UBC Theses and Dissertations
The molecular evolution of floral colour shifts in Lathyrus L. (Fabaceae) Xue , Xinxin
In this thesis, I used Lathyrus to study floral colour evolution. My work builds on that of pioneer geneticists who used the sweet pea (Lathyrus odoratus) for research in early 20th century England. They used the rich horticultural resource of inbred cultivars for genetic study. More recently, studies on flavonoid biochemistry and the phylogenetics of Lathyrus lend tools to place the colour shifts in an evolutionary and molecular context. In chapter 2, I show that the A1 locus is a missense mutation (332 G/A) in the substrate recognition site (SRS1) of flavonoid 3’,5’-hydroxylase (F3’5’H) and is associated with the pink mutant cultivar ‘Painted Lady’ (‘PL’). This single base pair substitution in the mutant F3’5’H is speculated to toggle the enzyme from primary F3’5’H activity to a relatively efficient F3’H, as shown in a heterologous transformation in Arabidopsis PAP1D (a mutant line that produces anthocyanin constitutively). In chapter 3, I constructed a multi-species coalescent tree using Bayesian inference and reconstructed the ancestral states for floral colour, life history trait (perennial or annual) and floral pattern. The ancestral states for Lathyrus are anthocyanin rich (AR), annual and concolourous. However, no correlation was found between the life history trait (which is linked to breeding systems) and the loss of anthocyanin colour in the petal, when corrected for phylogenetic independence. This suggests that in Lathyrus, autogamous species are as colourful as allogamous ones even though the latter are expected to need greater floral display. In chapter 4, I found that the lack of expression in dihydroflavonol reductase (DFR) was associated with a white mutant cultivar ‘Mrs Collier’ (‘MC’) of L. odoratus via a trans-regulatory machinery. Two transcription factors, the sweet pea orthologues of AN2 (MYB) and AN1 (bHLH) were also not expressed although neither was associated with the white phenotype in an F2-cosegregation analysis. This DFR silencing was also observed in another white mutant of the domesticated grass pea (L. sativus). In contrast, when unpigmented wild species originate under natural selection, DFR expression, if at all affected, is lowered rather than fully silenced, likely due to pleiotropic effects.
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