How Gene Duplication Led to the Evolution of Nectar Spurs

Tropaeolaceae flowers have a peculiar feature – a nectar spur formed by the swollen base of the floral tube, that growths on only one side of the flower. New research published in the Annals of Botany by Sebastián Martínez-Salazar and colleagues revealed that duplication of the gene TCP4 in the Tropaeolaceae lineage seems to have enabled the emergence of their unique nectar spurs.

The Tropaeolaceae family of flowering plants, native to South and Central America, are distinguished for their colorful, trumpet-shaped flowers. But what makes these flowers truly unique is their nectar spur, a long, slender projection that serves as a food source for pollinators. This nectar spur forms through late expansion and outpouching of the fused base region of the perianal tube (tissues surrounding the reproductive organs). Uniquely, the spur forms on the side of the tube facing towards the inflorescence stem, known as the adaxial side. However, the genetic mechanisms behind the evolution of this spur are yet unknown. 

The research team investigated the development of these spurs by tracking which genes are switched on and off throughout flower formation. Specifically, they focused on genes known to control differences between the adaxial and abaxial sides of plants, including TCP and KNOX transcription factors that regulate growth. They also tracked expression of the HISTONE H4 gene, a marker for cell division activity. 

Interestingly, they discovered a TCP4 homolog that acted simultaneously with spur initiation and elongation. Even more curiously, the Tropaeolaceae seemed to have experienced a duplication of their TCP4 gene early in their evolutionary history, leaving them with two nearly identical copies called TCP4L1 and TCP4L2.  

Comparing the activity of these copies in the model species Tropaeolum longifolium, researchers found both are active in surface tissues of developing inflorescences and young floral buds. However, only TCP4L2 showed restricted expression to the adaxial side of the floral tube, coinciding with the first signs of spur growth.  

Later on, only TCP4L2 was found within the nectar-producing spur tissue. It seems the TCP4L genes originally functioned broadly in epidermal development based on their early expression. However, after duplication, one of the copies (TCP4L2) took on the role of spur initiation and promotion. In their paper, Martínez-Salazar and colleagues say: “These results suggest that the Tropaeolaceae TCP4L2 gene copy might have acquired a role in spur initiation and nectary development after gene duplication, concordant with a neofunctionalization event.” 

Expanding this work to their sister plant group Akaniaceae, which also duplicated TCP4L independently but lack spurs, will help unravel how this strange feature evolved. Uncovering the genetic mechanisms transforming simple floral tubes into nectar-bearing spurs could provide insights into the origin of floral novelty. 

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Martínez-Salazar S., Kramer E. M., González F. and Pabón-Mora N. (2023) “Spatio-temporal expression of candidate genes for nectar spur development in Tropaeolum (Tropaeolaceae: Brassicales)” Annals of Botany.  Available at: https://doi.org/10.1093/aob/mcad164

Cover. Tropaeolum tuberosum. Image: Teresa Grau Ros / Wikimedia Commons