The architecture surrounding the sex locus is characterised as a synteny of genes for TPT/HMG/RNA helicase and the genomes of currently known Mucoralean fungi harbour this synteny. However, the details of the organisation of this synteny vary across species (Fig. 2a): (i) for example, in P. blakesleeanus, the sexP and sexM genes are divergently transcribed, whereas they are convergently transcribed in M. circinelloides, M. mucedo, R. oryzae and

S. megalocarpus; (ii) the arbA gene is incorporated between the sexP and tptA genes in R. oryzae or between sexM and tptA in S. megalocarpus; (iii) a repetitive element is found in the sex locus of the (+) mating type of P. blakesleeanus and (iv) partially divergent

Torin 1 ic50 rnhA genes are flanked by the sexM and sexP genes in S. megalocarpus. In addition, a comparison of the sex locus within the M. circinelloides subspecies complex revealed that the border of the sex locus spans the promoter of the tptA gene in M. circinelloides f. griseocyanus; on the other hand, the tptA promoter is not part of the sex locus in M. circinelloides f. lusitanicus or M. circinelloides f. circinelloides (Fig. 2b). Interestingly, the rnhA gene that flanks the sex locus in S. megalocarpus is adjacent to a glutathione oxidoreductase gene (glrA) that is divergent in the two mating type alleles, in which the (−) sex locus associated glrA is a pseudogene lacking the

first 676 bp Z-VAD-FMK in the first Tyrosine-protein kinase BLK exon, whereas the (+) sex locus associated glrA gene is intact.[27] Thus, the evolutionary trajectory of the sex locus has been punctuated by gene gain/loss, erosion or expansion of its borders, gene inversions, and invasions by repetitive elements. The divergent sex loci found in the Mucoralean fungi supports Ohno’s hypotheses on early stages and stepwise sex chromosome evolution (Fig. 3).[34] First, a sex determinant gene arises on an autosomal chromosome. Second, one allele undergoes a gene inversion that suppresses recombination, resulting in a pair of inverted genes that then diverge. The divergently transcribed sexP and sexM genes in P. blakesleeanus provide evidence for this step. The divergent but convergently transcribed sexP and sexM genes in the other Mucorales species may represent another round of gene inversion, or an ancestral step prior to gene inversion [reviewed in [35]]. Third, integration(s) of a repetitive element on the primitive sex chromosome occurs, and the repetitive element is then transposed into one sex allele and additional inversions between the two or more repetitive elements result in expansion of the sex locus throughout a substantial area of the chromosome. These later two steps are largely supported by the findings on the structure of the sex locus of P.