Mating Type Specific Roles During The Sexual Cycle Of Phycomyces Blakesleeanus
Phycomyces blakesleeanus is a filamentous fungus that belongs in the order Mucorales. It can propagate through both sexual and asexual reproduction. The asexual structures of Phycomyces called sporangiophores have served as a model for phototropism and many other sensory aspects. The MadA-MadB protein complex (homologs of WC proteins) is essential for phototropism. Light also inhibits sexual reproduction in P. blakesleeanus but the mechanism by which inhibition occurs has remained uncharacterized. In this study the role of the MadA-MadB complex was tested. Three genes that are required for pheromone biosynthesis or cell type determination in the sex locus are regulated by light, and require MadA and MadB. However, this regulation acts through only one of the two mating types, plus (+), by inhibiting the expression of the sexP gene that encodes an HMG-domain transcription factor that confers the (+) mating type properties. This suggests that the inhibitory effect of light on mating can be executed through the plus partner. These results provide an example of convergence in the mechanisms underlying signal transduction for mating in fungi. This study further investigated the role of mating type genes in the mating related process of mitochondrial inheritance. Due to isogamous fusion of cells in mating in this fungus, the initial expectation was that Phycomyces would demonstrate biparental inheritance but instead uniparental inheritance (UPI) was observed. Analysis of progeny from a series of genetic crosses between wild type strains of Phycomyces revealed a correlation between the individual genes in the mating type locus and UPI of mitochondria. Inheritance is from the (+) sex type and is associated with degradation of the mtDNA from the minus (−) parent. These findings suggest that UPI can be directly controlled by genes that determine sex identity, independent of cell size or the complexity of the genetic composition of a sex chromosome. Similar regulation may exist in other eukaryotes or was important at one stage of their evolution in establishing this system of inheritance. This investigation on the genetic regulation of mating and mating-related process of mitochondrial inheritance P. blakesleeanus demonstrates a prominent role of the sexP gene in both phenomena.
Table of Contents
Introduction -- Materials and methods -- The Mada-Madb photoreceptor complex inhibits mating by acting on the plus sex of phycomyces -- Uniparental mitochondrial inheritance in phycomyces -- Conclusions and future directions -- Appendix