Cytogenetic Characterization Of Scallop Chromosomes

In this study, the traditional banding techniques and fluorescence in situ hybridization (FISH) were used to characterize and identify the chromosomes of scallops. Genome components of interspecific hybrid F1 of Chlamys farreri×Mimachlamys nobilis and C. farreri×Argopecten i. irradians were also identified by genomic in situ hybridization (GISH). The main results are as follows:1. The C banding, NOR banding and DAPI banding distribution were analyzed in scallops chromosomes. C banding of A. i. irradians mainly distributed at telomeric and centromeric regions; some interstitial C-bands were also observed. The pattern of DAPI banding of A. i. irradians was almost consistent with that of C banding and also maily distributed at telomeric and interstitial regions. However, DAPI banding of Patinopecten yessoensis maily distributed at centromeric region and. Silver staining revealed that NORs were located on the short arms of subtelomeric chromosome 3 and telomeric chromosme 10 of A. i. irradians, and on the short arms of subtelomeric chromosome 10 of C. farreri. However, in C. farreri, one metaphase spread displayed an additional silver spot on the short arm of subtelocentric chromosome 12.2. Chromosomal localization of repeated sequences-rDNA(1) 18S-28S rDNA was assigned to one locus in C. farreri and M. nobilis, two loci in P. yessoensis and A. i. irradians. In C. farreri, 18S-28S rDNA was localized on the short arm of subtelocentric chromosome 10. In M. nobilis, 18S-28S rDNA was localized on the centromere of metacentric chromosome 1, which may reflect that the Robertsonian fusion has happened. In P. yessoensis, 18S-28S rDNA was localized on the short arm of subtelocentric chromosome 11 and 13. In A. i. irradians, 18S-28S rDNA were located on the short arms of subtelomeric chromosome 3 and telomeric chromosme 10. In general, low rDNA number reflects old evolutionary status. The two loci of 18S-28S rDNA in P. yessoensis and A. i. irradians may result from 18S-28S rDNA replication. As a result, among these four scallops in this study, C. farreri showed the oldest evolutionary status. (2) 5S rDNA was assigned to one locus in C. farreri and P. yessoensis, two loci in A. i. irradians. In C. farreri, 5S rDNA was localized on the long arm of subtelomeric chromosome 10. In P. yessoensis, 5S rDNA was localized on the long arm of subtelomeric chromosome 15. In A. i. irradians, 5S rDNA was mapped as two distinguishable loci on the long arm of subtelomeric chromosome 11, which was different from the previous report which showed only one 5S rDNA site in A. i. irradians (Wang and Guo 2004). The difference may result from the condensation state of chromosomes. It is reasonable to consider that there are two clusters of 5S rDNA and that when chromosomes are condensed only one signal is detected.(3) Sequential FISH on the same metaphase was used to examine the relationship of 18S-28S and 5S rDNA. The results showed that 18S-28S and 5S rDNA were located on three different pairs of chromosomes in A. i. irradians and three different pairs of chromosomes in P. yessoensis.3. Chromosomal localization of vertebrate telomeric sequences (TTAGGG)nVertebrate telomeric sequences, detected by FISH, were located on both ends of each chromosome of C. farreri, P. yessoensis and A. i. irradians. No interstitial signal was observed on any chromosomes, which may reflect that therer were no chromosomal rearrangements in these three scallops. The signal intensity varied among different chromosomes.4. Chromosome components analysis of hybrid scallopsGISH was employed to detect different chromosome components and identify donor chromatin in hybrids between C. farreri♀×M. nobilis♂. Results showed that more than 74 percentages of hybrids contain 35 chromosomes. 16 chromosomes were detected with green signals by M. nobilis-genome DNA as probes; 19 chromosomes were detected with green signals by C. farreri-genome DNA as probes corresponding to the sum of chromosomes from the haploid genomes of M. nobilis (n=16) and C. farreri (n=19). In this study, there was not any abnormal behaviour of chromosomes such as deletion, breakage, recombination and so on was detected. There was also not any haploid metaphases (n=16 or 19) was detected in hybrids. However, about 1-2% metaphases observed in this study was allopolyploid. GISH proved that they was made up of haploid genomes of M. nobilis (n=16) and triploid genomes of C. farreri (n=57).GISH was also employed to detect different chromosome components and identify donor chromatin in hybrids between C. farreri♀×A. i. irradians♂. Results showed that more than 68 percentages of hybrids contain 35 chromosomes. 16 chromosomes were detected with green signals by A. i. irradians-genome DNA as probes; 19 chromosomes were detected with green signals by C. farreri-genome DNA as probes, corresponding to the sum of chromosomes from the haploid genomes of A. i. irradians (n=16) and C. farreri-genome DNA. As a result, by the developmental stage of trochophore, most hybrids still had the karyotype of 2n=3m+5sm+15st+12t, according with theoretical expectation of hybrids, which crediblely proved to be a combination of haploid genomes of biparents in hybrids.