PgmNr D1436: Partial loss of function in the Drosophila melanogaster septin gene Sep5.

Authors:
R. S. O'Neill; D. V. Clark


Institutes
University of New Brunswick, Fredericton, New Brunswick, CA.


Keyword: other ( gene duplication )

Abstract:

Gene duplication is a major evolutionary process. One mechanism of gene duplication, retroduplication, generates a retrogene via retrotransposition of a parent gene’s mRNA. Retroduplication is thought to facilitate the evolution of novel functions by separating a parent gene’s coding sequence from its untranscribed regulatory region. Septins are a family of cytoskeletal proteins that form heter-oligomeric, rod-like complexes which can further assemble into higher-order structures. The septin family is divided into subgroups. The general model for septin complex assembly is that each position in the complex is occupied by a member of a specific subgroup, and thus subgroup members are thought to be interchangeable within the complex. Drosophila melanogaster has five septin genes, including Sep2 and its retrogene duplicate Sep5. Sep2 and Sep5 are the only D. melanogaster SEPT6 subgroup members. About 15% of amino acid sites in a multiple sequence alignment of Sep2 and Sep5 from 20 Drosophila species are conserved across orthologs but different between paralogs, suggesting functional diversification. We explored the functions of Sep2 and Sep5 to provide insight into the functional diversification of the complex-forming septin family. Sep5 mutants have no obvious phenotype. About half of Sep2 mutants die at the end of pupation when grown at 25°C, and those that emerge are sterile and have short eggs. About one fifth of Sep2 mutant egg chambers are fusions containing multiple germline cysts, suggesting a role for Sep2 in follicle cell encapsulation. A Sep5 transgene fails to rescue the Sep2 mutant egg chamber fusion phenotype, showing functional diversification of Sep2 and Sep5 proteins. Sep2 Sep5 double mutants lack imaginal discs and die as prepupae. Mosaic analysis showed that double mutant follicle cells can encapsulate germline cysts but fail to proliferate and die at mid-oogenesis, whereas double mutant germline cysts appear normal. Thus, Sep2 and Sep5 are redundant for proliferation of some cell types.  Sep2-GFP and Sep5-GFP localize similarly, including as double flanking rings at the germline ring canals, and at the follicle cell cortex.  Pnut localization, determined by immunostaining, is similar to Sep2-GFP and Sep5-GFP, and is maintained in both Sep2 and Sep5 single mutants but is lost in double mutants. The redundancy of Sep2 and Sep5 for Pnut recruitment suggests that they are interchangeable in and required for the formation of septin complexes. While we cannot rule out a unique function for Sep5 in an untested context, we suggest that Sep5 has undergone partial loss of its parent Sep2’s function; Sep5 has retained the ability to form part of the D. melanogaster septin complex, but has lost an ancestral follicle cell-specific function.



Flybase Genetic Index:
1. FlyBase gene symbol: Sep2; FBgn: FBgn0014029
2. FlyBase gene symbol: Sep5; FBgn: FBgn0026361
3. FlyBase gene symbol: pnut; FBgn: FBgn0013726