Molecular evolution of ribosomal intergenic spacers inOdontophrynus americanus2nand 4n(Amphibia: Anura)

Abstract

Ribosomal intergenic spacers (IGSs) of Odontophrynus americanus 2n and 4n were cloned, restriction mapped, and partially sequenced. Three distinct regions, namely α, β, and δ, were identified in the IGSs. The α and β regions flanked the 28S and 18S rRNA genes, respectively, conserving an identical restriction pattern at each ploidy level. The δ region, located between α and β, was highly variable in size and restriction pattern, enclosing different BamHI subrepeats (B-SR), 87- to 530-bp-long. Sequence analysis showed that B-SRs were composed mainly of different arrangements of similar blocks of sequences. Another family of repetitive sequences was found in the δ region, clustered inside large BamHI fragments. These subrepeats are 189-bp-long and, although very similar in diploid and tetraploid IGSs, show a pattern of concerted evolution. A hypothetical functional role for the 189-bp repeats is discussed in view of their predicted secondary structure and presence of potential E2 binding sites inside diploid subrepeats. Although the same structural elements were present both in diploid and tetraploid IGSs, the higher level of repeatability of tetraploid IGSs suggests that common ancestor sequences have undergone several rounds of amplification after O. americanus polyploidy.Key words: Odontophrynus americanus, amphibian polyploidy, ribosomal DNA, intergenic spacer, IGS subrepeats.