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Amita Saxena and Satesh Vasave

College of Fisheries, Gbpuat, Pantnagar 263145, India

The chromosomes are responsible for the mechanism of inheritance to express the genetic characteristics in successive generations. In 1902, W. S. Sutton state hypothesis that the chromosome provide the physical basis of heredity. Chromosomes are named from the fact that they stained preferentially with certain dyes and show suitable cellular material prepared for microscopic examination. They appear thread like at certain times and are composed of linear complexes of deoxyribonucleic acid (DNA), genetic material proper and histone proteins which have a supporting or structural role. All living organisms have chromosomes. Prokaryotes (bacteria and virus) are different from eukaryotes (plants and animals). The eukaryotic cell has nucleus that carries chromosome complements. The number of chromosomes per cell is the characteristics of the particular species but in case of fishes these chromosome numbers different within same species also observed. These chromosome numbers made up of maternal and paternal origin of two sets so called as diploid. Single set (egg or sperm) of chromosomes called as haploid. The chromosomes numbers varies in fishes.

The chromosome material is called chromatin which is of two types like euchromatin and heterochromatin. The euchromatin stains lightly and heterochromatin stains darkly. Euchromatin contains genes in a linear array like beds on a string and heterochromatin is genetically inert and acting in maintaining structural integrity of chromosomes and regulation of the gene expression. Heterochromatin made of highly repeated simple sequences of DNA. The part of chromosome at the end called telomere and the constriction called centromere. The telomeres are stable entities essential for maintaining the integrity of chromosome threads. The centromere controls the movement of chromosomes during cell division. Both the telomere and centromere are located in the heterochromatin. The chromosomes again divided as metacentric, telocentric and acrocentric based on the position of centromere in the chromosome. When centromere located at center, then it is called as metacentric, at or very near to one end called telocentric and acrocentric having one arm very longer than other. Intermediate cases are described by using the prefix ‘sub’. The fish chromosomes are comparatively small in size and have its own characterisrics. The NF value i. e. 'nombre fundamental' or number of chromosome arms is important because it gives the genetic content of a chromosome complement.

The chromosome staining is achieved by using dyes having affinity for DNA. But the different chromosome banding techniques shows the finer structure of chromosomes.

  1. Fluorescence banding

  2. Constitutive heterochromatin banding

  3. Giemsa banding

  4. Nucleolar organizers

  5. Late replicating DNA

  6. FISH (Fluorescent In- Situ Hybridization)

The importance of fish taxonomy is not only with description of new forms, but also with placing each form within taxonomic system that shows it’s relationships to other forms .For more than a century, systematists have sought to organize this diversity by studying aspects of their external and internal morphology which have been especially successful in defining species and in organizing these species into genera. These groupings have usually been confirmed when examined with cytogenetically approaches.

In the last few decades works have been focused on the field of cytogenetic investigation of fishes, especially in the area of systematics, mutagenesis and aquaculture. The karyotype is the chromosome complement of an individual or related group of individuals, as defined by chromosome size, morphology and number. Though for all somatic cells of all individuals of species, the number of chromosomes is used as an indicator of classification of species of chromosomes and interrelationships within families. The studies of these characters help to investigate the aquatic structure for the investigate the aquatic structure for the population of each species population in each habitat, so it can determine what species are related to each other in an accurate manner?. This may help to facilitate the hybridization between them in the future to improve the strains.

Cytogenetical studies on fish have been useful to provide information concerning evolutionary and taxonomic studies, as well as for the genetic improvement of commercial fish stocks (Gold, 1979). Several techniques have been devised to obtain mitotic chromosomes in fish, ranging from direct preparations to long-term cell culture (Denton, 1973, Ojima, 1982 and Alvarez et al., 1991, among others). Among these methodologies, in vivo procedures, usually time- and cost-saving, have been the most widespread (Egozcue, 1971, Gold, 1974 and Rivlin et al., 1985). However, to perform direct techniques, it is necessary to carry out a previous colchicine treatment of live animals for about 1 h. Very often, this is a not a feasible method, as in the case of delicate/fragile species after lengthy transportation or species requiring specially suited tanks (e.g., marine species). Consequently, most of the available cellular material can be lost (Ozouf-Costaz and Foresti, 1992 and Maddock and Schwartz, 1996).

In eukaryotes, chromosomes consist of a single molecule of DNA [Link to visual proof] associated with:

  • many copies of 5 kinds of histones. Histones are proteins rich in lysine and arginine residues and thus positively-charged. For this reason they bind tightly to the negatively-charged phosphates in DNA.

  • a small number of copies of many different kinds of non-histone proteins. Most of these are transcription factors that regulate which parts of the DNA will be transcribed into RNA.

Fishes have been the subject of an increasing number of cytogenetic investigations in the areas of systematics, mutagenesis and aquaculture. From about 20,000 - 40,000 fish species estimated to occur on this globe, the basic karyotypic characteristics[i.e., diploid chromosome number (2n) and number of chromosome arms (NF) are known for not more than 1700 species, which represent only about 9% of the total number of species available. Efforts have been made from time to time to update the karyotype list of available species.

Cyprinus carpio L. is a teleostean species having a tetraploid origin. Inspite of the fact that carp possesses a large number of very small chromosomes, it has been fairly well studied by the cytogenetic researchers. In a majority of these studies, a diploid chromosome number has been reported to be 2n = 100 in common carp (Raicu et al., 1972; Denton, 1973; Zan & Song, 1980; Blaxhall, 1983; Labat et al., 1983; Rab et al., 1989; Larka & Rishi, 1991; Anjum & Jankun, 1994; Anjum, 1995). Diploid chromosome number of native carp from Amur river has been divided into eight well-defined groups on the basis of their morphology and a standard karyotype has also been proposed (Rab et al., 1989). C-banding and Silver-NOR staining has also been employed in some cytogenetic studies on common carp (Takai & Ojima, 1982; Ruifang et al., 1985; Sola et al.,1986).

Neotropical fishes present a high chromosome diversity showing a wide diploid number variation range, including different levels of ploidies, sex chromosomes, chromosome supernumeraries, and several cases of polymorphisms, related particularly to heterochromatin and NOR sites. Two main general trends of chromosome diversification can be observed among neotropical fishes. First, several fish groups show a chromosome evolution relatively divergent from the point of view of the karyotypic macrostructure. Sister species show conspicuous differences in karyotype structure and most often also in the number of chromosomes.

On the other hand, there are fish groups in which chromosome evolution has been shown to be less divergent, and in this case whole families or even groups of families may share a common karyotype structure and equal number of chromosomes. Several fish groups appear conservative also with respect to the NOR bearing chromosomes. In this case, NOR chromosome location is invariable among species. In contrast, several other groups present wide NOR variability. Sister species may show quite diverse chromosomes bearing nucleolar organizing regions. The NOR and heterochromatin relationship is also very diverse among fishes and this may indicate organizational differences involving these chromosome segments. Thus, neotropical fish fauna presents great chromosome variability, verifiable also by NOR studies.

The karyotypes of five species of Scorpaenidae (genera Scorpenopsis, Dendrochirus and Pterois) from the Indian Ocean are characterized by a diploid set of 48 chromosomes (mainly acrocentric and/or subtelocentric) and by a NOR location on the small arm of a medium-sized pair. All the chromosomes stained uniformly with DAPI, whereas C-banding evidenced a small amount of heterochromatin. Despite the marked morphological differences among these species, the low degree of diversification of the sets with respect to the ancestral set of teleosts (2n = 48 acrocentric chromosomes) suggests that chromosome morphology has not undergone profound rearrangements during the evolution of these taxa. (chromosome no. structure, definitions of diff. types, diversity, manipulation, importance added)?

In general, cytogenetics studies of crustaceans are relatively few and very difficult to perform because their chromosome numbers are large (Zhang et al. 2003, Lee et al. 2004), chromosomes are small and theirs shapes are very variable including metacentric, submetacentric, and acrocentric chromosomes (Tan et al. 2004). Marine crustaceans such as the white shrimp Litopenaeus vannammei (Dumas & Campos-Ramos 1999) and the Chinese shrimp Fenneropenaeus chinensis (Zhang et al. 2003), as well as in marine bivalves such as the Japanese oyster Crassostrea gigas (Allen et al. 1989) were studied to get good results for manipulation of chromosomes. Consequently, information on the basic genetics of the tropical crayfish P. llamasi is necessary not only to reinforce its potential for aquaculture, but also for genetic improvements and conservation.

Loricariidae is one of the largest fish families of the world, with about 650 species separated into six subfamilies. To date, cytogenetic data on only 56 species of this family are available. The lowest diploid number, 2n=38, was observed in Ancistrus n.sp. 1 (Ancistrinae) and the highest diploid number, 2n=70, was observed in Rineloricarian sp. (Loricariinae). The nucleolar organizer regions (NORs) were seen at a terminal position in six species and at an interstitial position in two. The karyotypic analysis of Loricariinae and Ancistrinae species revealed that these groups exhibit a large diversity of diploid numbers, suggesting the occurrence of intense karyotypic evolution during their evolutionary history.

Although Reis et al. (2003) consider the Loricariidae as the largest family of catfishes in the world, little is known about the karyotypic organization in this group (Artoni and Ber­tollo, 2001). Meanwhile, some studies provide some cytogenetics information for Loricariinae (Scavone and Júlio Jr., 1994; Giuliano-Caetano, 1998; Artoni and Bertollo, 2001), Hypop­topomatinae (Andreata et al., 1992, 1993, 1994), Hypostominae (Artoni et al., 1998, 1999; Artoni and Bertollo, 1996, 1999, 2001; Alves et al., 2006), Ancistrini (Lara, 1998; Artoni and Bertollo, 2001) and Neoplecostominae (Alves, 2000; Kavalco et al., 2005).

In spite of the small amount of information compared with the number of the species already described for Loricariidae, the available data demonstrate that this is a group of great interest for cytogenetic studies, due not only to the variation in chromosome number, 2n = 36 in Rineloricaria latirostris (Giuliano-Caetano, 1998) to 2n = 96 in Upsilodus sp (Kavalco et al., 2005), but also to the occurrence of many chromosomal rearrangements, suggesting a divergent karyotypic evolution (Artoni and Bertollo, 2001).

Hypostomus is considered to be one of the most diversified groups of Neotropical fishes, and is one of the most studied genera from a cytogenetic point of view, showing a varia­tion in chromosome number from 2n = 54 in H. plecostomus (Muramoto et al., 1968, in Artoni and Bertollo, 2001) to 2n = 80 in Hypostomus sp E (Artoni and Bertollo, 1999).

The family Gobiidae is quite interesting because of its controversial morphological features (Fage, 1925; Arai and Sawada, 1974; Nishikawa et al., 1974) and its evolutionary stage which is not completely known. A high variability of chromosome number occurs within this group ranging from 2n = 40 to 2n = 62 (Solaetal.,1979). There still is no agreement as to the number of chromosomes characterizing Gobius paganellus. The diploid number 2n = 45 was proposed for a female specimen caught in the Thyrrhenian Sea (Cataudella et a!., 1973), n = 25 and 2n = 50 for male specimens caught in the Northern Adriatic Sea (Colombera and Rasotto, 1982), and 2n = 46 for eight male and female specimens caught in the Southern Mediterranean Sea near Spanish coasts (Thode et al., 1983). Thode et aL, (1983) claim that Gobius paganellus is characterized by male heterogamety (XY). The occurrence of a large metacentric chromosome in the female specimen investigated by Cataudella et al. (1973) does not agree with the mechanism of sex determination proposed for this species.

The Lutjanidae (snappers) is a group composed of 17 genera and 105 species of mostly reef-associated marine fishes, which are distributed in all the tropical and subtropical seas of the world (Nelson, 2006). The family is divided in four subfamilies. Three smaller subfamilies include the Paradichthyinae, with two monotypic genera (Symphorus and Symphorichthys), the Etelinae, with five genera (Aphareus, Aprion, Etelis, Pristipomoides and Rhandallichthys) and 19 species, and the Apsilinae, with four genera (Apsilus, Lipocheilus, Paracesio and Parapristipomoides) and 12 species (Nelson, 2006). The subfamily Lutjaninae is the largest, with three monotypic genera (Hoplopagrus, Ocyurus and Rhomboplites), the genera Macolor and Pinjalo with two species each, and the genus Lutjanus, which is the most speciose, with 64 species. In Venezuela, Cervigón (1993) recognizes six genera of Lutjanidae (Etelis, Pristipomoides, Apsilus, Ocyurus, Rhomboplites and Lutjanus ) including 15 species, 10 of which belong to the genus Lutjanus (L. analis, L. apodus, L. aya, L. bucanella, L. cyanopterus, L. griseus, L. jocu, L. mahogoni, L. purpureus, L. synagris and L. vivanus).

In spite of their high number and their ecological and economic importance, cytogenetic studies on Lutjanidae are scarce. In fact, among the 105 recognized species of Lutjanidae, barely five species have been karyotyped to date: Lutjanus argentimaculatus (Raghunath & Prasad, 1980), L. kasmira (Choudhury et al., 1979; Ueno & Takai, 2008), L. sanguineus (Rishi, 1973), L. russelli (Ueno & Ojima 1992), and L. quinquelineatus (Ueno & Takai, 2008). For most of them, only the chromosome number and morphology have been reported and there is no data regarding the chromosomal distribution and composition of the constitutive heterochromatin or numbers and locations of the major and minor ribosomal genes, which have proved to be useful markers in the investigation of the phylogenetic relationships among fish species within a family (Sola et al., 2007).

Studies on the chromosomes of fishes have not been successful or widespread as in other vertebrate groups. Fish karyotypes are generally characterized by a large number of small chromosomes, discouraging researchers from pursuing fish-karyotype analysis. Therefore karyological data on fish are available only for a small percentage (about 10%) of some 25 000 species taxonomically known so far. The Bagridae family of fish is the richest and most important of the teleostei class and its members are distributed throughout the world1. In the Bagridae family, the fish Mystus vittatus (Smith, 1945) is economically important and distributed in the semitemporal freshwater system of south India. Based upon the fish chromosome data, it seems that the chromosome number depends on the species in the Bagridae family, suggesting some major chromosome rearrangements which might have played a significant role during speciation and evolution of Bagridae. The family of Bagridae has received special attention in Asia; up to 40 species have been karyotyped so far. The fishes belonging to family Bagaridae have good nutritive value and a candidate species for aquaculture. The number of chromosomes varies between species in genus, Mystus. In M. vittatus the diploid chromosome number has been reported to be 2n = 586.


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