Drivers of C4 evolution in the grasses

Abstract

Biologists have long been fascinated by the biological complexity that exists today, with organisms displaying a remarkable diversity of adaptations that have enabled them to thrive in almost every environment. Many of the most impressive adaptations would be classified as complex traits, with structural and metabolic elements working in synergy. To evolve, multiple elements need to be rewired, which can be achieved by incremental modifications over successive generations. Understanding evolutionary steps leading to the emergence of a complex trait can help solve the evolutionary complexity puzzle. The C4 photosynthetic pathway is a complex adaptation involving multiple biochemical and anatomical changes that have allowed certain plant lineages to succeed in specific environmental conditions, including warm and arid climates. Here, we aimed to investigate the evolutionary modifications required to construct the C4 cycle, a complex trait. I initially focused on examining the remarkable photosynthetic variation in the grass Alloteropsis semialata, a species unique in having both C4 and non-C4 photosynthetic genotypes. In my research, I conducted a comparative analysis to identify leaf traits associated with the proportion of carbon fixed using the C4 cycle. The findings revealed that plants with higher C4 activity generally have a greater ratio of photosynthetically active bundle sheath tissue. Subsequently, I employed a genome-wide association study (GWAS) to identify several candidate genes associated with the strength of the C4 cycle and enhancing the proportion of bundle sheath tissue. Finally, in the final chapter of my research, I adopted a more comprehensive perspective to investigate the genetic precursors that may have enabled the repeated evolution of C4 photosynthesis in the PACMAD grasses. I found that genes associated with cell wall modifications and stomatal aperture were duplicated at the base of the PACMAD clade, potentially facilitating the repeated convergent evolution of Kranz anatomy. Collectively, all three data chapters highlight the importance of anatomical modifications in the emergence of the C4 cycle. Overall my thesis identifies several key changes required for C4 evolution, and the results may be relevant to engineering C4 in C3 species, such as rice.