Nori, a marine red alga, is one of the most profitable mariculture crops in the world. However, the biological properties of this macroalga are poorly understood at the molecular level. In this study, we determined the draft genome sequence of susabi-nori (Pyropia yezoensis) using next-generation sequencing platforms. For sequencing, thalli of P. yezoensis were washed to remove bacteria attached on the cell surface and enzymatically prepared as purified protoplasts. The assembled contig size of the P. yezoensis nuclear genome was approximately 43 megabases (Mb), which is an order of magnitude smaller than the previously estimated genome size. A total of 10,327 gene models were predicted and about 60% of the genes validated lack introns and the other genes have shorter introns compared to large-genome algae, which is consistent with the compact size of the P. yezoensis genome. A sequence homology search showed that 3,611 genes (35%) are functionally unknown and only 2,069 gene groups are in common with those of the unicellular red alga, Cyanidioschyzon merolae. As color trait determinants of red algae, light-harvesting genes involved in the phycobilisome were predicted from the P. yezoensis nuclear genome. In particular, we found a second homolog of phycobilisome-degradation gene, which is usually chloroplast-encoded, possibly providing a novel target for color fading of susabi-nori in aquaculture. These findings shed light on unexplained features of macroalgal genes and genomes, and suggest that the genome of P. yezoensis is a promising model genome of marine red algae.

No abstract

Stocking with fish from non-native populations may lead to spawning intrusion for native fish. We observed the spawning of ayu Plecoglossus altivelis with experimentally mixed fish groups, amphidromous •~ landlocked forms, and diploid •~ triploid. There was no difference in the courtship behavior among them, and cross-mating between forms and between ploidities occurred promiscuously. Spawning intrusion by transplanted ayu is considered to be inevitable, unless there is complete separation in the spawning period between transplanted and native fish.

Sexual differences in survival and changes in plasma ion levels after transfer from neutral water to acid water prepared with sulfuric acid were examined in medaka Oryzias latipes. In acid water at pH 3.5 and 3.8, survival of female fish was better than male. During exposure to acid water at pH 4.1, there was no mortality in either sex. Male fish showed a transient decrease in the plasma sodium level 24 h af ter transfer to pH 4.1, whereas there was no change in the female. The plasma level of sulfate was not affected in the female, but a significantly high level was consistently observed during the exposure in the male. In both sexes, the activity of gill Na+, K+-ATPase increased within 24 h after transfer to pH 4.1, and this high level was maintained for 1 month. Treatment with short day-photoperiod (8L: 16D) caused a cessation of spawning due to inactivation of the testis and ovary. Transfer to acid water caused a decline in plasma sodium only in male fish, but not in female even under a short photoperiod. These results indicate that female medaka are more tolerant to acidic environment, possibly with a greater os moregulatory ability than male fish.

Sexual dimorphism in the anal fin of ayu Plecoglossus altivelis was studied by serial sampling of ayu. Changes in the shape of the anal fin were triggered by sexual maturation in both sexes; however, secondary sexual characteristics developed drastically in males while they did so inconspicuously in females.The anal fin base became larger in both sexes during sexual maturation, but grew much faster in males than in females, and consequently males have larger anal fin bases than females. Secondary changes in the anal fin height occur only in males, and the change is negative, and consequently females have larger anal fin height than males. The relationship between the shape of the anal fin and the sex hormone levels in diploid and triploid ayu suggested that the development of secondary sex characteristics in the anal fin of ayu is controlled by androgens.