Karyotype evolution in Phalaris (Poaceae): The role of reductional dysploidy, polyploidy and chromosome alteration in a wide-spread and diverse genus

Abstract

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.

Fig 1. Chromosomes of Phalaris aquatica (A), P. caroliniana (B), P. minor (C), P. coerulescens (D), P. arundinacea (E), P. canariensis (F), and P. paradoxa (G) after in situ hybridisation with 5S rDNA (red signals, marked by open arrowheads), 45S rDNA (green signals, marked by filled arrowheads) and counterstaining with DAPI.

Fig 2. Idiograms of chromosome complements of diploid and tetraploid Phalaris species.

Chromosome pairs are arranged into groups of presumable homologues or homoeologues according to their 45S (green bands) and 5S rDNA (red bands) probe signals and karyotype features (length and symmetry). Chromosomes were designated below rendering their affiliation to the genomes A (blue), B (red) or C (green) and the chromosome number (1, 2, 3, …). sm—submetacentric, st—subtelocentric, no sign—metacentric.

Fig 3. Taxonomic classification [9], life form, chromosomal properties and genome size [38] of diploid and tetraploid Phalaris species on a ITS phylogram based on Bayesian inference [8].

Parsimony bootstrap values and Bayesian support are noted above and below the branches.

Fig 4. Possible scenario of reductional dysploidy in the genus Phalaris.

A: Chromosome prototypes (proto) of a fictive ancestral x = 7 genome A karyotype numbered according to the ideograms of P. brachystachys and P. canariensis in Fig 2; B: Pericentromeric break in proto-A7, end-to-end fusion with proto-A2 and proto-A6 and loss of centromere; C: Paracentric inversion of fused arms; D: Reductional dysploidy to an extant x = 6 karyotype with strong asymmetric chromosomes. m—metacentric, sm/st—submetacentric/subtelocentric.

Fig 5. Geographical distribution of different genomes A, B, and C in eight species of Phalaris and possible expansions routes and time of diversification within the genus according to Voshell & Hilu [10].