Hybrid origin of the invasive Spartina anglica inferred from chloroplast and nuclear ITS phylogenies

Abstract

Spartina anglica originated from interspecific hybridization between S. alterniflora and S. maritima which produced a sterile hybrid S. × townsendii. Subsequent whole genome duplication of the hybrid, gave rise to a vigorous allopolyploid S. anglica. In earlier studies, both parental nuclear sequences were detected in S. anglica. However, to date, parental copies of nuclear ribosomal DNA (nrDNA) have not been detected in a phylogenetic context. In the present study, we used the nuclear internal transcribed spacer (ITS) region in conjunction with chloroplast DNA (trnL-trnF, matK, and rbcL) to infer the hybrid origin of S. anglica invading Korean coasts. Our results revealed that the chloroplast DNA sequences of S. anglica were similar to those of S. alterniflora, which confirms its maternal origin. However, in the nuclear rDNA ITS region, we detected two highly divergent groups of ITS sequences that were divided between the two parent lineages. The maternal copies were indistinguishable from S. alterniflora, while the paternal repeats were highly differentiated from S. maritima and exhibited characteristics of ITS pseudogenes. This indicates that S. anglica has undergone rapid changes at the rDNA loci after the allopolyploid formation. Our study demonstrated the two divergent copies of ITS sequences, thus resolving the phylogenetic hybrid origin of S. anglica. This study offers further insights into the evolutionary history of S. anglica.