CONTEMPORARY STATE AND PROSPECTS OF RESEARCH OF PADDLEFISH (POLYODON SPATHULA) GENETIC STRUCTURE (REVIEW)

Abstract

Sturgeon species are the traditional source of black caviar and delicious meat. The number of their population has declined dramatically and, as a consequence, the anthropogenic pressure on natural paddlefish populations, as an alternative species for the production of valuable sturgeon products, increased. One of the ways to boost the production of these expensive delicacies is increasing freshwater aquaculture through the paddlefish cultivation in controlled conditions.

The American paddlefish (Polyodon spathula) is a freshwater species, representative of the family Polyodontidae, closely related to sturgeons in the order Acipenseriformes. This species of fish, as all Acipenseriformes, is a valuable source of expensive fish products in the internal and global commercial markets. American paddlefish is threatened species, primarily due to overfishing, habitat and spawning areas destruction, illegal harvesting and trade. The state of natural populations of the species requires comprehensive monitoring, management and control of harvesting, which was the reason to list paddlefish in Appendix II of the Convention CITES and the Red List of Threatened Species of the International Union for Conservation of Nature and Natural Resources. The high economic and commercial value of Acipenseriformes, including American paddlefish, promotes the growth of global scientific interest in studies of these species. There are many international organizations, such as the World Sturgeon Conservation Society, Wildlife Conservation Society, Natural Resources Defense Council, and scientific research institutes that are actively engaged in the study, preservation and restoration of the important representatives of modern fish fauna.

In the USA paddlefish artificial reproduction is carried out intensively in order to preserve and recover the natural stocks of this species in the periphery of its native range. Currently, the cultivation of paddlefish in terms of aquaculture is also practiced in Eastern Europe, including Ukraine. For efficient artificial reproduction and formation of reproductive paddlefish broodstocks it is important to use molecular-genetic methods for saving genetic biodiversity of natural and artificial populations of this species.

Presently, in the studies of the genetic structure of paddlefish in recent aquaculture there is a tendency of low genetic diversity in artificial population due to formation broodstocks with a limited number of stocking material that was imported from the United States. One of the most effective methods of controlling genetic processes in artificial populations is molecular-genetic research using microsatellite DNA-markers.

Microsatellite markers, or short tandem repeats (STR), are polymorphic DNA sequences in the genomes of eukaryotes containing repeated nucleotide sequences, typically from 2 to 6 nucleotides per unit. Due to high level of allelic variations microsatellites are used for evaluation of intraspecific genetic polymorphism, analysis the genetic structure of populations, studying linkage mapping and also for identification of organisms. The research of polymorphism of microsatellite DNA-markers is needed for studying the genetic diversity of broodstocks and understanding the causes of changes that occur in them.

Currently, the following microsatellite DNA-markers: Psp12, Psp18, Psp20, Psp21, Psp26, Psp28, Psp29 and Psp32 are used to monitor the efficiency of conservation and reproduction of paddlefish population. The development and implementation of these markers were started by team of scientists led by Heist (2002, 2008). As result of using mentioned DNA-markers intraspecific genetic polymorphism had been evaluated. In the future passportisation of paddlefish broodstocks and formation of pairs for breeding with alternative genotypes can be made for increasing of paddlefish stock material quality and monitoring the reproduction efficiency of this species in fish farms.

Based on the microsatellite analysis with mentioned DNA-markers, Zou and others have provided a detailed evaluation and made a comparison of the genetic structure of natural and artificial populations of paddlefish grown in Polish and Ukrainian fish farms. The features and differences formation of genetic biodiversity of natural paddlefish populations from geographically distant locations were investigated by Heist and others. The authors suggested that significant difference in allelic frequency developed recently due to the artificial regulation of river flow through to the construction of dams which limit natural exchange of genes. The results of comparative studies of intraspecific genetic polymorphism showed the absence of certain alleles in artificial populations that were identified in natural populations, indicating a decrease in genetic biodiversity of paddlefish as a consequence of prolonged artificial breeding practices.

Thus, a comprehensive genetic monitoring of American paddlefish populations using microsatellite DNA-markers has a significant meaning and perspective to optimize conservation and recover its population to historic ranges. Attempts should be made to preserve low frequency and unique alleles that can be lost due to reproductive isolation of natural populations and negative effects of closely related breeding in artificial conditions. Assessing genetic diversity of paddlefish is an important consideration for developing conservation plans. In this aspect, molecular genetic studies using microsatellite DNA-markers allow us to identify the differences and evaluate the status of paddlefish broodstocks and also to provide the theoretical and practical support of reproduction and conservation of this species.