Genome size variation and evolution in Veronica

Abstract

Background and aims: The amount of DNA per chromosome set is known to be a fairly constant characteristic of a species. Its interspecific variation is enormous, but the biological significance of this variation is little understood. Some of the characters believed to be correlated with DNA amount are alpine habitat, life history and breeding system. In the present study, the aim is to distinguish between direct causal connections and chance correlation of the amount of DNA in the genus Veronica.

Methods: Estimates of DNA amount were analysed for 42 members of Veroniceae in connection with results from a phylogenetic analysis of plastid trnL-F DNA sequences and tested correlations using standard statistical tests, phylogenetically independent contrasts and a model-based generalized least squares method to distinguish the phylogenetic effect on the results.

Key results: There appears to be a lower upper limit for DNA amount in annuals than in perennials. Most DNAC-values in Veroniceae are below the mean DNA C-value for annuals in angiosperms as a whole. However, the long-debated correlation of low genome size with annual life history is not significant (P = 0.12) using either standard statistical tests or independent contrasts, but it is significant with the generalized least squares method (P < 0.01).

Conclusions: The correlation of annual life history and low genome size found in earlier studies could be due to the association of annual life history and selfing, which is significantly correlated with low genome size using any of the three tests applied. This correlation can be explained by models showing a reduction in transposable elements in selfers. A significant correlation of higher genome sizes with alpine habitats was also detected.

Fig. 1.

Genome size mapped on the strict consensus tree of the phylogenetic analysis of the trnL-F sequence data (ACCTRAN optimization). Numbers on the side of branches with equivocal result give the range of values found among species to which this branch leads.

Fig. 2.

Strict consensus of 1000 most-parsimonious trees from the analysis of trnL-F sequence data. Numbers above the branches indicate estimated numbers of substitutions (ACCTRAN optimization) from one random most-parsimonious tree. Numbers below the branches indicate bootstrap percentages.

Fig. 3.

Distribution of known DNA C-values (A) and genome sizes (B) in Veroniceae.

Fig. 4.

Boxplot indicating variation of DNA C-value (A) and genome size (B) among non-alpine and alpine taxa of Veroniceae. The shaded box indicates the interquartile (25–75 %) range. The bar within the box indicates the mean value. ‘Whiskers’ below and above the box indicate the range of values within three times the value of the upper or lower edge of the box.

Fig. 5.

Boxplot indicating variation of DNA C-value (A) and genome size (B) among perennial and annual taxa of Veroniceae. The shaded box indicates the interquartile (25–75 %) range. The bar within the box indicates the mean value. ‘Whiskers’ below and above the box indicate range of within three times the value of the upper or lower edge of the box.

Fig. 6.

Boxplot indicating variation of DNA C-value (A) and genome size (B) among outcrossing and selfing taxa of Veroniceae. The shaded box indicates the interquartile (25–75 %) range. The bar within the box indicates the mean value. ‘Whiskers’ below and above the box indicate range of within three times the value of the upper or lower edge of the box.