The Evolution and Development of Nectar Spurs

Nectar spurs (tubular outgrowths of a floral organ which contain or give the appearance of containing nectar) are hypothesized to be a ‘key innovation’ which can lead to rapid speciation within a lineage, because they are involved in pollinator specificity. However, given that there are no conventional model plant species which possess a nectar spur, much is still unknown about the development and morphogenesis of spurs. This project aimed to determine the molecular and morphological processes underpinning nectar spur development. The well-known study system Antirrhinum majus (snapdragon), which possesses only a nectar pouch, called a gibba, was compared to the common toadflax Linaria vulgaris, a related species within the Antirrhineae which possesses a spur. The comparison between a species with a gibba and a species which possesses a spur allows insights into nectar spur development. A morphological framework was produced and used to inform a comparative transcriptome based on A. majus and L. vulgaris at three key developmental stages. This experiment provides a global view of the genes involved in nectar spur and gibba development and suggests new candidate genes for nectar spur outgrowth. The control of variation in nectar spur length at both a morphological and molecular level was also investigated, focusing on Linaria becerrae and Linaria clementei, sister species which have extremely long and short spurs respectively. A morphological characterisation (recording cell number and cell length across a range of developmental stages) was performed to determine whether the difference is due to cell expansion or cell division. We found that primarily cell number and therefore cell division drives an increase in spur length. This contrasts with previous studies in Aquilegia which have found that variation of nectar spur length is due to directed cell expansion (anisotropy) over a longer timeframe. A genetic approach to understanding genetic basis of spur length was pursued, by crossing L. becerrae and L. clementei, and producing an F2 and backcrosses. This work lays a groundwork for future studies and suggests that the genetic basis of spur length in Linaria may be complex. The morphological and genetic work to investigate control of spur length was complemented by a candidate gene approach. Candidate genes from previous work and suggested by the transcriptome were isolated in L. becerrae and L. clementei and expression in the ventral and dorsal petals tested via sqPCR.Biotechnology and Biological Sciences Research Council (BBSRC) Doctoral Training Partnership (DTP) studentship