The genetic architecture of the divaricate growth form : a QTL mapping approach in Sophora (Fabaceae) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Biology at Massey University, Manawatu, New Zealand
Divarication is a plant growth form described, in its simplest form, as a tree or shrub with interlaced branches, wide branch angles and small, widely spaced, leaves giving the appearance of a densely tangled shrub. This growth form is a unique feature in the New Zealand flora that is present in ~ 10% of the woody plant species, a much higher frequency than that of other regional floras. While several hypotheses have been developed to explain why this growth form has evolved multiple times within New Zealand, to our knowledge, no work has addressed the genetic basis of the divaricating form. Sophora is one of several genera in New Zealand that possesses divaricate species. Among the factors making this an ideal system for a genetic investigation of divarication is an existing F₂ population formed from reciprocal crosses between the divaricating S. prostrata and the non-divaricating S. tetraptera.
Using this segregating population and newly developed molecular markers, the first linkage maps for Sophora were generated, providing a new genetic resource in Sophora. These linkage maps allowed for quantitative trait locus (QTL) mapping for traits associated with the divaricate form in the segregating population. Multiple QTL were mapped to seven of the divaricate traits with many QTL co-locating for multiple traits, indicating that the divaricate growth form is genetically controlled by many loci, potentially including pleiotropic loci, that each contribute to the overall divaricate phenotype in Sophora.
The strigolactone biosynthesis and perception pathway is a good candidate for involvement in control of the divaricate form based on mutant phenotypes in Pisum that display similarities to the divaricate growth form, such as increased branching, shorter plant height and smaller leaves. QTL, for multiple traits, were mapped to two candidate genes investigated, RMS1 and RMS4. An amino acid replacement was identified in RMS1, in S. prostrata, that is predicted to be deleterious suggesting it may be non-functional in S. prostrata. These results support RMS1 as a strong candidate gene for future work on divarication. This study is the first to investigate the genetic architecture of the divaricate growth form and contributes to further understanding of this unique feature in the New Zealand flora and of plant architecture generally.