Reference: Bial. Bull. 207: 247-256. (December 20041 & 2004 Marine Biological Laboratory Chromosomal Rearrangement in Pectinidae Revealed by rRNA Loci and Implications for Bivalve Evolution YONGPING WANG 1 ' 2 AND XIMING GUO 1 * 1 Haskin Shellfish Research Laboratory. Institute of Marine and Coastal Sciences, Rutgers University, 6959 Miller Avenue, Port Norris, New Jersey 08349; and 2 Experimental Marine Biology Laboratory, Institute of Oceanologv, Chinese Academy of Sciences, 7 Nanluii Road, Qingdao, Shandong 266071, People 's Republic of China Abstract. Karyotype and chromosomal localization of major ( 18-5.8-28S) and minor (5S) ribosomal RNA genes were studied in two species of Pectinidae, zhikong (Chlann's farreri) and bay (Argopecten irradians irradians) scallops, using fluorescence in situ hybridization (FISH). C. farreri had a haploid number of 19 with a karyotype of 3m + 4sm + 7sm-st + 4st + 1 st-t, and A. i. irradians had a haploid number of 16 with a karyotype of 5st + 1 It. In C. farreri, the major and minor rRNA genes had one locus each and were mapped to the same chromosome Chromo-some 5. In A. i. irradians, the major rRNA genes had two loci, located on Chromosomes 4 and 8, and the 5S rRNA gene was found at a third chromosome Chromosome 10. Results of this and other studies indicate that karyotype of A. i. irradians (n = 16. 21 arms) is secondary and derived from an ancestral karyotype similar to that of C. farreri (n = 19, 38 arms) through considerable chromosomal loss and rearrangements. The ability to tolerate significant chromo-somal loss suggests that the modal karyotype of Pectinidae and possibly other bivalves with a haploid number of 19 is likely tetraploid; i.e., at least one genome duplication has occurred during the evolution of Bivalvia. Introduction Chromosomal changes, particularly polyploidy, have played a significant role in the evolution of plants, and most higher plants are recent polyploids (DeWit, 1980). Although polyploidy is relatively rare in animals, chromosomal changes are increasingly recognized as an important force in Received 5 April 2004; accepted 6 August 2004. * To whom correspondence should be addressed. E-mail: hsrl.rutgers.edu xguo@> animal evolution. It is hypothesized that two rounds of genomic duplication occurred during the evolution of ver-tebrates leading to humans (Furlong and Holland, 2002; Spring. 2002). Chromosomal rearrangements may play a role in reproductive isolation and speciation, by creating barriers to meiotic pairing and reducing the fitness of hy-brids (White, 1978; King, 1993). Genie theories, on the other hand, stress the importance of accumulation of genie mutations in reproductive isolation. Recent findings of ef-fects of chromosomal rearrangements on recombination have bridged the gap between the chromosomal and genie theories of reproductive isolation, arguing for a major role of chromosomal changes in speciation (Rieseberg, 2001; Navarro and Barton, 2003). However, the extent of chro-mosomal changes and their roles in speciation are poorly understood in many animal taxa, including marine bivalves. Many marine bivalves are sympatric broadcast spawners whose mechanisms of reproductive isolation are particularly interesting but largely unknown. Chromosomal studies may provide a unique perspective on the evolution of marine bivalves. Scallops, members of family Pectinidae. are widely dis-tributed in world oceans. Scallops are characterized by two mostly equal and round valves with a byssal notch and ctenolium on the right valve. Most scallops are free living and mobile, and inhabit the surface of subtidal bottoms. Some species may use byssal threads for temporary attach-ment or cement themselves permanently onto hard surfaces. The earliest representative of Pectinidae appeared in the Triassic period about 230 million years ago, but most groups did not emerge until the early Paleocene. or 65 million years ago (Waller, 1991 ). The family contains about 360 living species, and many of them are important for 247
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