Genome evolution in the genus Sorghum (Poaceae)

Abstract

Background and aims: The roles of variation in DNA content in plant evolution and adaptation remain a major biological enigma. Chromosome number and 2C DNA content were determined for 21 of the 25 species of the genus Sorghum and analysed from a phylogenetic perspective.

Methods: DNA content was determined by flow cytometry. A Sorghum phylogeny was constructed based on combined nuclear ITS and chloroplast ndhF DNA sequences.

Key results: Chromosome counts (2n = 10, 20, 30, 40) were, with few exceptions, concordant with published numbers. New chromosome numbers were obtained for S. amplum (2n = 30) and S. leiocladum (2n = 10). 2C DNA content varies 8.1-fold (1.27-10.30 pg) among the 21 Sorghum species. 2C DNA content varies 3.6-fold from 1.27 pg to 4.60 pg among the 2n = 10 species and 5.8-fold (1.52-8.79 pg) among the 2n = 20 species. The x = 5 genome size varies over an 8.8-fold range from 0.26 pg to 2.30 pg. The mean 2C DNA content of perennial species (6.20 pg) is significantly greater than the mean (2.92 pg) of the annuals. Among the 21 species studied, the mean x = 5 genome size of annuals (1.15 pg) and of perennials (1.29 pg) is not significantly different. Statistical analysis of Australian species showed: (a) mean 2C DNA content of annual (2.89 pg) and perennial (7.73 pg) species is significantly different; (b) mean x = 5 genome size of perennials (1.66 pg) is significantly greater than that of the annuals (1.09 pg); (c) the mean maximum latitude at which perennial species grow (-25.4 degrees) is significantly greater than the mean maximum latitude (-17.6) at which annual species grow.

Conclusions: The DNA sequence phylogeny splits Sorghum into two lineages, one comprising the 2n = 10 species with large genomes and their polyploid relatives, and the other with the 2n = 20, 40 species with relatively small genomes. An apparent phylogenetic reduction in genome size has occurred in the 2n = 10 lineage. Genome size evolution in the genus Sorghum apparently did not involve a 'one way ticket to genomic obesity' as has been proposed for the grasses.

Fig. 1.

Chromosomes of Sorghum species representing differences in number and size: (A) S. leiocladum (2n = 10); (B) S. brachypodum (2n = 10); (C) S. timorense (2n = 10); (D) S. nitidum (2n = 20); (E) S. laxiflorum (2n = 40); (F) S. bicolor (2n = 20). Scale bars = 10 μm.

Fig. 2.

2C DNA content and the maximum southern latitude inhabited by native Australian Sorghum species. Diamonds represent annual and circles perennial species. The y-axis is the maximum latitude south of native Australian species.

Fig. 3.

The strict consensus tree of six equally parsimonious solutions of 202 steps (CI = 0·792). Bootstrap support (%) for various nodes from 10 000 replications is indicated above the corresponding node. Only bootstrap values of ≥50 are shown. Numbers in parentheses above each node represent unambiguous nucleotide substitutions. The tree shows bootstrap support, 56 % and 58 % respectively, for lineages consisting of (a) S. bicolor through S. laxiflorum and (b) S. brachypodum through S. nitidum. The tree was rooted using Zea mays. The 2C DNA content, 2n chromosome number (in parenthesis), and x = 5 genome size are denoted next to each species. The DNA content for corn (4C = 10·31 pg) is for inbred line Va35 (Laurie and Bennett, 1985).