The Evolution of Haploid Chromosome Numbers in the Sunflower Family

Abstract

Chromosome number changes during the evolution of angiosperms are likely to have played a major role in speciation. Their study is of utmost importance, especially now, as a probabilistic model is available to study chromosome evolution within a phylogenetic framework. In the present study, likelihood models of chromosome number evolution were fitted to the largest family of flowering plants, the Asteraceae. Specifically, a phylogenetic supertree of this family was used to reconstruct the ancestral chromosome number and infer genomic events. Our approach inferred that the ancestral chromosome number of the family is n = 9. Also, according to the model that best explained our data, the evolution of haploid chromosome numbers in Asteraceae was a very dynamic process, with genome duplications and descending dysploidy being the most frequent genomic events in the evolution of this family. This model inferred more than one hundred whole genome duplication events; however, it did not find evidence for a paleopolyploidization at the base of this family, which has previously been hypothesized on the basis of sequence data from a limited number of species. The obtained results and potential causes of these discrepancies are discussed.

Keywords: ancestral chromosome number; duplication; dysploidy; polyploidy; probabilistic models; sunflower family.

Fig. 1.—

Chromosome number evolution of the Asteraceae, using the polymorphic data and according to the best-fitted model obtained. The main tribes were collapsed. Stars represent the expected chromosome number duplication events, while the circles at the tips of each branch represent the percentage (%) of the current chromosome numbers known for each tribe. Inside each circle (at interior nodes) the ancestral chromosome number with the highest probability is given. The differences observed between our study and previous ones (Barker et al. 2008, 2016) are highlighted as red and orange branches, respectively. Overall, our approach did not infer any paleoploidization events near the base of the family, nor any paleoploidization shared with the sister family, Calyceraceae. Also, no paleoploidization event was detected near the base of the tribe Mutisieae. Colors and shading are explained in the inset. For further and more precise information about the percentage of the current chromosome numbers for each tribe, please see the supplementary table S3, Supplementary Material online.