Evolution and function of spliced leader (SL) trans-splicing in the chordates and deuterostomes

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The goal of this research is to understand the evolution and function of a poorly understood gene expression mechanism - SL trans-splicing. SL trans-splicing is a form of RNA splicing, a complex mechanism that plays a key role in the expression of most genes in complex cells and organisms. However unlike conventional RNA splicing, which cuts out segments of a single RNA molecule and joins them together, trans-splicing involves the cutting and joining of two entirely independent RNA molecules. Conventional splicing is universal among complex organisms, but SL trans-splicing is not. Some groups do it, and some groups don't. The distribution of SL trans-splicing in the evolutionary tree of animal life is sporadic. This distribution indicates that either SL trans-splicing is an ancient mechanism that has been lost in many groups, or it is a mechanism that has arisen independenly several different times during animal evolution. One of the goals of this research is to establish which of those evolutionary hypotheses is correct. Our studies are focused on the division of the animal evolutionary tree, the deuterostomes, that includes our phylum - the chordates, and our particular group within the chordates - the vertebrates. Vertebrates such as mammals or birds do not do SL trans-splicing, but we recently discovered SL trans-splicing in a simple chordate group, the tunicates. We will determine whether other deuterostome and chordate groups apart from the tunicates carry out trans-splicing. The pattern will reveal whether trans-splicing was invented in the tunicates, or lost in the vertebrates. We will also carry out experimental studies in tunicates to clarify the genetic functions of trans-splicing. The information from this research will provide novel insight into the evolutionary dynamics of gene expression mechanisms and on the impact of evolutionary change in gene expression mechanisms on the evolution of the genome.