in a 10-L bucket with seawater. Numbers of larvae in each tank were estimated by counting in triplicate the number of larvae in 1 mL and extrapolating to the total holding volume. Then, D-stage larvae from each replicate were placed in a 200-L concrete tank. Initial density for all replicates in the experiment was set at 4 larvae/mL.
To minimize environmental effects, rearing conditions were maintained the same for all replicates at the larval, juvenile, and adult stages. The larvae were fed with k zhanjiangensis from days 2-6, and then with a mixture of I. zhanjiangensis and P. subcordiformis (1:1), with concentrations ranging from 15,000-50,000 cells/mL according to larvae aging. With larval growth, densities were kept the same by adjusting the water volume. Seawater used was sand filtrated and sterilized by UV radiation. Water was changed 30-50% every day. Water temperature was left to change with the outside environment, and salinity was kept at 28.9-29.8. After 9 days of larval culture, when about 30% of larvae developed to the eye stage in some replicates, spat collectors (polyethylene nets) were placed into the tanks. Metamorphosed and attached spat were kept in the concrete tanks until they reached 1 mm (on day 30), then they were placed in polyethylene bags and transferred to an outdoor nursery area in the sea. In the spat nursery, polyethylene bags were changed as spat aged; spat was sorted once every month to reduce density.
Three months later, all juveniles were transferred to 10-layer cages for grow-out. Densities were adjusted monthly, starting with 200 individuals per layer and ending with 30 individuals per layer.
Sampling, Measurement, and Inbreeding Depression Estimates For each replicate, the hatching success and survival at the larval, juvenile, and adult stages were estimated. The shell length and live weight were measured for juveniles and adults.
Hatching success was estimated as the ratio of the number of normal D-stage larvae to the number of fertilized eggs. Survival at the larval stage was estimated as the ratio of the number of surviving larvae on day 9 versus the number of D-stage larvae on day 1. Survival at the juvenile and adult stages was estimated as the ratio of the number of surviving juveniles on day 75 versus the number of juveniles on day 30, and the ratio of the number of surviving adults on day 230 versus the number of juveniles on day 75. For larvae/spat on day 1, 9 and 30, shell size of 40 individuals randomly sampled from each replicate was measured using a microscope (100x) equipped with an ocular micrometer. At the juvenile and adult stages, a random subsample of 30 scallops from each replicate was measured for shell size using a vernier caliper (0.02-mm accuracy) on days 75 and 230, respectively. Live weight of 30 individuals randomly sampled from each replicate for juveniles on day 75 and adults on day 230 was measured individually using an electronic balance.
The magnitude of inbreeding depression ([delta]) is calculated as the proportional decrement in the mean phenotypic value caused by self-fertilization in comparison with mass-mated offspring (Charlesworth & Charlesworth 1990, Crnokrak & Roff 1999):
[delta] = 1 - [W.sub.1]/[W.sub.0]
where, [W.sub.1] is the phenotypic value of self-fertilized offspring and [W.sub.0] is the phenotypic value of mass-mated offspring.
One-way analysis of variance was carried out to determine the differences in production traits. When there were significance differences, the means were further compared with Duncan's multiple range tests. Analyses were done using Statistical Program for Social Sciences (SPSS 12.0, Chicago, IL) software for Windows. Significance level for all analyses was set at P < 0.05.
Hatching and Survivorship
The observed hatching rates and survivorship at the larval, juvenile, and adult stages for the mass-spawned group (CG) and the 12 self-fertilized families (SF1-SF12) in the self-fertilized group are shown in Table 1. The hatching rate of CG (54.85%) was significantly larger than that of the self-fertilized group for 10 of the 12 self-fertilized families examined, and not significantly different from that of the other 2 self-fertilized families (SF2 and SF9). Hatching success varied from 22.73-56.67% among the 12 self-fertilized families (SF1-SF12), with an average of 35.84%. During the larval, juvenile, and adult stages, the survival of the mass-mated group was 53.54%, 77.93%, and 85.93%, respectively, which was always significantly higher than that of self-fertilized families at corresponding stages, except for SF2 and SF9. In addition, a significant difference in survival was found among the 12 self-fertilized families at all developmental stages.
The magnitude of inbreeding depression ([delta]) for traits in hatching and survivorship is shown in Table 2. The estimated inbreeding depression for hatching varied from 21.17-63.87%, with an average of 40.45%. The estimated mean value of inbreeding depression for survival at the larval, juvenile, and adult stages was 41.80%, 24.29%, and 18.71%, respectively.
Shell length and/or live weight at different ages for the 12 families (SF1-SF12) of the self-fertilized group and the mass-spawned group (CG) of the southern bay scallop are listed in Table 3. For newly hatched larvae on day 1, no significant difference in size was found between the self-fertilized families and the mass-spawned group. From day 9 to day 230, however, the shell length and live weight were significantly affected by the mating system (self-fertilization vs. mass mating). The shell length of the mass-mated group on days 9, 30, 75, and 230, and the live weight on days 75 and 230 were always significantly larger than that of the self-fertilized group for 9 of the 12 self-fertilized families examined, and not significantly different from that of the other 3 self-fertilized families (SF2, SF3, and SF9). In addition, a significant difference in shell length on days 9, 30, 75, and 230, and in live weight on days 75 and 230 was detected among the 12 self-fertilized families.
The magnitude of inbreeding depression (8) in shell length and live weight at different ages is presented in Table 4. The average inbreeding depression for shell length on days 9, 30, 75, and 230 was 10.36%, 25.90%, 21.08%, and 20.94%, respectively. The inbreeding depression for wet weight on days 75 and 230 was 42.21% and 32.85%, respectively.
Inbreeding is unavoidable in closed populations (Falconer & Mackay 1996, Pante et al. 2001). Unless managed strategically, inbreeding itself is almost universally harmful, and breeders normally seek to avoid levels that produce inbreeding depression (Falconer & Mackay 1996). Understanding the effects of inbreeding is critical to the long-term viability of shellfish breeding programs, especially as breeders attempt to develop selected lines in hatcheries with small, effective population sizes. For 10 of the 12 self-fertilized families of southern bay scallop A. irradians concentricus examined in the current study, inbreeding depression for production traits, hatching, survival, and growth was evident. Similar results have also been reported on A. irradians irradians, the northern subspecies of bay scallop (Stiles & Choromanski 1995, Zheng et al. 2008). These results were consistent with previous studies on 2 other hermaphroditic scallops: Pecten maximus (Beaumont & Budd 1983, Beaumont 1986) and A. circularis (Ibarra et al. 1995). In contrast, however, no evidence of inbreeding depression was found in either larvae or juveniles of 2 tropical hermaphroditic scallops: E. ziczac (Betancourt et al. 1994) and A. purpuratus (Winkler & Estevez 2003).
For simultaneous hermaphrodites, inbreeding depression is regarded as the most important selective force acting against self-fertilization, and maintaining cross-fertilization (Lande & Schemske 1985, Charlesworth & Charlesworth 1987). Several theoretical analyses predict that inbreeding depression under self-fertilization should be greater than 0.5 to maintain cross-fertilization (Maynard Smith 1978, Charlesworth & Charlesworth 1990). The levels of inbreeding depression for production traits we report here (Tables 2 and 4) were low compared with those found in previous studies in some simultaneous hermaphrodites (Jarne & Delay 1990, Chen 1993, Li & Pechenik 2007). One possible reason for this discord is the special breeding and mating history of the closed population of A. irradians concentricus studied here. After its introduction to the southern coast of China, the southern bay scallop has been successively cultured for nearly 2 decades (about 16 generations). All finite populations experience some degree of inbreeding (Pante et al. 2001, Falconer & Mackay 1996). Like other functional hermaphrodites, self-fertilization is a common occurrence during spawning, which can also result in some amount of inbreeding (Zheng et al. 2006). Mass spawning is used by farmers to produce scallop seed, and the use of limited parents in mating situations increases the chance of inbreeding. Theoretical genetic models showed that small amounts of "selfing" could reduce inbreeding depression (Lande & Schernske 1985, Charlesworth & Charlesworth 1990). High inbreeding depression is expected in cross-fertilizing species and relatively low inbreeding depression in self-fertilizing species (Maynard Smith 1978, Wells 1979, Lande & Schernske 1985, Charlesworth & Charlesworth 1987). Therefore, the normal occurrence of self-fertilization can explain the decline in the magnitude of inbreeding depression in the bay scallop population examined here. The magnitude of inbreeding depression detected in the current study is similar to that from a population of A. irradians irradians, which was introduced to the northern coast of China several decades ago (Zheng et al. 2008). In addition, the bay scallop is a simultaneous hermaphrodite; thus, partial self-fertilization was inevitable during mass spawning. The control group we used in the current study may exhibit an unknown amount of self-fertilization and may not be representative of a strict outcross population, which can also lead to an underestimation of inbreeding depression.
It is clear that the effects of inbreeding varied among families. For example, the offspring of 2 self-fertilized families, SF2 and SF9, did not show any effects of inbreeding for the traits that exhibited inbreeding depression in other families. The variety of responses and the range of variation in this study indicate that not all families were adversely affected by inbreeding. According to the partial dominance hypothesis, inbreeding depression results from the phenotypic expression of deleterious recessive or partially recessive alleles that are at least partially masked in the heterozygous condition. Inbreeding increases the proportion of individuals that are homozygous for such recessive alleles, allowing their full expression (Charlesworth & Charlesworth 1987, Crnokrak & Barrett 2002). Different families can harbor different proportions of deleterious alleles (mutation load) simply through random genetic drift (Frankham et al. 2002). It is possible that families SF2 and SF9 carried a lighter mutation load than other families and therefore suffered the least from inbreeding. In addition, some uncontrolled biological effects inherent in bay scallop may lead to the variety of responses. Different effects of inbreeding for different families have also been found in several marine invertebrates (Keys et al. 2004, Park et al. 2006, Li & Pechenik 2007). Results of the current study also indicate that inbreeding and heterosis may be exploited for strain development in the southern bay scallop A. irradians concentricus in the future.
This research is supported by Agricultural Science and Technology transfer funds of the Ministry of Science and Technology of China (2007GB2E000183), the Science and Technology Program of Guandong Province of China (2006B20201055), and the Program of Department of Education of Guangdong Province of China (2009B090300232). We also appreciate the helpful comments of the anonymous referees, which improved the manuscript.
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JIANYONG LIU, (1) ZHIGANG LIU (1),* AND XIAOZHEN SUN (2)
(1) Fisheries College, Guangdong Ocean University, 40 Jiefangdong Road, Zhanjiang, Guangdong 524025, China," (2) Zhanjiang Silver Wave Pearl Company, 40 Jiefangdong Road, Zhanjiang, Guangdong 524000, China
* Corresponding author. E-mail: email@example.com
TABLE 1. Hatching rate and survival at the larval, juvenile, and adult stages for the 12 families (SF1-SF12) and the mass-spawned group (CG) of the southern bay scallop A. irradians concentricus. Survival (%) Hatching Group Rate (%) Larva Juvenile SF1 43.21 (b) (1.22) 37.63 (b) (3.10) 65.78 (bc) (5.24) SF2 56.42 (a) (1.88) 53.64 (a) (3.20) 81.47 (a) (3.20) SF3 19.82 (d) (0.55) 31.98 (cd) (2.33) 44.21 (d) (5.59) SF4 22.87 (c) (1.33) 29.22 (d) (3.43) 59.44 (c) (3.57) SF5 42.83 (b) (1.24) 32.96 (bc) (2.17) 69.91 (b) (4.17) SF6 43.63 (b) (1.39) 36.83 (bc) (4.04) 64.85 (bc) (5.34) SF7 42.83 (b) (1.56) 34.02 (bc) (2.21) 66.02 (b) (4.04) SF8 23.36 (c) (1.34) 24.34 (d) (2.34) 50.65 (d) (3.33) SF9 56.67 (a) (1.69) 55.84 (a) (4.24) 78.87 (a) (4.25) SF10 23.14 (c) (1.11) 27.56 (d) (2.87) 61.11 (c) (3.63) SF11 42.23 (b) (1.52) 28.18 (d) (4.55) 58.18 (c) (3.33) SF12 22.73 (c) (1.23) 28.87 (d) (3.27) 49.87 (d) (5.25) CG 54.85 (a) (1.56) 53.54 (a) (2.66) 77.93 (a) (2.58) Survival (%) Group Adult SF1 77.67 (b) (2.22) SF2 86.48 (a) (3.90) SF3 63.09 (d) (4.58) SF4 66.09 (cd) (4.45) SF5 73.96 (bc) (3.37) SF6 70.53 (c) (3.32) SF7 79.02 (b) (2.66) SF8 67.65 (cd) (3.33) SF9 86.65 (a) (2.25) SF10 71.09 (c) (3.13) SF11 68.54 (cd) (5.01) SF12 60.85 (d) (4.22) CG 85.93 (a) (3.87) SDs are in parentheses. Within each column, means with the same superscripts are not statistically different (P > 0.05). TABLE 2. Inbreeding depression ([delta]) for hatching success and survival at the larval, juvenile, and adult stages of the southern bay scallop A. ivradians concentricus. Survival (%) Hatching Rate Group (%) Larva Juvenile Adult SF1 21.17 29.72 15.59 9.61 SF3 63.87 40.27 43.27 26.58 SF4 58.39 45.42 23.73 23.09 SF5 21.90 38.44 10.29 13.93 SF6 20.44 31.21 16.78 17.92 SF7 21.90 36.46 15.28 8.04 SF8 57.48 54.54 35.01 21.27 SF10 57.85 48.52 21.58 17.27 SF11 22.99 47.37 25.34 20.24 SF12 58.58 46.08 36.01 29.19 Average 40.45 41.80 24.29 18.71 As the hatching rate and survival of SF2 and SF9 were not significantly different from that of the CG, the inbreeding depression values of SF2 and SF9 were not listed. TABLE 3. Shell length (SL) and live weight (W) at different ages of the southern bay scallop A. irradians concentricus. Day 1 Day 9 Day 30 Group SL ([micro]m) SL ([micro]m) SL (mm) SF1 95.81 (2.14) 155.94 (cd) (3.26) 0.99 (bc) (0.08) SF2 97.62 (2.12) 169.78 (ab) (3.27) 1.23 (a) (0.11) SF3 96.42 (1.94) 175.50 (a) (3.23) 1.21 (ab) (0.10) SF4 96.47 (1.83) 154.06 (d) (4.27) 0.88 (c) (0.10) SF5 96.22 (2.14) 162.58 (bc) (4.26) 1.05 (b) (0.09) SF6 96.28 (2.15) 163.49 (b) (4.28) 0.95 (bc) (0.09) SF7 96.63 (2.21) 166.82 (b) (3.24) 1.04 (b) (0.08) SF8 95.43 (1.94) 149.85 (d) (2.32) 0.87 (c) (0.11) SF9 96.42 (1.21) 172.91 (a) (3.34) 1.21 (ab) (0.09) SF10 95.48 (2.73) 155.27 (c) (4.41) 0.86 (c) (0.07) SF11 96.62 (2.12) 157.36 (c) (4.22) 0.97 (bc) (0.07) SF12 95.2 (1.15) 149.14 (d) (4.32) 0.86 (b) (0.07) CG 97.52 (2.12) 175.34 (a) (4.24) 1.27 (a) (0.08) Day 75 Group SL (mm) W (g) SF1 18.34 (bc) (1.24) 1.59 (c) (0.24) SF2 20.42 (a) (1.20) 2.05 (ab) (0.20) SF3 20.14 (ab) (2.52) 2.22 (a) (0.22) SF4 14.66 (d) (2.07) 0.88 (d) (0.34) SF5 18.10 (bc) (1.17) 1.69 (bc) (0.17) SF6 18.16 (bc) (1.32) 1.69 (bc) (0.21) SF7 17.40 (bc) (2.06) 1.58 (c) (0.28) SF8 14.58 (d) (1.39) 0.80 (d) (0.39) SF9 20.21 (ab) (1.28) 1.94 (ab) (0.23) SF10 15.74 (cd) (1.64) 0.79 (d) (0.33) SF11 18.58 (bc) (1.33) 1.62 (b) (0.31) SF12 16.66 (cd) (2.23) 0.75 (d) (0.20) CG 21.43 (a) (1.32) 2.19 (a) (0.28) Day 230 Group SL (mm) W (g) SF1 40.96 (b) (2.12) 20.44 (b) (1.12) SF2 50.14 (a) (2.44) 27.24 (a) (1.33) SF3 51.64 (a)(2.49) 27.42 (a) (1.37) SF4 35.56 (c) (2.32) 14.24 (c) (1.21) SF5 40.62 (b) (2.10) 21.36 (b) (1.23) SF6 40.58 (b) (2.24) 21.42 (b) (1.24) SF7 40.24 (b) (2.23) 20.87 (b) (1.22) SF8 34.68 (c) (2.15) 14.22 (c) (1.28) SF9 51.68 (a) (2.20) 27.65 (a) (1.27) SF10 35.78 (c) (2.32) 14.92 (c) (1.14) SF11 41.86 (b) (2.40) 20.89 (b) (1.16) SF12 34.38 (c) (2.26) 14.32 (c) (1.21) CG 48.44 (a) (2.55) 26.92 (a) (1.45) SDs are in parentheses. Within each column, means with the same superscripts are not statistically different (P > 0.05). TABLE 4. Inbreeding depression ([delta]) in shell length (SL) and live weight (W) of the southern bay scallop A. irradians concentricus at different ages. Day 75 Day 230 Day 9 Day 30 Group SL ([micro]m) SL (mm) SL (mm) W (g) SL (mm) W (g) SF1 11.06 22.05 14.42 27.40 15.44 24.07 SF4 12.14 30.71 31.59 59.82 26.59 47.10 SF5 7.28 17.32 15.54 22.83 16.14 20.65 SF6 6.76 25.20 15.26 22.83 16.23 20.43 SF7 4.86 18.11 18.81 27.85 16.93 22.47 SF8 14.54 31.50 31.96 63.47 28.41 47.18 SF10 11.45 32.28 26.55 63.93 26.14 44.58 SF11 10.25 23.62 13.30 26.03 13.58 22.40 SF12 14.94 32.28 22.26 65.75 29.03 46.81 Average 10.36 25.90 21.08 42.21 20.94 32.85 As the shell length and live weight of SF2, SF3, and SF9 were not significantly different from that of CG, the inbreeding depression values of SF2, SF3 and SF9 were not listed.
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|Author:||Liu, Jianyong; Liu, Zhigang; Sun, Xiaozhen|
|Publication:||Journal of Shellfish Research|
|Date:||Apr 1, 2011|
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