We consider a modified 2-type Markov branching processes incorporating with both immigration and instantaneous resurrection. We prove that if the sum of the immigration rates is finite then no such ...structure can exist. The existence criterion is presented for the case that the sum of the resurrection rates is infinite. We construct some equivalent criteria that are far easier to verify. The recurrence and ergodicity criterion of the researching process are analysed in detail. The explicit expression for the equilibrium distribution is also presented.
The allotriploid technology has been widely applied to aquaculture worldwide. To overcome the deficiency of low growth, poor taste and high mortality of diploid hybrids between Crassostrea gigas and ...C. angulata in summer, two new types of triploid hybrids were produced by inhibiting the polar body II using cytochalasin B. The growth, survival and gonad traits of triploid hybrids (TGA – C. gigas ♀ × C. angulata ♂ and TAG – C. angulata ♀ × C. gigas ♂), diploid hybrids (GA – C. gigas ♀ × C. angulata ♂ and AG – C. angulata ♀ × C. gigas ♂) and purebred crosses (GG – C. gigas ♀ × C. gigas ♂ and AA – C. angulata ♀ × C. angulata ♂) were evaluated. A high triploidy induction rate (> 95%) was found by ploidy analysis. The fertilization and hatch rates of triploid hybrids were significantly (P < 0.05) lower than that of diploid crosses. During the larval stage, the triploid advantage (TA) was positive in growth, despite was negative in survival. The triploid hybrids exhibited superior performance during the grow-out stage. Across three culture environments (Rongcheng, Rushan and Huangdao), triploid hybrids performed better than diploid hybrids in growth with an increasing value of TA over time. The incremental survival rates of triploid hybrids, especially that of TGA, were significantly (P < 0.05) higher than that of diploid hybrids at Rushan and Huangdao in summer. The TA in cumulative survival rate ranged from 13.80% to 36.04% among environments. A positive yield advantage for diploid and triploid hybrids was observed at Rongcheng (mid-parent heterosis H: 114.11%; TA: 35.76%), Rushan (H: 148.22%; TA: 48.78%) and Huangdao (H: 138.96%; TA: 60.22%). Additionally, most of the triploid hybrids exhibited high sterility during reproduction phase. These findings demonstrated that triploid hybrids between C. gigas × C. angulata had obvious advantage in growth, survival and yield, which have important application value for commercial oyster cultivation in northern China.
•We obtained two new types of triploid hybrids with a high triploidy induction rate (> 95%) for the first time.•The reciprocal triploid hybrids exhibited positive advantage in growth, survival and yield compared to diploid lines.•Most of the triploid hybrids were virtually sterile during reproduction phase.•The triploid hybridC. gigas ♀ × C. angulata ♂ has important application value for commercial oyster cultivation.
•Effects of CNG injection timing studied under different pilot injection timings.•Effects of CNG injection timing studied under different pilot injection pressures.•Delayed CNG injection timing ...improves engine performance at low load in most cases.•Higher pilot injection pressure obtains better BTE and emissions except for NOx.•Advanced pilot injection timing obtains better BTE and emissions except for NOx.
In this paper, a common rail diesel research engine was converted to operate in dual-fuel mode and extensive experiments were conducted to investigate the effects of natural gas injection timing on the combustion and emissions performance under different pilot injection pressure and timing at low load conditions. The presented results include the cylinder pressure, heat release rate (HRR), ignition delay, combustion duration and brake thermal efficiency, as well as CO, HC and NOx emissions at different natural gas injection timing under pilot injection pressure (46 and 72MPa) and pilot injection timing (−8° and −17° ATDC) operation conditions at low load (BMEP=0.24MPa). The results indicated that retarded natural gas injection timing can achieve a stratified-like air–fuel mixture in cylinder under the different pilot injection conditions, which provided a method to improve the combustion performance and exhaust emissions at low load. Moreover, under higher pilot injection pressure (72MPa) conditions, better combustion performance, such as shorter ignition delay and combustion duration, higher brake thermal efficiency, were achieved; however, the exhaust emissions significantly increased compared with those under lower pilot injection pressure (46MPa). On the other hand, under the advanced pilot injection timing (−17° ATDC), the combustion performance was radically better, THC and CO emissions were lower but the NOx emissions were significantly higher compared with those under the regular pilot injection timing (−8° ATDC). This is attributed to faster flame propagation speed, better combustion phasing and higher volumetric efficiency. Consequently, employing appropriate natural gas injection timing accompanied with reasonable pilot injection parameters is critical to further improve combustion performance and exhaust emissions of a dual-fuel engine at low loads.
Allotriploid oysters between Crassostrea gigas and C. angulata have been utilized in commercial culture in China. The prevalence of this allotriploids is attributed to fast growth and extensive ...temperature adaptability, however its drawbacks are low meat ratio and slightly restored fecundity. To tackle these problems, two types of allotetraploids were produced in a previous study. Here, a comparative assessment of the growth, survival and reproductive traits was made for genetically improved allotriploids (based on allotetraploids), traditional allotriploids, and autotriploids (C. gigas and C. angulata). The results showed that the survival rates of larvae for all crosses were >80% on day 15. Genetically improved allotriploids survived better than autotriploids during juvenile and adult stages. The whole weights, wet meat weights, and meat ratios of genetically improved allotriploids were larger than traditional allotriploids on day 390 and day 510. The condition indexes, infertile percentages and fecundity of genetically improved allotriploids were lower than C. gigas autotriploids but higher than that of traditional allotriploids. At the end of the experiment, genetically improved allotriploids (GIT1) had the largest yield (56.79 kg) among all the six crosses. Overall, this study demonstrated that GIT1 exhibited superior growth and survival characteristics, as well as higher sterility compared with traditional allotriploids, which could be used as a potential variety in northern China.
•The genetically improved allotriploid oysters GIT1 survived better than autotriploids.•The total weights, wet meat weights, and meat ratios of GIT1 were larger than traditional allotriploids.•The condition indexes, infertile percentages and fecundity of GIT1 were higher than that of traditional allotriploids.•At harvest, GIT1 had the largest yield among all crosses.•The genetically improved allotriploids GIT1 can be used as a potential variety for oyster industry.
The various shell colors of bivalves not only result in a better visual perception but also show great commercial value as a highly potential breeding trait. Four shell color strains of Pacific ...oyster (Crassostrea gigas) characterized by black (SB), white (SW), gold (SG), and orange (SO) were bred purposefully for more than ten generations. In this study, whole-genome resequencing was performed on 20 oysters from each of the four strains, and a total of 14.67 million single nucleotide polymorphisms (SNPs) were obtained after stringent filtering, with an average sequencing depth of >10×. The population structure analysis provided support for the subdivision of all samples into four major genetic clusters corresponding to shell color phenotypes, indicating a high level of genomic differentiation driven by artificial selection. Then, a composite measure of fixation index (FST) and cross-population composite likelihood ratio (XP-CLR) allowed us to identify 336 to 414 candidate divergent regions (CDRs) in pairwise comparisons of different shell colors, with a total of 480 to 614 corresponding genes distributed. Of particular interest were tyrosinase (TYR) and perilipin2 (PLIN2) occurring with extremely high frequency, which participates in shell color determination by regulating melanin production and carotenoid metabolism, respectively. In addition, two genes, namely porphobilinogen deaminase (PBGD) and hephaestin (HEPH), may have a potential function in the accumulation of specific porphyrin in orange shells, while the kynurenine 3-monooxygenase (KMO) gene, known to be involved in ommochrome biosynthesis, may contribute to the darker coloration in black shells. Further, a total of 304 SNPs were extracted from the exons of candidate genes mentioned above, of which 25 loci had highly significant differences among populations (p-value ≤0.01). After Sanger sequencing in another population, 14 SNPs were successfully validated as specific loci closely linked to different shell colors of black, white, gold, and orange, respectively. All in all, the results of this study shed light on the molecular basis of pigmentation and facilitate the selective breeding of C. gigas with desired phenotypic color.
•Long-term artificial selection has caused large genetic differentiation among four shell color strains of C. gigas.•A composite measure of FST and XP-CLR identified divergent genomic regions associated with shell color selection.•Five pigment-related genes, namely TYR, PLIN, PBGD, HEPH, and KMO, were involved in the shell color formation of C. gigas.•A total of 14 SNPs were validated as specific loci linked to distinct shell colors of black, white, gold, and orange.
Pacific oysters, Crassostrea gigas, are one of the most widely farmed shellfish. To meet demand, the advantages of growth and increased sterility are currently being exploited in the commercial ...culture of triploid C. gigas. However, the extent of the triploid advantage can vary, environmental conditions and rearing methods. In the present study, the performance of diploid and triploid oysters was compared at two sites (Rongcheng and Rushan) to determine whether the triploidization would result in any advantage on production traits that could be utilized for aquaculture of this species. Triploid oysters were also produced using the wild broodstock and selected line to verify whether selectively bred oysters maintain their improved traits after triploidization. In this study, we found that the survival advantage of triploid is “site-dependent” and that Rongcheng's triploids did not exhibit significantly higher cumulative survival than diploids. Notably, incremental survival of triploid oysters was consistently higher than that of diploids during the second summer, which may be related to gonad development and spawning in diploids. At both sites, growth traits (shell height and individual weight) were always higher for triploids than in diploids in both the selected and control groups, which is strong evidence of a superior growth advantage for triploids. In addition, we found that the production performance (growth, survival and yield) of the selected group was consistently better than that of the control group (either diploid or triploid), suggesting that the superior traits of diploids can be converted to triploids through ploidy manipulation. Our results support the notion that yield can be further optimized by reducing mortality through a selective breeding program coupled with triploid production to increase growth rates. This study provides meaningful information on the potential of triploid C. gigas for oyster culture in northern China.
•Triploid oysters showed significant growth advantage in the two commercial culture sites.•The survival advantage of triploids (relative to diploids) is not observed at all sites and is “site-dependent”.•The excellent traits of selected diploids can be maintained in triploids.
The potential persistence and migration of 14 antibiotics comprising sulfonamides, fluoroquinolones, macrolides and tetracyclines were conducted using a 50-d recirculating flume study supported by ...batch attenuation experiments with spiked concentrations. The study demonstrated that photodegradation was the dominant attenuation process for these antibiotics in the water environment. The half-lives of 2–26 d were in order of sulfadiazine > sulfadimethoxine > sulfamerazine > sulfamethoxazole > sulfamethazine > sulfathiazole > ofloxacin > enrofloxacin > norfloxacin > ciprofloxacin > erythromycin > tetracycline > roxithromycin > oxytetracycline. These modest half-lives meant that the antibiotics were predicted to travel 30–400 km down a typical river before half the concentration would be lost. All antibiotics were detected on the surface sediment in the flume study. Under hyporheic exchange, some of them continually migrated into the deeper sediment and also the sediment pore water. All fluoroquinolones were detected in the sediments. The sulfonamides were detected in the pore water with relatively high concentrations and frequencies. Sulfadiazine, sulfamethazine and sulfathiazole in the upper layer pore water were found to be approaching equilibrium with the surface water. The high presence of sulfonamides in the pore water indicated that their high mobility and persistence potentially pose a risk to hyporheic zone.
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•Attenuation of 14 antibiotics was studied for 50 d in a simulated stream.•Persistence was in order of sulfonamides > quinolones > macrolides > tetracyclines.•Photodegradation was the dominant attenuation mechanism.•All quinolones were detected in the lower layer sediments at15 cm depth.•Sulfonamides were present in the sediment pore water with high concentrations.
Capsule: Natural attenuation of antibiotics in streams.
Mass mortalities of juvenile Pacific oysters (Crassostrea gigas) in aquaculture operations have been associated with sustained high temperatures during the summer months. Selective breeding programs ...to improve field survival in the C. gigas are expensive, labor-intensive, and often rely on prolonged field trials. Therefore, controlled challenge methods for improving selection response towards increased summer mortality resistance are required. The purpose of this study was to verify whether juvenile survival after heat challenges predicts field summer survival for full-sib families of C. gigas. We thermally challenged juveniles from 59 families using two separate methods (chronic heat stress and acute heat shock) and monitored their survival rates. Oysters were also deployed in two major oyster farms and then survival rates were compared between the two conditions. In this study, families from different genetic backgrounds differed in heat tolerance and field survival. Correlations between heat challenges and field trials were low to moderate and positive (0.321–0.346 for acute heat shock and 0.472–0.491 for chronic heat stress). Our results suggest that although current heat challenge trials cannot completely replace field trails in selective breeding programs, selection for heat tolerance is expected to indirectly improve field survival. An important outcome of this study is that heat challenges can discriminate some families that are resistant or susceptible to summer mortality at an early age, and that selection efficiency can be improved by identifying and eliminating poor-performing families prior to planting. In a separate experiment, we found that juvenile oysters had significantly higher summer survival when they were exposed to heat challenges than oysters that were not exposed earlier. We conclude that heat challenge of oysters is a useful method for assessing the survival capacity of hatchery-produced spat used by the oyster industry.
•This paper investigated the heat tolerance of juvenile oysters and compared these results with those obtained in the fields.•Heat challenges can discriminate some families that are resistant or susceptible to summer mortality at an early age.•Juvenile oysters had significantly higher summer survival when they were exposed to heat challenges than oysters that were not exposed earlier.
For aquatic species, long-term intense artificial selection inevitably leads to decreased genetic diversity and increased genotypic purity in the population, which may lead to a reduction in the ...adaptation of populations to their environment. Consequently, how to restore a high level of population genetic diversity becomes a critical issue in the breeding process. In this study, a complete diallel crosses was performed utilizing “Haida No. 1”, which had been selected for 10 generations, and the Orange-shell line of the Pacific oyster (Crassostrea gigas), which had been selected for 8 generations. The growth and survival traits of the crossed and self-crossed groups were systematically evaluated during the larval and grow-out stages. Meanwhile, the population genetic diversity of the four combinations was also assessed using 18 microsatellite loci and mitochondrial cytochrome oxidase I sequences (mtCOI). The growth and survival of two reciprocal groups were significantly larger than those of two parental lines. Meanwhile, the average allelic richness (Ar), observed heterozygosity (Ho), expected heterozygosity (He) and number of alleles (Na) of the hybrids was considerably larger than two purebreds. Moreover, significant reduction in average inbreeding coefficient Fis was detected in hybrids when compared to purebreds. The results indicate that crossbreeding between selected lines could not only obtain phenotypically superior descendants, but also increase the genetic diversity of C. gigas.
•The offspring obtained by crossing “Haida No.1” and Orange-shell lines of C. gigas exhibited significantly heterosis in both growth and survival.•The observed heterozygosity, expected heterozygosity and number of alleles of the two hybrids were significantly elevated compared to the parental lines.•Crossbreeding can not only obtain offspring with excellent performance, but also restore the genetic diversity of C. gigas to a higher level.
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