Roles Of Canonical MicroRNA Pathway In Female Drosophila Larval Gonad Development

The Drosophila female larval gonad morphogenesis mainly involves coordinated development of somatic and germ cell lineages that are essential for forming a correct number of niche-germline stem cell(GSC)units(ovarioles)in the adult ovary.At early larval stages,both somatic gonadal precursors(SGPs)and primordial germ cells(PGCs)proliferate.The number of PGCs increases from about 12 PGCs in each embryonic gonad to about 100 by the middle of third instar(ML3).Starting from the ML3,terminal filament cell(TFC)of the somatic lineage differentiation initiates,and 16~20 TF stacks are formed by late third larval instar(LL3).Once TFs and Cap cells induced at the base of TF stacks are formed at this late larval stage,PGCs that are not attached to the niche differentiate to form the germline cysts.Ecdysone,Insulin,Activin,BMP and EGFR signaling pathways form a regulatory network that orchestrates gonadal soma and germ line throughout larval development.Undoubtedly,identification and characterization of additional genes or machineries involved in this process will provide more insights into the underlying mechanisms.Canonical MicroRNA(miRNA)pathway is responsible for miRNA biosynthesis and functions,thereby functioning in a variety of cellular and biological processes.Particularly,while the core canonical miRNA pathway components Ago-1,Dcr-1,Drosha and Pasha are required for oocyte formation and germline division,in addition,Dcr-1,Loqs and Ago-1 have been found to regulate ovarian GSC cell fate.To investigate the possible function and mechanism of the canonical miRNA pathway in female larval gonad development,we carried out genetic analysis and molecular detection of relevant mutations.The results showed that drosha or pasha mutants display defective proliferation of PGCs,the precursors of GSCs prior to LL3 and promoted PGC differentiation at LL3.Further studies demonstrate that Drosha and Pasha act both cell-autonomously and non cell-autonomously in PGC differentiation control,and the molecular and genetic studies led us to propose that Drosha and Pasha at least partly through a BMP/Bam-independent pathway in this process.In the mean time,loss of Drosha or Pasha function perturbs somatic precursor development,causing defects in formation of TFs,as well as in TF precursor accumulation(proliferation)at early larval stages.In addition,we examined the gene mutation phenotype of other core canonical miRNA pathway components synchronously.Comparative analysis of the mutant phenotypes reveal that Dcr-1,Loqs and Ago-1 have similar effects as Drosha and Pasha that both somatic and germ cell lineages have developmental defects.The results above suggested a role of the canonical miRNA pathway in the larval gonad development.Given the key role of the canonical miRNA pathway in miRNA biogenesis,this study also analyzed and identified the functional miRNAs which may be involved in gonad development.High-throughput screening based on the microarray analysis and genetic studies identify a set of Drosha-controlled miRNAs including miR-8,miR-14,miR-33,miR-184,miR-317 and let-7-C that function in female larval gonad,and the miRNA gene mutations can lead to the abnormal development of somatic and germ cell lineages.Taken together,this study discovered that miRNA-mediated regulation is involved in the female Drosophila larval gonad development,and then confirmed that in addition to ecdysone and other systemic factors,as well as BMP and EGFR signaling pathways,the miRNA-mediated regulation also involved in gonad morphogenesis.More importantly,our studies provide an ideal layer of miRNA-mediated regulation underlies the development of organs.