Extensive intraindividual variation in plastid rDNA sequences from the holoparasite Cynomorium coccineum (Cynomoriaceae).

ABSTRACT

Ribosomal genes are considered to have a high degree of sequence conservation between species and also at higher taxonomic levels. In this paper we document a case where a single individual of Cynomorium coccineum (Cynomoriaceae), a nonphotosynthetic holoparasitic plant, contains highly divergent plastid ribosomal genes. PCR amplification a nearly complete ribosomal DNA cistron was performed using genomic DNA, the products cloned, and the 23S rDNA genes were sequenced from 19 colonies. Of these, five distinct types were identified. Fifteen of the sequences were nearly identical (11 or fewer differences) and these were designated Type I. The remaining types (II-V) were each represented by a single clone and differed from Type I by 93 to 255 changes. Compared with green vascular plants, we found that there are more substitutional differences in the 23S rDNA sequences within a single individual of Cynomorium than among all sequenced photosynthetic vascular plants. Several trends of molecular evolution observed in 16S rDNA from other holoparasitic angiosperms and heterotrophic green algae have been also observed in Cynomorium 23S rDNA. Higher-order structures were constructed for representatives of the five clone types, and in all cases these possessed complete complements of the major structural elements present in functional plastid 23S rRNAs. These data indicate that such molecules may be subject to purifying selection, thus providing indirect evidence that they have retained some degree of functionality. This intraindividual polymorphism is probably a case of plastid heteroplasmy but translocation of ribosomal cistrons to the nucleus or mitochondria has not been tested and therefore cannot be ruled out.