Abstract
Several genome duplications have been identified in the evolution of seed plants, providing unique systems for studying karyological processes promoting diversification and speciation. Knowledge about the number of ribosomal DNA (rDNA) loci, together with their chromosomal distribution and structure, provides clues about organismal and molecular evolution at various phylogenetic levels. In this work, we aim to elucidate the evolutionary dynamics of karyological and rDNA site-number variation in all known taxa of subtribe Vellinae, showing a complex scenario of ancestral and more recent polyploid events. Specifically, we aim to infer the ancestral chromosome numbers and patterns of chromosome number variation, assess patterns of variation of both 45S and 5S rDNA families, trends in site-number change of rDNA loci within homoploid and polyploid series, and reconstruct the evolutionary history of rDNA site number using a phylogenetic hypothesis as a framework. The best-fitting model of chromosome number evolution with a high likelihood score suggests that the Vellinae core showing x = 17 chromosomes arose by duplication events from a recent x = 8 ancestor. Our survey suggests more complex patterns of polyploid evolution than previously noted for Vellinae. High polyploidization events (6x, 8x) arose independently in the basal clade Vella castrilensis-V. lucentina, where extant diploid species are unknown. Reconstruction of ancestral rDNA states in Vellinae supports the inference that the ancestral number of loci in the subtribe was two for each multigene family, suggesting that an overall tendency towards a net loss of 5S rDNA loci occurred during the splitting of Vellinae ancestors from the remaining Brassiceae lineages. A contrasting pattern for rDNA site change in both paleopolyploid and neopolyploid species was linked to diversification of Vellinae lineages. This suggests dynamic and independent changes in rDNA site number during speciation processes and a significant lack of correlation between 45S and 5S rDNA evolutionary pathways.
Highlights
Angiosperms have a long and complex history of wholegenome duplication (WGD) rounds since their origin
Chromosome numbers of V. castriliensis (2n 1⁄4 102) and V. lucentina (2n 1⁄4 136) found in this study disagree with previous counts reporting lower numbers (Table 2)
For V. castriliensis, from which a very small population is known, in this study, we report 2n 1⁄4 102 and we cannot support the 2n 1⁄4 68 chromosome number reported for this species in Crespo et al (2005)
Summary
Angiosperms have a long and complex history of wholegenome duplication (WGD) rounds since their origin. All core eudicots probably shared a hexaploidization event (g-WGD) detected to date in asterids and rosids (Arabidopsis thaliana, Carica papaya, Populus trichocarpa, Cucumis sativus and Vitis vinifera; Amborella Genome Project 2013). Additional and more recent paleopolyploidy events characterize some plant lineages. This is the case of Brassicaceae, the mustard family, where two more polyploidization events (a-WGD and b-WGD) have been identified in Arabidopsis and are shared with other members of the order Brassicales (Bowers et al 2003). Other species from tribe Brassiceae have undergone a further whole-genome triplication since their divergence from Arabidopsis, an event dated between 13 and 17 Mya (Town et al 2006) or perhaps as early as 43 Mya (Beilstein et al 2010)
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