Diversity, ecology, and evolution in Phycomyces

Fungal Evolution: Mucor and Phycomyces See Double

Newly available genome sequences of two Mucoralean fungi, Phycomyces blakesleeanus and Mucor circinelloides, provide evidence for an ancient whole-genome duplication that contributed to the generation of expanded gene families. These fungi have robust responses to light that can be correlated with the expansion of gene networks involved in light sensing and signaling.

Development of a plasmid free CRISPR-Cas9 system for the genetic modification of Mucor circinelloides

Mucor circinelloides and other members of Mucorales are filamentous fungi, widely used as model organisms in basic and applied studies. Although genetic manipulation methods have been described for some Mucoral fungi, construction of stable integrative transformants by homologous recombination has remained a great challenge in these organisms. In the present study, a plasmid free CRISPR-Cas9 system was firstly developed for the genetic modification of a Mucoral fungus. The described method offers a rapid but robust tool to obtain mitotically stable mutants of M. circinelloides via targeted integration of the desired DNA. It does not require plasmid construction and its expression in the recipient organism. Instead, it involves the direct introduction of the guide RNA and the Cas9 enzyme and, in case of homology directed repair (HDR), the template DNA into the recipient strain. Efficiency of the method for non-homologous end joining (NHEJ) and HDR was tested by disrupting two different genes, i.e. carB encoding phytoene dehydrogenase and hmgR2 encoding 3-hydroxy-3-methylglutaryl-CoA reductase, of M. circinelloides. Both NHEJ and HDR resulted in stable gene disruption mutants. While NHEJ caused extensive deletions upstream from the protospacer adjacent motif, HDR assured the integration of the deletion cassette at the targeted site.

Fungal Sex: The Mucoromycota

Although at the level of resolution of genes and molecules most information about mating in fungi is from a single lineage, the Dikarya, many fundamental discoveries about mating in fungi have been made in the earlier branches of the fungi. These are nonmonophyletic groups that were once classified into the chytrids and zygomycetes. Few species in these lineages offer the potential of genetic tractability, thereby hampering the ability to identify the genes that underlie those fundamental insights. Research performed during the past decade has now established the genes required for mating type determination and pheromone synthesis in some species in the phylum Mucoromycota, especially in the order Mucorales. These findings provide striking parallels with the evolution of mating systems in the Dikarya fungi. Other discoveries in the Mucorales provide the first examples of sex-cell type identity being driven directly by a gene that confers mating type, a trait considered more of relevance to animal sex determination but difficult to investigate in animals. Despite these discoveries, there remains much to be gleaned about mating systems from these fungi.

The inhibition of mating in Phycomyces blakesleeanus by light is dependent on the MadA-MadB complex that acts in a sex-specific manner

Light is an environmental signal that influences reproduction in the Mucoromycotina fungi, as it does in many other species of fungi. Mating in Phycomyces blakesleeanus is inhibited by light, but the molecular mechanisms for this inhibition are uncharacterized. In this analysis, the role of the light-sensing MadA-MadB complex in mating was tested. The MadA-MadB complex is homologous to the Neurospora crassa White Collar complex. Three genes required for cell type determination in the sex locus or pheromone biosynthesis are transcriptionally-regulated by light and are controlled by MadA and MadB. This regulation acts through the plus partner, indicating that the inhibitory effect of light on mating is executed through only one of the two sexes. These results are an example whereby the mating types of fungi have acquired sex-specific properties beyond their role in conferring cell-type identity, and provide insight into how sex-determining chromosomal regions can expand the traits they control.

Cyclofarnesoids and methylhexanoids produced from β-carotene in Phycomyces blakesleeanus

The oxidative cleavage of β-carotene in the Mucorales produces three fragments of 18, 15, and 7 carbons, respective heads of three families of apocarotenoids: the methylhexanoids, the trisporoids, and the cyclofarnesoids (named after their 1,6-cyclofarnesane skeleton). The apocarotenoids are easily recognized because they are absent in white mutants unable to produce β-carotene. In cultures of Phycomyces blakesleeanus we detected thirty-two apocarotenoids by LC, UV absorbance, and MS. With additional IR and NMR we identified two methylhexanoids and the eight most abundant cyclofarnesoids. Four of them were previously-unknown natural compounds, including 4-dihydrocyclofarnesine S, the most abundant cyclofarnesoid in young cultures. We arranged the apocarotenoids of the Mucorales in a scheme that helps classifying and naming them and suggests possible metabolites and biosynthetic pathways. We propose specific biosynthetic pathways for cyclofarnesoids and methylhexanoids based on structural comparisons, the time course of appearance of individual compounds, and the bioconversion of β-apo-12-carotenol, an early precursor, to three more oxygenated cyclofarnesoids by the white mutants. Some of the reactions occur spontaneously in the increasingly acidic culture media. Mating increased the contents of methylhexanoids and cyclofarnesoids by ca. threefold in young cultures and ca. twelvefold in old ones (five days); cyclofarnesine S, the most abundant cyclofarnesoid in old cultures, increased over one hundredfold. We found no differences between the sexes and no activity as sexual pheromones, but we suggest that methylhexanoids and cyclofarnesoids could mediate species-specific physiology and behavior.