Effects of acute temperature and salinity changes, body length and starvation on the critical swimming speed of juvenile tiger puffer, Takifugu rubripes

The critical swimming speed (U _crit, cm s^-1) of juvenile tiger puffer Takifugu rubripes was determined under different temperatures (15, 21, 25 and 30 °C), salinities (5, 10, 20, 32 and 40), body lengths (3.32, 4.08, 5.06 and 5.74 cm) and starvation days (1, 3, 6 and 9 days). Acute temperature change, body length and starvation significantly influenced the U _crit of tiger puffers, whereas acute salinity change had no significant effect. The U _crit increased as the temperature increased from 15 to 30 °C. The U _crit increased as the body length increased from 3.32 to 5.74 cm, whereas relative critical swimming speed (U _crit', body length s^-1) decreased. The relationship between the body length (l, cm) and U _crit or U _crit' can be described by the quadratic model as U _crit = - 1.4088 l ² + 16.976 l - 11.64, R ² = 0.9698 (P < 0.01) or U _crit' = - 0.1937 l ² + 0.9504 l + 7.7666, R ² = 0.9493 (P < 0.01). The U _crit decreased as starvation days increased from 1 to 9 days. Low temperature and starvation can reduce the swimming ability of juvenile tiger puffers. Results can be of value in evaluating the swimming ability of juvenile tiger puffers, understanding ecological processes and improving the population enhancement of tiger puffers.

The genome of the largest bony fish, ocean sunfish (Mola mola), provides insights into its fast growth rate

BACKGROUND

The ocean sunfish (Mola mola), which can grow up to a length of 2.7 m and weigh 2.3 tons, is the world's largest bony fish. It has an extremely fast growth rate and its endoskeleton is mainly composed of cartilage. Another unique feature of the sunfish is its lack of a caudal fin, which is replaced by a broad and stiff lobe that results in the characteristic truncated appearance of the fish.

RESULTS

To gain insights into the genomic basis of these phenotypic traits, we sequenced the sunfish genome and performed a comparative analysis with other teleost genomes. Several sunfish genes involved in the growth hormone and insulin-like growth factor 1 (GH/IGF1) axis signalling pathway were found to be under positive selection or accelerated evolution, which might explain its fast growth rate and large body size. A number of genes associated with the extracellular matrix, some of which are involved in the regulation of bone and cartilage development, have also undergone positive selection or accelerated evolution. A comparison of the sunfish genome with that of the pufferfish (fugu), which has a caudal fin, revealed that the sunfish contains more homeobox (Hox) genes although both genomes contain seven Hox clusters. Thus, caudal fin loss in sunfish is not associated with the loss of a specific Hox gene.

CONCLUSIONS

Our analyses provide insights into the molecular basis of the fast growth rate and large size of the ocean sunfish. The high-quality genome assembly generated in this study should facilitate further studies of this 'natural mutant'.

Correlation with larval body size of mRNA levels of growth hormone, growth hormone receptor I and insulin-like growth factor I in larval torafugu Takifugu rubripes

The full-length of insulin-like growth factor (IGF) complementary (c)DNAs encoded by igf-I and igf-II from torafugu pufferfish Takifugu rubripes were cloned in the present study. The deduced amino acid sequences of the two genes showed c. 80% identity each with those of Igf-I and Igf-II from other teleosts, respectively. Two growth hormone (GH) receptors, ghr1 and ghr2, were also cloned in silico using the T. rubripes Fugu genome database. The transcripts of T. rubripes igf-I were detected in slow muscle, heart, skin, gill, liver and intestine but not in fast muscle, spleen and testis of adult fish, whereas those of igf-II were found in all tissues examined. Subsequently, the accumulated messenger (m)RNA levels of igf-I and igf-II were investigated in an F(2) population derived from a male of an apparent fast-growing T. rubripes strain and a wild female T. rubripes together with those of other growth-related genes encoding Gh, Ghr1 and Ghr2, and with those of prolactin (Prl) and leptin (Lep) previously reported. The accumulated mRNA levels of igf-I, gh and ghr1 were significantly correlated to growth rate at larval stages in the population, but not for those of igf-II, prl, ghr2 and lep. Although it is unclear whether or not this phenotype is directly related to the heredity of the fast-growing strain, the findings suggest that the expression of igf-I, gh and ghr1 is involved in the regulation of growth rate at larval stages in T. rubripes.

Takifugu obscurus is a euryhaline fugu species very close to Takifugu rubripes and suitable for studying osmoregulation

Background

The genome sequence of the pufferfish Takifugu rubripes is an enormously useful tool in the molecular physiology of fish. Euryhaline fish that can survive both in freshwater (FW) and seawater (SW) are also very useful for studying fish physiology, especially osmoregulation. Recently we learned that there is a pufferfish, Takifugu obscurus, common name "mefugu" that migrates into FW to spawn. If T. obscurus is indeed a euryhaline fish and shares a high sequence homology with T. rubripes, it will become a superior animal model for studying the mechanism of osmoregulation. We have therefore determined its euryhalinity and phylogenetic relationship to the members of the Takifugu family.

Results

The following six Takifugu species were used for the analyses: T. obscurus, T. rubripes, T. niphobles, T. pardalis, T. poecilonotus, and T. porphyreus. When transferred to FW, only T. obscurus could survive while the others could not survive more than ten days in FW. During this course of FW adaptation, serum Na⁺ concentration of T. obscurus decreased only slightly, but a rapid and large decrease occurred even in the case of T. niphobles, a peripheral fresh water species that is often seen in brackish river mouths. Phylogenetic analysis using nucleotide sequences of the mitochondrial 16S ribosomal RNA gene of each species indicated that the six Takifugu species are very closely related with each other.

Conclusion

T. obscurus is capable of adapting to both FW and SW. Its genomic sequence shares a very high homology with those of the other Takifugu species such that the existing Takifugu genomic information resources can be utilized. These properties make "mefugu", which has drawn little attention from animal physiologists until this study, a useful model animal for studying the molecular mechanism of maintaining body fluid homeostasis.

Two ribeye genes in teleosts: the role of Ribeye in ribbon formation and bipolar cell development

Ribeye is the only known protein specific to synaptic ribbon, but its function is unclear. We show that the teleost fish, Fugu and zebrafish, have two ribeye genes, ribeye a and ribeye b. Whole-mount in situ hybridization revealed that ribeye a is expressed in tissues containing synaptic ribbons, including the pineal gland, inner ear, and retina. Ribeye b is absent in the pineal gland. In the retina, ribeye a is expressed in both photoreceptors and bipolar cells, whereas ribeye b is detected only in photoreceptors. To study the function of Ribeye a in retina, we depleted it by morpholino antisense oligos. Fish deficient in Ribeye a lack an optokinetic response and have shorter synaptic ribbons in photoreceptors and fewer synaptic ribbons in bipolar cells. Their bipolar cells still target Syntaxin-3 proteins to the inner plexiform layer and have abundant vsx1 mRNA. However, they lack large synaptic terminals and show increased apoptosis. Rod bipolar cells are fewer in number and/or deficient in PKCalpha. Recovery of Ribeye a levels rescues the optokinetic response, increases the number of PKCalpha-positive bipolar cells, and stops apoptosis. We conclude that Ribeye a is important for late steps in bipolar cell development.