Genetics and plasticity of white leaf mark variegation in white clover (Trifolium repens L.)

Leaf variegation is common in plants and confers diverse adaptive functions. However, its genetic underpinnings remain largely unresolved; this is particularly true for variegation that arises through modified leaf tissue structure that affects light reflection. White clover is naturally polymorphic for structure-based white leaf mark variegation. It therefore provides a useful system in which to examine the genetic basis of this phenotype and to assess potential costs to photosynthetic efficiency resulting from modified leaf structures. In this study, we sought to map the loci controlling the white leaf mark in white clover and to evaluate the relationship between white leaf mark, leaf thickness and photosynthetic efficiency.

We generated a high-density genetic linkage map from an F3 mapping population, using reference genome-based single nucleotide polymorphism markers. White leaf mark was quantified through detailed phenotypic evaluations alongside leaf thickness to test how tissue thickness might affect the variegation phenotype. Mapping of quantitative trait loci was performed to characterize their genetic basis. Photosynthetic efficiency measurements were used to test for physiological trade-offs between variegation and photosynthetic output.

The V locus, a major gene responsible for the white leaf mark polymorphism, was mapped to the distal end of chromosome 5, and several modifier loci were also mapped that contribute additively to the intensity of variegation. The presence and intensity of white leaf mark were associated with greater leaf thickness; however, increased variegation did not affect photosynthetic efficiency detectably.

We have successfully mapped the major locus governing the white leaf mark in white clover, along with several modifier loci, revealing a complex basis for this structure-based variegation. The apparent absence of compromised photosynthesis in variegated leaves challenges the notion that variegation creates fitness trade-offs between photosynthetic efficiency and other adaptive functions. This finding suggests that other factors might maintain the white leaf mark polymorphism in white clover.