Septin evolution following gene duplication. Ryan S. O'Neill, Denise V. Clark. Biology, University of New Brunswick, Fredericton, New Brunswick, Canada.

   Septins are cytoskeletal components that assemble into oligomeric complexes and polymers, associate with cell membranes, actin filaments and microtubules, and can act as a scaffold for recruiting proteins and preventing diffusion of membrane bound proteins. In Drosophila, septins are involved in many biological processes, including cellularization during embryogenesis. Drosophila melanogaster has 5 septins: Sep1, Sep2, pnut, Sep4, and Sep5. Based on phylogenetic analysis, Sep1, pnut, and Sep4 are group 2B septins, whereas Sep2 and Sep5 are group 1B septins. Sep5 arose via retrotransposition of Sep2, between 62.2 and 62.9 Mya, and is not found outside of the subgenus Sophophora. Our work aims to assess functional redundancy and diversification of Sep2 and Sep5. Since retrotransposition does not duplicate transcriptional regulatory elements, we investigated the evolution of Sep2 and Sep5 expression patterns across the sequenced Drosophila species using in situ hybridization. Sep2 is expressed ubiquitously during embryogenesis; this pattern is conserved across species, regardless of the presence of Sep5, suggesting that Sep2 has maintained its ancestral function. The expression pattern of Sep5 has diversified across species, possibly indicating prolonged functional diversification. However, Sep5 is always co-expressed with ubiquitous Sep2, and both Sep2 and Sep5 interact with Sep1 and pnut, so it is not clear if these paralogs have different functions. Selection analyses of coding regions do not reveal clear patterns of positive selection acting specifically on either Sep2 or Sep5; most codons appear to be under pervasive purifying selection. However, multiple sequence alignment of Sep2 and Sep5 reveals that ~14% of amino acid positions were substituted early in the evolution of Sep5, and these amino acid differences between Sep2 and Sep5 are highly conserved; these differences include amino acids in the G1 and G3 GTPase domains, the Sep1 motif, and the N-terminal coiled coil domain.