Ribosomal DNA evolution and gene conversion in Nicotiana rustica

Area of Science:

• Plant genetics
• Molecular evolution
• Genomics

Background:

• Nicotiana rustica (2n=4x=48) is an allotetraploid species.
• Understanding its genomic origins and evolution is crucial for plant biology.
• Previous studies suggested its progenitors but lacked detailed molecular evidence.

Purpose of the Study:

• To confirm the parentage of Nicotiana rustica using genomic in situ hybridization.
• To investigate the organization and evolution of ribosomal DNA (rDNA) loci in N. rustica and its progenitors.
• To analyze the sequence types of the intergenic spacer (IGS) of 18-5.8-26S rDNA in N. rustica and its diploid ancestors.

Main Methods:

• Genomic in situ hybridization (GISH) to determine allotetraploid origins.
• Fluorescent in situ hybridization (FISH) to map rDNA loci.
• Restriction fragment length polymorphism (RFLP) analysis of rDNA IGS sequences.

Main Results:

• GISH confirmed N. rustica as an allotetraploid between N. paniculata (maternal P-genome) and N. undulata (paternal U-genome).
• FISH revealed N. rustica possesses the sum of rDNA loci from both parents, with evidence of amplification on U-genome chromosomes.
• RFLP analysis showed approximately 80% of IGS sequences in N. rustica are of the N. undulata type, suggesting interlocus sequence homogenization favoring the paternal genome.

Conclusions:

• N. rustica originated from hybridization between N. paniculata and N. undulata.
• Interlocus sequence homogenization has occurred in N. rustica's rDNA IGS, with a strong bias towards the paternal N. undulata type.
• The observed homogenization pattern aligns with findings in other Nicotiana allotetraploids, indicating a consistent evolutionary trend towards paternal genome dominance in rDNA sequence conversion.