Gynogenetic diploids were produced from the eggs of natural tetraploid loach Misgurnus anguillicaudatus (Pisces: Cobitidae) without any manipulation for chromosome duplication. When eggs of a four-year-old diploid gynogenetic individual were fertilized with spermatozoa of specimens from normal diploid and natural tetraploid lines, viable diploid and triploid progeny were produced, respectively. Thus, egg nucleus of the diploid gynogen is haploid. In the gonads of diploid progeny, diploid (2n = 50) and tetraploid (4n = 100) mitotic metaphases were observed. The majority of oocytes (76%) showed regular 25 bivalents as in normal diploids, but the other 16% showed a few univalents. The remaining 8% exhibited about 50 bivalents, suggesting chromosome duplication by premeiotic endomitosis. In the testes, a few spermatocytes (6%) showed normal 25 bivalents, but 86% contained various number of univalents and the remaining 8% contained about 50 bivalents. No peaks of spermatozoa were identified in the testis by flow cytometry. In the triploid progeny, triploid (3n = 75) and hexaploid (6n = 150) mitotic metaphases were observed in both ovaries and testes. Most meiotic figures (about 90%) contained approximately 25 bivalents and 25 univalents in both sexes; the rest contained approximately 75 or more bivalents. Spermatozoa were not identified in the testis by flow cytometry. Thus, the diploid males between the diploid gynogens and common diploid, and both sexes of triploids between the diploid gynogens and tetraploid, show aberrant meioses such as frequent formation of univalents, but the diploid females seem to be less affected.
The Cobitis hankugensis-Iksookimia longicorpa (formerly C. sinensis-C. longicorpus) complex in South Korea includes diploid and triploid forms of hybrid origin that are most often females. However, there are a few males that show no differences from the hybrid females in their general morphology except the presence of a lamina circularis on the pectoral fin. An examination of 354 adult hybrid specimens revealed that 3 % of the complex are male specimens. The testes of the rare males are normal in shape and external morphology. The gonadosomatic index of hybrid males is similar to that observed in males of the parental species and changes during the year in a similar pattern. However, the testicular cells differ from those of the parental species in their morphology. Especially sperm cells are very rare and show abnormalities in their morphology, therefore we conclude that the rare males are sterile. This result suggests that although males are present with a low frequency in the complex, this complex functionally is unisexual and depends on the males of the parental species for perpetuation.
The hexaploid barbel population of the species Labeobarbus fritschi
which lives in the Allal El Fassi dam reservoir in the north of Morocco showed an absence of males among a sample of 597 individuals analyzed in a first survey and among the 39 fish analyzed in this study. A possible explanation is that this population may be composed of females only and that
they reproduce either by gynogenesis or hybridogenesis, two processes triggered by hybridization with a sexually incompatible species (possibly the
sympatric barbel Luciobarbus setivimensis in this case). We used molecular markers to collect any information that could explain the phenomenon. To do this, numerous nuclear loci were screened (more than 50) in search of polymorphism. Genetic diversity was low as a possible consequence of clonal reproduction: only three allozymic and two intronic loci presented a sufficient level of polymorphism to be exploited. The existence of several copies of the same multilocus genotypes (1/3 of the fish) – with some showing a significantprobability of not being derived from sexual reproduction – provided evidence of unisexual reproduction. Deviations from panmixia as well as linkage equilibrium were also observed. Although preliminary, the data supports the hypothesis of the unisexual reproduction of females by way of gynogenesis rather than hybridogenesis.
The invasion success of gibel carp (Carassius gibelio) depends on demographic and competitive traits. The major biological trait responsible for the invasiveness of C. gibelio is the mode of reproduction. Apart from sexual reproduction, which is typical in fish, C. gibelio is a unique cyprinid species able to reproduce through asexual gynogenesis, which is also known as sperm-dependent parthenogenesis, observed in all-female populations. Though the sexual and asexual forms of C. gibelio co-exist widely in natural habitats, the gynogenetic form has the capacity to modulate the range of effective ecological niches, which may facilitate the process of invasion. In this paper, we reviewed current knowledge of the sexual and gynogenetic forms of gibel carp along with their physiological advantages, immunological traits, and ability to withstand different environmental conditions. As parasitic infection may directly alter the immunology of hosts, and also indirectly alter their investment in reproduction, we provide some insights into the role of parasites as one of the potential drivers facilitating the coexistence of asexual and sexual forms. We highlight evidence that gibel carp have been identified as a serious threat to native species; hence, its impact on the ecosystem is also discussed.