Rubio de Casas, R. | Search bioRxiv

bioRxiv2022

Lack of ITS sequence homogenization in congeneric plant species with different ploidy levels

The internal transcribed spacers (ITS) exhibit concerted evolution by the fast homogenization of these sequences at the intragenomic level. However, the rate and extension of this process are unclear and might be conditioned by the number and divergence of the different ITS copies. In some cases, such as hybrid species and polyploids, ITS sequence homogenization appears incomplete, resulting in multiple haplotypes within the same organism. Here, we studied the dynamics of concerted evolution in 85 individuals of seven plant species of the genus Erysimum (Brassicaceae) with multiple ploidy levels. We estimated the rate of concerted evolution and the degree of sequence homogenization separately for ITS1 and ITS2 and whether these varied with ploidy. Our results showed incomplete sequence homogenization, especially for polyploid samples, indicating a lack of concerted evolution in these taxa. Homogenization was usually higher in ITS2 than in ITS1, suggesting that concerted evolution operates more efficiently on the former. Furthermore, the hybrid origin of several species appears to contribute to the maintenance of high haplotype diversity, regardless of the level of ploidy. These findings indicate that sequence homogenization of ITS is a dynamic and complex process that might result in varying intra- and inter-genomic diversity levels.

evolutionary biology↗

bioRxiv2021

Hybridization and introgression are prevalent in Southern European Erysimum (Brassicaceae) species

Background and AimsHybridization is a common and important force in plant evolution. One of its outcomes is introgression - the transfer of small genomic regions from one taxon to another by hybridization and repeated backcrossing. This process is believed to be common in glacial refugia, where range expansions and contractions can lead to cycles of sympatry and isolation, creating conditions for extensive hybridization and introgression. Polyploidization is another genome-wide process with a major influence on plant evolution. Both hybridization and polyploidization can have complex effects on plant evolution. However, these effects are often difficult to understand in recently evolved species complexes. MethodsWe combined flow cytometry, transcriptomic and genomic analyses, and pollen-tube growth assays to investigate the consequences of polyploidization, hybridization, and introgression on the recent evolution of several Erysimum (Brassicaceae) species from the South of the Iberian Peninsula, a well-known glacial refugium. This species complex differentiated in the last 2Myr, and its evolution has been hypothesized to be determined mainly by polyploidization, interspecific hybridization, and introgression. Key ResultsOur results support a scenario of widespread hybridization involving both extant and "ghost" taxa. Several taxa studied here, most notably those with purple corollas, are polyploids, likely of allopolyploid origin. Moreover, hybridization in this group might be an ongoing phenomenon, as prezygotic barriers appeared weak in many cases. ConclusionsThe evolution of Erysimum spp. has been determined by hybridization to a large extent. The adaptive value of such genomic exchanges remains unclear, but our results indicate the importance of hybridization for plant diversification across evolutionary scales.

evolutionary biology↗