Genome size, quantitative genetics and the genomic basis for flower size evolution in Silene latifolia

Abstract

Background and aims: The overall goal of this paper is to construct an overview of the genetic basis for flower size evolution in Silene latifolia. It aims to examine the relationship between the molecular bases for flower size and the underlying assumption of quantitative genetics theory that quantitative variation is ultimately due to the impact of a number of structural genes.

Scope: Previous work is reviewed on the quantitative genetics and potential for response to selection on flower size, and the relationship between flower size and nuclear DNA content in S. latifolia. These earlier findings provide a framework within which to consider more recent analyses of a joint quantitative trait loci (QTL) analysis of flower size and DNA content in this species.

Key results: Flower size is a character that fits the classical quantitative genetics model of inheritance very nicely. However, an earlier finding that flower size is correlated with nuclear DNA content suggested that quantitative aspects of genome composition rather than allelic substitution at structural loci might play a major role in the evolution of flower size. The present results reported here show that QTL for flower size are correlated with QTL for DNA content, further corroborating an earlier result and providing additional support for the conclusion that localized variations in DNA content underlie evolutionary changes in flower size.

Conclusions: The search image for QTL should be broadened to include overall aspects of genome regulation. As we prepare to enter the much-heralded post-genomic era, we also need to revisit our overall models of the relationship between genotype and phenotype to encompass aspects of genome structure and composition beyond structural genes.

Fig. 1.

Floral dimensions measured in males and females of Silenelatifolia (from Meagher and Costich, 1994, fig. 1).

Fig. 2.

Response to selection on calyx diameter. The higher set of lines corresponds to females. For each sex within each panel, solid lines represent the control line (c), dashed lines represent female-based selection, and dotted lines represent male-based selection. Selection for increases or decreases in the character are indicated by ‘+’ and ‘−’ symbols (from Meagher, 1994, fig. 1).

Fig. 3.

A comparison between (A) the linear selection gradients (β values), and (B) the quadratic selection gradients (γ values) for the male and female plants in a natural Spanish population (Guadalupe) in 1997 and 1998. The 1997 male γ value for calyx diameter was the only result found to be significant (from Wright and Meagher, 2004, fig. 4).

Fig. 4.

Example flow cytometry output showing the frequency distribution of particle fluorescence in Silene latifolia nuclear DNA preparations. Chicken red blood cells (CRBC) were used as the internal calibration standard.

Fig. 5.

Frequency distributions of standardized QTL effects over the 469 AFLP loci.