Date Published: February 20, 2018
Publisher: Public Library of Science
Author(s): Grit Winterfeld, Hannes Becher, Stephanie Voshell, Khidir Hilu, Martin Röser, Lorenzo Peruzzi.
Karyotype characteristics can provide valuable information on genome evolution and speciation, in particular in taxa with varying basic chromosome numbers and ploidy levels. Due to its worldwide distribution, remarkable variability in morphological traits and the fact that ploidy change plays a key role in its evolution, the canary grass genus Phalaris (Poaceae) is an excellent study system to investigate the role of chromosomal changes in species diversification and expansion. Phalaris comprises diploid species with two basic chromosome numbers of x = 6 and 7 as well as polyploids based on x = 7. To identify distinct karyotype structures and to trace chromosome evolution within the genus, we apply fluorescence in situ hybridisation (FISH) of 5S and 45S rDNA probes in four diploid and four tetraploid Phalaris species of both basic numbers. The data agree with a dysploid reduction from x = 7 to x = 6 as the result of reciprocal translocations between three chromosomes of an ancestor with a diploid chromosome complement of 2n = 14. We recognize three different genomes in the genus: (1) the exclusively Mediterranean genome A based on x = 6, (2) the cosmopolitan genome B based on x = 7 and (3) a genome C based on x = 7 and with a distribution in the Mediterranean and the Middle East. Both auto- and allopolyploidy of genomes B and C are suggested for the formation of tetraploids. The chromosomal divergence observed in Phalaris can be explained by the occurrence of dysploidy, the emergence of three different genomes, and the chromosome rearrangements accompanied by karyotype change and polyploidization. Mapping the recognized karyotypes on the existing phylogenetic tree suggests that genomes A and C are restricted to sections Phalaris and Bulbophalaris, respectively, while genome B occurs across all taxa with x = 7.
It has become increasingly apparent that changes in chromosome number by polyploidy or dysploidy and structural rearrangements like inversions, deletions, or translocations play a substantial role in the evolution of many plant lineages [1, 2, 3, 4]. Further mechanisms like homoploid hybrization or polyploidization were considered as essential contributors to altered chromosomal behaviours [5, 6]. However, for a detailed historic assessment of chromosomal patterns within a group and their correlation with morphological innovations and geographic expansion, a valid phylogenetic hypothesis is essential . The genus Phalaris L. (Poaceae) stands out as an excellent system to study the types and extent of chromosomal changes and their role in species diversification and expansion due to its pattern of global species distribution, presence of different basic chromosome numbers and ploidy levels, varied morphological characters, and the availability of robust phylogenetic and phylogeographic hypotheses [8, 9, 10].
We uncover the following karyotypic evolutionary trends: (1) the chromosomal mechanisms of basic number reduction from x = 7 to x = 6 within the early diverging subgenus Phalaris and their outcomes for karyotype, genome size, morphological traits and habit form, (2) the role of chromosomal rearrangements, contribution of three different genomes and nature of polyploidy to the evolution of the genus, and (3) the significance of chromosomal reshuffling for species diversification, reproductive isolation, speciation, and geographic distribution.