Pollen polymorphism in Magnoliaceae and conservation of plant male germline regulators

In flowering plants twin sperm cells are produced by the male gametophyte to enable double fertilization, which is of vital importance for plant fertility. The process of male gametogenesis depends upon relatively poorly understood mechanisms of gamete cell production, but the male germline-specific R2R3 MYB transcription factor DUO1 is an essential factor in germ cell division and gamete differentiation in Arabidopsis. One major objective of the thesis was to assess the validity of Brewbaker’s evidence that Magnoliaceae possess exclusively bicellular pollen at anthesis. This analysis provided conclusive evidence of the coexistence of polymorphic (bicellular and tricellular) pollen in Magnoliaceae species. Further, the maturation of pollen under increased temperature regimes can significantly accelerate the rate of generative cell division in M. grandiflora. A second major aim was to identify sequence and functional conservation of DUO1 orthologs genes in angiosperms and bryophytes. This study provided further evidence that a supernumerary lysine residue in the DUO1 MYB domain is widely conserved and the sequences and expression of two DUO1 orthologs from moss (Physcomitrella patens) were validated. Arabidopsis, tomato and rice DUO1 orthologs were shown to transactivate Arabidopsis DUO1 target promoters. The expression of angiosperm DUO1 orthologs were able to complement the generative cell division and genetic transmission defects in duo1 mutants, demonstrating conservation of function of DUO1 orthologs in Arabidopsis. The final objective was to investigate the functional conservation of DAZ1/DAZ2–related proteins in other angiosperms. The AtDAZ1 orthologs from Brassica rapa (BrDAZ1) and tomato (SlDAZ1) were able to partially rescue the division and transmission defects of daz1/daz2 double mutant pollen, allowing daz1/daz2 homozygous lines to be established harbouring heterologous DAZ1 transgenes. Collectively, these results show conserved functions for both DUO1 and DAZ1/DAZ2 orthologs, highlighting the important role of the transcriptional network controlled by these male germline-specific regulators in from angiosperms.