Atypical flagella assembly and haploid genome coiling during male gamete formation in Plasmodium

Plasmodium spp. sexual reproduction occurs within the Anopheles mosquito and is essential for gametogenesis and onwards transmission to mammalian hosts. Upon activation, the male P. berghei gametocyte undergoes three rounds of inter-nuclear mitosis and assembles eight basal bodies and axonemes around the nucleus prior to ex-flagellation, resulting in 8 flagellated male gametes in 12-15 minutes. However there is little understanding of the 3D organisation of this rapid process of male gametogenesis. In this study we used serial block face scanning electron microscopy (SBF-SEM) and cellular electron tomography (ssET) of P. berghei microgametocytes to examine the 3D architecture of key structures during male gamete formation. Our data has revealed an exquisite organisation of axonemes coiling around the nucleus in opposite directions forming a central ‘axoneme band’ in microgametocytes. Furthermore, we discovered that the nucleus of microgametes is tightly coiled around the axoneme in an exquisitely complex structure whose formation starts before microgamete emergence during ex-flagellation. Our discoveries of the detailed 3D organisation of the flagellated microgamete and the haploid genome highlights some of the atypical mechanisms of axoneme assembly and haploid genome organisation during male gamete formation in the malaria parasite Plasmodium spp .