The obvious heterosis and genetic characters of intergeneric cross and backcross juveniles between blunt snout bream (Megalobrama amblycephala) and topmouth culter (Culter alburnus)

In this study, we used artificial insemination to generate hybrid groups of fish [MC‐F₁(MA♀×CA♂) and MC‐F₂(MC‐F₁♀×♂)] by intergeneric crosses of Megalobrama amblycephala (MA) and Culter alburnus (CA); sequential backcrosses [CAM‐B₁ (CA♀×MC‐F₁♂) and MCC‐B₁ (MC‐F₁♀×CA♂)] were also performed. All these hybrids showed high rates of fertilization, hatching and survival (p > 0.05). For genetic traits, compared with those of the M. amblycephala and C. alburnus parental lines (Table 1), the fertilization rate, hatching rate and 7‐day survival rate of MC‐F1(MA♀×CA♂), MC‐F2(MC‐F1♀×♂), CAM‐B1 (CA♀×MC‐F1♂) and MCC‐B1 (MC‐F1♀×CA♂) by artificial insemination exhibited similar high rates (p > 0.05). The morphology of the four hybrids MC‐F₁/F_2, CAM‐B₁ and MCC‐B₁ were intermediate between those of their parents. Compared with their parents of MA and CA, weight gain rate (WG), specific growth rate (SGR) and protein efficiency ratio (PER) of hybrids MC‐F₁/F₂, CAM‐B₁ and MCC‐B₁ were significantly (p < 0.05) increased and feed conversion ratio (FCR) was significantly (p < 0.05) decreased after 3 months feeding. Moreover, protein content of muscle for MC‐F₁/F₂, CAM‐B₁ and MCC‐B₁ was significantly (p < 0.05) higher and carbohydrate content of muscle was significantly (p < 0.05) lower than their parents. The females and males of the four hybrids had normal gonadal development. In this study, we successfully generated intergeneric and backcross hybridization lines with fertile potential among fish of the Cultrinae subfamily and these hybrids had obvious heterosis in terms of growth performance, feed utilization and muscle quality.     

Fertilization, hatching, survival and growth rates in reciprocal crosses of two strains of an African catfish Heterobranchus longifilis Valenciennes 1840 under controlled hatchery conditions

A diallelic cross between two strains [Layo strain (LS) and Noun strain (NS)] of the catfish Heterobranchus longifilis Valenciennes was carried out under controlled hatchery conditions to estimate their reproductive performance and aquaculture potential in terms of fertilizability, hatchability, survival, growth and heterosis. The average fertilization rate of all mating groups was as high as 90.4%, the fertilization rate of the purebred NS (95.2%) being significantly higher than that of the purebred LS or the reciprocal crosses (P < 0.05). The average hatching rate of all genetic groups was as high as 84.7%, the hatching rate of NS (89.4%) being significantly higher than that of the purebred LS or the reciprocal crosses (P < 0.05). The mean survival rate of all crosses from hatching up to the onset of exogenous feeding stood at 94.2%, without showing a significant difference (P > 0.05) between the crosses. During the larval rearing period, which extended from the onset of exogenous feeding up to 15 days of age, NS displayed a significantly lower growth rate (P < 0.001) than that of the purebred LS and their reciprocal hybrids. There was no significant difference (P > 0.05) in survival rate (mean 84.7%) between the four crosses at the end of the larval rearing period. During the juvenile rearing phase, the mean growth performance of all crossbreds was similar to that of the purebred LS and significantly different from that of the purebred NS (P < 0.001). The final individual weights attained by LS, NS, LS × NS and NS × LS were 3.31, 0.71, 3.97 and 3.66 g respectively. The increase in weight attained by the fast‐growing crossbred LS × NS was 562.2%, 20.4% and 8.7% more than that of NS, LS and NS × LS respectively. The survival rate of NS (57.1%) was significantly lower (P < 0.001) than that of all the rest of the crosses. The crossbreds displayed about 15.1% heterosis in mean body weight relative to the fast‐growing purebred LS. It was concluded that cross‐breeding of H. longifilis strains could be advantageous because of the hybrid vigour in the progeny.     

Comparison of the fillet yield, protein content and amino acid profile of Clarias gariepinus and the Clarias gariepinus 5 Heterobranchus longifilis hybrid

The fillet yield, total protein content and amino acid profile of the African sharptooth catfish Clarias gariepinus (Burchell) was compared with that of its hybrid with the African catfish Heterobranchus longifilis Valenciennes. The flesh of the C. gariepinus × H. longifilis hybrid had a protein content of 18.3%, compared with 18.6% for C. gariepinus. The amino acid profiles were similar for the two groups. The fillet yield of the C. gariepinus females was significantly lower than that of the male and female hybrids and C. gariepinus males. The difference in fillet yield was attributed to the high gonadosomatic index (GSI) of the C. gariepinus females, compared with the other groups. The overall fillet yield was 38.9% for C. gariepinus and 43.8% for the hybrid, which has important implications for African catfish production.     

Nutrient digestibility studies in Heteropneustes fossilis (Bloch), Clarias batrachus (Linnaeus) and C. gariepinus (Burchell)

The apparent digestibility coefficient (ADC %) for protein in catfishes, Clarias batrachus (Linnaeus) (16–18 cm; 45–50 g), C. gariepinus (Burchell) (21–22 cm; 60–65 g) and Heteropneustes fossilis (Bloch) (10–12 cm; 10–15 g) was estimated for nine feedstuffs of plant and animal origin. Fat digestibility was tested for C. batrachus and C. gariepinus, wherein the influence of feeding frequency on protein digestibility was also examined. The digestibility trials were conducted with triplicate groups of 25 fish each in 70‐L polyvinyl indoor flow‐through (1–1.5 L min^?1) circular tanks (water volume=55 L). Fish were fed to satiation daily, at 08:00 and 18:00 hours, and faecal matter was collected through slow siphoning. For studying the effect of feeding frequency, fish were fed the experimental diets at 08:00, 11:00, 14:00 and 17:00 hours. The protein digestibility of the feedstuffs varied significantly within each test species. However, the protein digestibility values for most of the ingredients tested were comparable between the three catfish species. The most efficient utilization of protein in these fishes was noted for soybean meal followed by lab‐prepared fishmeal. The lowest protein digestibility was recorded for rice bran. Similarly, the utilization of fat from the feedstuffs was also significantly different within each of the two species, C. batrachus and C. gariepinus. Fat digestibility of feed ingredients was similar between the two species. With respect to the fat, soybean meal, lab‐prepared fishmeal, full‐fat soybean meal and dried fish were more efficiently utilized, while the lowest fat digestibility was obtained for rice bran. A marked decline in protein utilization was observed in the two species where the feeding frequency was increased from two to four times a day.     

Production and Processing Trait Comparisons of Channel Catfish,Blue Catfish,and Their Hybrids Grown in Earthen Ponds

Fingerling HS‐5 channel catfish, Ictalurus punctatus, NWAC 103 channel catfish, D&B blue catfish, Ictalurus furcatus, HS‐5 female channel × D&B male blue catfish F₁ hybrids, and NWAC 103 female channel × D&B male blue catfish F₁ hybrids were stocked into twenty‐five 0.04‐ha earthen ponds at 12,500 fish/ha and grown for 277 d. Fish were fed daily at rates from 1.0 to 3.0% biomass based on feeding activity and temperature and adjusted weekly assuming a feed conversion ratio (FCR) of 1.8 and 100% survival. At harvest, 40 fish from each pond were sampled, and all other counted and weighed. Mean survival, growth rate indexes (a), FCR, and skin‐on fillet percentages were not significantly different. Mean harvest weights and net production were higher for HS‐5 channel and its hybrid than for the NWAC 103 channel, NWAC 103 hybrid, and D&B blue catfish, partially because of their larger mean stocking weights. D&B blue catfish was more uniform in size than NWAC 103 channel and NWAC 103 hybrid. D&B blue catfish was the easiest to seine. HS‐5 hybrids and NWAC 103 hybrids had lower mean head percentage and a better processing yield than their parent channel catfish.     

Comparative Growth and Survival of Reciprocal Hybrids Between Clarias gariepinus and Clarias anguillaris

Reciprocal, interspecific hybrids of Clarias gariepinus and Clarias anguillaris were produced via hypophysation and dry stripping. Two-week-old fry (0.06 ± 0.02 g) of the parental Clarias species and their interspecific hybrids were stocked in duplicated 1 m³ hapas suspended in outdoor earthen ponds (16 m²) for a rearing period of 56 days. Growth and survival of the hybrids was intermediate to the faster growing and better surviving C. gariepinus and the more slowly growing and more vulnerable C. anguillaris parental lines. The weight gain attained by C. gariepinus, C. anguillaris, C. gariepinus (♂)?×?C. anguillaris (♀), and C. anguillaris (♂)?×?C. gariepinus (♀) were 3.18 g, 2.99 g, 3.16 g and 3.05 g, respectively. Among hybrids, the growth of C. gariepinus (♂)?×?C. gariepinus (♀) was higher than the reciprocal C. anguillaris (♂)?×?C. gariepinus (♀). Heterosis for growth was a negligible 0.65% and for survival was negative (?5%). C. gariepinus appears to be the best species for aquaculture in terms of growth rate and survival.     

First Evidence for Hybrid Breakdown in the Backcross of Olive Flounder,Paralichthys olivaceus,and Summer Flounder,Paralichthys dentatus

Hybrid and parental backcross experiments were performed using olive flounder, Paralichthys olivaceus, summer flounder, Paralichthys dentatus, and their female F₁ hybrids to examine hybrid fitness in the backcross. Fertilization rate, hatching rate, and combined fitness measure (CFM, product of fertilization rate and hatching rate) were detected and the results showed that the CFM of hybrid and backcross generations was reduced significantly compared to pure olive flounder crosses. Genomic inheritance from parents to backcross progeny was also detected by amplified fragment length polymorphism (AFLP). Almost all AFLP bands observed in parents were presented in backcross progeny (backcross1, 218/227; backcross2, 265/282), although 9 and 17 parental bands were missing in backcross1 and backcross2, respectively. Novel bands (absent in parents) also occurred in progeny at the frequencies of 5.63% (13/231) in backcross1 and 3.28% (9/274) in backcross2. Additionally, 27.40 and 31.18% of AFLP markers deviated from expected Mendelian ratio in backcross1 and backcross2, respectively. The present study suggests that genetic incompatibilities may exist between olive flounder and summer flounder. Furthermore, possible implications of segregation distortion for reduced hybrid fitness in backcross generations are discussed.     

Determination of growth,colour and other traits in F1 hybrid of Amphiprion percula (male) × A. ocellaris (female)

This study reported the traits values such as colour, feed conversion ratio (FCR), specific growth rate (SGR) and fluorescence capacity of F₁ hybrids of Amphiprion percula (male) and A. ocellaris (female). The hybrids exhibited significant variation in FCR (3.63 ± 0.56) and SGR (3.63% ± 0.44) compared with the pure breeds, A. percula (3.12 ± 0.42; 2.80% ± 0.42) and A. ocellaris (3.17 ± 0.43; 3.02% ± 0.19). An exponential relationship was found between FCR and SGR in both the breeds. Image analysis displayed a better colour performance of hybrid than the pure breeds. Individual body parts of the hybrid and pure breeds showed significant colour variation between each other. However, colour contrast of whole body of hybrid was found closer to A. ocellaris in hue cone and towards A. percula in saturation and brightness values. Hence, hybrid displays combination colour reflexion of both the parents. The total pigment content of hybrid (65.71 μg g^?1 ± 2.81) was found higher than A. ocellaris (62.01 μg g^?1 ± 2.29) and A. percula (56.71 μg g^?1 ± 2.56). Further, the spectroflurometric analysis revealed that the both hybrid and pure breeds having poor fluorescence on skin pigmentation. A direct positive heterosis was observed on the SGR, FCR, total pigment and spawning frequency, while negative effect was noted on total length of newly hatched larvae (TL), fertilization rate (FrR), hatching rate (HR), deformation rate (DFR) and survival rate (SR). Hence, multiple cross‐breeding programmes will help in developing high‐quality traits in successive generations.     

Genetic basis of pectoral fin deformities in the African catfish Clarias gariepinus (Burchell, 1822), Heterobranchus longifilis (Valenciennes 1840) and their hybrids

Morphological aberrations of the pectoral fins in nine mating combinations involving Clarias gariepinus (Burchell, 1822), Heterobranchus longifilis (Valenciennes, 1840) and their hybrids were investigated to determine the level and genetic basis of occurrence. The highest mean percentage survival in a Clarias× Clarias group was 75%, whereas the least mean percentage survival was 2% in the same group. The least mean percentage survival (40.3%) in the remaining three groups occurred in the cross of female hybrid (right pectoral fin absent) × male C. gariepinus (right pectoral fin absent). A maximum of nine types of aberrations was observed in the four mating groups – double dorsal fin, curved posterior dorsal fin, spineless right pectoral fin, right pectoral fin absent, left pectoral fin absent, rudimentary pectoral fin, both pectoral fins absent, double anal fin and curved anterior dorsal fin. These nine aberration types were recorded in the Clarias× Clarias group, with a total frequency ranging from 7.14% to 75.00%. The least number of aberrations was observed in the hybrid ×Clarias group (double dorsal fin and both pectoral fins absent) with a frequency range of 1.47–5.55%. No aberration was observed in two crosses involving female hybrid (right or left pectoral fin absent) × female C. gariepinus (normal). The level of aberrations in some of these crosses indicates the involvement of genotype rather than the influence of environment.     

吉富罗非鱼与奥利亚罗非鱼自繁与杂交 F1遗传特性与抗病力分析

以吉富罗非鱼与奥利亚罗非鱼为繁育亲本,采用完全双列杂交繁育4组F1,将初始规格基本一致的4组罗非鱼饲养100 d后,运用“加性-显性”遗传分析模型,分析了4组F1罗非鱼8个生长相关性状杂种优势、遗传效应以及与性状间的相关性.结果表明:(1)F1群体平均优势为0.129 4～0.368 4.除尾柄长超亲优势较大外,其他性状的群体超亲优势较小或表现出负向超亲优势.(2)8个性状的广义遗传率(HB)为0.714 2～0.995 3,表明加性效应和显性效应对性状的遗传变异影响极显著(P＜0.01).除尾柄长外,其他性状的狭义遗传率(HN)介于0.469 4～0.737 9,表明加性遗传方差在表型方差中所占比率较高.(3)体质量、全长、体长、体高、体宽、头长、尾柄长、尾柄高性状之间表型相关在0.776 6～0.999 7范围内,而遗传相关在0.994 1～1.000 0之间,表明这些性状间都存在极显著的正相关.取样结束后,采用3.95×106 CFU/mL的海豚链球菌菌液进行腹腔感染,吉富罗非鱼自繁组F1代12 h后出现死亡,而奥利亚罗非鱼自繁组F1192 h后才出现死亡.384 h后,吉富罗非鱼自繁组F1死亡率为40％,正反交组F1分别为20％和23.3％,奥利亚罗非鱼自繁组F1死亡率最低,为6.67％.研究结果表明,除尾柄长外,杂交F1的其他性状不具备超亲优势,然而杂交可以提高选育后代的抗病力.     

Preliminary observations on growth and survival of Oreochromis spilurus x GIFT Oreochromis niloticus F1 reciprocal hybrids in fresh and seawater

Tilapia has a significant potential for culture in saline environments. There is an increasing demand among tilapia producers to develop a tilapia hybrid that can survive well in marine water conditions. This study compared mean weight (MWT), daily growth rate (DGR), specific growth rate (SGR), survival, feed conversion ratio (FCR), condition factor (K), production rate (PR) and rate of skin lesions, and tail and fin rot as well as eye cataract in the Genetically Improved Farmed Tilapia strain (GIFT) of the Nile tilapia, Oreochromis niloticus (NN) and the salt‐tolerant Oreochromis spilurus (SS), and their F₁ reciprocal hybrids; O. niloticus ♀ x O. spilurus ♂ (NS) and O. spilurus ♀ x GIFT O. niloticus ♂ (SN) in freshwater (0 g L^?1) and seawater (40 g L^?1). Fish (3.5 g) were stocked at 150‐fish m^?3 and fed with sea bream pellets (47% protein) for 180 days. Results showed that in seawater, the SN hybrid had the highest values for MWT (165.9 g), DGR (0.9 g fish^?1 day^?1), SGR (2.14% day^?1), survival (96.3%), PR (23.9 kg m^?3) and best FCR (1.53) followed by the NS hybrid and the NN parent. Both hybrids had significantly lower rates of skin lesions and fin and tail rot than the NN genotype. In freshwater, the NN had the highest values for MWT (255.1 g), DGR (1.40 g fish^?1 day^?1), SGR (2.38% day^?1), K (2.13%) and PR (34.9 kg m^?3) followed by the NS and SN hybrids. High estimate of heterosis for MWT (41.3%), DGR (42.5%), SGR (10.7%) and survival (22.1%) was obtained in the hybrids reared in seawater, indicating that a hybrid vigor was produced and the fast growth trait from the GIFT parent was successfully combined with the salinity tolerance trait from the O. spilurus parent. The better growth performance and survival of the SN hybrid in seawater indicate that this hybrid is more suitable for culture in seawater than its reciprocal hybrid.     

Reproductive ability of second generation ornamental (koi) carp (Cyprinus carpio L.) × goldfish (Carassius auratus L.) hybrids and characteristics of their offspring

The purpose of this study was to investigate the reproductive ability of second generation (F₂) koi (Cyprinus carpio L.) × goldfish (Carassius auratus L.) hybrids. Only diploid F₂ males and females were fertile and used in crosses. A significant increase was recorded in male fertility in F₂ versus F₁. In contrast with an earlier study in which only one fertile F₁ male was found, about 20% of F₂ males produced sperm. The observed reproductive ability of F₂ hybrids was similar to that demonstrated by the only fertile F₁ male and F₁ females. F₂ males produced diploid spermatozoa and generated triploids when crossed with koi females. All triploid fish in these progenies were males indicating that F₂ males had a sex chromosome constitution of XY. F₂ females produced diploid eggs and generated mostly triploids when crossed with koi males. In progenies obtained by crosses of F₂ males with F₁ and F₂ females, most of the surviving juveniles (63%–100%) were diploid; a minority of juveniles were aneuploid (ploidy ranged from 2.1n to 3.6n). Diploid fish in these progenies were presumably the result of spontaneous androgenesis and gynogenesis, by the same mechanisms observed earlier in progenies obtained by crossing the F₁ male with F₁ females.     

A comparative study on protein‐sparing effects among juvenile Erythroculter ilishaeformis line,Ancherythroculter nigrocauda line and their hybrid F1 fed diets with different protein to carbohydrate ratios

To investigate the mechanism underlying the difference of dietary protein requirements among Erythroculter ilishaeformis line, Ancherythroculter nigrocauda line and their hybrid F₁ (E. ilishaeformis♀×A. nigrocauda ♂), juvenile fish of these three species with initial body weight (5.86 ± 0.10 g) were fed the six isoenergy diets with different dietary protein/carbohydrate ratios (P/C ratios were 0.56, 0.78, 1.11, 1.61, 2.52 and 4.43, respectively) for 8 weeks. The results showed that the hybrid culter had the higher weight gain rate (WGR), protein efficiency ratio and protein deposition ratio compared with its parents lines; using WGR as a response criteria, the dietary carbohydrate tolerant level of juvenile E. ilishaeformis line, A. nigrocauda line and their hybrid was 24.3%, 28.5% and 29.7%, respectively, and the optimal P/C ratio was 1.69, 1.35, 1.22 and respectively. Dietary carbohydrate activated hepatic protein kinase B (Akt), dietary protein activated hepatic target of rapamycin (TOR) and the induction of mRNA expression of hepatic phosphoenolpyruvate carboxykinase and pyruvate kinase showed a similar change in hybrid F₁ and its parents. A. nigrocauda had the highest enzymes activities in intestinal protease and amylase, and hybrid culter had the highest intestinal lipase activities. Our results demonstrated that the hybrid culter has the better protein‐sparing effects by increasing carbohydrate metabolism than its parent lines.     

Progression of African Catfish Clarias gariepinus (Burchell) early stages challenged with two common pond fertilizers in incubation media

Early‐stage indicators of the African catfish Clarias gariepinus (Burchell) (egg fertilization and hatching rates, larval survival rate and time to hatch) were assessed in aqueous solutions of NPK 15‐15‐15 and layer manure. Recommended use rates (RUR) of NPK 15‐15‐15 and layer manure have volume‐equivalent of 15 mg/L and 28 mg/L respectively. Solutions containing *0.7RUR, *1.0RUR and *1.5RUR dilutions were used as treatments within‐fertilizer. There were significant differences in pH, dissolved oxygen (DO), total hardness and total metal levels in treatments of NPK (p < 0.0001) and layer manure (p < 0.0001). Fertilization, hatching rate and larval survival were significantly lower while incubation was significantly longer in treatments of NPK (p < 0.001) and layer manure (p < 0.0001) in comparison with control. In NPK treatments, the indicators correlated with pH (r ?0.96 to ?0.98, p < 0.0001), with 99.1%–99.6% (p < 0.0001) of changes of indicators explained by pH. In manure treatments, the indicators correlated with DO and pH (r = 0.92 to 0.998) (p < 0.0001), with 98.1%–99.7% (p < 0.0001) of changes explained by DO and pH. NPK and layer manure have deleterious effect on C. gariepinus early stages. The toxicity and explanatory physico‐chemical parameters should be considered when the fertilizers target aquatic productivity.     

近交对长牡蛎幼虫和稚贝生长与存活的影响

分别以2010年通过2个野生个体进行交配获得的A01全同胞家系和2011年通过A01家系子代进行交配获得A02全同胞家系为亲本,在2013年6月采用同时建立全同胞交配子一代F1(F=0.250)、全同胞交配子二代F2(F=0.375)及设置对照组(F=0)的方法,研究在相同环境条件下,不同实验组的受精率与孵化率以及近交对长牡蛎幼虫期、稚贝期生长和存活的影响,并初步探讨近交代数与近交衰退的关系。结果发现,各组的受精率均在90%以上,除F2组外其余2组的孵化率也在90%以上;幼虫阶段,F1组和F2组的壳高与壳长均从12日龄出现衰退(近交衰退系数,inbreeding depression coefficient,IDC0),且F2组壳高的近交衰退系数均小于同日龄F1组壳高的近交衰退系数;F1组和F2组的存活率在整个幼虫期间均出现衰退,且F1组和F2组存活率的近交衰退系数均随着幼虫日龄的增加而逐渐减小。稚贝阶段,F1组和F2组的平均壳高在各日龄均表现出近交衰退(IDC0),且F2组壳高的近交衰退系数均小于相同日龄F1组壳高的近交衰退系数;3个实验组的平均壳长在整个稚贝阶段无显著性差异;F1组和F2组存活率的衰退在不同日龄始终存在(IDC0),且随着稚贝日龄的增加其衰退程度逐渐加大。研究结果为长牡蛎选择育种和遗传改良提供了基础资料。     

Effect of water hardness on egg hatchability and larval viability of Clarias gariepinus

Studies were conducted to determine the effect of water hardness onClarias gariepinus egg hatchability and larval viability.The fertilized eggs were incubated at 28 °C and with waterhardnesses ranging from 10–700 mg/l CaCO₃. Themean hatching rate fluctuated between 42.31% at hardness of 10mg/land 64.66% at 2000 mg/l. Abnormalities in the larvae were observedbeyond 200 mg/l and increased with increase in water hardness. Thehighest larval survival of 71.05% was recorded at 60 mg/l waterhardness. Based on statistics performed with analysis of variance (ANOVA) andfurther compared with Duncan's multiple range test (p = 0.05), the resultsimplythat very soft water (0–10 mg/l) and very hard water (300mg/l and above) are not suitable for Clariasegg incubation and larval rearing. A water hardness of 30–60mg/l CaCO₃ is recommended for optimal normal hatching,high viability and maximum larval development of Clariasgariepinus.     

Temperature effects on sex differentiation of the reciprocal hybrids of Odontesthes bonariensis and Odontesthes hatcheri (Atherinopsidae)

The pejerrey Odontesthes bonariensis (Valenciennes 1835) and the Patagonian pejerrey Odontesthes hatcheri (Eigenmann 1909) are Atherinopsid species with commercial importance and potential for aquaculture. The hybrids of the two species are viable but their mode of sex determination is unknown. This study examined the gonadal histology and sex ratios of reciprocal hybrids that were reared at 15, 17, 21, 25 or 29 °C during the sex differentiation period. The genetic sex of hybrids from O. hatcheri fathers was inferred from a sex‐linked SNP marker. Both hybrids showed female‐biased sex ratios at the lowest temperature, female‐biased to balanced sex ratios at intermediate temperatures and balanced or male‐biased sex ratios at 29 °C, but unlike in purebred O. bonariensis, the lowest and highest temperatures did not yield monosex populations. The proportion of females in the offspring was affected more by parental genome than by hybrid combination. Female hybrids bearing the O. hatcheri Y chromosome showed temporary arrest of ovarian development that was rescued in adults. These results reveal strong interactions between genotype and temperature for sex determination and differentiation of the hybrids and provide important clues to understand the mechanisms of sex determination in these species.     

Selection of a new scallop strain,QN‐2, from the backcross of Peruvian scallop × bay scallop F1 hybrids with bay scallops

Cross‐breeding has been successfully applied in agriculture and aquaculture industries to improve important production traits. In our previous studies, we produced the F₁ hybrids with high heterosis in growth by crossing the bay scallops with the Peruvian scallops and selected a new strain, Bohai Red, from the F₁ hybrids. In this study, we selected another new strain from the backcross (BC₁) of the F₁ hybrids with bay scallops. The base cohort (G₀) was size‐selected from the BC₁, and the G₁, G₂, G₃ and G₄ cohorts were produced with size‐selected brood stocks from the G₀, G₁, G₂ and G₃ cohorts respectively. Our results showed that average shell height, shell length, shell width and whole body weight of the G₄ cohort were increased by 16.8%, 16.8%, 11.5% and 43.6%, respectively, compared with the unselected control group of bay scallop. This new strain has now become one of the major cultured Argopecten scallops in northern China waters and was later named QN‐2.     

The effect of cryoprotectants on the motility and fertilizing capacity of cryopreserved African catfish Clarias gariepinus (Burchell 1822) sperm

The effect of six cryoprotectants was investigated on the cryopreservation of African catfish Clarias gariepinus (Burchell) sperm. Fructose (6%) solution buffered with NaHCO₃‐CO₂ was used as the diluent in the experiments. Glycerol (5–11%), ethylene glycol, methanol and propylene glycol (5–15%) and, finally, dimethyl sulphoxide (DMSO) and dimethylacetamide (DMA) (10%) were tested using various equilibration times (2–30 min). Sperm was frozen in 250‐μL straws in a programmable freezer (Cryocell‐15, BLS, Hungary) from 3 °C to ?4 °C at 4 °C/min and from ?4 °C to ?80°C at 11 °C/min. Thawing was carried out in a 40 °C water bath for 5 s. Fertilization and hatching trials were performed only with DMSO and DMA using 200 and 100 μL of diluted sperm (100 and 50 μL of pure sperm) and the dry and the wet fertilization methods. Ethylene glycol, glycerol, methanol and propylene glycol yielded poor results. An average post‐thaw motility rate of 44.0 ± 9.7% and 22.6 ± 18.1% was achieved after 10 min equilibration using DMSO and DMA respectively. Highest average fertilization (86.8 ± 3.1%) and hatching (67.1 ± 11.9%) rates were achieved with DMA and DMSO, respectively, 200 μL of diluted sperm and the wet fertilization technique. The use of cryoprotectants increased the percentage of malformed larvae compared with the control groups. We found that DMA at a 10% concentration was equally as suitable for the cryopreservation of African catfish sperm as DMSO.     

Growth,salinity tolerance and microsatellite analysis of the F2 reciprocal hybrids of Oreochromis niloticus×Sarotherodon galilaeus at different salinities

Oreochromis niloticus (O), the sixth generation of Genetic Improvement of Farmed Tilapia, shows rapid growth but poor salt tolerance, while Sarotherodon galilaeus (S) exhibits opposite traits. To combine the traits, F₁ progeny was obtained through artificial fertilization. Fertile F₁ produced F₂ by natural spawning. The mean survival times, the median survival time (ST₅₀) or the survival rate of hybrids was greater than O. niloticus in a gradual or an acute salinity change. Plasma osmolarity, [Na⁺] and [Cl^?] of the hybrids fluctuated in 32 g L^?1 water during a 24‐h period, but eventually reached levels similar to fish in freshwater. O. niloticus♀×S. galilaeus♂ (OS F₂) or S. galilaeus♀×O. niloticus♂ (SO F₂) showed the fastest growth at 22.5 g L^?1, equal to about 78.2% or 69.7% of O. niloticus at 0 and 3.87 or 3.45 times that of S. galilaeus at their individual optimum growth. Growth in OS F₂ was 12% faster than SO F₂. Microsatellite analysis showed that F₂ had more alleles, a higher polymorphism information content and greater observed and expected heterozygosity than O. or S. Population differentiation was not detected between F₁ and F₂. All the results indicated that F₂ could be exploited for commercial production under saline conditions.