Genetic linkage map of the loach Misgurnus anguillicaudatus (Teleostei: Cobitidae)

Intra-ooplasmic injection of a multiple number of sperm to induce androgenesis and polyploidy in the dojo loach Misgurnus anguillicaudatus (Teleostei: Cobitidae)

SummaryPolyspermy was initiated by microinjecting a multiple number of sperm into the activated and dechorionated eggs of dojo loach Misgurnus anguillicaudatus (Teleostei: Cobitidae). A 10-nl sperm suspension from an albino (recessive trait) male (105, 106, 107 or 108 sperm ml -1) was microinjected into eggs from a wild-type female. Although the rates of embryos developing into the blastula stage in the injection group at the highest sperm concentration were similar to that of the control group, the hatching rates of the injection group were much lower. A large proportion of embryos that developed from the injected eggs was haploid and were mosaics containing haploid cells. Most of the haploid and mosaic embryos inherited only paternally derived alleles in the microsatellite markers (i.e. androgenesis was initiated by injecting multiple sperm). In contrast, some haploid embryos contained both paternal and maternal alleles despite haploidy, suggesting that they were mosaics consisting of cells with either paternal or maternal inheritance. The injected eggs displayed diploid, hypotriploid and triploid cells, all of which included both maternally and paternally derived alleles. One albino tetraploid with only paternal alleles was also observed from the injected eggs. These results suggested that part of the sperm microinjected into the ooplasm should form a male pronucleus/pronuclei, which could develop by androgenesis or could fuse with the female pronucleus/pronuclei. Therefore, microinjection of multiple sperm should be considered a potential technique to induce androgenesis and polyploidy.

Production of Tetraploid Gynogenetic Loach Using Diploid Eggs of Natural Tetraploid Loach, Misgurnus anguillicaudatus, Fertilized with UV-Irradiated Sperm of Megalobrama amblycephala without Treatments for Chromosome Doubling

The gynogenesis phenomenon in nature mainly appears in the reproduction of fish and invertebrates. So far, gynogenesis has been successfully induced in many fish species with the aid of some physical or chemical methods for chromosome doubling. However, few fish can produce gynogenetic progenies, genetically identical or similar to the somatic cells of the mothers, without a treatment for the doubling of chromosomes, which may be related to apomixis, premeiotic endoreduplication, or premeiotic endomitosis. At present, no studies are available about fish with normal ovarian structures producing gynogenetic progenies that could spontaneously double their chromosomes. According to the analyses of flow cytometry, chromosome number, and microsatellites, we found that, with the use of UV-irradiated sperm of blunt snout bream Megalobrama amblycephala, tetraploid loach Misgurnus anguillicaudatus produced tetraploid gynogenetic progenies without any treatments for the doubling of chromosomes. To determine the genetic relationships of gynogenetic progenies and their maternal parent, microsatellite genotyping was conducted. The results indicated that the reason for spontaneous chromosome duplication in gynogenetic progenies may be cytokinesis or inhibition of the extrusion of the second polar body. This is the first report on fish with normal ovarian structures that can produce gynogenetic progenies which spontaneously double their chromosomes and which are genetically identical or similar to the somatic cells of the mothers.

Genetic positioning of centromeres through half-tetrad analysis in gynogenetic diploid families of the Zhikong scallop (Chlamys farreri)

Centromere mapping is a powerful tool for improving linkage maps, investigating crossover events, and understanding chiasma interference during meiosis. Ninety microsatellite markers selected across all linkage groups (LGs) from a previous Chlamys farreri genetic map were studied in three artificially induced meiogynogenetic families for centromere mapping by half-tetrad analysis. Inheritance analyses showed that all 90 microsatellite loci conformed to Mendelian inheritance in the control crosses, while 4.4 % of the microsatellite loci showed segregation departures from an expected 1:1 ratio of two homozygote classes in meiogynogenetic progeny. The second division segregation frequency (y) of the microsatellites ranged from 0.033 to 0.778 with a mean of 0.332, confirming the occurrence of partial chiasma interference in this species. Heterogeneity of y is observed in one of 42 cases in which markers were typed in more than one family, suggesting variation in gene-centromere recombination among families. Centromere location was mostly in accordance with the C. farreri karyotype, but differences in marker order between linkage and centromere maps occurred. Overall, this study makes the genetic linkage map a more complete and informative tool for genomic studies and it will also facilitate future research of the structure and function of the scallop centromeres.

Construction of an integrated map of Haliotis diversicolor using microsatellite markers

Small abalone, Haliotis diversicolor, is naturally distributed along the coastal waters of East Asia from Japan to the Philippines. It is an economically important maricultured species in southern China and Taiwan. Genetic linkage maps for small abalone were constructed using a total of 308 simple sequence repeat markers including 297 novel markers. Segregation data on 96 progeny were genotyped using a pseudo-testcross strategy. Sixteen linkage groups were identified in both female and male maps, consistent with the haploid chromosome number. The female linkage map covered 758.3 cM, with an average interval of 5.2 cM. The male linkage map spanned a total genetic distance of 676.2 cM, with an average interval of 4.5 cM. An integrated map was constructed by incorporating the homologous parental linkage groups, resulting in 16 linkage groups with a total of 762.1 cM. Genome coverage of the integrated linkage map was approximately 80.7%. The genetic linkage maps of small abalone may facilitate marker-assisted selection and quantitative trait loci mapping.

Cold-shock eliminates female nucleus in fertilized eggs to induce androgenesis in the loach (Misgurnus anguillicaudatus), a teleost fish

BACKGROUND

Androgenesis (all-male inheritance) is generally induced by means of irradiating the eggs to inactivate the maternal genome, followed by fertilization with normal sperm. In fish, the conventional technique for induced androgenesis has been applied for rapid fixation to traits, recovery of cryopreserved genotypes, sex-control, etc. A new method of androgenesis that eliminates the need to irradiate the egg was proposed using the loach, Misgurnus anguillicaudatus (a teleost fish).

RESULTS

When the eggs of wild-type females were fertilized with sperm of albino or orange phenotype males and cold-shocked at 0 to 3°C for 60 min duration just after fertilization, generally more than 30% (with a peak of 100%) of the hatched progeny were androgenotes. While a few of them were the normal diploid, most of them turned out to be abnormal haploid. All-male inheritance was verified by the expression of the recessive color trait (albino or orange) and microsatellite genotypes comprising only paternally derived alleles. Nuclear behavior after the cold-shock treatment was traced by microscopic observation of DAPI (4'6-diamidino-2-phenylindole)-stained samples and hematoxylin-eosin stained histological sections, and the extrusion of egg (maternal) nucleus was observed in eggs treated in the optimum timing.

CONCLUSION

In this paper, we demonstrate that cold-shock treatment (at 0 and 3°C) of loach eggs for 60 min just after fertilization successfully induces androgenetic haploid development. The most likely mechanism of cold-shock induced androgenesis is an elimination of the egg nucleus together along with the second polar body and subsequent development of a decondensed sperm nucleus or male pronucleus.

Cloning and identification of a female-specific DNA marker in Paramisgurnus dabryanus

Paramisgurnus dabryanus (Cypriniformes; Cobitidae), has been an emerging aquaculture species in China since the 1990s. In this study, random amplified polymorphic DNA fingerprinting with 220 primers was used to identify a sex-specific DNA marker in pooled DNA and individual DNA samples from male and female P. dabryanus. One primer, S2115, produced a novel sex-specific DNA fragment found only in tested females. This female-specific fragment was 917 bp with 36% GC content, and was named Pdff1. To further validate the authenticity of this female-specific marker for sexing, two PCR primers (pdff1-F and -R) were designed according to the cloned female-specific sequence. Amplification showed bands specific for females. Dot blot and Southern blot hybridization experiments both displayed female specificity using this marker as the probe. Two other P. dabryanus populations were tested by dot blot hybridization with the Pdff1 probe. The hybridization signals were seen in 33 or 43% of males in addition to all females in the Jinan and Xichuan populations, respectively. We propose to use this sex-specific marker to rapidly and specifically identify the gender of P. dabryanus from the ancient Yellow River Wetland in Yanjin, Henan Province. Our results could assist in cloning sex-specific chromosomal regions.

Before maternal-zygotic transition ... There was morphogenetic function of nuclei

The idea of a major developmental transition that includes the activation of the embryonic genome has a long history. In the 1950-1960s this concept was developed to a large extent due to the efforts of Alexander Neyfakh, who described a specific type of deleterious effect resulting from X-ray irradiation of fish eggs. He interpreted the radiation-sensitive target as the nucleus and established the onset of the function of the zygotic genome, naming it the morphogenetic function of nuclei, what we call now the midblastula transition. Most of his studies were performed using the loach (Misgurnus fossilis), a European teleost. Neyfakh's efforts paved the way to understanding the whole phenomenon of the maternal-zygotic transition.

Meiotic hybridogenesis in triploid Misgurnus loach derived from a clonal lineage

Triploid loaches Misgurnus anguillicaudatus are derived from unreduced diploid gametes produced by an asexual clonal lineage that normally undergoes gynogenetic reproduction. Here, we have investigated the reproductive system of two types of triploids: the first type carried maternally inherited clonal diploid genomes and a paternally inherited haploid genome from the same population; the second type had the same clonal diploid genomes but a haploid genome from another, genetically divergent population. The germinal vesicles of oocytes from triploid females (3n=75) contained only 25 bivalents, that is, 50 chromosomes. Flow cytometry revealed that the majority of the progeny resulting from fertilization of eggs from triploid females with normal haploid sperm were diploid. This indicates that triploid females mainly produced haploid eggs. Microsatellite analyses of the diploid progeny of triploid females showed that one allele of the clonal genotype was not transmitted to haploid eggs. Moreover, the identity of the eliminated allele differed between the two types of triploids. Our results demonstrate that there is preferential pairing of homologous chromosomes as well as the elimination of unmatched chromosomes in the course of haploid egg formation, that is, meiotic hybridogenesis. Two distinct genomes in the clone suggest its hybrid origin.