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Genetics, Vol. 170, 1821-1826, August 2005, Copyright © 2005
doi:10.1534/genetics.104.039719
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* Laboratory of Aquatic Ecology, Katholieke Universiteit Leuven, B-3000 Leuven, Belgium
Roslin Institute, Roslin, Midlothian EH25 9PS, United Kingdom
Hellenic Centre for Marine Research Crete, Institute of Marine Biology and Genetics, 71500 Heraklion, Greece
CNR-Istituto di Scienze Marine, Biologia del Mare, Castello 1364/A, I-30122 Venezia, Italy
2 Corresponding author: Laboratory of Aquatic Ecology, Katholieke Universiteit Leuven, Charles de Bériotstraat 32, B-3000 Leuven, Belgium.
E-mail: filip.volckaert{at}bio.kuleuven.be
| ABSTRACT |
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Genetic management and selection in sea bass would benefit from the development of new genetic tools to assist in reducing inbreeding, lowering losses of farmed fish from infections, improving resistance to suboptimal environmental conditions, and accelerating growth rate. Marker-assisted selection (MAS) can provide a means for increasing response rates to selection, especially for traits such as fat content and shelf life that are difficult to record without killing the target fish (DEKKERS 2004). Species-specific linkage maps composed of highly polymorphic markers represent one of the genetic tools that are essential for conducting comprehensive searches for loci that affect phenotype(s) of interest, particularly those that link to commercially important traits (GARCíA DE LEóN et al. 1998; DEKKERS 2004).
Microsatellites, or single sequence repeats (SSRs), represent DNA sequences that show high levels of intraspecific allele polymorphism and are widely distributed within vertebrate genomes (TOTH et al. 2000). Microsatellite-based linkage maps have been constructed for agriculturally significant avian and mammalian species such as chicken (GROENEN et al. 2000), sheep (MADDOX et al. 2001), cattle (KAPPES et al. 1997), and pig (ROHRER et al. 1996). Among the economically important fish species, linkage maps based on microsatellite loci have been generated for the Atlantic salmon (GILBEY et al. 2004), Arctic char (WORAM et al. 2004), Japanese flounder (COIMBRA et al. 2003), rainbow trout (SAKAMOTO et al. 2000), channel catfish (GEOFFREY et al. 2001), and Nile tilapia (KOCHER et al. 1998). Here we present a first-generation microsatellite linkage map for the European sea bass that will provide a useful tool for selective breeding and mapping of economically important features in farmed sea bass populations. It will also benefit the evaluation of adaptive traits in natural populations (FARRALL 2004).
| MATERIALS AND METHODS |
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The cDNA library was constructed from a mixture of sea bass larval stages (details available upon request). A cDNA library from sea bass brain enriched with pituitary gland was kindly provided by A. Canario (Centro de Ciencias do Mar, Faro, Portugal). Nucleotide sequences of tandem-repeat-containing clones from microsatellite-enriched libraries, cDNA clones, and available sequences for genes and mRNA of D. labrax from the GenBank sequence database were used as a source for deriving putative microsatellites with the Tandem Repeat Finder v3.2.1 software (BENSON 1999). Flanking primers were designed using the PRIMER3.0 program (ROZEN and SKALETSKY 1998).
Microsatellites were amplified in a 10-µl volume PCR mixture containing 20 mM Tris-HCl, pH 8.4, 50 mM KCl, 1.01.5 mM MgCl2, 0.2 mM dNTPs, 1 unit of Taq DNA polymerase (Invitrogen Life Technologies, Carlsbad, CA), 50 ng of D. labrax genomic DNA, and 10 pmol of each locus-specific primer. Each forward primer has been labeled with the IRD-700 or IRD-800 fluorescent dye. PCR cycling was carried out on a GeneAmp PCR System 2700 thermocycler (Applied Biosystems, Foster City, CA) with the initial preheating for 96° at 3 min following 30 cycles of 30 sec at 96°, 30 sec at 54°60°, and 60 sec at 72° with a final extension for 7 min at 72°. MgCl2 concentrations and annealing temperatures (Ta) for each locus are given in supplemental SAppendix S1 at http://www.genetics.org/supplemental/. The sequence of new microsatellites was aligned vs. the sequence of D. labrax microsatellites already available in the nucleotide databases using the BLAST 2 SEQUENCES on-line tool (TATUSOVA and MADDEN 1999). For each PCR-amplifiable microsatellite, fluorescence-based genotyping of 21 unrelated sea bass individuals was performed using the Li-Cor 4200 DNA sequencer (LI-COR BioSciences, Lincoln, NE). Gel images were analyzed using GENEIMAGIR 4.2 software. Observed (Ho) and expected (He) heterozygosities and polymorphism information content (PIC) were computed using the POLYMORPHISM program (BOTSTEIN et al. 1980).
Linkage mapping analysis:
For linkage studies, the Venezia Fbis family, consisting of both parents and 50 full-sib progeny (biparental diploids), was genotyped (CHISTIAKOV et al. 2004). Parents were wild and originated from the northern Adriatic Sea. Detection of linkage and map building made use of the CRI-MAP 3.0 software (GREEN et al. 1990). The assignment of markers to linkage groups was performed by clustering markers showing pairwise LOD scores of
4.0. This value was increased from the commonly used threshold of 3.0 to minimize the risk of false linkage being accepted in the large number of two-way tests being performed (OTT 1991). Where there were three or more markers in a linkage group, comprehensive maps (KEATS et al. 1991) were derived by selecting the most likely order from those possible by looking at all possible orders of up to six adjacent loci (using the FLIPS6 option in CRI-MAP). Additional markers were added to the map in their most likely position, with the resulting map order being rechecked by looking at all possible orders of up to six adjacent loci in a window that was slid across the whole linkage group until all linked markers had been added to the map. Maps were built by assuming that the recombination rate was the same in both sexes, but orders were also checked allowing the recombination rate to differ between sexes. In no case was a better order found in this latter situation. Once the most likely order had been derived, sex-average and sex-specific linkage distances were estimated for each linkage group assuming the KOSAMBI (1944) mapping function.
| RESULTS |
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In total, 1830 recombinant clones were obtained in both libraries. Of them, 1162 clones contained SSR motifs. Primer pairs were designed for 470 clones and, of these, 277 amplified a PCR product of the expected size. A total of 171 loci showed polymorphism in a population sample of 21 unrelated sea bass individuals. Thus, testing 1830 clones from microsatellite libraries finally yielded 171 (9.3%) polymorphic SSR loci of D. labrax.
We searched for microsatellite nucleotide sequences in 124 nonredundant cDNA clones (109 larval and 15 brain). For 20 SSR-containing cDNA clones, we developed 23 pairs of PCR primers. Testing 17 primer pairs yielded PCR products of the expected size. Finally, 11 of these 17 markers showed polymorphism in the population sample. The polymorphic microsatellites were designated as DLA0223EDLA0234E (SAppendix S1 at http://www.genetics.org/supplemental/). All polymorphic markers were found in anonymous clones, for which no homology with known nucleotide sequences was found.
We also screened for SSR nucleotide sequences of known genes and mRNAs of D. labrax in the GenBank database. We selected eight SSR-containing DNA sequences encoding cytochrome P450 aromatase (CYP19; AJ318516) and recombination activating protein 1 (RAG1; AF137203), somatolactin (AJ277390), ß-actin (AJ537421), interleukin 1ß (AJ269472), peptide Y (AJ005380), cytochrome P450 aromatase (AY138522), and HMG-CoA reductase (AY424801). Microsatellites located within the RAG1 gene (VENKATESH et al. 1999) and at the 3'-untranslated region of mRNAs encoding aromatase and peptide Y were found to be polymorphic. The polymorphic markers were designated DLA0222RAG1, DLA0237PY, and DLA0238CYP19 (SAppendix S1 at http://www.genetics.org/supplemental/). Therefore, a total of 14 type I polymorphic microsatellites were developed from cDNA clones and known genes of sea bass.
Characteristics of polymorphic microsatellites:
To date, a total of 200 polymorphic SSRs have been characterized from genomic DNA of D. labrax. These contain 53 markers previously reported by GARCíA DE LEóN et al. (1995), CASTILHO and MCANDREW (1998), CIFTCI et al. (2002), TSIGENOPOULOS et al. (2003), and CHISTIAKOV et al. (2004) and 146 microsatellites developed in this study. In addition, one locus, SaGT41b, isolated from the gilthead sea bream, Sparus aurata, has been used successfully on genomic DNA of sea bass (BATARGIAS et al. 1999).
For the 200 SSRs, the number of alleles ranged from 2 to 17, with an average of 7.7 alleles per marker (SAppendix S1 at http://www.genetics.org/supplemental/). The observed heterozygosity ranged from 0.048 to 1.0 (average of 0.718) and the PIC from 0.048 to 0.923 (average of 0.676). A subset of 112 (56%) of 200 microsatellites is highly informative, with an expected heterozygosity of 0.75 and higher and with a PIC value of 0.7 and higher. The 200 SSRs are unique sequences, which have been aligned against each other with no significant similarities being found.
Most of these microsatellites consist of dinucleotide tandem repeats. However, there are also 36 tetra-, 4 hexa-, 4 tri-, and 2 pentanucleotide loci. One marker, DLA0198, actually represents a variable number of tandem repeats because it contains repeat units with an 11-bp consensus sequence.
Linkage mapping:
The map reference family was polymorphic for 174 microsatellite loci including 11 type I markers and 163 type II SSRs. A null allele was detected in the genotype of the male parent for the loci DLA0023K, DLA0026, and DLA0249. For three markers (DLA0125, DLA0198, and DLA0205), a null allele was identified in the genotype of the female parent. We carefully took these cases into consideration to avoid mapping errors.
The 162 mapped markers were assembled into 25 linkage groups, with an average of six SSRs per linkage group, ranging in length from 1.0 to 147.2 cM (Figure 1). Twenty-one linkage groups comprised three and more loci whereas the remaining 4 groups each contained two SSRs.
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The current sex-averaged map spans 814.5 cM of the European sea bass genome. In the map, the intermarker distance ranged from 0 to 32.1 cM, with an average of 5.03 cM. Sixty percent of the intervals between markers varied from 0 to 5 cM, 19% ranged from 5 to 10 cM, 15% varied from 10 to 20 cM, and 6% were >20 cM. The map contains 138 genetically separated markers. A total of 12 microsatellites remained unlinked.
Sex-specific maps have also been constructed. The length of the male map is 567.4 cM, with an average intermarker distance of 3.5 cM (Figure 2) whereas the female map is 905.9 cM long, with an average intermarker spacing of 5.59 cM (Figure 3). The male map comprises linkage groups ranging in length from 0 to 128.7 cM (Figure 2) while the female map contains linkage groups ranging in length from 0 to 178.7 cM (Figure 3). The female map length is 1.6-fold that of the male map. The sex-averaged map is intermediate in length between the sex-specific maps and is 1.44 times as long as the male map.
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| DISCUSSION |
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The recombination rate has been known to differ between sexes in numerous vertebrates, with a higher prevalence of recombination in females compared to males. F:M recombination rate ratios lie between 1.0 and 2.0 in higher vertebrates [e.g., human (DIB et al. 1996), mouse (DIETRICH et al. 1996), dog (MELLERSH et al. 1997), and pig (MIKAWA et al. 1999)]. In lower vertebrates, the F:M ratio is known to exceed 2.0; in zebrafish, rainbow trout, and Atlantic salmon, the F:M ratio was detected to be 2.74:1, 3.25:1, and 8.26:1, respectively (SAKAMOTO et al. 2000; SINGER et al. 2002; COIMBRA et al. 2003; MOEN et al. 2003). In Japanese flounder, the recombination rate was unusually high in males (7.4 times) compared to females (COIMBRA et al. 2003). In sea bass, the calculated map length is 1.6-fold higher in females than in males. Therefore it represents a value typical for most vertebrate species, for which sex recombination rate has been measured. However, in sea bass, this ratio is the lowest among those reported among marine fishes.
The current linkage map is sufficiently dense to be successfully applied to the mapping of QTL. In addition, both mapped genes (CERDA-REVERTER et al. 1998; DALLA VALLE et al. 2002) might be involved in economically significant traits such as sex determination, response to stress, and growth rate. The future identification of sex-determining loci will support the use of MAS in sea bass for the feminization of farmed populations, breeding of monosex cultures, and acceleration of male growth rates (DEKKERS 2004). Further enrichment of the linkage map with coding markers will be a prerequisite for comparative mapping with those bony fish species for which a high-density map has been available (NARUSE et al. 2000; WOODS et al. 2000).
| ACKNOWLEDGEMENTS |
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| FOOTNOTES |
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1 These authors contributed equally to this article. ![]()
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