Abstract
In wild populations of the Japanese endemic freshwater fish Minami-medaka (Oryzias latipes), genetic disturbances caused by crossing with artificially introduced exotic populations, especially an orange-red commercial variety of medaka commonly called himedaka, have been reported throughout Japan. Minami-medaka populations in the Kyushu-Ryukyu Islands region include many genetically distinct groups, indicating that the region is important for the conservation of genetic diversity in Minami-medaka populations. In this study, genetic analyses using two DNA markers (one for a mitochondrial gene, the other for a nuclear gene) were conducted to reveal the current status of genetic disturbance in wild Minami-medaka populations in the Kyushu-Ryukyu Islands region. In 125 individuals from 18 populations analyzed, one individual, from the northern Kyushu Islands, was found to carry alleles originating from the himedaka strain. Based on the results of all available surveys, including those of the present study, a total of six introgressed populations are currently known in the region. All other populations are considered unaffected. The native genetic diversity of Minami-medaka populations in the Kyushu-Ryukyu Islands region is comparatively well preserved; results of surveys throughout the rest of the range of the species indicate that almost half of all Minami-medaka populations have been introgressed by non-native genes. Conservation measures and regular genetic monitoring are recommended to preserve the genetic diversity of this species.
Highlights
Genetic disturbances primarily attributable to the artificial introduction of alien species or populations have been known to decrease genetic diversity in wild populations, and to drive decreases in the long-term survival of native species (Frankham, Ballou, & Briscoe, 2002; Allendorf, Luikart, & Aitken, 2013).Minami-medaka (Oryzias latipes; Asai, Senou, & Hosoya, 2011), an endemic Japanese freshwater fish, is listed as “Vulnerable” in the Red List of Japan due to degradation and predation pressures caused by invasive alien species (Japanese Ministry of Environment, 2015)
Allozyme and mitochondrial DNA studies showed that wild Minami-medaka populations have diverged into nine regional groups (Sakaizumi, 1990; Takehana, Nagai, Matsuda, Tsuchiya, & Sakaizumi, 2003)
Additional sequencing of the cytb gene of this individual indicated that it was closely related to the native mitotype B23, from which it differed by only one nucleotide. Another himedaka-phenotype individual collected from site Ky5 was homozygous for the b allele; it had the native mitotype B24 (Figure 2; Table 1)
Summary
Minami-medaka (Oryzias latipes; Asai, Senou, & Hosoya, 2011), an endemic Japanese freshwater fish, is listed as “Vulnerable” in the Red List of Japan due to degradation and predation pressures caused by invasive alien species (Japanese Ministry of Environment, 2015). In addition to decreasing population sizes caused by invasive species, genetic disturbances in Minami-medaka populations, caused by crossing with artificially introduced exotic populations, have been reported throughout Japan. The primary cause of genetic disturbances in wild Minami-medaka populations is the accidental or intentional introduction of an orange-red commercial variety of medaka commonly called himedaka, which is farmed and sold as an aquarium fish and as a live feeding fish (Nakao, Iguchi, Koyama, Nakai, & Kitagawa, 2017). The Kyushu-Ryukyu Islands region includes six of these regional groups
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