Analysis on karyotype of Platax teira

Abstract: As the primary carrier of biological and genetic material information, chromosomes are the archives of natural evolution. The conclusions obtained by karyotype analysis research methods are essential reference materials in fish genetic breeding. The karyotype of chromosome is a significant basis and key for cytogenetic studies. The number and morphology of chromosomes are speciesspecific. Chromosome karyotype can reflect the overall and structural characteristics at the chromosomal level as a crucial theoretical basis and a critical experimental standard in genetic research. Chromosome karyotype analysis has essential reference value and practical significance for understanding the evolutionary history of organisms and exploring the relationship between species. The longfin batfish (Platax teira) is a marine fish that lives nearshore in tropical and temperate regions. It is native to the IndoEastern Pacific, from the Red Sea and East Africa to Papua New Guinea, north to the Ryukyus, and south to Australia, New Zealand, and Melanesia. Juveniles inhabit shallow inner reefs and mangrove areas, while adults inhabit lagoons and coral reefs at 70 m, feeding on algae, zooplankton and benthic invertebrates. P. teira is ornamental, and the adult fish has high nutritional and economic value. As the market demand for adult P.teira increases, the supply of fry does not meet the demand, so there is an urgent need to research the artificial breeding technology of P.teira. The problems of artificial breeding are strengthening the nutrition of parents, cultivating highquality fertilized eggs, improving the hatching rate, increasing the survival rate of the young, and controlling the phenomenon of mutual mutilation of the young. In this study, samples were pretreated with phytohaemagg lutinin (PHA, 0.75% NaCl prepare the concentration of 2.5 mg·mL-1), colchicine (0.75% NaCl prepare the concentration of 125 mg·mL-1) injection, different tissue soaking methods, and 110 metaphase cell karyotype of 8 juvenile fishes were analyzed by Giemsa staining using the cold drop method. The position of the mitotic sites was confirmed, and the relative lengths and arm ratios of each chromosome were measured and calculated, and they were paired. The chromosomes were divided into four groups according to the arm ratios: 1) metacentric chromosomes (m), with arm ratios ranging from 1.0 to 1.70; 2) submetacentric chromosomes (sm), with arm ratios ranging from 1.71 to 3.00; 3) subtelocentric chromosomes (st), with arm ratios ranging from 3.01 to 7.00; 4) telocentric chromosomes (t), with arm ratios ≥7.00. Results showed that there were 48 chromosomes in 95 division items (86.36%), all of which were telocentric chromosomes, and the relative length of chromosomes ranged from (1.90±0.123) to (4.67±0.220). 11 mitotic terms (10%) had less than 48 chromosomes, and 4 mitotic terms (3.64%) had more than 48 chromosomes. Among them, 15 were from group 2 gill and fin tissues, accounting for 13.6%, of which 12 were from gill tissues, accounting for 10.9%; 3 were from fin tissues, accounting for 2.7%. This indicated that the direct injection method of P. teira chromosome preparation was superior to the tissue immersion method under the present experimental conditions. In a given taxonomic order, the fish belonged to the primitive type when the telomeric chromosomes occupied most of the chromosomes, while the fish was said to be the specialized type when it had more central or sub-central chromosomes; and the taxa with fewer chromosome arms were more likely to be primitive than the taxa with more chromosome arms. In this study, we found that all 24 pairs of chromosomes of P.teira were telomeric (ttype), their chromosome numbers were excessively convergent, and the number of chromosome arms was 48, so P.teira belonged to a highly evolved taxon based on the above classification. The chromosome types of fish were speciesspecific, and the evolution level of species was mainly consistent with their chromosome types. Through comparative analysis of experimental data, it was concluded that the evolution of P.teira was consistent with the typical karyotype of high fish, and the polyploid phenomenon was not found in the chromosome of P.teira, nor was the appearance of heteromorphic sex chromosome and satellite. However, caution should be exercised when comparing the number of mitotic chromosomes between or within species studied, as the determination of confidence intervals might depend on the degree of chromosome shrinkage. Successful chromosome production presupposed access to tissues with rapid cell division and high metabolic activity, and the cephalic kidney, as an essential hematopoietic and immune organ of scleractinians, was preferred for production because of its rapid cell division rate. The present experiment showed that filming could also be accomplished by soaking the fin and gill tissues, but the results of filming differed between fin and gill tissues, with gill tissues being slightly better than fin tissues, probably because the fins of P.teira were too large and the cell division was slower, which affected the results of the experiment. A chromosome is all biological heredity, variation and the material basis for the development and evolution, The chromosome karyotype is significant not only for understanding the laws of biological, genetic composition and genetic variation and development mechanism but also for predicting and identifying the result of interspecific hybridization and polyploidy breeding, genetic mechanism, understanding gender. It is also of great reference value to determine the number of genomes of organisms and study the origin of species and their relationship with each other, their evolutionary status, classification, and race relations. Therefore, this study provides essential information and experimental enrichment for the cytogenetic study of P.teira, and reference data and practical integration for germplasm identification of P.teira through a large number of experimental data analyses, also provides a theoretical basis for future research on P.teira.

Keywords: Platax teira; chromosome; karyotype