四个奥利亚罗非鱼群体的微卫星分析

应用筛选到的19对微卫星引物,对四个不同来源的奥利亚罗非鱼（Oreochromis aureus）群体（奥利亚罗非鱼83系、奥利亚罗非鱼02系、奥利亚罗非鱼05系和红色奥利亚罗非鱼）的基因组DNA进行PCR扩增,分析其群体遗传结构和亲缘关系。根据几个群体在19个位点上的PCR扩增图谱,统计计算各群体的遗传多样性指数。四个群体的平均观测遗传杂合度值在0．154—0．391间;平均预期杂合度在0．181—0．428间;平均多态信息含量值在0．1513—0．3882间,说明它们的遗传多样性水平较低。遗传偏离指数D的评估结果显示这4个群体有多个位点存在不同程度的Hardy—Weinberg遗传平衡偏离。运用MicroChecker软件进行零等位基因预测,结果显示除红色奥利亚罗非鱼群体外,其他3个群体中均可能存在零等位基因位点。各群体零等位基因的位点数分别为：83系1个,02系3个,05系7个,红奥群体为0。零等位基因位点的存在可能是导致位点发生Hardy—Weinberg遗传平衡偏离的原因之一。4个群体中,05系群体与83系群体间的遗传相似性系数最高（0．9422）,遗传距离最小（0．0596）,说明两者亲缘关系最近;83系群体与红奥群体的遗传相似性系数最低（0．6977）,遗传距离最大（0．3599）,可推断两者亲缘关系最远。根据群体间的遗传距离采用UPGMA法进行聚类,结果表明：83系首先与05系聚类为一支,然后与02系群体聚类,最后与红奥群体聚类。聚类结果说明红奥群体与其他三个群体亲缘关系最远;83系群体与05系群体亲缘关系最近,与02系群体次之。     

Toxicity and genotoxicity of wastewater from gasoline stations

The toxicity and genotoxicity of wastewater from eight gasoline stations in Brasília, Brazil's capital city, was studied by assessing chromosomal aberrations, chromosomal malsegregation and the mitotic index in Alliumcepa root cells, and the occurrence of micronucleus and nuclear abnormalities in peripheral erythrocytes of tilapia fish (Oreochromis niloticus). The content of gasoline station effluents was also analyzed based on several physico-chemical parameters. None of the wastewater samples was genotoxic to A. cepa root cells, although cell proliferation was significantly inhibited, especially at the highest concentrations. Likewise, no micronuclei were observed in O. niloticus peripheral erythrocytes, even after exposure to high concentrations, but there was an increase in the number of nuclear abnormalities and fish mortality. These results show that although the effluent from gasoline stations is processed by an oil/water separation system before being discharged into the main sewage system, the wastewater still contains toxic compounds.     

Studies on Bacillus subtilis and Lactobacillus acidophilus, as potential probiotics, on the immune response and resistance of Tilapia nilotica (Oreochromis niloticus) to challenge infections

The probiotic activity of two bacteria (Bacillus subtilis and Lactobacillus acidophilus) was evaluated by its effect on the immune response of Nile tilapia (Oreochromis niloticus), beside its protective effect against challenge infections. Furthermore, their in-vitro inhibitory activity was evaluated. The in-vitro antimicrobial assay showed that Bacillus subtilis and Lactobacillus acidophilus inhibited the growth of A. hydrophila. The B. subtilis inhibited the development of P. fluorescens while L. acidophilus inhibited the growth of Strept. iniae. The B. subtilis and L. acidophilus proved harmless when injected in the O. niloticus. The feed, containing a mixture of B. subtilis and L. acidophilus or B. subtilis alone, showed significantly greater numbers of viable cells than feed containing L. acidophilus only after 1, 2, 3 and 4 weeks of storage at 4 degrees C and 25 degrees C. The survival rate and the body-weight gain were significantly increased in the fish given B. subtilis and L. acidophilus for one and two months after application. The hematocrit values showed a significant increase in the group that received the mixture of B. subtilis and L. acidophilus compared with the control group. The nitroblue tetrazolium (NBT) assay, neutrophil adherence and lysozyme activity, showed a significant increase in all the probiotic-treated groups after 1 and 2 months of feeding, when compared with the untreated control group. The serum bactericidal activity was high in the group that was given a mixture of the two bacteria. The relative level of protection (RLP) was significantly higher against A. hydrophila, in the bacterial mixture treated group and against P. fluorescens in the L. acidophilus treated group, after one month of the feeding trial. A significantly higher RLP, against A. hydrophila or P. fluorescens, was noticed after 2 months of the feeding trial in the group given a mixture of the two bacteria, and against Strept. iniae in the group fed a diet containing L. acidophilus.     

Effects of different food restrictions on somatic and otolith growth in Nile tilapia reared under controlled conditions

.Nile tilapia Oreochromis niloticus , initial age 12 days, were given an unrestricted (NR) or restricted (R) ration over 93 days which resulted in fish of very different sizes although the body condition factor ( K ) and the viscero-somatic index ( I _V) remained almost unchanged. In a second stage (64 days) each group (NR & R) was divided into three subgroups that were subjected to 0 (NR0, R0), 15 (NR15, R15) and 30 (NR30, R30) days of food restriction, respectively. The impact of the different treatments on the somatic growth during the second stage of the experiment had an effect, with a highly significant difference between the mean ± S.D. masses ( M _T) in the different subgroups (NR0= 115.0 ± 26.6 g; NR15 = 94.8 ± 24.9 g; NR30 = 56.3 ± 28 g; R0 = 76.4 ± 20.1 g; R15 = 72.l ± 17.6 g; R30 = 43.6 ± 17.2 g). Similarly, K and I _V decreased. Irrespective of the initial feeding condition, the width of the otolith microincrements started to decrease at the end of the first or second day of restricted feeding. In the subgroups given a restricted food ration for 30 days (NR30 and R30), this decrease reached a plateau at about day 30, which was maintained even when the restriction had ended. This slowed growth did not lead to any marked halt in microincrement formation, since there were no significant differences (ANOVA; P >0.05) in the numbers of increments counted in the various subgroups. The results show that in 153 day old fish, a period of severe food restriction, even if prolonged (15 to 30 days), had no influence on the timing of the laying down of microincrements but only affected their growth.     

Microsatellites from genes show polymorphism in two related Oreochromis species

Six microsatellites were detected in the IGF‐II, GH2, prolactin I and insulin genes of Mozambique tilapia (Oreochromis mossambicus) and Nile tilapia (Oreochromis niloticus) by screening DNA sequences deposited to GenBank. Genotyping of 24 individuals from each species revealed that all six microsatellites were polymorphic in both species. In Mozambique tilapia the number of alleles ranged from 3 to 17 (average 9.8), whereas in Nile tilapia the range was 4–21 (average 10.5). The range of expected heterozygosity values were 0.44–0.95 (average 0.79) in Mozambique tilapia, and 0.52–0.96 (average 0.73) in Nile tilapia.     

Large extent of mitochondrial DNA transfer from Oreochromis aureus to O. niloticus in West Africa

Introgressive hybridization has an important evolutionary significance in terms of gene diversity and speciation. Among the major groups of vertebrates, fish show a strong propensity to hybridize. In order to highlight the possible occurrence of gene flow between two tilapia species, Oreochromis niloticus and O. aureus, a comparison of allozyme and mitochondrial DNA (mtDNA) polymorphism was performed on sympatric and allopatric populations of these two species. Nuclear data were congruent with the morphological identification of O. niloticus and O. aureus populations. In opposition, the mtDNA analysis resulted in two strictly differentiated groups which did not follow the morphological and nuclear DNA classification. The first group consisted of East African O. niloticus populations and the second included all the O. aureus populations and the West African O. niloticus populations. Moreover, in some cases, the same sequences were detected in both species. These data strongly support a differential introgression of mtDNA from O. aureus to O. niloticus involving all the West African area. This work points out the risk of misinterpretation of mtDNA or nuclear DNA data when only one single class of marker is used.     

Social facilitation of learning in mixed-species schools of common carp Cyprinus carpio L. and Nile tilapia Oreochromis niloticus (L.)

The process of learning to associate a visual cue with food was studied in groups of common carp Cyprinus carpio and Nile tilapia Oreochromis niloticus by means of a classical conditioning paradigm. In the first phase of the study, fishes were conditioned in either single-species or mixed-species groups to associate a blinking blue light with food. In the second phase, group composition was altered so that only single species were reconditioned, composed of individuals previously paired with the same or the other species. In the first phase, in the single-species groups, Nile tilapia rapidly formed an association between the visual cue and food, whereas common carp failed to do so. In mixed-species groups, both Nile tilapia and common carp associated a visual cue with food. In the second phase of the study, Nile tilapia retained that ability irrespective of whether they had previously been trained in single-species or mixed-species groups, common carp previously trained with Nile tilapia, were not repelled by the blinking blue light and gradually approached it from a distance, whereas common carp previously trained in homogenous groups continued to be repelled by the blinking light. Fish arrivals and departures from the feeding zones as well as fish position in the testing apparatus were analysed and contrasted in single-species and mixed-species groups outside the training sessions, revealing mutual impacts of one species on the other in mixed-species groups. The findings of this study demonstrated interspecific social facilitation of learning to associate a visual cue with food.     

Observations on limnological conditions associated with a fish kill of Oreochromis niloticus in Lake Chivero following collapse of an algal bloom

Possible causes of deaths of Oreochromis niloticus in Lake Chivero were examined in relation to changes in limnological conditions monitored over a 25‐month period. The fish deaths coincided with the collapse of an algal bloom that had developed and builtup in the lake for 8 months. Chlorophyll a and dissolved oxygen increased to average concentrations of 42.4 μg l^?1 and 10.9 mg l^?1 respectively prior to the collapse of the bloom. Dissolved oxygen decreased when the bloom started to die off and coincided with the fish deaths when the average surface dissolved oxygen concentration in the lake was 3.9 mg l^?1 and was at a depth of 5 m <2 mg l^?1. Mortality probably resulted from depressed oxygen levels caused by the high oxygen demand from the massive algal die‐off and released algal toxins. This is the first time that die‐off of algae has been linked to fish‐kills in Lake Chivero as occurs in other hypereutrophic systems.