The common carp is one of the main aquaculture species in the world. Despite this, most of the production is carried out using unselected strains. Selective breeding for fast growth has not proven to be effective in this species, but other traits (disease resistance, shape) could be successfully selected for. Most heritability estimates in the literature are unreliable due to environmental biases, but complementary results from population genetics and comparison of strains seem to indicate that there should be a potential for selective breeding in this species, including selection for growth rate, provided the base populations are variable enough (e.g. synthetic strains). New techniques such as parentage assignment with microsatellites and use of doubled haploid progenies may help describe much more accurately, without environmental bias, the genetic determination of traits of interest in the carp. This could be a new opportunity to design efficient breeding programs in this important species.

The development of techniques for production of gynogenetic, androgenetic, polyploid, and monosex progenies in common carp (Cyprinus carpio L.) is described from a chronological perspective. Gynogenetic progenies were obtained either by suppression of the second meiotic division in eggs (meiotic gynogenesis) or by suppression of the first mitotic division in haploid embryos (mitotic gynogenesis). As a rule, gynogenetic progenies of common carp were all-female, revealing female homogamety (females—XX, males—XY) in this species. Induced gynogenesis results in increased homozygosity; the rate of increase depends on the type of gynogenesis. Inbreeding coefficient (F) for one generation of meiotic gynogenesis in common carp is about 0.6, while diploids obtained by mitotic gynogenesis are homozygous for all genes (F = 1.0). Mitotic gynogenesis was used for production of clones in common carp. In androgenetic progenies of common carp, YY males were identified, that after crossing with normal females (XX) produced all-male progenies. Triploids of common carp are characterized by a significant reduction in gonad development (especially ovaries). However, the reduction in gonad development did not result in an increase of somatic growth rate of fish. The procedure for androgen treatment to induce phenotypic sex reversal in genotypic females (XX) was elaborated. All-female progenies of common carp were produced on a large scale by crossing normal females (XX) with hormonally sex-reversed males (XX). Rearing of all-female progenies in conditions when fish normally reach sexual maturity before reaching of market size increased production yield by 7–8%. In a few cases distant hybridization resulted in polyploidy of fish without application of any physical treatment. The ability of hybrid females between crucian carp (Carassius auratus) and common carp to produce diploid (with unreduced chromosome number) gametes resulted in opportunities to produce triploid and tetraploid hybrid progenies.

The first successful case of transgenic fish was achieved in 1984. It is in a model system that the integration and expression of recombinant human growth hormone (hGH) in host red common carp (Cyprinus carpio, red var.) have been thoroughly studied. Recently, the integration sites have been recovered and characterized. Compared with non-transgenic peers, hGH-transgenic fish are prior in dietary utilization and growth performance. In view of bio-safety and bio-ethics, an "all-fish" construct CAgcGH, grass carp growth hormone fused with common carp β-actin promoter, has been generated and transferred into Yellow River carp (C. carpio, local strain in Yellow River) fertilized eggs. Under middle-scale trial, CAgcGH-transgenics show higher growth rate and food conversion efficiency than the controls, which is consistent to laboratory findings. To avoid the potential impact of transgenic fish on the environment, a sterile strain of transgenic triploid fish has been successfully produced. The "all-fish" transgenic common carp is also approved safe enough as daily food, according to a test based on the pathological principles of new medicines issued by the Ministry of Health of China. The "all-fish" transgenic common carp with growth enhancement is now ready for market, but looking for governmental authorization.

Domesticated/captive stocks and wild/feral populations of common carp from Europe, Central Asia and East/South-East Asia were examined for allozyme (23 populations), microsatellite (11 populations) and mitochondrial DNA (21 populations) variation. Allozyme variability (1.06–1.81 alleles per locus, expected heterozygosity 0.006–0.136 at 16 loci) was much lower than microsatellite variability (2.5–14.0 alleles per locus, expected heterozygosity 0.426–0.887 at four loci). Differences in variability between domesticated/captive stocks and wild-caught ones were more pronounced at microsatellite loci than at allozyme loci, suggesting that microsatellites are better suited to detect population bottlenecks and loss of variation due to inbreeding. All but one European population were fixed for a single composite mtDNA haplotype, which also dominated in Central Asia but was completely missing in East/South-East Asia, indicating a single origin of European carp in Central Asia. All three classes of genetic markers clustered populations into two highly divergent groups: Europe/Central Asia and East/South-East Asia. Hierarchical partition of genetic diversity showed that for microsatellite loci most of variation was due to the within-population component while the highest proportion of mtDNA variation and substantial proportion of allozyme variation was accounted for by differences between geographical regions. Genetic data support the subspecies status of C. c. carpio assigned to the European carp and C. c. haematopterus assigned to the East/South-East Asian carp but do not justify a separate subspecies status (C. c. aralensis) for the Central Asian carp. As demonstrated for a wild/feral carp population from R. Danube, Germany, the genetic markers used in our study may be effectively applied to detect mixing and introgression of intra-species units in the presence of sufficient genetic differentiation.

Regular observation of survival of the carp breeding lines constituting a living gene bank at the Institute of Ichthyobiology and Aquaculture in Golysz (Poland) over a period of at least 15 years showed different survival rates for various lines. In this study, we have examined the polymorphism of the major histocompatibility complex (MHC) gene class II B in nine carp lines. The class II B gene encodes for the part of the MHC class II molecule which presents peptides from pathogens and protein antigens that are present in the extracellular milieu and have been taken up into the endocytic vesicles of antigen-presenting cells. Polymerase chain reaction was used to amplify Cyca-DAB gene fragments comprising part of exon 1, complete intron 1 and almost complete exon 2. Exon 2 encodes for the β1 domain which is the most polymorphic fragment of MHC class II molecules. Single-strand conformational polymorphism (SSCP) was applied to detect different MHC class II B haplotypes. The analysis revealed the presence of seven different haplotypes occurring with various frequencies.

Proteases and protease inhibitors were detected in the seminal plasma, blood plasma, skin mucus and spermatozoa. Their molecular weights were estimated using SDS-PAGE under non-reducing conditions. The results demonstrate that the two main bands of anti-proteinase activity (APA) detected earlier in common carp seminal plasma with molecular weight of approximately 47 and 58 kDa are also present in other fluids. The intensity of staining was highest in blood and seminal plasma. The intensity in skin mucus was visible, but in a sperm extract it was faint. An additional fast migrating band (30 kDa) was observed only in seminal plasma. Highest APA was found in blood and seminal plasma followed by skin mucus. The activity in sperm extracts was low. The two serine-like proteases with molecular weight of 79 and 189 kDa in seminal and blood plasma have also been found. In skin mucus, protease of 79 kDa was also present. Metalloproteinases with molecular weights of 61 and 69 kDa were found in seminal and blood plasma but metalloproteinases of 44 and 38 kDa were observed only in seminal plasma. Although metalloproteinases, of molecular weight ranging between 61 and 75 kDa, were also visible in a sperm extract, the experimental approach used in this study did not allow unequivocal identification of unique proteases of spermatozoa. Blood plasma contains a serine protease and protease inhibitor not present in seminal plasma. Skin mucus also showed the profile of three unique proteases (two EDTA stimulated and one metalloproteinase). These results indicate that the analysis of proteases and their inhibitors makes it possible to distinguish contamination of milt with either blood or skin mucus. The physiological role of the detected protease-inhibitory system in fish seminal plasma is still unknown. It is possible that the protease inhibitor and proteases, unique for seminal plasma, are involved in a specific function of milt or testis (e.g. the control of spermatogenesis).

The exposure of freshly spawned, immotile carp sperm to hypoosmotic media triggers the initiation of calcium-dependent flagellar motility. Intracellular calcium concentration has been thought to be the critical component in motility initiation, possibly acting through a novel signalling pathway. The sensitivity of sperm cells to changes of osmolality of the environment raises the question whether a mechanoregulated osmosensitive calcium pathway is involved in the activation mechanism of carp sperm motility. The sperm cells are in a depolarized state in the seminal plasma (Ψ = –2.6 ± 3 mV) and they hyperpolarize upon hypoosmosis-induced activation of motility (Ψ = –29 ± 4 mV). The intracellular sodium [Na+]i, potassium [K+]i and calcium [Ca2+]i ion concentrations were determined in quiescent cells, and at 20, 60 and 300 s after activation. The [Na+]i and [K+]i of the quiescent cells were similar to the [Na+]e and [K+]e of the seminal plasma. Following hypoosmotic shock-induced motility, both [Na+]i and [K+]i decreased to one-fourth of the initial concentration. The [Ca2+]i doubled at initiation of the motility of the sperm cells and remained unchanged for 5 min. Bepridil (50–250 μM), a blocker of the Na+/Ca2+ exchanger, blocked carp sperm motility reversibly. Gadolinium, a blocker of stretch-activated channels (10–20 μM), inhibited sperm motility in a dose-dependent manner and its effect was reversible. Hypoosmotic shock fluidized the membrane and gadolinium treatment made it more rigid in both quiescent cells and hypotonic shock treated but immotile sperm cells. Based on these observations, it is suggested that, besides the well-known function of potassium and calcium channels, stretch-induced conformational changes of membrane proteins are also involved in the sperm activation mechanism of common carp.

This study summarizes optimization of techniques for common carp artificial propagation including improvements of activation solution (AS), the process of insemination, and elimination of egg stickiness. The optimum gamete ratio for good fertilization and hatching rate ranged from 8490 to 23672 spermatozoa per egg, when dechlorinated tap water was used. Optimal ratio between eggs (weight in g) and AS (in ml) was defined as 1:1 to 1:2. Different concentrations of AS such as NaCl from 0 to 34 mM (0–68 mOsmol kg–1) did not change fertilization and hatching rates. An AS adopted for carp spermatozoa (45 mM NaCl, 5 mM KCl, 30 mM Tris–HCl, pH 8) was compared with other saline AS; only the 51 mM (102 mOsmol kg–1) NaCl solution decreased fertilization and hatching rate. The AS containing 20 mM Tris–HCl at pH 9 increased fertilization and hatching rates compared to dechlorinated tap water of pH 7 or to AS of pH 6 and 7. Adhesiveness from the eggs was successfully removed by incubation in Alcalase DX (PLN 04715) using two successive steps with different enzyme concentrations. The first step with an enzyme concentration of 2 ml l–1 was applied from 8 to 20 min after fertilization. Later in a second step, the best time for application of alcalase enzyme at a concentration of 20 ml l–1 was for 45 and 60 s at 20 min after fertilization leading to fertilization and hatching rates of 80–87%. The α-Chymotrypsin (EC 3.4.21.1. Merck) was also found effective for elimination of stickiness. Results withα-Chymotrypsin enzyme indicate that the response to success in elimination of stickiness is highly variable mainly due to differences in the environment, quality of water and carp strains.

Experiments were carried out to investigate the effect of five extenders (sucrose, glucose, fructose, KCl and a saline carp sperm extender) and two cryoprotectants (dimethyl-sulfoxide (DMSO) and methanol) on the cryopreservation of common carp sperm. Freezing of sperm using glucose extender and methanol as cryoprotectant resulted in the highest post-thaw motility, fertilization as well as hatching rates (63 ± 9%, 74 ± 15% and 67 ± 17% vs. 87 ± 5%, 84 ± 14% and 69 ± 14% using fresh sperm, respectively). In general, sugar-based extenders combined with methanol as cryoprotectant yielded higher motility, fertilization and hatching rates than ionic extenders in combination with DMSO. The jelly-like agglutination observed after thawing in samples frozen with sugar-based extenders did not reduce fertilization and hatching rates. Frozen–thawed sperm samples were able to successfully fertilize 10 g (8000) eggs.

Anti-proteinase activity was demonstrated in the seminal plasma of cyprinid fish species (bream, chub, ide, dace, asp, goldfish, roach, common carp) using electrophoretic techniques combined with a detection method based on inhibition of bovine trypsin. We found species-specific protease inhibitors in the seminal plasma of cyprinids. At least three bands of protease inhibitors with different migration rates could be identified by native PAGE. Higher variability was characterized for bands with slower migration rates. Visualization of inhibitors after SDS-PAGE under non-reducing conditions allowed estimation of their molecular weights. Apparent molecular weights were within the range of 51–59 and 47–54 kDa for the bands with slower and moderate migration rates, respectively. The molecular weight of fast migration bands for roach and common carp were estimated to 23 and 30 kDa, respectively. Inhibitors of common carp seminal plasma differed in their affinity toward serine proteases. Three inhibitors in common carp seminal plasma could be visualized using cod and bovine trypsin, but only two inhibitors (of high molecular weight) were recognized with chymotrypsin. There were differences in anti-proteinase activity and seminal plasma protein concentration in relation to the origin of common carp seminal plasma (breeding lines) and time of milt collection (spawning vs. post-spawning season).