Genome size variation and evolution in allotetraploid Arabidopsis kamchatica and its parents, Arabidopsis lyrata and Arabidopsis halleri

Abstract

Polyploidization and subsequent changes in genome size are fundamental processes in evolution and diversification. Little is currently known about the extent of genome size variation within taxa and the evolutionary forces acting on this variation. Arabidopsis kamchatica has been reported to contain both diploid and tetraploid individuals. The aim of this study was to determine the genome size of A. kamchatica, whether there is variation in ploidy and/or genome size in A. kamchatica and to study how genome size has evolved. We used propidium iodide flow cytometry to measure 2C DNA content of 73 plants from 25 geographically diverse populations of the putative allotetraploid A. kamchatica and its parents, Arabidopsis lyrata and Arabidopsis halleri. All A. kamchatica plants appear to be tetraploids. The mean 2C DNA content of A. kamchatica was 1.034 pg (1011 Mbp), which is slightly smaller than the sum of its diploid parents (A. lyrata: 0.502 pg; A. halleri: 0.571 pg). Arabidopsis kamchatica appears to have lost ∼37.594 Mbp (3.6 %) of DNA from its 2C genome. Tetraploid A. lyrata from Germany and Austria appears to have lost ∼70.366 Mbp (7.2 %) of DNA from the 2C genome, possibly due to hybridization with A. arenosa, which has a smaller genome than A. lyrata. We did find genome size differences among A. kamchatica populations, which varied up to 7 %. Arabidopsis kamchatica ssp. kawasakiana from Japan appears to have a slightly larger genome than A. kamchatica ssp. kamchatica from North America, perhaps due to multiple allopolyploid origins or hybridization with A. halleri. However, the among-population coefficient of variation in 2C DNA content is lower in A. kamchatica than in other Arabidopsis taxa. Due to its close relationship to A. thaliana, A. kamchatica has the potential to be very useful in the study of polyploidy and genome evolution.

Keywords: 2C DNA content; Allotetraploid; Arabidopsis halleri ssp. gemmifera; Arabidopsis kamchatica; Arabidopsis lyrata; C-value; flow cytometry; genome size; genome size variation.

Figure 1.

Map of collection localities of plants used for flow cytometry.

Figure 2.

Fluorescence intensity histograms (PE-A) for (A) tetraploid A. kamchatica (2C = 4x = 32), (B) diploid A. h. gemmifera (2C = 2x = 16) and (C) diploid A. lyrata (2C = 2x = 16). Arabidopsis leaves show extensive endopolyploidy (Galbraith et al. 1991), and the 2C, 4C and 8C peaks are indicated, along with the soybean standard (std). The mean fluorescence of the smallest peak (2C) relative to the soybean peak was used to estimate 2C DNA content.

Figure 3.

Estimates of 2C DNA content (pg) of each taxon and the 95 % confidence intervals of the estimates. Letters indicate significant differences (P< 0.05) based on Tukey's post hoc comparisons.

Figure 4.

Estimates of 1Cx (haploid) genome size (pg) of each taxon. The error bars are 95 % confidence intervals of the estimates. Letters indicate significant differences (P < 0.05) based on Tukey's post hoc comparisons.

Figure 5.

Genome size diversity in Arabidopsis taxa, measured as CV in 2C DNA content. Only diploid A. thaliana and A. l. petraea are included because there were too few tetraploids to estimate CV (two from each taxon).