Cryptic and repeated “allopolyploid” speciation within Allium przewalskianum Regel. (Alliaceae) from the Qinghai-Tibet Plateau

Abstract

Polyploidization has contributed greatly to current plant diversity. Allopolyploid speciation, which can rapidly overcome meiosis abnormalities, is a common and repeated process in numerous genera. However, most polyploids within a single morphological/taxonomic species have been considered autopolyploids, which were assumed to arise through spontaneous genome doubling and/or following the fusion of unreduced diploid gametes. It remains untested whether these intraspecific polyploids may also be the result of ‘allopolyploid’ hybridizations between differentiated diploid populations and whether such speciation has also occurred repeatedly. In this study, we examined the diploid–tetraploid species Allium przewalskianum on the Qinghai-Tibet Plateau (QTP) to test this cryptic speciation hypothesis under morphological stasis, based on three sets of nuclear genetic data (AFLP, ITS, and CHS) and niche modeling. Our analyses of the three datasets together revealed that the diploid populations across the northeastern to southeastern QTP exhibit strong geographical differentiation. Based on the shared nuclear lineages fixed in the different diploids, three tetraploid groups in the northern, southern, and eastern QTP were identified as having originated independently, through “allopolyploid” hybridization between the differentiated diploids. Ecological niche modeling based on ecological variables suggested distinct niche differentiation between two tetraploid groups in the northern and southern QTP and also between these and the diploid populations. Furthermore, they also differed in their responses to past climate changes. These findings together suggested that at least two tetraploid groups had originated independently through hybridizations between the differentiated diploid populations. Our results highlight the cryptic allopolyploid speciation underlying a single morphological species, which mirrors well the repeated allopolyploid speciations within the genus. This allopolyploid speciation may prevail within the diploid–polyploid species complex that is classified as a single morphological species; indeed, the underlying cryptic speciation and evolutionary dynamics are certainly more complex than previously assumed.