NOR-Chromosome morphology and evidence for rDNA selection in phylactolaemates

Abstract

Silver-stained chromosome spreads from the phylactolaemate ectoprocts Fredericella indica, Fredericella browni, Plumatella repens, Pectinatella magnifica and Asajirella gelatinosa show that each of these species has a single pair of NORs. In the first four species each NOR is immediately adjacent to the centromere of a large metacentric chromosome. Giemsa-stained preparations from Cristatella mucedo show centromeric gapping in a similar large metacentric, indicating the same type of NOR-chromosome. In Asajirella gelatinosa the two NORs are on a pair of smaller chromosomes, and are not immediately adjacent to a centromere. A close NOR-centromere association would inhibit meiotic crossing over in the NOR region. Since in phylactolaemates reproduction is primarily asexual, often by formation of statoblasts, there is potential intraclonal selection for favorable ribosomal DNA (rDNA) mutations, including from simultaneous germination of otherwise genetically identical statoblasts within a limited area and competition between cell lines within statoblasts. Inhibition of meiotic crossing over within the NOR would maintain its integrity during periods when intraclonal selection would not be occurring. This suggests that a more rapid rate of rDNA evolution is possible in phylactolaemates than in most other organisms, and is consistent with a derivation of this group from the morphologically similar phoronids, despite phylogenetic interpretations of rDNA sequence data that suggest the two groups are not closely related.