Molecular evolution of nuclear ribosomal DNA in Aeschynanthus and Streptocarpus (Gesneriaceae)

• Main Author: Denduangboripant, Jessada
• Published: University of Edinburgh 2001
• Subjects: 580
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.649480

Molecular phylogenetics using internal transcribed spacer (ITS) sequences of 50 <i>Aeschynanthus </i>species was carried out using PCR cloning and RNA secondary-structure analysis techniques to help solve ITS intra-individual sequence variation problems in this genus. It is suggested that intra-individual variation in <i>Aeschynanthus</i> is the result of low molecular drive and the clone variation, although persistent, is found not to predate the divergence of <i>Aeschynanthus </i>species. RNA secondary-structure analysis reveals an insertion/deletion hotspot in the ITS2 sequences that is difficult to align, especially on the terminal part of arm 1 which is probably functionally superfluous. By contrast the base of the arm is relatively conserved and may function as an exonuclease recognition site. The phylogeny of <i>Aeschynanthus</i> reveals two major clades with different but overlapping geographic distributions: one occurs primarily in mainland SE Asia and the other in Malesia. This pattern is interpreted as indicating an ancient vicariance event followed by dispersal and plate fusion. The phylogeny also reflects the classification based on seed testa and seed appendage morphology; that is, clade I has some species of section <i>Microtrichium</i> forming a basal group with sections <i>Haplotrichium</i> sens. str., a putative section <i>X, Diplotrichium, </i>and <i>Polytrichium</i> being polyphyletic or paraphyletic, and clade II has section <i>Aeschynanthus</i> nested within the paraphyletic, basal <i>Microtrichium.</i> In <i>Streptocarpus</i>, a length difference between ITS2 of <i>S. dunnii </i>and <i>S. rexii</i> rDNA was used as a genetic marker to study the inheritance of rDNA in hybrids between the two species. The <i>Streptocarpus</i> rDNA gene is found to have additive inheritance without any detected recombination and there are apparently two rDNA loci. A PCR silent allele was detected as a band-intensity difference in <i>S. dunnii</i> ITS2. This phenomenon was reproducible and persistent in backcrosses. I suggest that an rDNA pseudogene is possibly present as an allele at one locus of <i>S. dunnii.</i>