Changes in environmental conditions can result in remarkable variations in the expression of phenotypes and the induction of differentiation processes. An important factor distinguishing Saccharomyces cerevisiae from many other yeast species is the role of environmental signals in the induction of sexual processes. Natural isolates of S. cerevisiae are primarily diploid (diplontic lifestyle). An environmental cue (nitrogen starvation) is required to induce them to sporulate, but the resulting haploids can then go through mating-type switching and mating without any additional environmental input. In contrast, many other yeasts such as the methylotrophic species Hansenula polymorpha are primarily haploid (haplontic lifestyle). In these species mating-type switching, mating, and sporulation are coordinately induced by nitrogen limitation. Mating-type switching in H. polymorpha occurs through a two-locus chromosomal inversion mechanism that is ancestral to the well-characterized three-locus MAT/HML/HMR mechanism in S. cerevisiae. Although nitrogen limitation has been identified as the primary nutritional signal, the molecular pathway from this signal to mating-type switching in H. polymorpha is unknown. We surveyed H. polymorpha strains and found one strain that is unable to switch mating-types. This strain was crossed with a switching strain and bulk segregant analysis revealed a frameshift in the transcription factor EFG1, homolog of S. cerevisiae PHD1 and SOK2, as the causative mutation. Although EFG1 has a well-characterized role as a key regulator of filamentous growth and the white-opaque phenotypic switch in Candida albicans, it was not previously known to have a direct role in mating in any species. Through functional genetics and mRNA-seq analysis, we have found that EFG1 is part of a transcriptional network that integrates components of the S. cerevisiae mating and C. albicans differentiation pathways to control mating-type switching and mating responses in H. polymorpha.