Species diversity and speciation mechanisms in Spiranthes (Orchidaceae)

Abstract

Interspecific hybridization followed by polyploidization is an important speciation mechanism in angiosperms. Orchids are well known for their species richness. We investigate allopolyploid speciation mechanism in Spiranthes, a small orchid genus with several polyploid species and confirmed allopolyploids, such as S. hongkongensis in Asia and S. diluvialis in North America. Our previous work has shown parental additivity and fixed heterozygosity at allozyme loci in the allotetraploid S. hongkongensis. However, responses of repeated rDNA sequences to allopolyploidization are different, as a result of fast concerted evolution. Using mitochondrial DNA, we had earlier shown that the widely occurring Asian species S. sinensis, is the maternal parent of S. hongkongensis. The putative paternal progenitor might be the European species S. spiralis as its range extends eastwards to the Himalayas in western Asia. We hypothesize that the Asian allopolyploid species S. hongkongensis and others might have evolved through natural hybridization in the Himalayan mountain ranges where the white flowered S. spiralis and the pink flowered S. sinensis occur in sympatry. Using nuclear ribosomal ITS markers together with several cpDNA markers, we test the hypotheses about allopolyploid speciation in Asian Spiranthes. We examined rDNA polymorphism and the extent and direction of concerted evolution in the clones of the ITS region in the putative allopolyploid individuals and their diploid progenitors. Among 131 sequenced clones, most clones from S. hongkongensis were similar to the pink-flowered S. sinensis, its maternal diploid progenitor. However, we identified two different rDNA lineages in three individuals from two different populations of S. hongkongensis that were similar to a white-flowered species from India and also occurring in the Himalayas. The data indicate concerted evolution has not proceeded to completion in the allopolyploid S. hongkongensis populations; and the rDNA repeats of the paternal progenitor tend to be lost or converted in different allopolyploids of independent origin, indicating a directional evolution of the rDNA in favor of the maternal genome. Furthermore, our phylogenetic studies of the putative diploid progenitors and allopolyploids uncovered a new species, previously mistaken as synonymous to S. sinensis or S. spiralis. The evolutionary implications of geographical isolation and recurrent allopolyploidization in generating species diversity are discussed. Further studies of the genomic and transcriptomic changes in different allopolyploids can provide new insights into this mode of speciation in flowering plants.