How Female Dojo Loach Fishes Clone Themselves

A team of scientists in Japan has shed light on the mechanism by which female dojo loach fishes achieve clonal reproduction.

AsianScientist (Aug. 17, 2018) – Researchers in Japan have clarified the mechanism behind clonal reproduction of female dojo loach fish, providing insight into the ancestral origin of the clonal population. Their findings are published in Chromosome Research.

The dojo loach (Misgurnus anguillicaudatus) is a bottom-dwelling freshwater fish native to East Asia. The majority of them are sexually reproducing male and female fish. Their non-reproductive (somatic) cells contain a full set of 50 chromosomes—25 from each parent—while their reproductive (egg and sperm) cells contain 25 chromosomes.

However, a population of female clones of the species can be found in Hokkaido Island and other areas of Japan. Unlike the sexually reproducing female population, both their somatic and reproductive eggs contain 50 chromosomes, assuring their clonal reproduction. How the reproductive process leads to 50 chromosomes in egg cells has been unclear.

To better understand this mechanism, a research team led by Dr. Masamichi Kuroda and Associate Professor Takafumi Fujimoto of Hokkaido University’s Graduate School of Fisheries Sciences in Japan developed DNA probes to track the chromosomes in the dojo loach’s somatic and reproductive cells. Previous studies have suggested that the female clone population arose when two genetically distinct groups within the species, called A and B for simplicity, mated. Kuroda and his colleagues developed a fluorescent DNA probe that binds to specific chromosomal regions derived from type B.

The fluorescent signals indicated that somatic cells of the female clones have 25 chromosomes derived from type B, providing evidence that their ancestral origin arose when type A and B mated. The researchers then looked into the process of egg production using the DNA probes. In the sexually reproducing dojo loach, reproductive cells divided through the normal process of meiosis, in which a single cell containing a full set of 50 chromosomes produces one egg containing 25 chromosomes. This requires one round of chromosomal doubling.

In the female clones, the team found that the chromosomal material doubles twice so that when the reproductive cell divides, each resultant egg cell contains a full set of 50 chromosomes. Fish sperm activates these egg cells to start developing into embryos, without the sperm’s genetic material being incorporated.

“This is the first time that ‘cytogenetic’ evidence has been found for this type of chromosomal duplication in a unisexual, ray-finned fish. Further study could help develop a seedling production that can produce a large population of clone fish with desirable characteristics,” said Fujimoto.

The article can be found at: Kuroda et al. (2018) Clonal Reproduction Assured by Sister Chromosome Pairing in Dojo Loach, a Teleost Fish.


Source: Hokkaido University; Photo: Takafumi Fujimoto.
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