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CLAN
We are pleased to focus on The Great Exploration of Clones in this issue of Carp and Loach by renowned fish biologists in Hokkaido University with great knowledge in their chosen fields.
Carp and loach are easily differentiated by morphology, but they are members of the same order, the Cypriniformes. In the order Cypriniformes, there are two families: Cyprinidae and Cobitidae. Interestingly, clonal reproduction is observed in crucian carp in Cyprinidae and dojo loach in Cobitidae.
Why, and how, does clonal reproduction occur in those fish? We are proud to introduce new research by fish biologists in Hokkaido University, who have been trying to solve this mystery in the coming breeding season for both crucian carp and dojo loach.
Those fish biologists, who have been unraveling the mysteries of the cloning of the crucian carp and dojo loach, step by step, are from related laboratories originating from the Laboratory of Embryology and Genetics in the Faculty of Fisheries, Hokkaido University. In particular, Professor Emeritus ARAI Katsutoshi, Professor Emeritus YAMAHA Etsuro, and Associate Professor FUJIMOTO Takafumi, who have made significant contributions to the elucidation of clonal reproduction, have summarized the results of a series of studies on the genetics of the crucian carp and dojo loach.
The discovery of clonal individuals of crucian carp, in Onuma Lake, near Hakodate, has initiated a large scale study involving chromosome manipulation techniques related to fish breeding and sex control, and the national project on developing surrogate propagation in teleost, which is one of core study area for the formation of center of excellence (COE) in the field of fisheries science in Japan. The existence of the crucian carp and dojo loach clones continues to weave the bonds between related research groups and is the driving force behind the importance of the group.
The quest for clonal individuals of crucian carp and dojo loach fits well with the "frontier spirit" that is one of the basic principles involved in Hokkaido University's education and research. It also led to the publication of extremely original, high quality research results, that reflect highly on Hokkaido University and have a major impact on society generally. In addition, we can work with many talented people who carry on the university's philosophy through such research projects.
The mystery of clonal reproduction in crucian carp and dojo loach is unlikely to have been fully solved. We will continue to look forward to and await new findings in the genetics of crucian carp and dojo loach.
FoM Editorial
30 June 2023 posted
Genetics of Crucian Carp and Dojo Loach
Crucian carp and dojo loach inhabit streams all over Japan, and belong to Cypriniformes. They are well-known and familiar fish, as song in the nursery rhyme “Dojokko-funakko”. On the other hand, these fish contain "clonal" individuals that are very useful for genetic research. In many organisms, offspring inherit the genome from their parents according to Mendel’s law. Some crucian carp and dojo loach also produce offspring according to Mendel's laws, but some produce clonal offspring genetically identical to the mother. How did these cloned individuals arise? What is the biological significance of generating clonal individuals? We, the researchers, have uncovered some of the mechanisms of clonal reproduction. Here we introduce some information about clonal reproduction that we have discovered from crucian carp and dojo loach.
We introduce two text books below described for further readings to study genetics of crucian carp and dojo loach.
Arai K, Fujimoto T, Yamaha E. 2017. Chapter 6. Chromosome manipulation and breeding and Chapter 10. Hybrid and breeding. Fish genetics breeding science、Nakajima et al. eds. Tohoku Univ. Press、ISBN978-4-86163-270-9C3062. In Japanese.
Arai K and Fujimoto T. 2019. Sex control in aquaculture vol. 1. Chapter 6、Chromosome manipulation techniques and applications to aquaculture. Wang et al. (eds)、John Wiley and Dons Ltd.
FUJIMOTO Takafumi・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
30 June 2023 posted
Genetics of Crucian Carp (Carassius auratus langsdorfii)-1: Crucian Carp Population in Japan
In the Japanese crucian carp, it is known that female individuals are more common than males. Dr. Hiroshi Kobayashi at Japan Women's University investigated this phenomenon using crucian carp from Jonuma-pond, Gunma Prefecture, Japan. As a result, he found crucian carp individuals with 150-chromosomes and ones with about 200-chromosomes, in addition to ones with 100-chromosomes. Most individuals with about 150- and about 200-chromosome were female. In addition, he also found that, when the eggs from individuals with about 150-chromosomes were fertilized with sperm from various other species, they developed into normal crucian carp. This result indicates that such offspring can develop without genetic involvement from the male. In other words, they inherit the genome only from their mother, resulting in an all-female population of offspring. This phenomenon is known as gynogenesis. Photo 1 shows a male crucian carp collected from Jonuma-pond.
Ploidy variation is the name of the phenomenon when individuals have a higher number of chromosomes in their cells than normal. In the ploidy variation, the cell size increases according to the increase of the number of chromosomes. Therefore, the ploidy levels can be determined by comparing the size of the red blood cells taken from each individual (left side of Photo 2). Alternatively, the ploidy of individual chromosomes can be accurately determined by measuring the amount of cell DNA using modern, advanced measuring instruments (flow cytometry) (right side of Photo 2). Of course, one can also check the number of chromosomes by preparing a chromosome specimen, but this is a time-consuming process.
YAMAHA Etsuro・Hokkaido University・Professor Emeritus
30 June 2023 posted
Genetics of Crucian Carp (Carassius auratus langsdorfii)-2: Verification of Natural Clone
As the offspring from triploid crucian carp develop from eggs without the genetic involvement of the father, they inherit genes solely from their mother. If the mother's genes are all inherited by her children, the sisters might be genetically uniform individuals (clones). If they are clonal individuals, donor grafts can be accepted from their siblings. This has been demonstrated by scale transplantation by Dr. Hiroshi Onozato, formerly of the Faculty of Fisheries, Hokkaido University (later at the Aquaculture Research Institute, Fisheries Agency, and Shinshu University). You can see the process of scales-transplantation in the attached movie (Movie 1). If the donor and host are clones, the transplanted scales will survive for a long time after transplantation. You can see accepted scales with more black melanophores than the surrounding scales (Photo 3 top). On the other hand, the scale transplanted from a genetically different goldfish were rejected (Photo 3 bottom). These results indicate that triploid females produce genetically homogeneous eggs and that the resultant individuals are all genetic clones.
Movie 1. Transplantation of a scale.
Although scale grafting is laborious and time-consuming to identify clones, Dr. Onozato collected many Gin-buna from various parts of Hokkaido and revealed how many clonal populations were present in various ponds, swamps and lakes. For example, in Onuma-lake, Nanae Town, near Hakodate, he found that about 80% of the triploids came from only five clones. The population of crucian carp on Okushiri Island in the Sea of Japan, west of Hokkaido, consisted of only one clone. They were all clonal sisters. Currently, DNA analysis can identify such clones.
YAMAHA Etsuro・Hokkaido University・Professor Emeritus
30 June 2023 posted
Genetics of Crucian Carp (Carassius auratus langsdorfii)-3: Trial of Artificial Clone Induction 1 - Haploid
Dr. Onozato wondered if it could be possible to induce artificially gynogenesis naturally observed in natural triploid crucian carp. To make it happen, he had to solve two problems. First, it was necessary to eliminate the genetic involvement of the father at the fertilization. He deveveloped a method in which the father's sperm were irradiated with ultraviolet (UV) light and then fertilized the eggs. An appropriate amount of UV irradiation retains sperm motility but inactivates genetic contribution to the eggs. Therefore, eggs fertilized with UV irradiated sperm are activated, and developed under only the maternal half of the genome. When actually fertilized by the UV-sperm, the egg exhibited haploid (one-ploidy) syndrome, a deformity with small eyes and a curved body (bottom of Photo 4).
These haploids exhibit a variety of morphologies, ranging from those that are close to the control diploid to those which are different (Photo 5). In animals, most haploid individuals are known to be lethal. In Photo 6, you can see the external appearance in some examples of char (1-3), masu salmon (4-6), and rainbow trout (7-9). Normally, the diploid embryos are normal, as shown on the left, but when fertilized by UV-irradiated sperm, they exhibit malformations as shown in the middle row. The chromosomes of these malformed embryos have only half of their original chromosomes, as shown in the right column.
YAMAHA Etsuro・Hokkaido University・Professor Emeritus
30 June 2023 posted
Genetics of Crucian Carp (Carassius auratus langsdorfii)-4: Trial of Artificial Clone Induction 2 - Disruption of Mitotic Spindle
Although individuals that develop only from the mother's genome have been obtained, as described above, they cannot be used for seedlings because of lethal individual. Dr. Onozato then developed a method for duplicating the genome of haploid individual obtained from the fertilization between eggs and UV-irradiated sperm.
In the case of teleost, just prior to fertilization, eggs are at the stage of second maturation division of meiosis and have the same number of chromosomes as a diploid. When fertilization occurs, the second maturation division begins, while half of the chromosomes are pushed out of the egg as the second polar body. If the mechanism of pushing out half of the chromosomes is destroyed, all maternal chromosomes remain in the egg, resulting in diploids. The left-side picture of photo 7 shows the spindle of 2nd maturation division with diploid chromosomes in the egg just before fertilization. The black dots in the center are chromosomes. When fertilization occurs, the threads (spindle threads; microtubule) on both sides of the chromosomes pull half of the chromosomes out of the egg (upper direction). When the egg is exposed to high-pressure, high-water temperature, or low-water temperature, the threads break down and all chromosomes remain in the egg. The picture on right side shows the chromosomes of an egg exposed to high pressure (750 atm for 7 min).
In fact, normal individuals without haploid syndrome have been produced from the eggs, which were fertilized with the UV-sperm and exposed to high-pressure, high-water temperature, and low-water temperature. In addition, in fish species with the XX-XY type of sex-determination system, it is expected that all female individuals are produced since these individuals have only female genes in the egg.
YAMAHA Etsuro・Hokkaido University・Professor Emeritus
30 June 2023 posted
Genetics of Crucian Carp (Carassius auratus langsdorfii)-5: Chimera Between Crucian Carp and Goldfish and Its Descendants
Eggs from triploid crucian carp can develop normally after fertilization with UV-irradiated sperm. Therefore, an interesting study has been performed on the embryology in the triploid crucian carp as a material.
In the goldfish, the same species of crucian carp, the blastomeres at the blastula stage can be transferred from one individual to another, because they have pluripotency of differentiation. Photo 8 shows an experiment in which the upper halves were cut out and transplanted interchangeably between red-stained and non-stained embryos. A short time after the operation, the grafts of the exchanged embryos held together and develop intact as if nothing had happened.
When this transfer of blastoderms is carried out between blastulas of a diploid goldfish and a triploid crucian carp, a chimeric individual with cells of both species can be produced (Photo 9). When eggs are collected from these chimeric individuals, large and small eggs are included (Photo 10A and B). At this time, when fertilized by a sperm with UV-irradiation, the small eggs become malformed, showing haploid syndrome (Photo 10C), while the large eggs develop normally (Photo 10C). Flow cytometry of the DNA content of the large and small eggs revealed that the former was triploid and the latter diploid, respectively. This experiment indicates that two genetically different types of eggs can be obtained from a single individual. This experiment led to the development of the "surrogate propagation" in teleost.
YAMAHA Etsuro・Hokkaido University・Professor Emeritus
30 June 2023 posted
Loach Genetics 1: Phylogeny and Taxonomy
The dojo loach Misgurnus anguillicaudatus is a pure freshwater fish of the family Cobitidae and is commonly found in rice paddies, ponds, and rivers throughout Japan (Photo behind of the section CLAN). Dojo loach have long been used as a valuable source of protein in rural areas in Japan, but they are also supplied as a highly prized delicacy in such dishes as "Yanagawa nabe" and "Kabayaki," and in recent years the price of domestic products has increased. Although the Japanese dojo loach has been regarded as a single species entity, its classification is confusing and unclear. This is due to the high probability that the dojo loach contains some cryptic and undescribed species that differ genetically, and the presence of invasive alien fish makes taxonomical studies much more complicated. In addition, they exhibit not only polyploidy (variations in numbers of chromosome sets), but also unique clonal reproduction, originating from the past hybridization event(s). The dojo loach is a common fish, as is the crucian carp, but both share the similar atypical reproductive mode, clonal development.
A study of allele frequencies of 12 polymorphic enzyme loci in more than 900 specimens from 44 sites across Japan showed that dojo loach from the Abashiri region of eastern Hokkaido differed significantly from those from other regions. The other specimens were then divided into groups distributed in central and southern Hokkaido, Tohoku, Kinki, Chugoku, and Shikoku, and in Kanto and Kou-Shin-Etsu area. Similar results were obtained from mitochondrial DNA sequencing, which divided the Japanese dojo loach into two groups: eastern Hokkaido (Group A) and others (Group B). Group B was further divided into B1, which is widely distributed in central Hokkaido, southern Hokkaido, Tohoku, western Honshu, and Shikoku, and B2, which is distributed in Kanto, Kou-Shin-Etsu, and Kyushu. This result correlates with the analysis of nuclear gene RAG1 and other genes. Consequently, there are three groups of dojo loach with interspecific genetic differences.
Recently, a specimen from southern Sakhalin was described by Russian researchers as a new species, M. chipisaniensis, closely related to M. nikolskyi from Siberia and Primorsky Krai. The species was reported to share a common sequence with the mitochondrial DNA of Group A and also share several nuclear genes. Whether Group A loaches are taxonomically the same as M. chipisaniensis remains to be determined, but some a certain illustrated book on Japanese freshwater fish already refers to the Group A loach as "Kitadojo" in Japanese and “Northern weather loach” in English.
Classification, nomenclature, and phylogeny of Misgurnus loaches are still confusing and inconclusive, but it is possible that B1 is a native Japanese loach and B2 is continental (the original description of the loach is from the Zhoushan Archipelago of China). The natural tetraploid loach that often appears in the Japanese fish markets described in the next section belongs to B2. In addition to diploid loach, natural triploid and tetraploid individuals occur in China. Other related species to M. anguillicaudatus include M. mohoity and M. bipartitus, but the latter is considered a synonym of the former, and its phylogenetic relationship to each loach group is not clear.
Exotic mud loach or large-scale loach living in the continent and Korean peninsula (Photo behind of this section) can be distinguished by its morphology. Moreover, karyotype (chromosome number and shape) assists species identification, because mud loach or large-scale loach has 2n = 48 chromosomes (Photo 11), while Japanese dojo loach has 2n = 50 chromosomes (Photo 12). Although the scientific names M. mizolepis and M. dabryanus have been applied to the exotic mud loach and large-scale loach, some have suggested that the former is a synonym of the latter. However, it has been reported that there are two genetically distinct biotypes within invasive loach species found in Japan. Therefore, it is difficult to determine which specimen should be given which species name. Such a situation adds further confusion. On the other hand, M. fossilis in Europe can be separated from other Misgurnus species by morphology and larger chromosome number (2n = 100).
The genus Misgurnus is not highly morphologically differentiated between species, and thus, it is difficult to identify species using taxonomical keys, especially in the juvenile and small size stages. If cross-breeding occurs between non-native and native loach, resultant viable hybrid offspring makes species and hybrid identification much more difficult. Various DNA markers are currently being developed to reliably identify each group/species of the dojo loach and related non-native loaches.
ARAI Katsutoshi・Hokkaido University・Professor Emeritus
30 June 2023 posted
Loach Genetics 2: Polyploidy
Chromosome number of Japanese dojo loach have been reported as 2n = 50 since the 1960s, but in 1979, Prof. Yoshio Ojima (Kwansei Gakuin University) reported dojo loach with 75 and 100 chromosomes in wild and commercial specimens, in addition to wild-type diploids with 2n = 50. As described below, triploid loach (3n = 75) (Photo 13、courtesy of Dr. M. Kuroda) can be artificially produced by chromosome manipulation, but they occur in certain areas at a relatively high frequency in Japan. On the other hand, dojo loach with 100 chromosomes are thought to be tetraploid, but Prof. Ojima concluded that they are genetic diploids (2n = 100) consisting of two sets of homologous chromosomes paired together, based on their chromosome morphology.
Generally, it is not easy to conclude whether dojo loach with 100 chromosomes are genetic diploids with two sets of homologous chromosomes or tetraploids with four sets. Most species in the family Cyprinidae have 2n = 50, but both common carp Cyprinus carpio and goldfish Carassius auratus have twice the number of chromosomes (2n = 100). As shown in the previous chapter, "Genetics of Carassius auratus-3," gynogenetic development can be artificially induced by fertilization of eggs (1n when meiosis is complete) of wild-type diploids (2n) with genetically inactivated UV-irradiated sperm. The resultant progeny produced by this manipulation are gynogenetic haploids with only one set of maternally derived chromosomes, and all of these haploid progeny died due to abnormalities referred as haploid syndrome. When artificial gynogenesis is induced in carp and goldfish, all the offspring show inviable abnormalities due to haploid syndrome, indicating presence of only one set of homologous chromosomes in the eggs as well as of two sets in somatic cells. Therefore, although the number of chromosomes of carp and goldfish is twice (2n = 100) of other carp species (2n = 50) in the family Cyprinidae, they are not concluded to be a genetic tetraploid with four sets of chromosomes (4n = 100), but a diploid with two sets of homologous chromosomes (2n = 100).
Are the dojo loach with 100 chromosomes genetic diploid (2n = 100) or tetraploid (4n = 100)? When mature eggs of the dojo loach with 100 chromosomes were subjected to induce artificial gynogenesis, normal viable dojo loach fry appeared. Furthermore, when UV-inactivated eggs were fertilized with sperm of males with 100 chromosomes, resultant androgenetic progeny developed normally clearly indicate that the dojo loach with 100 chromosomes are genetic tetraploid (4n = 100) with four sets of homologous chromosomes, because resultant viable gynogenetic and androgenetic progeny had two sets of chromosome, brought by 2n eggs or 2n sperm formed by tetraploid parents (4n = 100).
Triploids with a chromosome number of 75 often appear in commercial fish market specimens along with tetraploids. Triploidy can be induced by chromosome manipulation (inhibition of the second polar body release), and such artificial triploids are frequently sterile (female: retarded ovarian development, male: aneuploid spermatogenesis). However, natural triploid loaches may result from the mating between tetraploid and diploid loach. In fact, when triploids were produced by artificial mating between diploids and tetraploids, males were sterile (retarded testicular development, aneuploid spermatogenesis), while females simultaneously produced both isogenic 3n eggs and regular 1n eggs. These 3n and 1n egg formations occurred in a same female, although their frequencies were individually different, and the 3n eggs were larger in size and genetically identical or very similar to the maternal triploids mother. Isogenecity can be confirmed in aritificially induced gynogenetic triploids from the large 3n eggs by multilocus DNA fingerprinting using minisatellites probes, which was already used to identify clonal crucian carp population (Photo 14).
Flow cytometry has been intensively conducted to examine the presence or absence of tetraploid loach in Japanese wild populations, but to date, no tetraploid loach have been found. However, triploid loach have been found with relatively high frequency in some areas. In loach specimens obtained from the fish market, we often found not only triploid and tetraploid loach, but also higher polyploids with high DNA content corresponding to pentaploid and hexaploid, hyper-tetraploid and hyper-triploid, and unusual mosaics comprising cells with different ploidy status. In addition to the above results, the naturally occurring tetraploid loach is widely distributed in the Yangtze River basin in mainland China, where normal reproduction is reported in both sexes (female and male). Therefore, these polyploid loach specimens found in Japan, especially in fish markets, are considered non-native. The fact that non-native loaches from the continent are now widely distributed in Japan, is a serious problem from the viewpoints of the preservation of indigenous ecosystem in Japanese water systems.
ARAI Katsutoshi・Hokkaido University・Professor Emeritus
30 June 2023 posted
Loach Genetics 3: Clonal Loach
In the Abashiri region of eastern Hokkaido, Japan, triploid loach (3n = 75) can be found at a relatively high frequency. To investigate the cause of high frequent occurrence, mature loach (2n = 50) were collected from this area and eggs were obtained from them. Eggs were then inseminated with (1) loach sperm (normal breeding), (2) goldfish sperm (inter-specific hybridization), and (3) UV-irradiated sperm(induced gynogenesis). Four of the six females produced loach fry from the experiment (1), but only inviable hybrid embryos from experiment (2), and inviable gynogenetic haploid embryos from experiment (3). However, the remaining two females surprisingly produced normal loach progeny not only from experiment (1), but also from experiments (2) and (3). These viable dojo loach progeny showed DNA fingerprints identical to their mother and were also isogenic among the sib progeny. Thus, it is concluded that some diploid dojo loach individuals in the Abashiri area produce unreduced 2n eggs that are genetically identical to their mothers, and these resultant eggs gynogenetically develop without any genetic contribution of sperm to generate clonal progeny. Consequently, genetically identical clonal lineages are maintained by natural gynogenesis.
Clonal dojo loach individuals form isogenic 2n eggs. Then, clonal 2n eggs borrow sperm from normally reproducing gonochoristic wild-type diploids living in the same location to conduct gynogenetic reproduction to maintain clonal lineages. We have also found that clone-derived triploids arise when clonal 2n eggs accidentally incorporate 1n sperm nucleus. The high frequency of triploid individuals in the Abashiri area is well explained by the existence of a clonal lineage, where clone 2n eggs, which normally develop gynogenetically, happen to produce triploids by 1n sperm incorporation. Later, clonal dojo loach lineages were found not only in the Abashiri region of eastern Hokkaido, but also later on in Notojima Island, in Ishikawa Prefecture.
ARAI Katsutoshi・Hokkaido University・Professor Emeritus
30 June 2023 posted
Loach Genetics 4: The Origin of Clonal Dojo Loach and Its Reproductive Mechanism
Mitochondrial analysis revealed that the clonal lineages have mitochondria derived from Group A, and in the nucleus, genes from Group A and Group B1 were heterozygous. This indicates that the clonal dojo loach originated from a hybridization between a female of Group A ancestry and a male of Group B1 ancestry. Recently, a molecular cytogenetic method (FISH, fluorescence in situ hybridization) was successfully developed, and then, it differentially stained Group A-derived chromosomes and Group B-derived chromosomes, using the repeatitive sequences ManDraA and ManDraB, respectively, as group-specific probes (Photo 15、courtesy of Dr. M. Kuroda). FISH also revealed that clonal dojo loach (2n = 50) had 25 Group A-derived chromosomes and 25 Group B-derived chromosomes. The A-derived chromosomes possessed by the clone differed somewhat from the current Group A loach chromosomes. This indicates that the clones originated from past, rather than present, hybridization event. In fact, the present clones lay isogenic 2n eggs which reproduce by natural gynogenesis by borrowing sperm from sympatric, sexually reproducing Group A diploid males, just to trigger development. However, artificial hybrid females created by crossing between contemporary Group A and B loach produced not only 2n eggs but also a variety of abnormal eggs. In addition, these 2n eggs did not undergo gynogenesis; the 2n eggs incorporated sperm to form clone-derived triploids. No males appeared in the clonal diploids, but the artificial hybrid males exhibited sterility.
Animals that normally reproduce with both females and males (e.g., loach diploids, crucian carp diploids) produce gametes (eggs and sperm) with half the number of somatic chromosomes by the process of meiosis. The homologous chromosomes from the mother and the homologous chromosomes from the father replicate and form bivalents by pairing (synapsis), and the subsequent two divisions produce a haploid gamete (egg and sperm) (Background figure upper). In contrast, eggs with the same 2n number of chromosomes and maternally derived isogenic genotypes are produced by a mechanism different from this normal process. The clone-loach genome is composed of chromosomes of different origin (non-homologous), originating from groups A and B. Normally, these non-homologous chromosomes cannot find a partner to form bivalents during meiosis. Such a situation usually results in abortion of meiosis and/or unusual gametogenesis. In clonal dojo loach, however, all chromosomes are doubled before entering meiosis, and the resultant sister chromatids behave as if they are homologous chromosomes, resulting in replication, pairing (synapsis), and crossing over (recombination). Two successive divisions then proceed to form unreduced 2n eggs identical to the mother's genotype (Background figure lower). This process of clonal unreduced 2n egg formation is called “premeiotic endomitosis”. When clones (all-female) are artificially sex-reversed to clonal males, genetically identical 2n sperm are formed by the same mechanism. Such a mechanism in clonal loach has been elucidated by labeling and tracing chromosomes derived from Group A and Group B chromosomes in meiosis by the FISH method. The unreduced 3n egg formation in triploids derived from the cross-breeding between diploids and tetraploids described in the previous section is underwent by a similar mechanism, resulting in the formation of isogenic clonal 3n eggs.
ARAI Katsutoshi・Hokkaido University・Professor Emeritus
30 June 2023 posted
Loach Genetics 5: Clone-Derived Triploids and Their Atypical Reproduction
The 2n eggs laid by clonal dojo loach gynogenetically develop and maintain clonal lineages with maternally derived isogenic genotypes. However, when the 2n eggs accidentally incorporate 1n sperm, triploid-individuals appear. In other words, triploids are generated when clones produce 2n eggs, which drew attention to the Abashiri region of eastern Hokkaido as a location with a high frequency of the occurrence of triploids, and then, it led to the discovery of clonal loach.
Clone-derived triploids reproduce in a unique manner, which similar to the small 1n egg formation by triploid females derived from the above-mentioned inter-ploidy cross-breeding between diploids and tetraploids. Clonal loaches are known to be of hybrid origin between Group A and Group B (female A x male B), and the sexually reproducing loach in eastern Hokkaido is Group A. Therefore, if the clonal 2n eggs take in 1n sperm from Group A loach, the triploid will have the genomic (chromosome-set) constitution with ABA. Chromosomes from group B cannot participate in meiosis because of their low homology to group A chromosomes, and thus, they are eliminated. As a result, the remaining 2n chromosomes exclusively derived from group A undergo normal meiosis to form 1n eggs with genetic variations. This process is called “meiotic hybridogenesis” and is considered to be the same mechanism as the formation of 1n eggs in triploid females derived from the inter-ploidy mating of between diploid and tetraploid loaches.
ARAI Katsutoshi・Hokkaido University・Professor Emeritus
30 June2023 posted
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