Patterns of concerted evolution of the rDNA family in a natural population of Zhikong scallop, Chlamys farreri.

Using denaturing high-performance liquid chromatography (DHPLC), we screened the insertion/deletion (indel) polymorphism in the internal transcribed spacer (ITS) sequences of 40 individuals from a natural population of Zhikong scallop (Chlamys farreri). Surprisingly, only 7.5% of individuals were homogeneous in ITS constitution, while the others (92.5%) were heterozygous. Based on different peak types in DHPLC analysis, seven individuals were randomly chosen to investigate indel polymorphism in the ITS sequences within individuals. Furthermore, indel polymorphism in the ITS sequences of single sperms was also investigated in more individuals belonging to different peak types. Based on these results, we concluded that rapid intrachromosomal recombination drove homogenization of rDNA arrays and interchromosomal recombination might contribute to form new variants, and that it may be less rare than previously thought although it was much less frequent than intrachromosomal recombination in the homogenization process. Further, we proposed an expanded model for concerted evolution of the rDNA family in a natural population of C. farreri. A pathway in the new model which homogenizes a variant unit, beginning with two-peak type individuals and ends with two-peak type individuals, is a larruping pathway in the natural population of C. farreri. As the highest proportion in natural populations, two-peak individuals with equal peak areas can be viewed as being in a steady and balanced state which is maintained by rapid intrachromosomal recombination.